remit

What's Remit?

remit is intended to be a small set of functionality used to create simple microservices that don't need to be aware of one-another's existence.

It uses RabbitMQ at its core to manage service discovery-like behaviour without the need to explicitly connect one service to another.

Pre-requisites

To use remit you'll need:

• A RabbitMQ server
• Node v4.x.x
• npm

Installation

Once your RabbitMQ server's up and running, simply use npm to install remit!

npm install remit

Simple usage

remit makes use of four simple commands: req (request), res (respond), emit and listen.

• req requests data from a defined endpoint which, in turn, is created using res
• listen waits for messages emitted from anywhere in the system.

A connection to your AMQP server's required before you can get going, but you can easily do that!

const remit = require('remit')({
	name: 'my_service', // this is required for a service that has a listener
	url: 'amqp://localhost'
})

After that, the world is yours! Here are some basic examples of the four commands mentioned above.

// API
remit.req('add', {
	first: 2,
	second: 7
}, function (err, data) {
	console.log('The result is ' + data)
})

// Server
remit.res('add', function (args, done) {
	done(null, (args.first + args.second))

	remit.emit('something.happened', args)
})

// Listener 1
remit.listen('something.happened', function (args) {
	console.log(args)
})

// Listener 2
remit.listen('something.#', function (args) {
	console.log('Something... did something...')
})

/*
	1. The API requests the 'add' endpoint.
	2. The Server responds with the result of the sum.
	3. The API logs 'The result is 9'.
	4. The Server emits the 'something.happened' event.
	5. Listener 1 logs the arguments the API sent.
	6. Listener 2 logs 'Something... did something...'.
*/

Key examples

There are two methods for sending messages with remit: request or emit.

A request implies that the requester wants a response back, whereas using an emission means you wish to notify other services of an event without requiring their input.

Let's start with a simple authentication example. We'll set up an API that our user can request to log in.

// Import remit and connect to our AMQP server
const remit = require('remit')()

// Import whatever HTTP API creator we want
const api = require('some-api-maker')

// Set up a route using our API creator
api.get('/login', function (req, res) {
	// Send a request via remit to the 'user.login' endpoint
	remit.req('user.login', {
		username: req.username,
		password: req.password
	}, function (err, data) {
		//If there's something wrong...
		if (err) return res.failure(err)

		// Otherwise, woohoo! We're logged in!
		return res.success(data.user)
	})
})

Awesome! Now we'll set up the authentication service that'll respond to the request.

// Import remit and connect to our AMQP server
const remit = require('remit')()

// Respond to 'user.login' events
remit.res('user.login', function (args, done) {
	// If it's not Mr. Bean, send back an error!
	if (args.username !== 'Mr. Bean') return done('You\'re not Mr. Bean!')

	// Otherwise, let's "log in"
	done(null, {
		username: 'Mr. Bean',
		birthday: '14/06/1961'
	})
})

Done. That's it. Our API service will request an answer to the user.login endpoint and our server will respond. Simples.

Let's now say that we want a service to listen out for if it's a user's birthday and send them an email if they've logged in on that day! With most other systems, this would require adding business logic to our login service to explicitly call some birthday service and check, but not with remit.

At the end of our authentication service, let's add an emission of user.login.success.

// Respond to 'user.login' events
remit.res('user.login', function (args, done) {
	// If it's not Mr. Bean, send back an error!
	if (args.username !== 'Mr. Bean') return done('You\'re not Mr. Bean!')

	// Otherwise, let's "log in"
	let user = {
		username: 'Mr. Bean',
		birthday: '14/06/1961'
	}

	done(null, user)

	// After we've logged the user in, let's emit that everything went well!
	remit.emit('user.login.success', { user })
})

Now that we've done that, any other services on the network can listen in on that event and react accordingly!

Let's make our birthday service.

const remit = require('remit')({
	name: 'birthday'	
})

const beanmail = require('send-mail-to-mr-bean')

remit.listen('user.login.success', function (args) {
	let today = '14/06/1961'

	if (today === args.user.birthday) {
		beanmail.send()
	}
})

Sorted. Now every time someone logs in successfully, we run a check to see if it's their birthday.

Emissions can be hooked into by any number of different services, but only one "worker" per service will receive each emission.

So let's also start logging every time a user performs any action. We can do this by using the # wildcard.

const remit = require('remit')({
	name: 'logger'
})

let user_action_counter = 0

remit.listen('user.#', function (args) {
	user_action_counter++
})

Improvements

remit's in its very early stages. Basic use is working well, but here are some features I'm looking at implementing to make things a bit more diverse.

• Ability to specify exchange per connection, endpoint or event
• Cleaner error handling (along with some standards)
• Removal of all use of process.exit()
• Connection retrying when losing connection to the AMQ
• Use promises instead of callbacks
• Warnings for duplicate req subscriptions
• Better handling of req timeouts
• Ability for emissions to receive (multiple) results from listeners if required (I really want to use generators for this)
• Obey the JSON-RPC 2.0 spec
• Tests!