Serverless-cqrs NPM

About

serverless-cqrs is collection tools to help you get started with building a fully functioning backend based on the principles of CQRS, Event Sourcing, Domain Driven Design, and Onion Architecture.

The goal is twofold:

Write as little boilerplate code as possible.
Clean service boundaries so that parts can be easily tested and swapped out.

Readme

https://serverless-cqrs.gitbook.io/serverless-cqrs
https://medium.com/@yonah.forst/introducing-serverless-cqrs-24471045c08d
https://medium.com/@yonah.forst/getting-started-with-serverless-cqrs-part-2-2ea4ac114439

Quickstart

Install

npm i --save serverless-cqrs
npm i --save serverless-cqrs.memory-adapter

Usage

To start, you need Actions and a Reducer. So let's write simple ones:

// actions.js
const actions = {
  addTodo: (state, payload) => {
    if (!payload.title) throw new Error('titleMissing')
    
    return [{
      type: 'TodoAdded',
      title: payload.title,
      at: Date.now(),
    }]
  }
}

module.exports = actions

// reducer.js
const initialState = {
  todos: []
}

const reducer = (state, event) => {
  switch (event.type) {
    case 'TodoAdded':
      return {
        todos: [
          ...state.todos,
          { title: event.title },
        ]
      }
      
    default:
      return state
  }
}

module.exports = (events, state=initialState) => events.reduce(reducer, state)

Above we have a basic action and reducer.

The action, addTodo, does some basic validation to check the presence of a title and if it succeeds, returns a new event with the type TodoAdded. When that event is run through the reducer, a new todo is appended to the list.

Next, we build an adapter to help us persist the events.

// adapter.js
const memoryAdapterBuilder = require('serverless-cqrs.memory-adapter')
module.exports = memoryAdapterBuilder.build({ 
  entityName: 'todo'
})

This adapter will let us persist events and read-model projections in memory.
Finally, we use these to build our read and write model.

// index.js
const {
  writeModelBuilder,
  readModelBuilder,
} = require('serverless-cqrs')

const actions = require('./actions')
const reducer = require('./reducer')
const adapter = require('./adapter')

module.exports.writeModel = writeModelBuilder.build({
  actions,
  reducer,
  adapter,
})

module.exports.readModel = readModelBuilder.build({
  reducer,
  adapter,
  eventAdapter: adapter,
})

That's it!