ddd-js

Basic / boilerplate JS classes & functions.

We're still on alpha here, APIs may change any time.

Quick Start

The example application will be a small chat that allows sending messages under an alias. Implementation consists of:

• value objects for validation of the author's name and the chat text
• a root entity that accepts commands, validates input and returns the proper events
• a read model keeping an API representation of all messages sent
• an API that takes commands and gives access to the read model

src/ValueObjects.js

const { NonEmptyStringValue } = require('ddd-js')

class Author extends NonEmptyStringValue {}
class ChatText extends NonEmptyStringValue {}

module.exports = { Author, ChatText }

src/Entities/Message.js

const { RootEntity, DateTime } = require('ddd-js')
const { Author, ChatText } = require('./ValueObjects') // see Value Objects

class Message extends RootEntity {
  setup () {
    this.registerCommand(
      'Message.sendMessage',
      command => this.sendMessage(command.payload.author, command.payload.chatText, command.time)
    )
  }

  sendMessage (author, chatText, time) {
    // validate the input through value objects - this will throw an error if a value is invalid, rejecting the command
    new Author(author)
    new ChatText(chatText)
    new DateTime(time)

    // if all good, return the event
    return [this.createEvent('Message.messageSent', { author, chatText, commandTime: time })]
  }
}

module.exports = Message

src/ReadModel/Messages.js

const { ReadModel } = require('ddd-js')

class Messages extends ReadModel {
  setup () {
    this.messages = []
    this.registerEvent(
      'Message.messageSent',
      event => this.messageSent(event.payload.author, event.payload.chatText, event.payload.commandTime)
    )
  }

  messageSent (author, chatText, commandTime) { this.messages.push({ author, chatText, time: commandTime }) }

  get messages () { return this.messages }
}

module.exports = Messages

src/app.js

const bunyan = require('bunyan')
const { Runner } = require('ddd-js')
const Message = require('./Entities/Message') // see Root Entity
const Messages = require('./ReadModels/Messages') // see Read Model
const logger = bunyan.createLogger({ name: 'chat' })

Runner.createWithExpress(logger, '../eventstore.json')
  .attachRootEntity(Message)
  .attachReadModel('/messages', Messages, 'messages')
  .replayHistory()
  .then(runner => runner.startServer(8000))

Run node src/app.js - There's your API! You can now POST commands to http://localhost:8000/command and GET messages from http://localhost:8000/messages.

Command Example:

POST /command
Host: localhost:8000
Content-Type: application/json

{"name":"Message.sendMessage","time":"2019-12-08 16:06:37","payload":{"author":"Bob","chatText":"Hey, has anyone seen Jack recently!?"}}

Entity Versioning & Optimistic Lock

To utilize optmistic lock functionality, base your entity class on BaseEntity and register a function that returns all entities that will be affected by a command.

The CommandDispatcher will now monitor entity versions and block sending events if versions become inconsistent.

Example: To assure that the fuel tank of a car in our car park cannot be stored with a valie below zero, we'll return the affected car entity in the affectedEntitiesHandler for the `removeFuel` command.

const { RootEntity, BaseEntity } = require('ddd-js')

class Car extends BaseEntity {
  constructor (fuelLevel) {
    super()
    this.fuelLevel = fuelLevel
  }
}

class CarPool extends RootEntity {
  constructor () { this.cars = {} }

  setup () {
    this.registerCommand(
    'removeFuel',                                             // command name
    command => this.removeFuel(carId, liters)),               // command handler function
    command => { return [this.cars[command.payload.carId]] }, // function returning affected entities per command
    5                                                         // number of retries for optimistic lock until giving up
  }

  removeFuel (carId, liters) {
    if (this.cars[carId].fuelLevel - liters < 0) throw new Error('This is more than is left in the tank.')
    return [this.createEvent('fuelRemoved', { carId, liters })]
  }
}

Sagas

To support transactions over multiple aggregates while respecting bounded contexts a saga can be used. To do so, extend the Saga class, register a command that triggers it and add tasks and their according rollback tasks and then let the Saga run.

Example: If someone rents a car, it should be marked unavailable. At the same time, the customer's credit card should be debited. Only if both actions succeed the process is considered complete. If the car cannot be reserved the customer should get their money back. If the amount can't be debited the car should be freed again.

const { Saga } = require('ddd-js')

class RentCar extends Saga {
  setup () {
    this.registerCommand('rentCar', async command => {
      // prepare a new run of the Saga and get an identifier for that
      const id = this.provision()

      this.addTask(
        id, 'Car',                                                         // Saga ID and entity name
        { ...command, name: 'reserveCar', time: new Date().toJSON() },     // command to be sent
        () => ({ ...command, name: 'freeCar', time: new Date().toJSON() }) // roll back handler if any other task fails
      )

      this.addTask(
        id, 'Payment',
        { ...command, name: 'debitAmount', time: new Date().toJSON() },
        () => ({ ...command, name: 'payAmount', time: new Date().toJSON() })
      )

      await this.run(id)

      return [] // a saga could return its own events after it has finished
    })
  }
}

Success Of A Saga

A saga will succeed only if every task succeeded. It will then emit the events that were returned by the root entities.

Failure Of A Saga

A saga will fail if

• one or more commands failed to be executed
• one or more commands timed out

It will then send "rollback" commands to every root entity that succeeded or timed out.