red-rover

red-rover

A JavaScript event publisher/subscriber built on Redis' great pub/sub platform. RedRover allows applications and services to communicate events without foreknowledge of integrating systems. This decoupling allows for improved application simplicity through isolation.

RedRover has three simple mechanisms for publishing events

1. Simple Publish and Subscribe Applications have the ability to publish an event, including relevant event data. Any subscribers would recieve the event and data.
2. Group Subscribe Applications in a load balanced or clustered configuration could subscribe to published events as a group. This unique feature of RedRobin makes it so that only one member of the group receives the event.
3. Commands Commands allow for bi-directional pub/sub. They provide a return channel while still allowing publishers and subscribers to be completely unaware of eachother.

RedRover is ready for use. It comes complete with documentation, a full suite of tests, and performance benchmarks that you can run in your own environment.

A JavaScript event publisher/subscriber backed by Redis.

Red Rover, Red Rover, send user:created right over

Why?

Well I am glad that you asked. Event buses like redis-eventemitter, message-exchange and redis-event-bus are a bit too simplistic. They do not give you event ids so you cannot easily respond to specific events. Nor do they have the notion of groups. So every event goes to every subscriber. This sounds reasonable, but it is common to have several load-balanced servers running an application, and having all of them respond to an event is not always the desired behavior. On the other hand bus.io is a full application framework. Sometimes you just want to publish events.

Installation

You must have node and npm installed

npm install --save @kuali/red-rover

Usage

Using red-rover is easy. After requiring it, instantiate it by passing in your configuration. Omitting a configuration will attempt to connect to a local redis on the default port of 6379. Optional failSilently boolean flag may be added to allow red-rover to swallow any errors.

const RedRover = require('@kuali/red-rover')
const redRover = new RedRover(config, failSilently)

Create publishers and/or subscribers.

const publisher = redRover.publisher()
const subscriber = redRover.subscriber()

Publish

publisher.publish('user:created', { id: 56789, name: 'John' })

Subscribe

subscriber.subscribe('user:created')
  .then(() => {
    subscriber.on('message', (channel, message) => {
      console.log(message)
    })
  })

Group Subscribe

Allows delivery to only one of many subscribers.

const subscriber = redRover.subscriber('production-group')
subscriber.subscribe('user:created')
  .then(() => {
    subscriber.on('message', (channel, message) => {
      console.log(message)
    })
  })

In the circumstance when you desire a single event delivery to a group of subscribers you can create a subscriber group that will act as a single subscriber. This is particularly useful if you have load-balanced servers but you only want a single action to take place.

In the following example, 3 subscribers are set up, two of them part of the same group. Each of these subscribers could be in different processes or even different servers (as long as they point to the same redis instance).

Two of the subscribers are signed up with group1, and the other is a part of group2. When an event goes out that all three of them are listening to, only one of the subscribers from each group will pick up the event.

const redRover = require('@kuali/red-rover')()

const subscriber1 = redRover.subscriber('group1')
const subscriber2 = redRover.subscriber('group1')
const subscriber3 = redRover.subscriber('group2')
const publisher = redRover.publisher()

subscriber1.on('message', (channel, message) => {
  console.log('user created by 1', payload)
})

subscriber2.on('message', (channel, message) => {
  console.log('user created by 2', payload)
})

subscriber3.on('message', (channel, message) => {
  console.log('user created by 3', payload)
})

subscriber.subscribe('user:created')
  .then(() => {
    publisher.publish('user:created', {
      email: '[e-mail]'
    })
  })

// Result
// user created by 1 { email: '[e-mail]' }
// user created by 3 { email: '[e-mail]' }

Config options

const RedRover = require('@kuali/red-rover')
const redRover = new RedRover({
  host: '127.0.0.1',
  port: 6379   
})

The complete set of configuration options.

API

redRover.publisher()

Returns a publisher

redRover.subscriber(group)

Returns a subscriber. Passing the optional group parameter restricts the message delivery to a single member of the group.

• group (String) - The group name to use

Publisher

publisher.publish(channel[, event])

Publishes a message the the passed channel.

• channel (String) - The channel on which to publish
• event (Mixed) - The payload to send through the event. Note that this should be serializable into JSON. Any functions or circular references get removed.

publisher.quit()

Quits this publisher. No more events may be sent.

Subscriber

subscriber.subscribe(channel)

Subscribes to a channel. Returns a promise that is fulfilled when the subscriber is ready to receive events. Subscribers can subscribe to many channels.

• channel (String) - The channel on which to listen

subscriber.psubscribe(channel)

Subscribes to a channel using a pattern. Returns a promise that is fulfilled when the subscriber is ready to receive events. Subscribers can subscribe to many channels.

• channel (String) - The channel on which to listen

subscriber.on(eventType, handler)

Sets up a subscription hander on a subscriber. Events on all channels on which the subscriber is listening will call the handler.

• eventType (String) - This string should be 'message' or 'pmessage'.
• handler (Function) - The handler to the event. It should have two arguments, the channel and the payload that the event emits.

Once

redRover.once(channel[, group])

Subscribes to a channel for a single event. Once an event is received the promise is fulfilled and the subscription is cleaned up.

• channel (String) - The channel on which to listen
• group (String) - Optional.

Commands

RedRover commands leverage publishers and subscribers to allow for bi-directional communication. The Sender creates a second channel on which responses can be sent. The Receiver uses the second channel to return a response to the Sender.

Although commands support groups, they still have no idea how many Receivers might be listening or respond. This is necessary to enable complete decoupling. The only way to ensure that a Sender receieve a single response is to have all receivers be members of the same group. This is unenforceable through code in the current version.

Sender

Send a event and expect responses

redRover.sender(channel)
  // the returned promise is fulfilled when channel subscriptions are setup
  .then((sender) => {
    // send an event for which you expect a response
    sender.send(event, (response) => {
      // handle returned response
    })
    // call stop when you are done
    sender.stop()
  })

Receiver

Respond to sender events

redRover.receiver(channel, group)
  // the returned promise is fulfilled when channel subscriptions are setup
  .then((receiver) => {
    // receive an event for which you will send a response
    receiver.receive((event) => {
      // act on event - return your response or a promise
    })
    // call stop when you are done
    receiver.stop()
  })