diff --git a/pom.xml b/pom.xml
index a8ffa1a..0f7a615 100644
--- a/pom.xml
+++ b/pom.xml
@@ -95,6 +95,19 @@
             <artifactId>spring-boot-starter-test</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-web</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+        </dependency>
+
     </dependencies>
 
     <build>
diff --git a/src/main/java/com/devexperts/account/Account.java b/src/main/java/com/devexperts/account/Account.java
index fb2a3af..ccc2bd2 100644
--- a/src/main/java/com/devexperts/account/Account.java
+++ b/src/main/java/com/devexperts/account/Account.java
@@ -1,6 +1,14 @@
 package com.devexperts.account;
 
+import lombok.Builder;
+import lombok.Data;
+import lombok.Getter;
+import org.apache.commons.lang3.builder.EqualsBuilder;
+import org.apache.commons.lang3.builder.HashCodeBuilder;
+
+@Builder(builderMethodName = "of")
 public class Account {
+
     private final AccountKey accountKey;
     private final String firstName;
     private final String lastName;
@@ -32,4 +40,38 @@ public Double getBalance() {
     public void setBalance(Double balance) {
         this.balance = balance;
     }
+
+    public void increaseBalance(double amount) {
+        this.balance += amount;
+    }
+
+    public void decreaseBalance(double amount){
+        this.balance -= amount;
+    }
+
+    public long getAccountId(){
+        return accountKey.getAccountId();
+    }
+
+    @Override
+    public String toString() {
+        return accountKey.toString();
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        Account account = (Account) o;
+        return new EqualsBuilder()
+                .append(getAccountKey(), account.getAccountKey())
+                .isEquals();
+    }
+
+    @Override
+    public int hashCode() {
+        return new HashCodeBuilder(17, 37)
+                .append(getAccountKey())
+                .toHashCode();
+    }
 }
diff --git a/src/main/java/com/devexperts/account/AccountKey.java b/src/main/java/com/devexperts/account/AccountKey.java
index 1b0a233..6ca2fcb 100644
--- a/src/main/java/com/devexperts/account/AccountKey.java
+++ b/src/main/java/com/devexperts/account/AccountKey.java
@@ -1,5 +1,7 @@
 package com.devexperts.account;
 
+import lombok.EqualsAndHashCode;
+
 /**
  * Unique Account identifier
  *
@@ -7,6 +9,8 @@
  * NOTE: we suspect that later {@link #accountId} is not going to be uniquely identifying an account,
  * as we might add human-readable account representation and some clearing codes for partners.
  * */
+
+@EqualsAndHashCode
 public class AccountKey {
     private final long accountId;
 
@@ -17,4 +21,13 @@ private AccountKey(long accountId) {
     public static AccountKey valueOf(long accountId) {
         return new AccountKey(accountId);
     }
+
+    public long getAccountId() {
+        return accountId;
+    }
+
+    @Override
+    public String toString() {
+        return "AccountKey{accountId=" + accountId + '}';
+    }
 }
diff --git a/src/main/java/com/devexperts/rest/AbstractAccountController.java b/src/main/java/com/devexperts/rest/AbstractAccountController.java
index dea5a3c..9e96b51 100644
--- a/src/main/java/com/devexperts/rest/AbstractAccountController.java
+++ b/src/main/java/com/devexperts/rest/AbstractAccountController.java
@@ -1,7 +1,8 @@
 package com.devexperts.rest;
 
+import com.devexperts.rest.pojo.TransferOperationDTO;
 import org.springframework.http.ResponseEntity;
 
 public abstract class AbstractAccountController {
-    abstract ResponseEntity<Void> transfer(long sourceId, long targetId, double amount);
+    abstract ResponseEntity<?> transfer(TransferOperationDTO transferOperation);
 }
diff --git a/src/main/java/com/devexperts/rest/AccountController.java b/src/main/java/com/devexperts/rest/AccountController.java
index b300282..0d30f42 100644
--- a/src/main/java/com/devexperts/rest/AccountController.java
+++ b/src/main/java/com/devexperts/rest/AccountController.java
@@ -1,14 +1,43 @@
 package com.devexperts.rest;
 
+import com.devexperts.account.Account;
+import com.devexperts.rest.pojo.TransferOperationDTO;
+import com.devexperts.service.AccountService;
+import lombok.AllArgsConstructor;
+import lombok.extern.apachecommons.CommonsLog;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.server.ResponseStatusException;
 
+import javax.validation.Valid;
+import javax.validation.constraints.NotNull;
+
+@CommonsLog
 @RestController
-@RequestMapping("/api")
+@AllArgsConstructor
+@RequestMapping("/api/operations")
 public class AccountController extends AbstractAccountController {
 
-    public ResponseEntity<Void> transfer(long sourceId, long targetId, double amount) {
-        return null;
+    private final AccountService accountService;
+
+    @PostMapping(value = "/transfer")
+    public ResponseEntity<?> transfer(@RequestBody @Valid @NotNull TransferOperationDTO transferOperation) {
+
+        Account source = accountService.getAccount(transferOperation.getSourceId()),
+                target = accountService.getAccount(transferOperation.getTargetId());
+
+        try {
+            accountService.transfer(source, target, transferOperation.getAmount());
+        } catch (RuntimeException exception) {
+            log.error("Couldn't complete transfer", exception);
+            throw new ResponseStatusException(HttpStatus.INTERNAL_SERVER_ERROR, exception.getMessage(), exception);
+        }
+
+        log.info("Transfer complete.");
+        return ResponseEntity.ok("nice!");
     }
 }
diff --git a/src/main/java/com/devexperts/rest/pojo/TransferOperationDTO.java b/src/main/java/com/devexperts/rest/pojo/TransferOperationDTO.java
new file mode 100644
index 0000000..934ab58
--- /dev/null
+++ b/src/main/java/com/devexperts/rest/pojo/TransferOperationDTO.java
@@ -0,0 +1,25 @@
+package com.devexperts.rest.pojo;
+
+import com.devexperts.rest.validation.ExistingAccountId;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+import javax.validation.constraints.NotNull;
+import javax.validation.constraints.Positive;
+
+@Data
+@AllArgsConstructor
+@NoArgsConstructor
+public class TransferOperationDTO {
+
+    @ExistingAccountId
+    private Long sourceId;
+
+    @ExistingAccountId
+    private Long targetId;
+
+    @NotNull
+    @Positive
+    private Double amount;
+}
diff --git a/src/main/java/com/devexperts/rest/validation/AccountIdValidator.java b/src/main/java/com/devexperts/rest/validation/AccountIdValidator.java
new file mode 100644
index 0000000..b09290f
--- /dev/null
+++ b/src/main/java/com/devexperts/rest/validation/AccountIdValidator.java
@@ -0,0 +1,26 @@
+package com.devexperts.rest.validation;
+
+import com.devexperts.service.AccountService;
+import lombok.AllArgsConstructor;
+import org.springframework.http.HttpStatus;
+import org.springframework.web.server.ResponseStatusException;
+
+import javax.validation.ConstraintValidator;
+import javax.validation.ConstraintValidatorContext;
+
+@AllArgsConstructor
+public class AccountIdValidator implements ConstraintValidator<ExistingAccountId, Long> {
+
+    private final AccountService accountService;
+
+    @Override
+    public boolean isValid(Long id, ConstraintValidatorContext context) {
+        if(id == null)
+            throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Account id cannot be null");
+        else if(id <= 0)
+            throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Account id must be above 0");
+        else if(accountService.getAccount(id) == null)
+            throw new ResponseStatusException(HttpStatus.NOT_FOUND, String.format("Account [%s] not found", id));
+        else return true;
+    }
+}
diff --git a/src/main/java/com/devexperts/rest/validation/ExistingAccountId.java b/src/main/java/com/devexperts/rest/validation/ExistingAccountId.java
new file mode 100644
index 0000000..1d15231
--- /dev/null
+++ b/src/main/java/com/devexperts/rest/validation/ExistingAccountId.java
@@ -0,0 +1,24 @@
+package com.devexperts.rest.validation;
+
+import javax.validation.Constraint;
+import javax.validation.Payload;
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+@Documented
+@Retention(RUNTIME)
+@Target({FIELD, PARAMETER})
+@Constraint(validatedBy = AccountIdValidator.class)
+public @interface ExistingAccountId {
+
+    String message() default "Account not exists.";
+
+    Class<?>[] groups() default {};
+
+    Class<? extends Payload>[] payload() default {};
+}
diff --git a/src/main/java/com/devexperts/service/AccountServiceImpl.java b/src/main/java/com/devexperts/service/AccountServiceImpl.java
index 91261ba..5bb4c45 100644
--- a/src/main/java/com/devexperts/service/AccountServiceImpl.java
+++ b/src/main/java/com/devexperts/service/AccountServiceImpl.java
@@ -4,13 +4,19 @@
 import com.devexperts.account.AccountKey;
 import org.springframework.stereotype.Service;
 
-import java.util.ArrayList;
+import javax.annotation.PostConstruct;
+import java.util.Arrays;
+import java.util.Comparator;
 import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+import static com.devexperts.service.TransferHelper.validateBalance;
 
 @Service
 public class AccountServiceImpl implements AccountService {
 
-    private final List<Account> accounts = new ArrayList<>();
+    private final Map<AccountKey, Account> accounts = new ConcurrentHashMap<>();
 
     @Override
     public void clear() {
@@ -19,19 +25,46 @@ public void clear() {
 
     @Override
     public void createAccount(Account account) {
-        accounts.add(account);
+        accounts.put(account.getAccountKey(), account);
     }
 
     @Override
     public Account getAccount(long id) {
-        return accounts.stream()
-                .filter(account -> account.getAccountKey() == AccountKey.valueOf(id))
-                .findAny()
-                .orElse(null);
+        return accounts.get(AccountKey.valueOf(id));
+    }
+
+
+    @PostConstruct
+    public void fillAccounts() {
+        Account
+        one = Account.of()
+                .accountKey(AccountKey.valueOf(1))
+                .balance(5d)
+                .firstName("Daisy")
+                .lastName("Duck")
+                .build(),
+        two = Account.of()
+                .accountKey(AccountKey.valueOf(2))
+                .balance(500000d)
+                .firstName("Donald")
+                .lastName("Duck")
+                .build();
+
+        accounts.put(one.getAccountKey(), one);
+        accounts.put(two.getAccountKey(), two);
     }
 
     @Override
     public void transfer(Account source, Account target, double amount) {
-        //do nothing for now
+        List<Account> locks = Arrays.asList(source, target); //consider ids sequential
+        locks.sort(Comparator.comparingLong(Account::getAccountId));
+
+        synchronized (locks.get(0)) {
+            synchronized (locks.get(1)) {
+                validateBalance(source, target, amount);
+                source.decreaseBalance(amount);
+                target.increaseBalance(amount);
+            }
+        }
     }
 }
diff --git a/src/main/java/com/devexperts/service/TransferHelper.java b/src/main/java/com/devexperts/service/TransferHelper.java
new file mode 100644
index 0000000..d87230e
--- /dev/null
+++ b/src/main/java/com/devexperts/service/TransferHelper.java
@@ -0,0 +1,12 @@
+package com.devexperts.service;
+
+import com.devexperts.account.Account;
+
+public class TransferHelper {
+    public static void validateBalance(Account source, Account target, double amount) throws RuntimeException{
+        if(source.getBalance() < amount)
+            throw new RuntimeException(String.format("[%s] has not enough money", source.getFirstName()));
+        if(source.equals(target))
+            throw new IllegalArgumentException("Cannot transfer to the same account");
+    }
+}
diff --git a/src/main/resources/accounts.sql b/src/main/resources/accounts.sql
new file mode 100644
index 0000000..026e67f
--- /dev/null
+++ b/src/main/resources/accounts.sql
@@ -0,0 +1,8 @@
+CREATE TABLE accounts
+(
+    id              BIGINT                      NOT NULL,
+    first_name      VARCHAR(255)                NOT NULL,
+    last_name       VARCHAR(255)                NOT NULL,
+    balance         BIGINT                      NOT NULL,
+    CONSTRAINT accounts_pkey2 PRIMARY KEY (id)
+);
\ No newline at end of file
diff --git a/src/main/resources/select.sql b/src/main/resources/select.sql
new file mode 100644
index 0000000..014eef3
--- /dev/null
+++ b/src/main/resources/select.sql
@@ -0,0 +1,5 @@
+select source.first_name, concat(coalesce(sum(transfer.amount), 0), ' $') as total_sent
+from accounts source inner join transfers transfer on source.id = transfer.source_id
+where transfer.transfer_time > '2011-01-01'
+group by source.id
+having coalesce(sum(transfer.amount), 0) > 1000;
\ No newline at end of file
diff --git a/src/main/resources/transfers.sql b/src/main/resources/transfers.sql
new file mode 100644
index 0000000..235f4ce
--- /dev/null
+++ b/src/main/resources/transfers.sql
@@ -0,0 +1,12 @@
+CREATE TABLE transfers
+(
+    id                  BIGINT                              NOT NULL,
+    source_id           BIGINT                              NOT NULL,
+    target_id           BIGINT                              NOT NULL,
+    amount              BIGINT                              NOT NULL,
+    transfer_time       TIMESTAMP WITHOUT TIME ZONE         NOT NULL,
+    CONSTRAINT transfers_pkey2 PRIMARY KEY (id)
+);
+
+ALTER TABLE transfers ADD CONSTRAINT transfers_source_id FOREIGN KEY (source_id) REFERENCES accounts (id) ON UPDATE CASCADE ON DELETE RESTRICT;
+ALTER TABLE transfers ADD CONSTRAINT transfers_target_id FOREIGN KEY (target_id) REFERENCES accounts (id) ON UPDATE CASCADE ON DELETE RESTRICT;
\ No newline at end of file