From 1aee49be9a87ff77edb8e0924bb5ae9d20b8e204 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Fri, 21 Feb 2025 21:58:37 +0900
Subject: [PATCH 01/22] Refactor codes

---
 .../codepair/v1/models/model_hello_request.go |  1 -
 backend-go/internal/core/handlers.go          | 26 -----------
 backend-go/internal/core/hello/handler.go     | 45 ++++++++++++-------
 backend-go/internal/core/hello/model.go       |  1 +
 backend-go/internal/core/hello/repository.go  |  7 ++-
 backend-go/internal/core/hello/server.go      | 15 +++++++
 backend-go/internal/core/hello/service.go     | 27 ++++++-----
 .../internal/infra/database/mongo/hello.go    | 37 ++++++++++++---
 backend-go/internal/server/routes.go          | 12 -----
 backend-go/internal/server/server.go          |  6 +--
 10 files changed, 97 insertions(+), 80 deletions(-)
 delete mode 100644 backend-go/internal/core/handlers.go
 create mode 100644 backend-go/internal/core/hello/server.go
 delete mode 100644 backend-go/internal/server/routes.go

diff --git a/backend-go/api/codepair/v1/models/model_hello_request.go b/backend-go/api/codepair/v1/models/model_hello_request.go
index 453ccf26..13ff94c2 100644
--- a/backend-go/api/codepair/v1/models/model_hello_request.go
+++ b/backend-go/api/codepair/v1/models/model_hello_request.go
@@ -1,7 +1,6 @@
 package models
 
 type HelloRequest struct {
-
 	// New nickname to say hello
 	Nickname string `json:"nickname"`
 }
diff --git a/backend-go/internal/core/handlers.go b/backend-go/internal/core/handlers.go
deleted file mode 100644
index 1a2da3d3..00000000
--- a/backend-go/internal/core/handlers.go
+++ /dev/null
@@ -1,26 +0,0 @@
-package core
-
-import (
-	"github.com/yorkie-team/codepair/backend/internal/core/hello"
-	"github.com/yorkie-team/codepair/backend/internal/infra/database/mongo"
-)
-
-type Handlers struct {
-	Hello *hello.Handler
-}
-
-// NewHandlers creates a new handlers.
-func NewHandlers() *Handlers {
-	// Repositories
-	helloRepository := mongo.NewHelloRepository()
-
-	// Services
-	helloService := hello.NewService(helloRepository)
-
-	// Handlers
-	helloHandler := hello.NewHandler(helloService)
-
-	return &Handlers{
-		Hello: helloHandler,
-	}
-}
diff --git a/backend-go/internal/core/hello/handler.go b/backend-go/internal/core/hello/handler.go
index 5dc96208..214d3991 100644
--- a/backend-go/internal/core/hello/handler.go
+++ b/backend-go/internal/core/hello/handler.go
@@ -1,6 +1,9 @@
 package hello
 
 import (
+	"fmt"
+	"strconv"
+
 	"github.com/labstack/echo/v4"
 
 	"github.com/yorkie-team/codepair/backend/api/codepair/v1/models"
@@ -8,32 +11,40 @@ import (
 )
 
 type Handler struct {
-	helloService *Service
-}
-
-// NewHandler creates a new handler for hello.
-func NewHandler(service *Service) *Handler {
-	return &Handler{
-		helloService: service,
-	}
+	service *Service
 }
 
-// HelloCodePair returns a hello message for a given CodePairVisitor.
-func (h *Handler) HelloCodePair(e echo.Context) error {
+// createHello returns a hello message for a given CodePairVisitor.
+func (h *Handler) createHello(c echo.Context) error {
 	req := new(models.HelloRequest)
 
-	if err := http.BindAndValidateRequest(e, req); err != nil {
-		return err
+	if err := http.BindAndValidateRequest(c, req); err != nil {
+		return http.NewErrorResponse(c, fmt.Errorf("invalid request: %w", err))
+	}
+
+	if err := h.service.createHello(CodePairVisitor{Nickname: req.Nickname}); err != nil {
+		return http.NewErrorResponse(c, fmt.Errorf("failed to create hello message for visitor %s: %w", req.Nickname, err))
 	}
 
-	helloMessage, err := h.helloService.HelloCodePair(e, CodePairVisitor{
-		Nickname: req.Nickname,
+	return http.NewOkResponse(c, models.HelloResponse{
+		Message: fmt.Sprintf("Hello, %s!", req.Nickname),
 	})
+}
+
+// readHello returns a hello message for a given CodePairVisitor.
+func (h *Handler) readHello(c echo.Context) error {
+	idStr := c.Param("id")
+	id, err := strconv.Atoi(idStr)
+	if err != nil {
+		return http.NewErrorResponse(c, fmt.Errorf("invalid ID format: %s", idStr))
+	}
+
+	nickname, err := h.service.readNickname(id)
 	if err != nil {
-		return http.NewErrorResponse(e, err)
+		return http.NewErrorResponse(c, fmt.Errorf("failed to find hello message for visitor with ID %d: %w", id, err))
 	}
 
-	return http.NewOkResponse(e, models.HelloResponse{
-		Message: helloMessage,
+	return http.NewOkResponse(c, models.HelloResponse{
+		Message: fmt.Sprintf("Hello, %s!", nickname),
 	})
 }
diff --git a/backend-go/internal/core/hello/model.go b/backend-go/internal/core/hello/model.go
index 3acf1bd1..dbde36c1 100644
--- a/backend-go/internal/core/hello/model.go
+++ b/backend-go/internal/core/hello/model.go
@@ -3,5 +3,6 @@ package hello
 // CodePairVisitor is a visitor for CodePair
 type CodePairVisitor struct {
 	// Nickname is the nickname of the visitor
+	ID       int
 	Nickname string
 }
diff --git a/backend-go/internal/core/hello/repository.go b/backend-go/internal/core/hello/repository.go
index 5a718599..85c256d5 100644
--- a/backend-go/internal/core/hello/repository.go
+++ b/backend-go/internal/core/hello/repository.go
@@ -1,6 +1,9 @@
 package hello
 
 type Repository interface {
-	// ReadHelloMessageFor reads a hello message for a given CodePairVisitor
-	ReadHelloMessageFor(codePairVisitor CodePairVisitor) (string, error)
+	// FindHelloMessage find a hello message for a given CodePairVisitor
+	FindHelloMessage(id int) (CodePairVisitor, error)
+
+	// CreateHelloMessage creates a hello message for given CodePairVisitor
+	CreateHelloMessage(codePairVisitor CodePairVisitor) error
 }
diff --git a/backend-go/internal/core/hello/server.go b/backend-go/internal/core/hello/server.go
new file mode 100644
index 00000000..ea456698
--- /dev/null
+++ b/backend-go/internal/core/hello/server.go
@@ -0,0 +1,15 @@
+package hello
+
+import "github.com/labstack/echo/v4"
+
+// New creates a new handler for hello.
+func New(e *echo.Echo, repo Repository) {
+	svc := &Service{
+		repository: repo,
+	}
+	handler := &Handler{
+		service: svc,
+	}
+	e.GET("/hello", handler.readHello)
+	e.POST("/hello", handler.createHello)
+}
diff --git a/backend-go/internal/core/hello/service.go b/backend-go/internal/core/hello/service.go
index 98777f4d..c717941f 100644
--- a/backend-go/internal/core/hello/service.go
+++ b/backend-go/internal/core/hello/service.go
@@ -1,29 +1,28 @@
 package hello
 
 import (
-	"github.com/labstack/echo/v4"
-
-	"github.com/yorkie-team/codepair/backend/internal/transport/http"
+	"fmt"
 )
 
 type Service struct {
-	helloRepository Repository
+	repository Repository
 }
 
-// NewService creates a new service for hello.
-func NewService(repository Repository) *Service {
-	return &Service{
-		helloRepository: repository,
+// createHello returns a hello message for a given CodePairVisitor
+func (s *Service) createHello(codePairVisitor CodePairVisitor) error {
+	if err := s.repository.CreateHelloMessage(codePairVisitor); err != nil {
+		return fmt.Errorf("failed to create hello message for visitor %v: %w", codePairVisitor, err)
 	}
+
+	return nil
 }
 
-// HelloCodePair returns a hello message for a given CodePairVisitor
-func (s *Service) HelloCodePair(e echo.Context, codePairVisitor CodePairVisitor) (string, error) {
-	helloMessage, err := s.helloRepository.ReadHelloMessageFor(codePairVisitor)
+// readNickname returns a hello message for a given CodePairVisitor
+func (s *Service) readNickname(id int) (string, error) {
+	hello, err := s.repository.FindHelloMessage(id)
 	if err != nil {
-		e.Logger().Fatal(err)
-		return "", http.ErrInternalServerError
+		return "", fmt.Errorf("failed to find hello message for ID %d: %w", id, err)
 	}
 
-	return helloMessage, nil
+	return hello.Nickname, nil
 }
diff --git a/backend-go/internal/infra/database/mongo/hello.go b/backend-go/internal/infra/database/mongo/hello.go
index 52aa5d7c..16d23eec 100644
--- a/backend-go/internal/infra/database/mongo/hello.go
+++ b/backend-go/internal/infra/database/mongo/hello.go
@@ -1,14 +1,41 @@
 package mongo
 
-import "github.com/yorkie-team/codepair/backend/internal/core/hello"
+import (
+	"time"
+
+	"github.com/yorkie-team/codepair/backend/internal/core/hello"
+)
+
+type CodePairVisitor struct {
+	ID        int
+	Nickname  string
+	CreatedAt time.Time
+	UpdatedAt time.Time
+}
+
+func (c *CodePairVisitor) ToDomain() hello.CodePairVisitor {
+	return hello.CodePairVisitor{
+		ID:       c.ID,
+		Nickname: c.Nickname,
+	}
+}
 
 type HelloRepository struct{}
 
 // NewHelloRepository creates a new HelloRepository.
-func NewHelloRepository() HelloRepository {
-	return HelloRepository{}
+func NewHelloRepository() *HelloRepository {
+	return &HelloRepository{}
+}
+
+func (h *HelloRepository) FindHelloMessage(id int) (hello.CodePairVisitor, error) {
+	he := &CodePairVisitor{
+		ID:       id,
+		Nickname: "find",
+	}
+	return he.ToDomain(), nil
 }
 
-func (h HelloRepository) ReadHelloMessageFor(codePairVisitor hello.CodePairVisitor) (string, error) {
-	return "Hello, " + codePairVisitor.Nickname + "!", nil
+func (h *HelloRepository) CreateHelloMessage(codePairVisitor hello.CodePairVisitor) error {
+
+	return nil
 }
diff --git a/backend-go/internal/server/routes.go b/backend-go/internal/server/routes.go
deleted file mode 100644
index d9fc1ec2..00000000
--- a/backend-go/internal/server/routes.go
+++ /dev/null
@@ -1,12 +0,0 @@
-package server
-
-import (
-	"github.com/labstack/echo/v4"
-
-	"github.com/yorkie-team/codepair/backend/internal/core"
-)
-
-// RegisterRoutes registers routes for the server.
-func RegisterRoutes(e *echo.Echo, handlers *core.Handlers) {
-	e.POST("/hello", handlers.Hello.HelloCodePair)
-}
diff --git a/backend-go/internal/server/server.go b/backend-go/internal/server/server.go
index 4da21f18..583ca960 100644
--- a/backend-go/internal/server/server.go
+++ b/backend-go/internal/server/server.go
@@ -8,7 +8,8 @@ import (
 	"github.com/labstack/echo/v4"
 
 	"github.com/yorkie-team/codepair/backend/internal/config"
-	"github.com/yorkie-team/codepair/backend/internal/core"
+	"github.com/yorkie-team/codepair/backend/internal/core/hello"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database/mongo"
 )
 
 type CodePair struct {
@@ -18,8 +19,7 @@ type CodePair struct {
 
 // New creates a new CodePair server.
 func New(e *echo.Echo, conf *config.Config) *CodePair {
-	handlers := core.NewHandlers()
-	RegisterRoutes(e, handlers)
+	hello.New(e, mongo.NewHelloRepository())
 
 	cp := &CodePair{
 		config: conf,

From d6e733323aeab804006dcf67806e5ad4aee88091 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Fri, 21 Feb 2025 22:18:44 +0900
Subject: [PATCH 02/22] Lint

---
 backend-go/internal/infra/database/mongo/.gitkeep | 1 -
 backend-go/internal/infra/database/mongo/hello.go | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)
 delete mode 100644 backend-go/internal/infra/database/mongo/.gitkeep

diff --git a/backend-go/internal/infra/database/mongo/.gitkeep b/backend-go/internal/infra/database/mongo/.gitkeep
deleted file mode 100644
index 8b137891..00000000
--- a/backend-go/internal/infra/database/mongo/.gitkeep
+++ /dev/null
@@ -1 +0,0 @@
-
diff --git a/backend-go/internal/infra/database/mongo/hello.go b/backend-go/internal/infra/database/mongo/hello.go
index 16d23eec..b94d9306 100644
--- a/backend-go/internal/infra/database/mongo/hello.go
+++ b/backend-go/internal/infra/database/mongo/hello.go
@@ -35,7 +35,7 @@ func (h *HelloRepository) FindHelloMessage(id int) (hello.CodePairVisitor, error
 	return he.ToDomain(), nil
 }
 
-func (h *HelloRepository) CreateHelloMessage(codePairVisitor hello.CodePairVisitor) error {
+func (h *HelloRepository) CreateHelloMessage(_ hello.CodePairVisitor) error {
 
 	return nil
 }

From 50783b956a44c286c141f16a34a976a37e414bd1 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Sun, 23 Feb 2025 11:25:05 +0900
Subject: [PATCH 03/22] Add docker-compose

---
 backend-go/docker/docekr-compose.yml | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)
 create mode 100644 backend-go/docker/docekr-compose.yml

diff --git a/backend-go/docker/docekr-compose.yml b/backend-go/docker/docekr-compose.yml
new file mode 100644
index 00000000..0e207cfe
--- /dev/null
+++ b/backend-go/docker/docekr-compose.yml
@@ -0,0 +1,22 @@
+version: "3.8"
+
+services:
+  mongo:
+    build:
+      context: ./mongodb_replica
+      args:
+        MONGO_VERSION: 4
+    environment:
+      MONGO_REPLICA_HOST: 127.0.0.1
+      MONGO_REPLICA_PORT: 27017
+      MONGO_INITDB_DATABASE: "codepair"
+      MONGO_COMMAND: "mongo"
+    ports:
+      - "27017:27017"
+    restart: unless-stopped
+    healthcheck:
+      test:
+        ["CMD", "mongo", "admin", "--port", "27017", "--eval", "db.adminCommand('ping').ok"]
+      interval: 5s
+      timeout: 2s
+      retries: 20

From 38a9a3e2e022f4debd132afdc743a9aa15e76f9d Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Sun, 23 Feb 2025 11:25:57 +0900
Subject: [PATCH 04/22] Add mongo connection

---
 backend-go/go.mod                             |  1 +
 backend-go/go.sum                             |  2 ++
 .../internal/infra/database/mongodb/client.go | 33 +++++++++++++++++++
 3 files changed, 36 insertions(+)
 create mode 100644 backend-go/internal/infra/database/mongodb/client.go

diff --git a/backend-go/go.mod b/backend-go/go.mod
index 1c898c67..676db2eb 100644
--- a/backend-go/go.mod
+++ b/backend-go/go.mod
@@ -33,6 +33,7 @@ require (
 	github.com/subosito/gotenv v1.6.0 // indirect
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
 	github.com/valyala/fasttemplate v1.2.2 // indirect
+	go.mongodb.org/mongo-driver/v2 v2.0.1 // indirect
 	go.uber.org/atomic v1.9.0 // indirect
 	go.uber.org/multierr v1.9.0 // indirect
 	golang.org/x/crypto v0.32.0 // indirect
diff --git a/backend-go/go.sum b/backend-go/go.sum
index 21b9d179..9e610a21 100644
--- a/backend-go/go.sum
+++ b/backend-go/go.sum
@@ -75,6 +75,8 @@ github.com/valyala/bytebufferpool v1.0.0 h1:GqA5TC/0021Y/b9FG4Oi9Mr3q7XYx6Kllzaw
 github.com/valyala/bytebufferpool v1.0.0/go.mod h1:6bBcMArwyJ5K/AmCkWv1jt77kVWyCJ6HpOuEn7z0Csc=
 github.com/valyala/fasttemplate v1.2.2 h1:lxLXG0uE3Qnshl9QyaK6XJxMXlQZELvChBOCmQD0Loo=
 github.com/valyala/fasttemplate v1.2.2/go.mod h1:KHLXt3tVN2HBp8eijSv/kGJopbvo7S+qRAEEKiv+SiQ=
+go.mongodb.org/mongo-driver/v2 v2.0.1 h1:mhB/ZJkLSv6W6LGzY7sEjpZif47+JdfEEXjlLCIv7Qc=
+go.mongodb.org/mongo-driver/v2 v2.0.1/go.mod h1:w7iFnTcQDMXtdXwcvyG3xljYpoBa1ErkI0yOzbkZ9b8=
 go.uber.org/atomic v1.9.0 h1:ECmE8Bn/WFTYwEW/bpKD3M8VtR/zQVbavAoalC1PYyE=
 go.uber.org/atomic v1.9.0/go.mod h1:fEN4uk6kAWBTFdckzkM89CLk9XfWZrxpCo0nPH17wJc=
 go.uber.org/multierr v1.9.0 h1:7fIwc/ZtS0q++VgcfqFDxSBZVv/Xo49/SYnDFupUwlI=
diff --git a/backend-go/internal/infra/database/mongodb/client.go b/backend-go/internal/infra/database/mongodb/client.go
new file mode 100644
index 00000000..0328eef5
--- /dev/null
+++ b/backend-go/internal/infra/database/mongodb/client.go
@@ -0,0 +1,33 @@
+package mongodb
+
+import (
+	"context"
+	"fmt"
+
+	"go.mongodb.org/mongo-driver/v2/mongo"
+	"go.mongodb.org/mongo-driver/v2/mongo/options"
+	"go.mongodb.org/mongo-driver/v2/mongo/readpref"
+
+	"github.com/yorkie-team/codepair/backend/internal/config"
+)
+
+// Dial creates an instance of Mongo and dials the given MongoDB.
+func Dial(conf *config.Mongo) (*mongo.Client, error) {
+	client, err := mongo.Connect(
+		options.Client().
+			ApplyURI(conf.ConnectionURI).
+			SetConnectTimeout(conf.ConnectionTimeout),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("connect to mongo: %w", err)
+	}
+
+	ctxPing, cancel := context.WithTimeout(context.Background(), conf.PingTimeout)
+	defer cancel()
+
+	if err := client.Ping(ctxPing, readpref.Primary()); err != nil {
+		return nil, fmt.Errorf("ping mongo: %w", err)
+	}
+
+	return client, nil
+}

From 19a0bc32559fe94aed3eb95b9570617375aa4e34 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Sun, 23 Feb 2025 13:55:00 +0900
Subject: [PATCH 05/22] Refactor

- remove repository in `core`
- `core` imports `database`
- `mongodb` imports `database`
- add CRUD operations about `hello`
---
 backend-go/cmd/codepair/main.go               |   5 +-
 backend-go/internal/core/hello/handler.go     |  52 +++++--
 backend-go/internal/core/hello/model.go       |   8 --
 backend-go/internal/core/hello/repository.go  |   9 --
 backend-go/internal/core/hello/server.go      |  16 ++-
 backend-go/internal/core/hello/service.go     |  48 +++++--
 backend-go/internal/infra/database/hello.go   |  30 ++++
 .../internal/infra/database/mongo/hello.go    |  41 ------
 .../internal/infra/database/mongodb/hello.go  | 130 ++++++++++++++++++
 backend-go/internal/server/server.go          |  13 +-
 10 files changed, 258 insertions(+), 94 deletions(-)
 delete mode 100644 backend-go/internal/core/hello/model.go
 delete mode 100644 backend-go/internal/core/hello/repository.go
 create mode 100644 backend-go/internal/infra/database/hello.go
 delete mode 100644 backend-go/internal/infra/database/mongo/hello.go
 create mode 100644 backend-go/internal/infra/database/mongodb/hello.go

diff --git a/backend-go/cmd/codepair/main.go b/backend-go/cmd/codepair/main.go
index 71b26bb6..94fd8142 100644
--- a/backend-go/cmd/codepair/main.go
+++ b/backend-go/cmd/codepair/main.go
@@ -24,7 +24,10 @@ func main() {
 		e.Logger.Fatal(err)
 	}
 
-	cp := server.New(e, conf)
+	cp, err := server.New(e, conf)
+	if err != nil {
+		e.Logger.Fatal(err)
+	}
 
 	if err := cp.Start(); err != nil {
 		e.Logger.Fatal(err)
diff --git a/backend-go/internal/core/hello/handler.go b/backend-go/internal/core/hello/handler.go
index 214d3991..60bcce55 100644
--- a/backend-go/internal/core/hello/handler.go
+++ b/backend-go/internal/core/hello/handler.go
@@ -2,7 +2,6 @@ package hello
 
 import (
 	"fmt"
-	"strconv"
 
 	"github.com/labstack/echo/v4"
 
@@ -10,41 +9,64 @@ import (
 	"github.com/yorkie-team/codepair/backend/internal/transport/http"
 )
 
+// Handler handles HTTP requests for hello messages.
 type Handler struct {
 	service *Service
 }
 
-// createHello returns a hello message for a given CodePairVisitor.
+// createHello handles the POST /hello endpoint.
+// It creates a new visitor record and returns a hello message.
 func (h *Handler) createHello(c echo.Context) error {
 	req := new(models.HelloRequest)
 
 	if err := http.BindAndValidateRequest(c, req); err != nil {
 		return http.NewErrorResponse(c, fmt.Errorf("invalid request: %w", err))
 	}
-
-	if err := h.service.createHello(CodePairVisitor{Nickname: req.Nickname}); err != nil {
-		return http.NewErrorResponse(c, fmt.Errorf("failed to create hello message for visitor %s: %w", req.Nickname, err))
+	if err := h.service.createHello(req); err != nil {
+		return http.NewErrorResponse(c, err)
 	}
-
 	return http.NewOkResponse(c, models.HelloResponse{
 		Message: fmt.Sprintf("Hello, %s!", req.Nickname),
 	})
 }
 
-// readHello returns a hello message for a given CodePairVisitor.
+// readHello handles the GET /hello/:id endpoint.
+// It retrieves a visitor record by its id and returns a hello message.
 func (h *Handler) readHello(c echo.Context) error {
-	idStr := c.Param("id")
-	id, err := strconv.Atoi(idStr)
-	if err != nil {
-		return http.NewErrorResponse(c, fmt.Errorf("invalid ID format: %s", idStr))
-	}
-
+	id := c.Param("id")
 	nickname, err := h.service.readNickname(id)
 	if err != nil {
-		return http.NewErrorResponse(c, fmt.Errorf("failed to find hello message for visitor with ID %d: %w", id, err))
+		return http.NewErrorResponse(c, fmt.Errorf("failed to find hello message for visitor with ID %s: %w", id, err))
 	}
-
 	return http.NewOkResponse(c, models.HelloResponse{
 		Message: fmt.Sprintf("Hello, %s!", nickname),
 	})
 }
+
+// updateHello handles the PUT /hello/:id endpoint.
+// It updates an existing visitor record and returns a confirmation message.
+func (h *Handler) updateHello(c echo.Context) error {
+	id := c.Param("id")
+	req := new(models.HelloRequest)
+	if err := http.BindAndValidateRequest(c, req); err != nil {
+		return http.NewErrorResponse(c, fmt.Errorf("invalid request: %w", err))
+	}
+	if err := h.service.updateHello(id, req); err != nil {
+		return http.NewErrorResponse(c, err)
+	}
+	return http.NewOkResponse(c, models.HelloResponse{
+		Message: fmt.Sprintf("Hello updated, new nickname: %s", req.Nickname),
+	})
+}
+
+// deleteHello handles the DELETE /hello/:id endpoint.
+// It deletes a visitor record by its id and returns a confirmation message.
+func (h *Handler) deleteHello(c echo.Context) error {
+	id := c.Param("id")
+	if err := h.service.deleteHello(id); err != nil {
+		return http.NewErrorResponse(c, err)
+	}
+	return http.NewOkResponse(c, map[string]string{
+		"message": fmt.Sprintf("Visitor with id %s deleted", id),
+	})
+}
diff --git a/backend-go/internal/core/hello/model.go b/backend-go/internal/core/hello/model.go
deleted file mode 100644
index dbde36c1..00000000
--- a/backend-go/internal/core/hello/model.go
+++ /dev/null
@@ -1,8 +0,0 @@
-package hello
-
-// CodePairVisitor is a visitor for CodePair
-type CodePairVisitor struct {
-	// Nickname is the nickname of the visitor
-	ID       int
-	Nickname string
-}
diff --git a/backend-go/internal/core/hello/repository.go b/backend-go/internal/core/hello/repository.go
deleted file mode 100644
index 85c256d5..00000000
--- a/backend-go/internal/core/hello/repository.go
+++ /dev/null
@@ -1,9 +0,0 @@
-package hello
-
-type Repository interface {
-	// FindHelloMessage find a hello message for a given CodePairVisitor
-	FindHelloMessage(id int) (CodePairVisitor, error)
-
-	// CreateHelloMessage creates a hello message for given CodePairVisitor
-	CreateHelloMessage(codePairVisitor CodePairVisitor) error
-}
diff --git a/backend-go/internal/core/hello/server.go b/backend-go/internal/core/hello/server.go
index ea456698..f16a6efa 100644
--- a/backend-go/internal/core/hello/server.go
+++ b/backend-go/internal/core/hello/server.go
@@ -1,15 +1,21 @@
 package hello
 
-import "github.com/labstack/echo/v4"
+import (
+	"github.com/labstack/echo/v4"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database"
+)
 
-// New creates a new handler for hello.
-func New(e *echo.Echo, repo Repository) {
+// New creates a new handler for hello endpoints and registers the routes.
+func New(e *echo.Echo, repo database.Hello) {
 	svc := &Service{
-		repository: repo,
+		repo: repo,
 	}
 	handler := &Handler{
 		service: svc,
 	}
-	e.GET("/hello", handler.readHello)
+
 	e.POST("/hello", handler.createHello)
+	e.GET("/hello/:id", handler.readHello)
+	e.PUT("/hello/:id", handler.updateHello)
+	e.DELETE("/hello/:id", handler.deleteHello)
 }
diff --git a/backend-go/internal/core/hello/service.go b/backend-go/internal/core/hello/service.go
index c717941f..af2cbff1 100644
--- a/backend-go/internal/core/hello/service.go
+++ b/backend-go/internal/core/hello/service.go
@@ -2,27 +2,53 @@ package hello
 
 import (
 	"fmt"
+
+	"github.com/yorkie-team/codepair/backend/api/codepair/v1/models"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database"
 )
 
+// Service provides business logic for handling hello messages.
 type Service struct {
-	repository Repository
+	repo database.Hello
 }
 
-// createHello returns a hello message for a given CodePairVisitor
-func (s *Service) createHello(codePairVisitor CodePairVisitor) error {
-	if err := s.repository.CreateHelloMessage(codePairVisitor); err != nil {
-		return fmt.Errorf("failed to create hello message for visitor %v: %w", codePairVisitor, err)
+// createHello creates a new visitor record based on the provided hello request.
+func (s *Service) createHello(req *models.HelloRequest) error {
+	visitor := database.Visitor{
+		Nickname: req.Nickname,
+	}
+	if err := s.repo.CreateVisitor(visitor); err != nil {
+		return fmt.Errorf("failed to create hello message for visitor %v: %w", req, err)
 	}
-
 	return nil
 }
 
-// readNickname returns a hello message for a given CodePairVisitor
-func (s *Service) readNickname(id int) (string, error) {
-	hello, err := s.repository.FindHelloMessage(id)
+// readNickname retrieves the nickname of a visitor record by its unique identifier.
+func (s *Service) readNickname(id string) (string, error) {
+	visitor, err := s.repo.FindVisitor(id)
 	if err != nil {
-		return "", fmt.Errorf("failed to find hello message for ID %d: %w", id, err)
+		return "", fmt.Errorf("failed to find hello message for ID %s: %w", id, err)
 	}
+	return visitor.Nickname, nil
+}
 
-	return hello.Nickname, nil
+// updateHello updates an existing visitor record with new data from the hello request.
+func (s *Service) updateHello(id string, req *models.HelloRequest) error {
+	visitor := database.Visitor{
+		ID:       id,
+		Nickname: req.Nickname,
+		// Note: CreatedAt is not modified here; the repository is responsible for setting UpdatedAt.
+	}
+	if err := s.repo.UpdateVisitor(visitor); err != nil {
+		return fmt.Errorf("failed to update hello message for visitor with ID %s: %w", id, err)
+	}
+	return nil
+}
+
+// deleteHello removes a visitor record by its unique identifier.
+func (s *Service) deleteHello(id string) error {
+	if err := s.repo.DeleteVisitor(id); err != nil {
+		return fmt.Errorf("failed to delete hello message for visitor with ID %s: %w", id, err)
+	}
+	return nil
 }
diff --git a/backend-go/internal/infra/database/hello.go b/backend-go/internal/infra/database/hello.go
new file mode 100644
index 00000000..52b1663a
--- /dev/null
+++ b/backend-go/internal/infra/database/hello.go
@@ -0,0 +1,30 @@
+package database
+
+import "time"
+
+// Visitor represents a visitor record in the hello repository.
+type Visitor struct {
+	ID        string    `bson:"_id,omitempty"` // Auto-generated by MongoDB if empty.
+	Nickname  string    `bson:"nickname"`      // Visitor's nickname.
+	CreatedAt time.Time `bson:"created_at"`    // Timestamp when the record was created.
+	UpdatedAt time.Time `bson:"updated_at"`    // Timestamp when the record was last updated.
+}
+
+// Hello defines the interface for CRUD operations on visitor records.
+type Hello interface {
+	// CreateVisitor creates a new visitor record.
+	// It accepts a Visitor object and returns an error if creation fails.
+	CreateVisitor(visitor Visitor) error
+
+	// FindVisitor retrieves a visitor record by its unique identifier.
+	// It returns the Visitor and an error if the record cannot be found.
+	FindVisitor(id string) (Visitor, error)
+
+	// UpdateVisitor updates an existing visitor record.
+	// It accepts a Visitor object (including its ID) and returns an error if the update fails.
+	UpdateVisitor(visitor Visitor) error
+
+	// DeleteVisitor removes a visitor record by its unique identifier.
+	// It returns an error if the deletion operation fails.
+	DeleteVisitor(id string) error
+}
diff --git a/backend-go/internal/infra/database/mongo/hello.go b/backend-go/internal/infra/database/mongo/hello.go
deleted file mode 100644
index b94d9306..00000000
--- a/backend-go/internal/infra/database/mongo/hello.go
+++ /dev/null
@@ -1,41 +0,0 @@
-package mongo
-
-import (
-	"time"
-
-	"github.com/yorkie-team/codepair/backend/internal/core/hello"
-)
-
-type CodePairVisitor struct {
-	ID        int
-	Nickname  string
-	CreatedAt time.Time
-	UpdatedAt time.Time
-}
-
-func (c *CodePairVisitor) ToDomain() hello.CodePairVisitor {
-	return hello.CodePairVisitor{
-		ID:       c.ID,
-		Nickname: c.Nickname,
-	}
-}
-
-type HelloRepository struct{}
-
-// NewHelloRepository creates a new HelloRepository.
-func NewHelloRepository() *HelloRepository {
-	return &HelloRepository{}
-}
-
-func (h *HelloRepository) FindHelloMessage(id int) (hello.CodePairVisitor, error) {
-	he := &CodePairVisitor{
-		ID:       id,
-		Nickname: "find",
-	}
-	return he.ToDomain(), nil
-}
-
-func (h *HelloRepository) CreateHelloMessage(_ hello.CodePairVisitor) error {
-
-	return nil
-}
diff --git a/backend-go/internal/infra/database/mongodb/hello.go b/backend-go/internal/infra/database/mongodb/hello.go
new file mode 100644
index 00000000..9aeb2c4c
--- /dev/null
+++ b/backend-go/internal/infra/database/mongodb/hello.go
@@ -0,0 +1,130 @@
+package mongodb
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"go.mongodb.org/mongo-driver/v2/bson"
+	"go.mongodb.org/mongo-driver/v2/mongo"
+
+	"github.com/yorkie-team/codepair/backend/internal/config"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database"
+)
+
+// ColVisitor is the name of the collection storing visitor records.
+const ColVisitor = "hello_visitors"
+
+// HelloRepository implements the CRUD operations for the hello service.
+type HelloRepository struct {
+	conf   *config.Mongo
+	client *mongo.Client
+}
+
+// NewHelloRepo creates a new instance of HelloRepository.
+func NewHelloRepo(conf *config.Mongo, client *mongo.Client) *HelloRepository {
+	return &HelloRepository{
+		conf:   conf,
+		client: client,
+	}
+}
+
+// CreateVisitor inserts a new visitor record into the database,
+// letting MongoDB auto-generate the _id field.
+func (r *HelloRepository) CreateVisitor(visitor database.Visitor) error {
+	now := time.Now()
+	visitor.CreatedAt = now
+	visitor.UpdatedAt = now
+	// Ensure ID is empty so that MongoDB will auto-generate it.
+	visitor.ID = ""
+
+	result, err := r.collection().InsertOne(context.Background(), visitor)
+	if err != nil {
+		return fmt.Errorf("failed to create visitor: %w", err)
+	}
+
+	// Optionally, update the visitor with the generated id.
+	if oid, ok := result.InsertedID.(bson.ObjectID); ok {
+		visitor.ID = oid.Hex()
+	}
+	return nil
+}
+
+// FindVisitor retrieves a visitor record by its id.
+func (r *HelloRepository) FindVisitor(id string) (database.Visitor, error) {
+	// Convert string id to ObjectID.
+	oid, err := bson.ObjectIDFromHex(id)
+	if err != nil {
+		return database.Visitor{}, fmt.Errorf("invalid id format: %w", err)
+	}
+
+	// Use an intermediate type to correctly decode the auto-generated ObjectID.
+	var resultModel struct {
+		ID        bson.ObjectID `bson:"_id"`
+		Nickname  string        `bson:"nickname"`
+		CreatedAt time.Time     `bson:"created_at"`
+		UpdatedAt time.Time     `bson:"updated_at"`
+	}
+
+	filter := bson.M{"_id": oid}
+	err = r.collection().FindOne(context.Background(), filter).Decode(&resultModel)
+	if err != nil {
+		return database.Visitor{}, fmt.Errorf("failed to find visitor: %w", err)
+	}
+
+	return database.Visitor{
+		ID:        resultModel.ID.Hex(),
+		Nickname:  resultModel.Nickname,
+		CreatedAt: resultModel.CreatedAt,
+		UpdatedAt: resultModel.UpdatedAt,
+	}, nil
+}
+
+// UpdateVisitor updates an existing visitor record.
+func (r *HelloRepository) UpdateVisitor(visitor database.Visitor) error {
+	oid, err := bson.ObjectIDFromHex(visitor.ID)
+	if err != nil {
+		return fmt.Errorf("invalid id format: %w", err)
+	}
+
+	visitor.UpdatedAt = time.Now()
+	filter := bson.M{"_id": oid}
+	update := bson.M{
+		"$set": bson.M{
+			"nickname":   visitor.Nickname,
+			"updated_at": visitor.UpdatedAt,
+		},
+	}
+
+	result, err := r.collection().UpdateOne(context.Background(), filter, update)
+	if err != nil {
+		return fmt.Errorf("failed to update visitor: %w", err)
+	}
+	if result.MatchedCount == 0 {
+		return fmt.Errorf("no visitor found with id %s", visitor.ID)
+	}
+	return nil
+}
+
+// DeleteVisitor removes a visitor record by its id.
+func (r *HelloRepository) DeleteVisitor(id string) error {
+	oid, err := bson.ObjectIDFromHex(id)
+	if err != nil {
+		return fmt.Errorf("invalid id format: %w", err)
+	}
+
+	filter := bson.M{"_id": oid}
+	result, err := r.collection().DeleteOne(context.Background(), filter)
+	if err != nil {
+		return fmt.Errorf("failed to delete visitor: %w", err)
+	}
+	if result.DeletedCount == 0 {
+		return fmt.Errorf("no visitor found with id %s", id)
+	}
+	return nil
+}
+
+// collection returns a reference to the MongoDB collection for visitor records.
+func (r *HelloRepository) collection() *mongo.Collection {
+	return r.client.Database(r.conf.DatabaseName).Collection(ColVisitor)
+}
diff --git a/backend-go/internal/server/server.go b/backend-go/internal/server/server.go
index 583ca960..45a561fe 100644
--- a/backend-go/internal/server/server.go
+++ b/backend-go/internal/server/server.go
@@ -9,7 +9,7 @@ import (
 
 	"github.com/yorkie-team/codepair/backend/internal/config"
 	"github.com/yorkie-team/codepair/backend/internal/core/hello"
-	"github.com/yorkie-team/codepair/backend/internal/infra/database/mongo"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database/mongodb"
 )
 
 type CodePair struct {
@@ -18,14 +18,19 @@ type CodePair struct {
 }
 
 // New creates a new CodePair server.
-func New(e *echo.Echo, conf *config.Config) *CodePair {
-	hello.New(e, mongo.NewHelloRepository())
+func New(e *echo.Echo, conf *config.Config) (*CodePair, error) {
+	db, err := mongodb.Dial(conf.Mongo)
+	if err != nil {
+		return nil, fmt.Errorf("failed to dial mongo: %w", err)
+	}
+
+	hello.New(e, mongodb.NewHelloRepo(conf.Mongo, db))
 
 	cp := &CodePair{
 		config: conf,
 		echo:   e,
 	}
-	return cp
+	return cp, nil
 }
 
 // Start starts the server.

From a1c22746be08b46be1eb905e444dd32bbd609e53 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Sun, 23 Feb 2025 13:55:34 +0900
Subject: [PATCH 06/22] Renew openapi spec

- add CURD operations to `hello`
---
 api/backend-openapi-spec.json                 | 116 +++++++++++++++++-
 .../codepair/v1/models/model_hello_request.go |   1 +
 2 files changed, 111 insertions(+), 6 deletions(-)

diff --git a/api/backend-openapi-spec.json b/api/backend-openapi-spec.json
index 9ce83346..9e1f9214 100644
--- a/api/backend-openapi-spec.json
+++ b/api/backend-openapi-spec.json
@@ -3,10 +3,9 @@
   "paths": {
     "/hello": {
       "post": {
-        "operationId": "helloCodePair",
-        "summary": "Example API",
-        "description": "Sample API endpoint for CodePair",
-        "parameters": [],
+        "operationId": "createHello",
+        "summary": "Create Hello Message",
+        "description": "Creates a new hello message for a visitor.",
         "requestBody": {
           "required": true,
           "content": {
@@ -18,8 +17,113 @@
           }
         },
         "responses": {
-          "default": {
-            "description": "",
+          "200": {
+            "description": "Hello message created successfully.",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/HelloResponse"
+                }
+              }
+            }
+          }
+        },
+        "tags": [
+          "Hello"
+        ]
+      }
+    },
+    "/hello/{id}": {
+      "get": {
+        "operationId": "readHello",
+        "summary": "Get Hello Message",
+        "description": "Retrieves a hello message for the visitor with the given ID.",
+        "parameters": [
+          {
+            "name": "id",
+            "in": "path",
+            "description": "Unique identifier for the visitor",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Hello message retrieved successfully.",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/HelloResponse"
+                }
+              }
+            }
+          }
+        },
+        "tags": [
+          "Hello"
+        ]
+      },
+      "put": {
+        "operationId": "updateHello",
+        "summary": "Update Hello Message",
+        "description": "Updates the hello message for the visitor with the given ID.",
+        "parameters": [
+          {
+            "name": "id",
+            "in": "path",
+            "description": "Unique identifier for the visitor",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/HelloRequest"
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "Hello message updated successfully.",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/HelloResponse"
+                }
+              }
+            }
+          }
+        },
+        "tags": [
+          "Hello"
+        ]
+      },
+      "delete": {
+        "operationId": "deleteHello",
+        "summary": "Delete Hello Message",
+        "description": "Deletes the hello message for the visitor with the given ID.",
+        "parameters": [
+          {
+            "name": "id",
+            "in": "path",
+            "description": "Unique identifier for the visitor",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Hello message deleted successfully.",
             "content": {
               "application/json": {
                 "schema": {
diff --git a/backend-go/api/codepair/v1/models/model_hello_request.go b/backend-go/api/codepair/v1/models/model_hello_request.go
index 13ff94c2..453ccf26 100644
--- a/backend-go/api/codepair/v1/models/model_hello_request.go
+++ b/backend-go/api/codepair/v1/models/model_hello_request.go
@@ -1,6 +1,7 @@
 package models
 
 type HelloRequest struct {
+
 	// New nickname to say hello
 	Nickname string `json:"nickname"`
 }

From 16bdc06dcb987bf19b9349f6feb6c84ef2402fc0 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Mon, 24 Feb 2025 14:13:28 +0900
Subject: [PATCH 07/22] Use collection as singleton

---
 .../internal/infra/database/mongodb/hello.go  | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/backend-go/internal/infra/database/mongodb/hello.go b/backend-go/internal/infra/database/mongodb/hello.go
index 9aeb2c4c..4689cc82 100644
--- a/backend-go/internal/infra/database/mongodb/hello.go
+++ b/backend-go/internal/infra/database/mongodb/hello.go
@@ -17,15 +17,13 @@ const ColVisitor = "hello_visitors"
 
 // HelloRepository implements the CRUD operations for the hello service.
 type HelloRepository struct {
-	conf   *config.Mongo
-	client *mongo.Client
+	collection *mongo.Collection
 }
 
 // NewHelloRepo creates a new instance of HelloRepository.
 func NewHelloRepo(conf *config.Mongo, client *mongo.Client) *HelloRepository {
 	return &HelloRepository{
-		conf:   conf,
-		client: client,
+		collection: client.Database(conf.DatabaseName).Collection(ColVisitor),
 	}
 }
 
@@ -38,7 +36,7 @@ func (r *HelloRepository) CreateVisitor(visitor database.Visitor) error {
 	// Ensure ID is empty so that MongoDB will auto-generate it.
 	visitor.ID = ""
 
-	result, err := r.collection().InsertOne(context.Background(), visitor)
+	result, err := r.collection.InsertOne(context.Background(), visitor)
 	if err != nil {
 		return fmt.Errorf("failed to create visitor: %w", err)
 	}
@@ -67,7 +65,7 @@ func (r *HelloRepository) FindVisitor(id string) (database.Visitor, error) {
 	}
 
 	filter := bson.M{"_id": oid}
-	err = r.collection().FindOne(context.Background(), filter).Decode(&resultModel)
+	err = r.collection.FindOne(context.Background(), filter).Decode(&resultModel)
 	if err != nil {
 		return database.Visitor{}, fmt.Errorf("failed to find visitor: %w", err)
 	}
@@ -96,7 +94,7 @@ func (r *HelloRepository) UpdateVisitor(visitor database.Visitor) error {
 		},
 	}
 
-	result, err := r.collection().UpdateOne(context.Background(), filter, update)
+	result, err := r.collection.UpdateOne(context.Background(), filter, update)
 	if err != nil {
 		return fmt.Errorf("failed to update visitor: %w", err)
 	}
@@ -114,7 +112,7 @@ func (r *HelloRepository) DeleteVisitor(id string) error {
 	}
 
 	filter := bson.M{"_id": oid}
-	result, err := r.collection().DeleteOne(context.Background(), filter)
+	result, err := r.collection.DeleteOne(context.Background(), filter)
 	if err != nil {
 		return fmt.Errorf("failed to delete visitor: %w", err)
 	}
@@ -123,8 +121,3 @@ func (r *HelloRepository) DeleteVisitor(id string) error {
 	}
 	return nil
 }
-
-// collection returns a reference to the MongoDB collection for visitor records.
-func (r *HelloRepository) collection() *mongo.Collection {
-	return r.client.Database(r.conf.DatabaseName).Collection(ColVisitor)
-}

From 23d5e4f770ee6234239929241f3800467499b0a1 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Mon, 24 Feb 2025 19:33:13 +0900
Subject: [PATCH 08/22] Use `ID` and register codec

- for mongoDB use `database` type directly
---
 backend-go/internal/core/hello/service.go     |  6 +--
 backend-go/internal/infra/database/hello.go   | 10 ++--
 backend-go/internal/infra/database/id.go      |  3 ++
 .../internal/infra/database/mongodb/client.go |  3 +-
 .../internal/infra/database/mongodb/hello.go  | 53 ++++---------------
 .../infra/database/mongodb/registry.go        | 47 ++++++++++++++++
 6 files changed, 70 insertions(+), 52 deletions(-)
 create mode 100644 backend-go/internal/infra/database/id.go
 create mode 100644 backend-go/internal/infra/database/mongodb/registry.go

diff --git a/backend-go/internal/core/hello/service.go b/backend-go/internal/core/hello/service.go
index af2cbff1..d79d71f9 100644
--- a/backend-go/internal/core/hello/service.go
+++ b/backend-go/internal/core/hello/service.go
@@ -25,7 +25,7 @@ func (s *Service) createHello(req *models.HelloRequest) error {
 
 // readNickname retrieves the nickname of a visitor record by its unique identifier.
 func (s *Service) readNickname(id string) (string, error) {
-	visitor, err := s.repo.FindVisitor(id)
+	visitor, err := s.repo.FindVisitor(database.ID(id))
 	if err != nil {
 		return "", fmt.Errorf("failed to find hello message for ID %s: %w", id, err)
 	}
@@ -35,7 +35,7 @@ func (s *Service) readNickname(id string) (string, error) {
 // updateHello updates an existing visitor record with new data from the hello request.
 func (s *Service) updateHello(id string, req *models.HelloRequest) error {
 	visitor := database.Visitor{
-		ID:       id,
+		ID:       database.ID(id),
 		Nickname: req.Nickname,
 		// Note: CreatedAt is not modified here; the repository is responsible for setting UpdatedAt.
 	}
@@ -47,7 +47,7 @@ func (s *Service) updateHello(id string, req *models.HelloRequest) error {
 
 // deleteHello removes a visitor record by its unique identifier.
 func (s *Service) deleteHello(id string) error {
-	if err := s.repo.DeleteVisitor(id); err != nil {
+	if err := s.repo.DeleteVisitor(database.ID(id)); err != nil {
 		return fmt.Errorf("failed to delete hello message for visitor with ID %s: %w", id, err)
 	}
 	return nil
diff --git a/backend-go/internal/infra/database/hello.go b/backend-go/internal/infra/database/hello.go
index 52b1663a..134fb19c 100644
--- a/backend-go/internal/infra/database/hello.go
+++ b/backend-go/internal/infra/database/hello.go
@@ -1,10 +1,12 @@
 package database
 
-import "time"
+import (
+	"time"
+)
 
 // Visitor represents a visitor record in the hello repository.
 type Visitor struct {
-	ID        string    `bson:"_id,omitempty"` // Auto-generated by MongoDB if empty.
+	ID        ID        `bson:"_id,omitempty"` // Auto-generated by MongoDB if empty.
 	Nickname  string    `bson:"nickname"`      // Visitor's nickname.
 	CreatedAt time.Time `bson:"created_at"`    // Timestamp when the record was created.
 	UpdatedAt time.Time `bson:"updated_at"`    // Timestamp when the record was last updated.
@@ -18,7 +20,7 @@ type Hello interface {
 
 	// FindVisitor retrieves a visitor record by its unique identifier.
 	// It returns the Visitor and an error if the record cannot be found.
-	FindVisitor(id string) (Visitor, error)
+	FindVisitor(id ID) (Visitor, error)
 
 	// UpdateVisitor updates an existing visitor record.
 	// It accepts a Visitor object (including its ID) and returns an error if the update fails.
@@ -26,5 +28,5 @@ type Hello interface {
 
 	// DeleteVisitor removes a visitor record by its unique identifier.
 	// It returns an error if the deletion operation fails.
-	DeleteVisitor(id string) error
+	DeleteVisitor(id ID) error
 }
diff --git a/backend-go/internal/infra/database/id.go b/backend-go/internal/infra/database/id.go
new file mode 100644
index 00000000..ed35c895
--- /dev/null
+++ b/backend-go/internal/infra/database/id.go
@@ -0,0 +1,3 @@
+package database
+
+type ID string
diff --git a/backend-go/internal/infra/database/mongodb/client.go b/backend-go/internal/infra/database/mongodb/client.go
index 0328eef5..a67a4d3c 100644
--- a/backend-go/internal/infra/database/mongodb/client.go
+++ b/backend-go/internal/infra/database/mongodb/client.go
@@ -16,7 +16,8 @@ func Dial(conf *config.Mongo) (*mongo.Client, error) {
 	client, err := mongo.Connect(
 		options.Client().
 			ApplyURI(conf.ConnectionURI).
-			SetConnectTimeout(conf.ConnectionTimeout),
+			SetConnectTimeout(conf.ConnectionTimeout).
+			SetRegistry(NewRegistry()),
 	)
 	if err != nil {
 		return nil, fmt.Errorf("connect to mongo: %w", err)
diff --git a/backend-go/internal/infra/database/mongodb/hello.go b/backend-go/internal/infra/database/mongodb/hello.go
index 4689cc82..90b2c642 100644
--- a/backend-go/internal/infra/database/mongodb/hello.go
+++ b/backend-go/internal/infra/database/mongodb/hello.go
@@ -33,60 +33,30 @@ func (r *HelloRepository) CreateVisitor(visitor database.Visitor) error {
 	now := time.Now()
 	visitor.CreatedAt = now
 	visitor.UpdatedAt = now
-	// Ensure ID is empty so that MongoDB will auto-generate it.
-	visitor.ID = ""
 
-	result, err := r.collection.InsertOne(context.Background(), visitor)
+	_, err := r.collection.InsertOne(context.Background(), visitor)
 	if err != nil {
 		return fmt.Errorf("failed to create visitor: %w", err)
 	}
 
-	// Optionally, update the visitor with the generated id.
-	if oid, ok := result.InsertedID.(bson.ObjectID); ok {
-		visitor.ID = oid.Hex()
-	}
 	return nil
 }
 
 // FindVisitor retrieves a visitor record by its id.
-func (r *HelloRepository) FindVisitor(id string) (database.Visitor, error) {
-	// Convert string id to ObjectID.
-	oid, err := bson.ObjectIDFromHex(id)
-	if err != nil {
-		return database.Visitor{}, fmt.Errorf("invalid id format: %w", err)
-	}
-
-	// Use an intermediate type to correctly decode the auto-generated ObjectID.
-	var resultModel struct {
-		ID        bson.ObjectID `bson:"_id"`
-		Nickname  string        `bson:"nickname"`
-		CreatedAt time.Time     `bson:"created_at"`
-		UpdatedAt time.Time     `bson:"updated_at"`
-	}
-
-	filter := bson.M{"_id": oid}
-	err = r.collection.FindOne(context.Background(), filter).Decode(&resultModel)
-	if err != nil {
+func (r *HelloRepository) FindVisitor(id database.ID) (database.Visitor, error) {
+	var visitor database.Visitor
+	filter := bson.M{"_id": id}
+	if err := r.collection.FindOne(context.Background(), filter).Decode(&visitor); err != nil {
 		return database.Visitor{}, fmt.Errorf("failed to find visitor: %w", err)
 	}
 
-	return database.Visitor{
-		ID:        resultModel.ID.Hex(),
-		Nickname:  resultModel.Nickname,
-		CreatedAt: resultModel.CreatedAt,
-		UpdatedAt: resultModel.UpdatedAt,
-	}, nil
+	return visitor, nil
 }
 
 // UpdateVisitor updates an existing visitor record.
 func (r *HelloRepository) UpdateVisitor(visitor database.Visitor) error {
-	oid, err := bson.ObjectIDFromHex(visitor.ID)
-	if err != nil {
-		return fmt.Errorf("invalid id format: %w", err)
-	}
-
 	visitor.UpdatedAt = time.Now()
-	filter := bson.M{"_id": oid}
+	filter := bson.M{"_id": visitor.ID}
 	update := bson.M{
 		"$set": bson.M{
 			"nickname":   visitor.Nickname,
@@ -105,13 +75,8 @@ func (r *HelloRepository) UpdateVisitor(visitor database.Visitor) error {
 }
 
 // DeleteVisitor removes a visitor record by its id.
-func (r *HelloRepository) DeleteVisitor(id string) error {
-	oid, err := bson.ObjectIDFromHex(id)
-	if err != nil {
-		return fmt.Errorf("invalid id format: %w", err)
-	}
-
-	filter := bson.M{"_id": oid}
+func (r *HelloRepository) DeleteVisitor(id database.ID) error {
+	filter := bson.M{"_id": id}
 	result, err := r.collection.DeleteOne(context.Background(), filter)
 	if err != nil {
 		return fmt.Errorf("failed to delete visitor: %w", err)
diff --git a/backend-go/internal/infra/database/mongodb/registry.go b/backend-go/internal/infra/database/mongodb/registry.go
new file mode 100644
index 00000000..feb2a58d
--- /dev/null
+++ b/backend-go/internal/infra/database/mongodb/registry.go
@@ -0,0 +1,47 @@
+package mongodb
+
+import (
+	"reflect"
+
+	"github.com/yorkie-team/codepair/backend/internal/infra/database"
+
+	"go.mongodb.org/mongo-driver/v2/bson"
+)
+
+var tID = reflect.TypeOf(database.ID(""))
+
+func iDEncoder(_ bson.EncodeContext, vw bson.ValueWriter, val reflect.Value) error {
+	if val.Type() != tID {
+		return bson.ValueEncoderError{Name: "iDEncoder", Types: []reflect.Type{tID}, Received: val}
+	}
+
+	oid, err := bson.ObjectIDFromHex(val.String())
+	if err != nil {
+		return err
+	}
+
+	return vw.WriteObjectID(oid)
+}
+
+func iDDecoder(_ bson.DecodeContext, vr bson.ValueReader, val reflect.Value) error {
+	if val.Type() != tID {
+		return bson.ValueDecoderError{Name: "iDDecoder", Types: []reflect.Type{tID}, Received: val}
+	}
+
+	oid, err := vr.ReadObjectID()
+	if err != nil {
+		return err
+	}
+
+	val.SetString(oid.Hex())
+	return nil
+}
+
+func NewRegistry() *bson.Registry {
+	rb := bson.NewRegistry()
+
+	rb.RegisterTypeEncoder(tID, bson.ValueEncoderFunc(iDEncoder))
+	rb.RegisterTypeDecoder(tID, bson.ValueDecoderFunc(iDDecoder))
+
+	return rb
+}

From d8858e8e4c47da96d412caf49e22e7905a3a735a Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Mon, 24 Feb 2025 19:47:44 +0900
Subject: [PATCH 09/22] go mod tidy

- add mongo driver
---
 backend-go/go.mod | 11 +++++++++--
 backend-go/go.sum | 50 +++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 51 insertions(+), 10 deletions(-)

diff --git a/backend-go/go.mod b/backend-go/go.mod
index 676db2eb..2760c9c4 100644
--- a/backend-go/go.mod
+++ b/backend-go/go.mod
@@ -3,10 +3,12 @@ module github.com/yorkie-team/codepair/backend
 go 1.21
 
 require (
+	github.com/go-playground/validator/v10 v10.24.0
 	github.com/labstack/echo/v4 v4.13.3
 	github.com/labstack/gommon v0.4.2
 	github.com/spf13/viper v1.19.0
 	github.com/stretchr/testify v1.10.0
+	go.mongodb.org/mongo-driver/v2 v2.0.1
 )
 
 require (
@@ -15,8 +17,9 @@ require (
 	github.com/gabriel-vasile/mimetype v1.4.8 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
-	github.com/go-playground/validator/v10 v10.24.0 // indirect
+	github.com/golang/snappy v0.0.4 // indirect
 	github.com/hashicorp/hcl v1.0.0 // indirect
+	github.com/klauspost/compress v1.17.2 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/magiconair/properties v1.8.7 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
@@ -33,12 +36,16 @@ require (
 	github.com/subosito/gotenv v1.6.0 // indirect
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
 	github.com/valyala/fasttemplate v1.2.2 // indirect
-	go.mongodb.org/mongo-driver/v2 v2.0.1 // indirect
+	github.com/xdg-go/pbkdf2 v1.0.0 // indirect
+	github.com/xdg-go/scram v1.1.2 // indirect
+	github.com/xdg-go/stringprep v1.0.4 // indirect
+	github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78 // indirect
 	go.uber.org/atomic v1.9.0 // indirect
 	go.uber.org/multierr v1.9.0 // indirect
 	golang.org/x/crypto v0.32.0 // indirect
 	golang.org/x/exp v0.0.0-20230905200255-921286631fa9 // indirect
 	golang.org/x/net v0.34.0 // indirect
+	golang.org/x/sync v0.10.0 // indirect
 	golang.org/x/sys v0.29.0 // indirect
 	golang.org/x/text v0.21.0 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect
diff --git a/backend-go/go.sum b/backend-go/go.sum
index 9e610a21..3af7575c 100644
--- a/backend-go/go.sum
+++ b/backend-go/go.sum
@@ -8,16 +8,22 @@ github.com/fsnotify/fsnotify v1.7.0 h1:8JEhPFa5W2WU7YfeZzPNqzMP6Lwt7L2715Ggo0nos
 github.com/fsnotify/fsnotify v1.7.0/go.mod h1:40Bi/Hjc2AVfZrqy+aj+yEI+/bRxZnMJyTJwOpGvigM=
 github.com/gabriel-vasile/mimetype v1.4.8 h1:FfZ3gj38NjllZIeJAmMhr+qKL8Wu+nOoI3GqacKw1NM=
 github.com/gabriel-vasile/mimetype v1.4.8/go.mod h1:ByKUIKGjh1ODkGM1asKUbQZOLGrPjydw3hYPU2YU9t8=
+github.com/go-playground/assert/v2 v2.2.0 h1:JvknZsQTYeFEAhQwI4qEt9cyV5ONwRHC+lYKSsYSR8s=
+github.com/go-playground/assert/v2 v2.2.0/go.mod h1:VDjEfimB/XKnb+ZQfWdccd7VUvScMdVu0Titje2rxJ4=
 github.com/go-playground/locales v0.14.1 h1:EWaQ/wswjilfKLTECiXz7Rh+3BjFhfDFKv/oXslEjJA=
 github.com/go-playground/locales v0.14.1/go.mod h1:hxrqLVvrK65+Rwrd5Fc6F2O76J/NuW9t0sjnWqG1slY=
 github.com/go-playground/universal-translator v0.18.1 h1:Bcnm0ZwsGyWbCzImXv+pAJnYK9S473LQFuzCbDbfSFY=
 github.com/go-playground/universal-translator v0.18.1/go.mod h1:xekY+UJKNuX9WP91TpwSH2VMlDf28Uj24BCp08ZFTUY=
 github.com/go-playground/validator/v10 v10.24.0 h1:KHQckvo8G6hlWnrPX4NJJ+aBfWNAE/HH+qdL2cBpCmg=
 github.com/go-playground/validator/v10 v10.24.0/go.mod h1:GGzBIJMuE98Ic/kJsBXbz1x/7cByt++cQ+YOuDM5wus=
-github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
-github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/golang/snappy v0.0.4 h1:yAGX7huGHXlcLOEtBnF4w7FQwA26wojNCwOYAEhLjQM=
+github.com/golang/snappy v0.0.4/go.mod h1:/XxbfmMg8lxefKM7IXC3fBNl/7bRcc72aCRzEWrmP2Q=
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/hashicorp/hcl v1.0.0 h1:0Anlzjpi4vEasTeNFn2mLJgTSwt0+6sfsiTG8qcWGx4=
 github.com/hashicorp/hcl v1.0.0/go.mod h1:E5yfLk+7swimpb2L/Alb/PJmXilQ/rhwaUYs4T20WEQ=
+github.com/klauspost/compress v1.17.2 h1:RlWWUY/Dr4fL8qk9YG7DTZ7PDgME2V4csBXA8L/ixi4=
+github.com/klauspost/compress v1.17.2/go.mod h1:ntbaceVETuRiXiv4DpjP66DpAtAGkEQskQzEyD//IeE=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
@@ -75,30 +81,58 @@ github.com/valyala/bytebufferpool v1.0.0 h1:GqA5TC/0021Y/b9FG4Oi9Mr3q7XYx6Kllzaw
 github.com/valyala/bytebufferpool v1.0.0/go.mod h1:6bBcMArwyJ5K/AmCkWv1jt77kVWyCJ6HpOuEn7z0Csc=
 github.com/valyala/fasttemplate v1.2.2 h1:lxLXG0uE3Qnshl9QyaK6XJxMXlQZELvChBOCmQD0Loo=
 github.com/valyala/fasttemplate v1.2.2/go.mod h1:KHLXt3tVN2HBp8eijSv/kGJopbvo7S+qRAEEKiv+SiQ=
+github.com/xdg-go/pbkdf2 v1.0.0 h1:Su7DPu48wXMwC3bs7MCNG+z4FhcyEuz5dlvchbq0B0c=
+github.com/xdg-go/pbkdf2 v1.0.0/go.mod h1:jrpuAogTd400dnrH08LKmI/xc1MbPOebTwRqcT5RDeI=
+github.com/xdg-go/scram v1.1.2 h1:FHX5I5B4i4hKRVRBCFRxq1iQRej7WO3hhBuJf+UUySY=
+github.com/xdg-go/scram v1.1.2/go.mod h1:RT/sEzTbU5y00aCK8UOx6R7YryM0iF1N2MOmC3kKLN4=
+github.com/xdg-go/stringprep v1.0.4 h1:XLI/Ng3O1Atzq0oBs3TWm+5ZVgkq2aqdlvP9JtoZ6c8=
+github.com/xdg-go/stringprep v1.0.4/go.mod h1:mPGuuIYwz7CmR2bT9j4GbQqutWS1zV24gijq1dTyGkM=
+github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78 h1:ilQV1hzziu+LLM3zUTJ0trRztfwgjqKnBWNtSRkbmwM=
+github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78/go.mod h1:aL8wCCfTfSfmXjznFBSZNN13rSJjlIOI1fUNAtF7rmI=
+github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
 go.mongodb.org/mongo-driver/v2 v2.0.1 h1:mhB/ZJkLSv6W6LGzY7sEjpZif47+JdfEEXjlLCIv7Qc=
 go.mongodb.org/mongo-driver/v2 v2.0.1/go.mod h1:w7iFnTcQDMXtdXwcvyG3xljYpoBa1ErkI0yOzbkZ9b8=
 go.uber.org/atomic v1.9.0 h1:ECmE8Bn/WFTYwEW/bpKD3M8VtR/zQVbavAoalC1PYyE=
 go.uber.org/atomic v1.9.0/go.mod h1:fEN4uk6kAWBTFdckzkM89CLk9XfWZrxpCo0nPH17wJc=
 go.uber.org/multierr v1.9.0 h1:7fIwc/ZtS0q++VgcfqFDxSBZVv/Xo49/SYnDFupUwlI=
 go.uber.org/multierr v1.9.0/go.mod h1:X2jQV1h+kxSjClGpnseKVIxpmcjrj7MNnI0bnlfKTVQ=
-golang.org/x/crypto v0.31.0 h1:ihbySMvVjLAeSH1IbfcRTkD/iNscyz8rGzjF/E5hV6U=
-golang.org/x/crypto v0.31.0/go.mod h1:kDsLvtWBEx7MV9tJOj9bnXsPbxwJQ6csT/x4KIN4Ssk=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
 golang.org/x/crypto v0.32.0 h1:euUpcYgM8WcP71gNpTqQCn6rC2t6ULUPiOzfWaXVVfc=
 golang.org/x/crypto v0.32.0/go.mod h1:ZnnJkOaASj8g0AjIduWNlq2NRxL0PlBrbKVyZ6V/Ugc=
 golang.org/x/exp v0.0.0-20230905200255-921286631fa9 h1:GoHiUyI/Tp2nVkLI2mCxVkOjsbSXD66ic0XW0js0R9g=
 golang.org/x/exp v0.0.0-20230905200255-921286631fa9/go.mod h1:S2oDrQGGwySpoQPVqRShND87VCbxmc6bL1Yd2oYrm6k=
-golang.org/x/net v0.33.0 h1:74SYHlV8BIgHIFC/LrYkOGIwL19eTYXQ5wc6TBuO36I=
-golang.org/x/net v0.33.0/go.mod h1:HXLR5J+9DxmrqMwG9qjGCxZ+zKXxBru04zlTvWlWuN4=
+golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
+golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
+golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug+ECip1KBveYUHfp+8e9klMJ9c=
 golang.org/x/net v0.34.0 h1:Mb7Mrk043xzHgnRM88suvJFwzVrRfHEHJEl5/71CKw0=
 golang.org/x/net v0.34.0/go.mod h1:di0qlW3YNM5oh6GqDGQr92MyTozJPmybPK4Ev/Gm31k=
+golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.10.0 h1:3NQrjDixjgGwUOCaF8w2+VYHv0Ve/vGYSbdkTa98gmQ=
+golang.org/x/sync v0.10.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220811171246-fbc7d0a398ab/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.28.0 h1:Fksou7UEQUWlKvIdsqzJmUmCX3cZuD2+P3XyyzwMhlA=
-golang.org/x/sys v0.28.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
 golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
+golang.org/x/term v0.0.0-20210927222741-03fcf44c2211/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
+golang.org/x/text v0.3.8/go.mod h1:E6s5w1FMmriuDzIBO73fBruAKo1PCIq6d2Q6DHfQ8WQ=
 golang.org/x/text v0.21.0 h1:zyQAAkrwaneQ066sspRyJaG9VNi/YJ1NfzcGB3hZ/qo=
 golang.org/x/text v0.21.0/go.mod h1:4IBbMaMmOPCJ8SecivzSH54+73PCFmPWxNTLm+vZkEQ=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
+golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 h1:YR8cESwS4TdDjEe65xsg0ogRM/Nc3DYOhEAlW+xobZo=
 gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=

From 2bf24bc72e1cf15482317e6bd6d0f3e45b716817 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Mon, 24 Feb 2025 19:49:39 +0900
Subject: [PATCH 10/22] Fix typo correction

---
 backend-go/docker/{docekr-compose.yml => docker-compose.yml} | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 rename backend-go/docker/{docekr-compose.yml => docker-compose.yml} (100%)

diff --git a/backend-go/docker/docekr-compose.yml b/backend-go/docker/docker-compose.yml
similarity index 100%
rename from backend-go/docker/docekr-compose.yml
rename to backend-go/docker/docker-compose.yml

From c575a60a020f84f75924b6bbfa7592bd0469e322 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Mon, 24 Feb 2025 20:02:15 +0900
Subject: [PATCH 11/22] Fix create returns id

---
 backend-go/internal/core/hello/handler.go           |  5 +++--
 backend-go/internal/core/hello/service.go           |  9 +++++----
 backend-go/internal/infra/database/hello.go         |  2 +-
 backend-go/internal/infra/database/mongodb/hello.go | 12 ++++++++----
 4 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/backend-go/internal/core/hello/handler.go b/backend-go/internal/core/hello/handler.go
index 60bcce55..a71cf435 100644
--- a/backend-go/internal/core/hello/handler.go
+++ b/backend-go/internal/core/hello/handler.go
@@ -22,11 +22,12 @@ func (h *Handler) createHello(c echo.Context) error {
 	if err := http.BindAndValidateRequest(c, req); err != nil {
 		return http.NewErrorResponse(c, fmt.Errorf("invalid request: %w", err))
 	}
-	if err := h.service.createHello(req); err != nil {
+	id, err := h.service.createHello(req)
+	if err != nil {
 		return http.NewErrorResponse(c, err)
 	}
 	return http.NewOkResponse(c, models.HelloResponse{
-		Message: fmt.Sprintf("Hello, %s!", req.Nickname),
+		Message: fmt.Sprintf("Hello, id:%s, nickname: %s!", id, req.Nickname),
 	})
 }
 
diff --git a/backend-go/internal/core/hello/service.go b/backend-go/internal/core/hello/service.go
index d79d71f9..f1ebf9fa 100644
--- a/backend-go/internal/core/hello/service.go
+++ b/backend-go/internal/core/hello/service.go
@@ -13,14 +13,15 @@ type Service struct {
 }
 
 // createHello creates a new visitor record based on the provided hello request.
-func (s *Service) createHello(req *models.HelloRequest) error {
+func (s *Service) createHello(req *models.HelloRequest) (string, error) {
 	visitor := database.Visitor{
 		Nickname: req.Nickname,
 	}
-	if err := s.repo.CreateVisitor(visitor); err != nil {
-		return fmt.Errorf("failed to create hello message for visitor %v: %w", req, err)
+	visitor, err := s.repo.CreateVisitor(visitor)
+	if err != nil {
+		return "", fmt.Errorf("failed to create hello message for visitor %v: %w", req, err)
 	}
-	return nil
+	return string(visitor.ID), nil
 }
 
 // readNickname retrieves the nickname of a visitor record by its unique identifier.
diff --git a/backend-go/internal/infra/database/hello.go b/backend-go/internal/infra/database/hello.go
index 134fb19c..1eba7d33 100644
--- a/backend-go/internal/infra/database/hello.go
+++ b/backend-go/internal/infra/database/hello.go
@@ -16,7 +16,7 @@ type Visitor struct {
 type Hello interface {
 	// CreateVisitor creates a new visitor record.
 	// It accepts a Visitor object and returns an error if creation fails.
-	CreateVisitor(visitor Visitor) error
+	CreateVisitor(visitor Visitor) (Visitor, error)
 
 	// FindVisitor retrieves a visitor record by its unique identifier.
 	// It returns the Visitor and an error if the record cannot be found.
diff --git a/backend-go/internal/infra/database/mongodb/hello.go b/backend-go/internal/infra/database/mongodb/hello.go
index 90b2c642..e1f142a0 100644
--- a/backend-go/internal/infra/database/mongodb/hello.go
+++ b/backend-go/internal/infra/database/mongodb/hello.go
@@ -29,17 +29,21 @@ func NewHelloRepo(conf *config.Mongo, client *mongo.Client) *HelloRepository {
 
 // CreateVisitor inserts a new visitor record into the database,
 // letting MongoDB auto-generate the _id field.
-func (r *HelloRepository) CreateVisitor(visitor database.Visitor) error {
+func (r *HelloRepository) CreateVisitor(visitor database.Visitor) (database.Visitor, error) {
 	now := time.Now()
 	visitor.CreatedAt = now
 	visitor.UpdatedAt = now
 
-	_, err := r.collection.InsertOne(context.Background(), visitor)
+	res, err := r.collection.InsertOne(context.Background(), visitor)
 	if err != nil {
-		return fmt.Errorf("failed to create visitor: %w", err)
+		return database.Visitor{}, fmt.Errorf("failed to create visitor: %w", err)
 	}
 
-	return nil
+	if oid, ok := res.InsertedID.(bson.ObjectID); ok {
+		visitor.ID = database.ID(oid.String())
+	}
+
+	return visitor, nil
 }
 
 // FindVisitor retrieves a visitor record by its id.

From b83839520f8b9a59374988d26fdb9d031868bed9 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Mon, 24 Feb 2025 20:08:21 +0900
Subject: [PATCH 12/22] lint

---
 .../internal/infra/database/mongodb/registry.go    | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/backend-go/internal/infra/database/mongodb/registry.go b/backend-go/internal/infra/database/mongodb/registry.go
index feb2a58d..ac8845ad 100644
--- a/backend-go/internal/infra/database/mongodb/registry.go
+++ b/backend-go/internal/infra/database/mongodb/registry.go
@@ -1,11 +1,12 @@
 package mongodb
 
 import (
+	"fmt"
 	"reflect"
 
-	"github.com/yorkie-team/codepair/backend/internal/infra/database"
-
 	"go.mongodb.org/mongo-driver/v2/bson"
+
+	"github.com/yorkie-team/codepair/backend/internal/infra/database"
 )
 
 var tID = reflect.TypeOf(database.ID(""))
@@ -17,10 +18,13 @@ func iDEncoder(_ bson.EncodeContext, vw bson.ValueWriter, val reflect.Value) err
 
 	oid, err := bson.ObjectIDFromHex(val.String())
 	if err != nil {
-		return err
+		return fmt.Errorf("convert hex to ObjectID: %w", err)
 	}
 
-	return vw.WriteObjectID(oid)
+	if err = vw.WriteObjectID(oid); err != nil {
+		return fmt.Errorf("write ObjectID: %w", err)
+	}
+	return nil
 }
 
 func iDDecoder(_ bson.DecodeContext, vr bson.ValueReader, val reflect.Value) error {
@@ -30,7 +34,7 @@ func iDDecoder(_ bson.DecodeContext, vr bson.ValueReader, val reflect.Value) err
 
 	oid, err := vr.ReadObjectID()
 	if err != nil {
-		return err
+		return fmt.Errorf("read ObjectID: %w", err)
 	}
 
 	val.SetString(oid.Hex())

From 87e344193942a3093a8a1c15bd7fb598ad49a0f3 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Mon, 24 Feb 2025 20:10:43 +0900
Subject: [PATCH 13/22] lint

---
 backend-go/internal/core/hello/server.go | 1 +
 1 file changed, 1 insertion(+)

diff --git a/backend-go/internal/core/hello/server.go b/backend-go/internal/core/hello/server.go
index f16a6efa..1d6bb8c1 100644
--- a/backend-go/internal/core/hello/server.go
+++ b/backend-go/internal/core/hello/server.go
@@ -2,6 +2,7 @@ package hello
 
 import (
 	"github.com/labstack/echo/v4"
+
 	"github.com/yorkie-team/codepair/backend/internal/infra/database"
 )
 

From f98f32e2c49ec8f1ec9483a3c41525890ca977e3 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Tue, 25 Feb 2025 10:53:29 +0900
Subject: [PATCH 14/22] Define common error in database

---
 backend-go/internal/infra/database/errors.go  | 17 +++++++++++++
 backend-go/internal/infra/database/id.go      |  3 +++
 .../internal/infra/database/mongodb/client.go |  6 ++---
 .../internal/infra/database/mongodb/hello.go  | 24 ++++++++++++-------
 4 files changed, 39 insertions(+), 11 deletions(-)
 create mode 100644 backend-go/internal/infra/database/errors.go

diff --git a/backend-go/internal/infra/database/errors.go b/backend-go/internal/infra/database/errors.go
new file mode 100644
index 00000000..3ccda488
--- /dev/null
+++ b/backend-go/internal/infra/database/errors.go
@@ -0,0 +1,17 @@
+package database
+
+import "errors"
+
+var (
+	// ErrDocumentNotFound No matching document found
+	ErrDocumentNotFound = errors.New("document not found")
+
+	// ErrDuplicatedKey Duplicate key error (e.g., unique constraint violation)
+	ErrDuplicatedKey = errors.New("duplicate key error")
+
+	// ErrNetwork Network connectivity issue
+	ErrNetwork = errors.New("network error")
+
+	// ErrDisconnected Database connection lost
+	ErrDisconnected = errors.New("database disconnected")
+)
diff --git a/backend-go/internal/infra/database/id.go b/backend-go/internal/infra/database/id.go
index ed35c895..84f52829 100644
--- a/backend-go/internal/infra/database/id.go
+++ b/backend-go/internal/infra/database/id.go
@@ -1,3 +1,6 @@
 package database
 
+// ID is a string type used as an identifier in the database.
+// It is designed for encoding and decoding MongoDB Object IDs.
+// This allows us to use the database type directly with MongoDB encoders and decoders.
 type ID string
diff --git a/backend-go/internal/infra/database/mongodb/client.go b/backend-go/internal/infra/database/mongodb/client.go
index a67a4d3c..3a623b05 100644
--- a/backend-go/internal/infra/database/mongodb/client.go
+++ b/backend-go/internal/infra/database/mongodb/client.go
@@ -2,13 +2,13 @@ package mongodb
 
 import (
 	"context"
-	"fmt"
 
 	"go.mongodb.org/mongo-driver/v2/mongo"
 	"go.mongodb.org/mongo-driver/v2/mongo/options"
 	"go.mongodb.org/mongo-driver/v2/mongo/readpref"
 
 	"github.com/yorkie-team/codepair/backend/internal/config"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database"
 )
 
 // Dial creates an instance of Mongo and dials the given MongoDB.
@@ -20,14 +20,14 @@ func Dial(conf *config.Mongo) (*mongo.Client, error) {
 			SetRegistry(NewRegistry()),
 	)
 	if err != nil {
-		return nil, fmt.Errorf("connect to mongo: %w", err)
+		return nil, database.ErrDisconnected
 	}
 
 	ctxPing, cancel := context.WithTimeout(context.Background(), conf.PingTimeout)
 	defer cancel()
 
 	if err := client.Ping(ctxPing, readpref.Primary()); err != nil {
-		return nil, fmt.Errorf("ping mongo: %w", err)
+		return nil, database.ErrNetwork
 	}
 
 	return client, nil
diff --git a/backend-go/internal/infra/database/mongodb/hello.go b/backend-go/internal/infra/database/mongodb/hello.go
index e1f142a0..6ec5605c 100644
--- a/backend-go/internal/infra/database/mongodb/hello.go
+++ b/backend-go/internal/infra/database/mongodb/hello.go
@@ -2,6 +2,7 @@ package mongodb
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"time"
 
@@ -36,7 +37,10 @@ func (r *HelloRepository) CreateVisitor(visitor database.Visitor) (database.Visi
 
 	res, err := r.collection.InsertOne(context.Background(), visitor)
 	if err != nil {
-		return database.Visitor{}, fmt.Errorf("failed to create visitor: %w", err)
+		if mongo.IsDuplicateKeyError(err) {
+			return database.Visitor{}, database.ErrDuplicatedKey
+		}
+		return database.Visitor{}, fmt.Errorf("create visitor: %w", err)
 	}
 
 	if oid, ok := res.InsertedID.(bson.ObjectID); ok {
@@ -48,10 +52,14 @@ func (r *HelloRepository) CreateVisitor(visitor database.Visitor) (database.Visi
 
 // FindVisitor retrieves a visitor record by its id.
 func (r *HelloRepository) FindVisitor(id database.ID) (database.Visitor, error) {
-	var visitor database.Visitor
+	visitor := database.Visitor{}
 	filter := bson.M{"_id": id}
-	if err := r.collection.FindOne(context.Background(), filter).Decode(&visitor); err != nil {
-		return database.Visitor{}, fmt.Errorf("failed to find visitor: %w", err)
+
+	err := r.collection.FindOne(context.Background(), filter).Decode(&visitor)
+	if errors.Is(err, mongo.ErrNoDocuments) {
+		return database.Visitor{}, database.ErrDocumentNotFound
+	} else if err != nil {
+		return database.Visitor{}, fmt.Errorf("find visitor: %w", err)
 	}
 
 	return visitor, nil
@@ -70,10 +78,10 @@ func (r *HelloRepository) UpdateVisitor(visitor database.Visitor) error {
 
 	result, err := r.collection.UpdateOne(context.Background(), filter, update)
 	if err != nil {
-		return fmt.Errorf("failed to update visitor: %w", err)
+		return fmt.Errorf("update visitor: %w", err)
 	}
 	if result.MatchedCount == 0 {
-		return fmt.Errorf("no visitor found with id %s", visitor.ID)
+		return database.ErrDocumentNotFound
 	}
 	return nil
 }
@@ -83,10 +91,10 @@ func (r *HelloRepository) DeleteVisitor(id database.ID) error {
 	filter := bson.M{"_id": id}
 	result, err := r.collection.DeleteOne(context.Background(), filter)
 	if err != nil {
-		return fmt.Errorf("failed to delete visitor: %w", err)
+		return fmt.Errorf("delete visitor: %w", err)
 	}
 	if result.DeletedCount == 0 {
-		return fmt.Errorf("no visitor found with id %s", id)
+		return database.ErrDocumentNotFound
 	}
 	return nil
 }

From 903b8709a78b40221854d6c12a76f70a95e3bce8 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Tue, 25 Feb 2025 10:56:40 +0900
Subject: [PATCH 15/22] initializing struct reference

- ref `https://github.com/uber-go/guide/blob/master/style.md#initializing-struct-references`
---
 backend-go/internal/core/hello/handler.go | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/backend-go/internal/core/hello/handler.go b/backend-go/internal/core/hello/handler.go
index a71cf435..f54279f6 100644
--- a/backend-go/internal/core/hello/handler.go
+++ b/backend-go/internal/core/hello/handler.go
@@ -17,7 +17,7 @@ type Handler struct {
 // createHello handles the POST /hello endpoint.
 // It creates a new visitor record and returns a hello message.
 func (h *Handler) createHello(c echo.Context) error {
-	req := new(models.HelloRequest)
+	req := &models.HelloRequest{}
 
 	if err := http.BindAndValidateRequest(c, req); err != nil {
 		return http.NewErrorResponse(c, fmt.Errorf("invalid request: %w", err))
@@ -48,7 +48,7 @@ func (h *Handler) readHello(c echo.Context) error {
 // It updates an existing visitor record and returns a confirmation message.
 func (h *Handler) updateHello(c echo.Context) error {
 	id := c.Param("id")
-	req := new(models.HelloRequest)
+	req := &models.HelloRequest{}
 	if err := http.BindAndValidateRequest(c, req); err != nil {
 		return http.NewErrorResponse(c, fmt.Errorf("invalid request: %w", err))
 	}

From 659fd66c82fc8e3969c0e90baed9b10a9a505489 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Tue, 25 Feb 2025 11:00:06 +0900
Subject: [PATCH 16/22] Add interface compliance

---
 backend-go/internal/infra/database/mongodb/hello.go | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/backend-go/internal/infra/database/mongodb/hello.go b/backend-go/internal/infra/database/mongodb/hello.go
index 6ec5605c..fadb3fe1 100644
--- a/backend-go/internal/infra/database/mongodb/hello.go
+++ b/backend-go/internal/infra/database/mongodb/hello.go
@@ -21,6 +21,8 @@ type HelloRepository struct {
 	collection *mongo.Collection
 }
 
+var _ database.Hello = (*HelloRepository)(nil)
+
 // NewHelloRepo creates a new instance of HelloRepository.
 func NewHelloRepo(conf *config.Mongo, client *mongo.Client) *HelloRepository {
 	return &HelloRepository{

From 9e5199d29326fb73684b6a595be824e78fa2ba01 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Tue, 25 Feb 2025 11:34:54 +0900
Subject: [PATCH 17/22] Add index for visitor nickname

---
 .../internal/infra/database/mongodb/client.go |  9 ++++-
 .../internal/infra/database/mongodb/hello.go  |  3 --
 .../infra/database/mongodb/indexes.go         | 38 +++++++++++++++++++
 backend-go/internal/server/server.go          |  2 +-
 4 files changed, 47 insertions(+), 5 deletions(-)
 create mode 100644 backend-go/internal/infra/database/mongodb/indexes.go

diff --git a/backend-go/internal/infra/database/mongodb/client.go b/backend-go/internal/infra/database/mongodb/client.go
index 3a623b05..848aa306 100644
--- a/backend-go/internal/infra/database/mongodb/client.go
+++ b/backend-go/internal/infra/database/mongodb/client.go
@@ -7,12 +7,13 @@ import (
 	"go.mongodb.org/mongo-driver/v2/mongo/options"
 	"go.mongodb.org/mongo-driver/v2/mongo/readpref"
 
+	"github.com/labstack/echo/v4"
 	"github.com/yorkie-team/codepair/backend/internal/config"
 	"github.com/yorkie-team/codepair/backend/internal/infra/database"
 )
 
 // Dial creates an instance of Mongo and dials the given MongoDB.
-func Dial(conf *config.Mongo) (*mongo.Client, error) {
+func Dial(conf *config.Mongo, logger echo.Logger) (*mongo.Client, error) {
 	client, err := mongo.Connect(
 		options.Client().
 			ApplyURI(conf.ConnectionURI).
@@ -30,5 +31,11 @@ func Dial(conf *config.Mongo) (*mongo.Client, error) {
 		return nil, database.ErrNetwork
 	}
 
+	if err := ensureIndexes(context.Background(), client.Database(conf.DatabaseName)); err != nil {
+		return nil, err
+	}
+
+	logger.Infof("MongoDB connected, URI: %s, DB: %s", conf.ConnectionURI, conf.DatabaseName)
+
 	return client, nil
 }
diff --git a/backend-go/internal/infra/database/mongodb/hello.go b/backend-go/internal/infra/database/mongodb/hello.go
index fadb3fe1..5dc6bf1c 100644
--- a/backend-go/internal/infra/database/mongodb/hello.go
+++ b/backend-go/internal/infra/database/mongodb/hello.go
@@ -13,9 +13,6 @@ import (
 	"github.com/yorkie-team/codepair/backend/internal/infra/database"
 )
 
-// ColVisitor is the name of the collection storing visitor records.
-const ColVisitor = "hello_visitors"
-
 // HelloRepository implements the CRUD operations for the hello service.
 type HelloRepository struct {
 	collection *mongo.Collection
diff --git a/backend-go/internal/infra/database/mongodb/indexes.go b/backend-go/internal/infra/database/mongodb/indexes.go
new file mode 100644
index 00000000..afd1834a
--- /dev/null
+++ b/backend-go/internal/infra/database/mongodb/indexes.go
@@ -0,0 +1,38 @@
+package mongodb
+
+import (
+	"context"
+	"fmt"
+
+	"go.mongodb.org/mongo-driver/v2/bson"
+	"go.mongodb.org/mongo-driver/v2/mongo"
+	"go.mongodb.org/mongo-driver/v2/mongo/options"
+)
+
+// ColVisitor is the name of the collection storing visitor records.
+const ColVisitor = "hello_visitors"
+
+type collectionInfo struct {
+	name    string
+	indexes []mongo.IndexModel
+}
+
+var collectionInfos = []collectionInfo{
+	{
+		name: ColVisitor,
+		indexes: []mongo.IndexModel{{
+			Keys:    bson.D{{Key: "nickname", Value: 1}},
+			Options: options.Index().SetUnique(true),
+		}},
+	},
+}
+
+func ensureIndexes(ctx context.Context, db *mongo.Database) error {
+	for _, info := range collectionInfos {
+		_, err := db.Collection(info.name).Indexes().CreateMany(ctx, info.indexes)
+		if err != nil {
+			return fmt.Errorf("create indexes: %w", err)
+		}
+	}
+	return nil
+}
diff --git a/backend-go/internal/server/server.go b/backend-go/internal/server/server.go
index 45a561fe..25547881 100644
--- a/backend-go/internal/server/server.go
+++ b/backend-go/internal/server/server.go
@@ -19,7 +19,7 @@ type CodePair struct {
 
 // New creates a new CodePair server.
 func New(e *echo.Echo, conf *config.Config) (*CodePair, error) {
-	db, err := mongodb.Dial(conf.Mongo)
+	db, err := mongodb.Dial(conf.Mongo, e.Logger)
 	if err != nil {
 		return nil, fmt.Errorf("failed to dial mongo: %w", err)
 	}

From d749e10df4808e4f9c484aba11f650b7cb41b3a0 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Tue, 25 Feb 2025 11:58:14 +0900
Subject: [PATCH 18/22] Use database error in service

---
 backend-go/internal/core/hello/service.go | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/backend-go/internal/core/hello/service.go b/backend-go/internal/core/hello/service.go
index f1ebf9fa..11676a41 100644
--- a/backend-go/internal/core/hello/service.go
+++ b/backend-go/internal/core/hello/service.go
@@ -1,6 +1,7 @@
 package hello
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/yorkie-team/codepair/backend/api/codepair/v1/models"
@@ -19,6 +20,9 @@ func (s *Service) createHello(req *models.HelloRequest) (string, error) {
 	}
 	visitor, err := s.repo.CreateVisitor(visitor)
 	if err != nil {
+		if errors.Is(err, database.ErrDuplicatedKey) {
+			return "", fmt.Errorf("nickname '%s' already exists: %w", req.Nickname, err)
+		}
 		return "", fmt.Errorf("failed to create hello message for visitor %v: %w", req, err)
 	}
 	return string(visitor.ID), nil
@@ -28,6 +32,9 @@ func (s *Service) createHello(req *models.HelloRequest) (string, error) {
 func (s *Service) readNickname(id string) (string, error) {
 	visitor, err := s.repo.FindVisitor(database.ID(id))
 	if err != nil {
+		if errors.Is(err, database.ErrDocumentNotFound) {
+			return "", fmt.Errorf("visitor with ID '%s' not found: %w", id, err)
+		}
 		return "", fmt.Errorf("failed to find hello message for ID %s: %w", id, err)
 	}
 	return visitor.Nickname, nil
@@ -38,9 +45,12 @@ func (s *Service) updateHello(id string, req *models.HelloRequest) error {
 	visitor := database.Visitor{
 		ID:       database.ID(id),
 		Nickname: req.Nickname,
-		// Note: CreatedAt is not modified here; the repository is responsible for setting UpdatedAt.
 	}
+
 	if err := s.repo.UpdateVisitor(visitor); err != nil {
+		if errors.Is(err, database.ErrDocumentNotFound) {
+			return fmt.Errorf("cannot update: visitor with ID '%s' not found: %w", id, err)
+		}
 		return fmt.Errorf("failed to update hello message for visitor with ID %s: %w", id, err)
 	}
 	return nil
@@ -49,6 +59,9 @@ func (s *Service) updateHello(id string, req *models.HelloRequest) error {
 // deleteHello removes a visitor record by its unique identifier.
 func (s *Service) deleteHello(id string) error {
 	if err := s.repo.DeleteVisitor(database.ID(id)); err != nil {
+		if errors.Is(err, database.ErrDocumentNotFound) {
+			return fmt.Errorf("cannot delete: visitor with ID '%s' not found: %w", id, err)
+		}
 		return fmt.Errorf("failed to delete hello message for visitor with ID %s: %w", id, err)
 	}
 	return nil

From fa04b9be278096b9daf3a701d6c53da6cc59c792 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Tue, 25 Feb 2025 11:59:12 +0900
Subject: [PATCH 19/22] Lint

---
 backend-go/internal/infra/database/mongodb/client.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/backend-go/internal/infra/database/mongodb/client.go b/backend-go/internal/infra/database/mongodb/client.go
index 848aa306..b47fe956 100644
--- a/backend-go/internal/infra/database/mongodb/client.go
+++ b/backend-go/internal/infra/database/mongodb/client.go
@@ -3,11 +3,11 @@ package mongodb
 import (
 	"context"
 
+	"github.com/labstack/echo/v4"
 	"go.mongodb.org/mongo-driver/v2/mongo"
 	"go.mongodb.org/mongo-driver/v2/mongo/options"
 	"go.mongodb.org/mongo-driver/v2/mongo/readpref"
 
-	"github.com/labstack/echo/v4"
 	"github.com/yorkie-team/codepair/backend/internal/config"
 	"github.com/yorkie-team/codepair/backend/internal/infra/database"
 )

From a0dec495f3e25450255eaf313696d32ed933f128 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Tue, 25 Feb 2025 13:07:17 +0900
Subject: [PATCH 20/22] Rename `New` -> `Register`

- `New` is some strange because it doesn't return anything
---
 backend-go/internal/core/hello/server.go | 4 ++--
 backend-go/internal/server/server.go     | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/backend-go/internal/core/hello/server.go b/backend-go/internal/core/hello/server.go
index 1d6bb8c1..0e888ebb 100644
--- a/backend-go/internal/core/hello/server.go
+++ b/backend-go/internal/core/hello/server.go
@@ -6,8 +6,8 @@ import (
 	"github.com/yorkie-team/codepair/backend/internal/infra/database"
 )
 
-// New creates a new handler for hello endpoints and registers the routes.
-func New(e *echo.Echo, repo database.Hello) {
+// Register creates a new handler for hello endpoints and registers the routes.
+func Register(e *echo.Echo, repo database.Hello) {
 	svc := &Service{
 		repo: repo,
 	}
diff --git a/backend-go/internal/server/server.go b/backend-go/internal/server/server.go
index 25547881..f694d3ca 100644
--- a/backend-go/internal/server/server.go
+++ b/backend-go/internal/server/server.go
@@ -24,7 +24,7 @@ func New(e *echo.Echo, conf *config.Config) (*CodePair, error) {
 		return nil, fmt.Errorf("failed to dial mongo: %w", err)
 	}
 
-	hello.New(e, mongodb.NewHelloRepo(conf.Mongo, db))
+	hello.Register(e, mongodb.NewHelloRepo(conf.Mongo, db))
 
 	cp := &CodePair{
 		config: conf,

From e6714a636ee620e3b055be0bba7841c063b510fc Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Wed, 26 Feb 2025 11:20:47 +0900
Subject: [PATCH 21/22] Apply kokodak's suggestions

---
 backend-go/internal/core/hello/server.go      |  2 +-
 backend-go/internal/core/hello/service.go     | 13 ++++----
 .../infra/database/{ => entity}/id.go         |  2 +-
 .../internal/infra/database/entity/visitor.go | 11 +++++++
 backend-go/internal/infra/database/hello.go   | 32 -------------------
 .../infra/database/mongodb/registry.go        |  4 +--
 .../database/mongodb/{hello.go => visitor.go} | 23 +++++++------
 backend-go/internal/infra/database/visitor.go | 24 ++++++++++++++
 8 files changed, 57 insertions(+), 54 deletions(-)
 rename backend-go/internal/infra/database/{ => entity}/id.go (93%)
 create mode 100644 backend-go/internal/infra/database/entity/visitor.go
 delete mode 100644 backend-go/internal/infra/database/hello.go
 rename backend-go/internal/infra/database/mongodb/{hello.go => visitor.go} (75%)
 create mode 100644 backend-go/internal/infra/database/visitor.go

diff --git a/backend-go/internal/core/hello/server.go b/backend-go/internal/core/hello/server.go
index 0e888ebb..2c1a3b45 100644
--- a/backend-go/internal/core/hello/server.go
+++ b/backend-go/internal/core/hello/server.go
@@ -7,7 +7,7 @@ import (
 )
 
 // Register creates a new handler for hello endpoints and registers the routes.
-func Register(e *echo.Echo, repo database.Hello) {
+func Register(e *echo.Echo, repo database.Visitor) {
 	svc := &Service{
 		repo: repo,
 	}
diff --git a/backend-go/internal/core/hello/service.go b/backend-go/internal/core/hello/service.go
index 11676a41..297cb5d8 100644
--- a/backend-go/internal/core/hello/service.go
+++ b/backend-go/internal/core/hello/service.go
@@ -6,16 +6,17 @@ import (
 
 	"github.com/yorkie-team/codepair/backend/api/codepair/v1/models"
 	"github.com/yorkie-team/codepair/backend/internal/infra/database"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database/entity"
 )
 
 // Service provides business logic for handling hello messages.
 type Service struct {
-	repo database.Hello
+	repo database.Visitor
 }
 
 // createHello creates a new visitor record based on the provided hello request.
 func (s *Service) createHello(req *models.HelloRequest) (string, error) {
-	visitor := database.Visitor{
+	visitor := entity.Visitor{
 		Nickname: req.Nickname,
 	}
 	visitor, err := s.repo.CreateVisitor(visitor)
@@ -30,7 +31,7 @@ func (s *Service) createHello(req *models.HelloRequest) (string, error) {
 
 // readNickname retrieves the nickname of a visitor record by its unique identifier.
 func (s *Service) readNickname(id string) (string, error) {
-	visitor, err := s.repo.FindVisitor(database.ID(id))
+	visitor, err := s.repo.FindVisitor(entity.ID(id))
 	if err != nil {
 		if errors.Is(err, database.ErrDocumentNotFound) {
 			return "", fmt.Errorf("visitor with ID '%s' not found: %w", id, err)
@@ -42,8 +43,8 @@ func (s *Service) readNickname(id string) (string, error) {
 
 // updateHello updates an existing visitor record with new data from the hello request.
 func (s *Service) updateHello(id string, req *models.HelloRequest) error {
-	visitor := database.Visitor{
-		ID:       database.ID(id),
+	visitor := entity.Visitor{
+		ID:       entity.ID(id),
 		Nickname: req.Nickname,
 	}
 
@@ -58,7 +59,7 @@ func (s *Service) updateHello(id string, req *models.HelloRequest) error {
 
 // deleteHello removes a visitor record by its unique identifier.
 func (s *Service) deleteHello(id string) error {
-	if err := s.repo.DeleteVisitor(database.ID(id)); err != nil {
+	if err := s.repo.DeleteVisitor(entity.ID(id)); err != nil {
 		if errors.Is(err, database.ErrDocumentNotFound) {
 			return fmt.Errorf("cannot delete: visitor with ID '%s' not found: %w", id, err)
 		}
diff --git a/backend-go/internal/infra/database/id.go b/backend-go/internal/infra/database/entity/id.go
similarity index 93%
rename from backend-go/internal/infra/database/id.go
rename to backend-go/internal/infra/database/entity/id.go
index 84f52829..ddfd6144 100644
--- a/backend-go/internal/infra/database/id.go
+++ b/backend-go/internal/infra/database/entity/id.go
@@ -1,4 +1,4 @@
-package database
+package entity
 
 // ID is a string type used as an identifier in the database.
 // It is designed for encoding and decoding MongoDB Object IDs.
diff --git a/backend-go/internal/infra/database/entity/visitor.go b/backend-go/internal/infra/database/entity/visitor.go
new file mode 100644
index 00000000..ad8bc593
--- /dev/null
+++ b/backend-go/internal/infra/database/entity/visitor.go
@@ -0,0 +1,11 @@
+package entity
+
+import "time"
+
+// Visitor represents a visitor record in the hello repository.
+type Visitor struct {
+	ID        ID        `bson:"_id,omitempty"` // Auto-generated by MongoDB if empty.
+	Nickname  string    `bson:"nickname"`      // Visitor's nickname.
+	CreatedAt time.Time `bson:"created_at"`    // Timestamp when the record was created.
+	UpdatedAt time.Time `bson:"updated_at"`    // Timestamp when the record was last updated.
+}
diff --git a/backend-go/internal/infra/database/hello.go b/backend-go/internal/infra/database/hello.go
deleted file mode 100644
index 1eba7d33..00000000
--- a/backend-go/internal/infra/database/hello.go
+++ /dev/null
@@ -1,32 +0,0 @@
-package database
-
-import (
-	"time"
-)
-
-// Visitor represents a visitor record in the hello repository.
-type Visitor struct {
-	ID        ID        `bson:"_id,omitempty"` // Auto-generated by MongoDB if empty.
-	Nickname  string    `bson:"nickname"`      // Visitor's nickname.
-	CreatedAt time.Time `bson:"created_at"`    // Timestamp when the record was created.
-	UpdatedAt time.Time `bson:"updated_at"`    // Timestamp when the record was last updated.
-}
-
-// Hello defines the interface for CRUD operations on visitor records.
-type Hello interface {
-	// CreateVisitor creates a new visitor record.
-	// It accepts a Visitor object and returns an error if creation fails.
-	CreateVisitor(visitor Visitor) (Visitor, error)
-
-	// FindVisitor retrieves a visitor record by its unique identifier.
-	// It returns the Visitor and an error if the record cannot be found.
-	FindVisitor(id ID) (Visitor, error)
-
-	// UpdateVisitor updates an existing visitor record.
-	// It accepts a Visitor object (including its ID) and returns an error if the update fails.
-	UpdateVisitor(visitor Visitor) error
-
-	// DeleteVisitor removes a visitor record by its unique identifier.
-	// It returns an error if the deletion operation fails.
-	DeleteVisitor(id ID) error
-}
diff --git a/backend-go/internal/infra/database/mongodb/registry.go b/backend-go/internal/infra/database/mongodb/registry.go
index ac8845ad..bcb7c7d8 100644
--- a/backend-go/internal/infra/database/mongodb/registry.go
+++ b/backend-go/internal/infra/database/mongodb/registry.go
@@ -6,10 +6,10 @@ import (
 
 	"go.mongodb.org/mongo-driver/v2/bson"
 
-	"github.com/yorkie-team/codepair/backend/internal/infra/database"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database/entity"
 )
 
-var tID = reflect.TypeOf(database.ID(""))
+var tID = reflect.TypeOf(entity.ID(""))
 
 func iDEncoder(_ bson.EncodeContext, vw bson.ValueWriter, val reflect.Value) error {
 	if val.Type() != tID {
diff --git a/backend-go/internal/infra/database/mongodb/hello.go b/backend-go/internal/infra/database/mongodb/visitor.go
similarity index 75%
rename from backend-go/internal/infra/database/mongodb/hello.go
rename to backend-go/internal/infra/database/mongodb/visitor.go
index 5dc6bf1c..22af7c34 100644
--- a/backend-go/internal/infra/database/mongodb/hello.go
+++ b/backend-go/internal/infra/database/mongodb/visitor.go
@@ -11,6 +11,7 @@ import (
 
 	"github.com/yorkie-team/codepair/backend/internal/config"
 	"github.com/yorkie-team/codepair/backend/internal/infra/database"
+	"github.com/yorkie-team/codepair/backend/internal/infra/database/entity"
 )
 
 // HelloRepository implements the CRUD operations for the hello service.
@@ -18,8 +19,6 @@ type HelloRepository struct {
 	collection *mongo.Collection
 }
 
-var _ database.Hello = (*HelloRepository)(nil)
-
 // NewHelloRepo creates a new instance of HelloRepository.
 func NewHelloRepo(conf *config.Mongo, client *mongo.Client) *HelloRepository {
 	return &HelloRepository{
@@ -29,7 +28,7 @@ func NewHelloRepo(conf *config.Mongo, client *mongo.Client) *HelloRepository {
 
 // CreateVisitor inserts a new visitor record into the database,
 // letting MongoDB auto-generate the _id field.
-func (r *HelloRepository) CreateVisitor(visitor database.Visitor) (database.Visitor, error) {
+func (r *HelloRepository) CreateVisitor(visitor entity.Visitor) (entity.Visitor, error) {
 	now := time.Now()
 	visitor.CreatedAt = now
 	visitor.UpdatedAt = now
@@ -37,35 +36,35 @@ func (r *HelloRepository) CreateVisitor(visitor database.Visitor) (database.Visi
 	res, err := r.collection.InsertOne(context.Background(), visitor)
 	if err != nil {
 		if mongo.IsDuplicateKeyError(err) {
-			return database.Visitor{}, database.ErrDuplicatedKey
+			return entity.Visitor{}, database.ErrDuplicatedKey
 		}
-		return database.Visitor{}, fmt.Errorf("create visitor: %w", err)
+		return entity.Visitor{}, fmt.Errorf("create visitor: %w", err)
 	}
 
 	if oid, ok := res.InsertedID.(bson.ObjectID); ok {
-		visitor.ID = database.ID(oid.String())
+		visitor.ID = entity.ID(oid.String())
 	}
 
 	return visitor, nil
 }
 
 // FindVisitor retrieves a visitor record by its id.
-func (r *HelloRepository) FindVisitor(id database.ID) (database.Visitor, error) {
-	visitor := database.Visitor{}
+func (r *HelloRepository) FindVisitor(id entity.ID) (entity.Visitor, error) {
+	visitor := entity.Visitor{}
 	filter := bson.M{"_id": id}
 
 	err := r.collection.FindOne(context.Background(), filter).Decode(&visitor)
 	if errors.Is(err, mongo.ErrNoDocuments) {
-		return database.Visitor{}, database.ErrDocumentNotFound
+		return entity.Visitor{}, database.ErrDocumentNotFound
 	} else if err != nil {
-		return database.Visitor{}, fmt.Errorf("find visitor: %w", err)
+		return entity.Visitor{}, fmt.Errorf("find visitor: %w", err)
 	}
 
 	return visitor, nil
 }
 
 // UpdateVisitor updates an existing visitor record.
-func (r *HelloRepository) UpdateVisitor(visitor database.Visitor) error {
+func (r *HelloRepository) UpdateVisitor(visitor entity.Visitor) error {
 	visitor.UpdatedAt = time.Now()
 	filter := bson.M{"_id": visitor.ID}
 	update := bson.M{
@@ -86,7 +85,7 @@ func (r *HelloRepository) UpdateVisitor(visitor database.Visitor) error {
 }
 
 // DeleteVisitor removes a visitor record by its id.
-func (r *HelloRepository) DeleteVisitor(id database.ID) error {
+func (r *HelloRepository) DeleteVisitor(id entity.ID) error {
 	filter := bson.M{"_id": id}
 	result, err := r.collection.DeleteOne(context.Background(), filter)
 	if err != nil {
diff --git a/backend-go/internal/infra/database/visitor.go b/backend-go/internal/infra/database/visitor.go
new file mode 100644
index 00000000..bf39108e
--- /dev/null
+++ b/backend-go/internal/infra/database/visitor.go
@@ -0,0 +1,24 @@
+package database
+
+import (
+	"github.com/yorkie-team/codepair/backend/internal/infra/database/entity"
+)
+
+// Visitor defines the interface for CRUD operations on visitor records.
+type Visitor interface {
+	// CreateVisitor creates a new visitor record.
+	// It accepts a Visitor object and returns an error if creation fails.
+	CreateVisitor(visitor entity.Visitor) (entity.Visitor, error)
+
+	// FindVisitor retrieves a visitor record by its unique identifier.
+	// It returns the Visitor and an error if the record cannot be found.
+	FindVisitor(id entity.ID) (entity.Visitor, error)
+
+	// UpdateVisitor updates an existing visitor record.
+	// It accepts a Visitor object (including its ID) and returns an error if the update fails.
+	UpdateVisitor(visitor entity.Visitor) error
+
+	// DeleteVisitor removes a visitor record by its unique identifier.
+	// It returns an error if the deletion operation fails.
+	DeleteVisitor(id entity.ID) error
+}

From 8efa5f809b5a54b9b5e6b0c6639dd12ffa78ca45 Mon Sep 17 00:00:00 2001
From: Changyu Moon <window9u@konkuk.ac.kr>
Date: Wed, 26 Feb 2025 11:43:08 +0900
Subject: [PATCH 22/22] Remove handler comment

---
 backend-go/internal/core/hello/handler.go | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/backend-go/internal/core/hello/handler.go b/backend-go/internal/core/hello/handler.go
index f54279f6..0b124741 100644
--- a/backend-go/internal/core/hello/handler.go
+++ b/backend-go/internal/core/hello/handler.go
@@ -14,8 +14,6 @@ type Handler struct {
 	service *Service
 }
 
-// createHello handles the POST /hello endpoint.
-// It creates a new visitor record and returns a hello message.
 func (h *Handler) createHello(c echo.Context) error {
 	req := &models.HelloRequest{}
 
@@ -31,8 +29,6 @@ func (h *Handler) createHello(c echo.Context) error {
 	})
 }
 
-// readHello handles the GET /hello/:id endpoint.
-// It retrieves a visitor record by its id and returns a hello message.
 func (h *Handler) readHello(c echo.Context) error {
 	id := c.Param("id")
 	nickname, err := h.service.readNickname(id)
@@ -44,8 +40,6 @@ func (h *Handler) readHello(c echo.Context) error {
 	})
 }
 
-// updateHello handles the PUT /hello/:id endpoint.
-// It updates an existing visitor record and returns a confirmation message.
 func (h *Handler) updateHello(c echo.Context) error {
 	id := c.Param("id")
 	req := &models.HelloRequest{}
@@ -60,8 +54,6 @@ func (h *Handler) updateHello(c echo.Context) error {
 	})
 }
 
-// deleteHello handles the DELETE /hello/:id endpoint.
-// It deletes a visitor record by its id and returns a confirmation message.
 func (h *Handler) deleteHello(c echo.Context) error {
 	id := c.Param("id")
 	if err := h.service.deleteHello(id); err != nil {