RedisEventBroker

A simple redis event broker implementation.

This event broker is thought to be used in an event sourcing evironment. It should be used as a module inside a microservice application. This means that to be effective and fault tolerant needs multiple instaces of the microservice application that it's using it.

Advantages

• No need to deploy and manage another dedicated service which works as an event broker manager.
• Fault tolerance stricly related to the fault tolerance of the microservice which is using it.

Drawbacks

• Not recommend for really large instaces number and very-intensive event communication because it can lead to too many (unnecessary) calls to redis that I'm not sure if redis can manage them. This is a very small experimental but perfectly working project. It's really easy to integrate and use it but a good scalability it's not assured.

How it works

Each subscriber to a certain topic subscribes using RedisEvents to that topic.

When redisEventBroker.publishEvent(event) is called RedisEventBroker fetch every subscriber of that event's topic and push atomically the serialized event to every subscriber's publishedList and publish a redis event with the topic name as a message.

Each subscriber, receiveing a new redis event, calls redisEventBroker.pick(cb) processing the event in the callback function.

The "event picking" is done atomically and consists of:

• Removing the first event from the publishedList and pushing it in the processingList
• Publish a new processingNotification redis event.

After processing the event, the event is deleted from the processingList.

In case of processing failure or microservice's instace failure the event will be reinserted in the publishedList and another instance will process it.

This is achieved because on the processingNotification event, each instance of the microservice register a new timeout of 100ms (for now it's an arbitrary value, it will be customizable). After that if the processing event is still in the processingList will be atomically removed and reinserted at the front of the publishedList. During the removing and reinsertion a new eventPublished redis event is raised atomically so that other microservice instances are notified.

Installation

Run: npm install redis-event-broker --save

How to use it

Important note: in order to publish and pick the events in the right lists it's required that process.env.MICROSERVICE_NAME environment variable is initialized. Otherwise this module will throw an error.

Require

const redisEventBroker = require('redis-event-broker')();

The module create a new forked process (called checker process) responsible to check that lost events in processing state are re-enqued in the publishedList.

Note: every time the function RedisEventBroker is called no new instances of redis client are created and no new checker processes are created.

Event publishing

To publish an event:

redisEventBroker.publishEvent(event);

The event object must have the following fields:

• streamId
• topic

Other optional fields:

• message
• payload

Event picking

To pick an event:

redisEventBroker.pick(cb);

Note: every asyncronous job in the callback must be awaited or returned in a promise. Otherwise on unsuccessful event processing the event will be handled as successful.

Topic subscription

To subscribe to a new topic:

redisEventBroker.subscribe(topic);

Topic's "new event" event handling

To react to a new event notification:

redisEventBroker.onNotification(cb);

You can combine redisEventBroker.onNotification with redisEventBroker.pick, to pick new messages on notification (let's call it picking on notification from now on).

Example

redisEventBroker.onNotification(() => {
    redisEventBroker.pick((event) => {
        if (event.topic === first_topic) {
            // Do stuff...
        } else if (event.topic === second_topic) {
            // Do other stuff...
        }
    });
});

Currently is not possible to register callbacks based on events topics and messages. If you don't like to have a big if-else tree you can:

const emitter = new (require('events'))();

redisEventBroker.onNotification(() => {
    redisEventBroker.pick((event) => {
        emitter.emit(`${event.topic}:${event.message}`, event);
    });
});

emitter.on('topic1:entityCreated', (event) => {});
emitter.on('topic2:entityCreated', (event) => {});

// Don't to this:
emitter.on('topic3:entityCreated', (event) => {
    setImmediate(() => {
        // Do stuff...
    });
});

Since EventEmitter callbacks are called syncronously everything is ok for the pick callback.

Picking on notification

From 1.3.x you can get new event on notification like this:

redisEventBroker.pickOnNotification(event => {
    // Do stuff...
})

Example

const emitter = new (require('events'))();

redisEventBroker.pickOnNotification(event => {
    emitter.emit(`${event.topic}:${event.message}`, event);
});

emitter.on('topic1:entityCreated', (event) => {});
emitter.on('topic2:entityCreated', (event) => {});

Todo

• Nothing to do (for now).