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@fanoutio/connect-eventstream

connect-eventstream

Utility library to facilitate the creation of endpoints that implement the server-sent events (SSE) protocol to stream events to clients, provided as a connect-compatible middleware.

Sucn an endpoint can be consumed in web browsers using EventSource.

Since this library is connect-compatible, it is usable with frameworks such as the following:

• connect
• Express
• Next.js *

Additionally, this library is GRIP-aware for scaling. In fact, when running on serverless environments such as Next.js, you will almost always want to use GRIP to hold the stream open while your application publishes to it in short-lived connections.

Supported GRIP servers include:

• Pushpin
• Fanout Cloud

Author: Katsuyuki Ohmuro [e-mail]

Based on Previous work by Justin Karneges [e-mail], Benjamin Goering [e-mail]

Initialize

Construct a ConnectEventStream object. This object's constructor takes an optional object that has grip and gripPrefix.

grip is optional and can be any of the following:

1. null. This is the default, and GRIP will not be used.
2. a string. This will be parsed using parseGripUri. The common use case would be to pass in process.env.GRIP_URL.
3. an object that has control_uri, control_iss, and key, used to initialize a GRIP publisher.
4. an array of objects described in 3.
5. an instantiated Publisher object from @fanoutio/grip. Publishing through connect-eventstream will then end up publishing to all channels on that Publisher object.

gripPrefix is optional and defaults to 'events-' if not specified. This can be used to namespace GRIP events.

import { ConnectEventStream } from "@fanoutio/connect-eventstream";
const connectEventStream = new ConnectEventStream({grip:process.env.GRIP_URI});

You need to create this object once as a singleton and then refer to it from all routes, as events sent over the publisher will only be seen by requests listening on the same instance. This also means that if your application has several processes running, published events will only go to HTTP Connections on the process that publishes the message. To scale to more than one web server process, you'll need to use GRIP, and make sure you publish each event from one place.

To use in Express:

Add Routes

Add routes, and use connectEventStream to create handlers. For this you have two options:

1. Call connectEventStream as a function, and pass in a string or array of strings. These strings will be used as the names of the channel(s) to listen to. Any tokens in the channel names delimited by { and } will be replaced by their corresponding values from route parameters.

import { ConnectEventStream } from "@fanoutio/connect-eventstream";
export const CHANNEL_NAME = 'test';
const connectEventStream = new ConnectEventStream({grip:process.env.GRIP_URI});

// localhost:7999/api/events (listens on 'test' because it is literal string passed in)
app.get('/api/events', connectEventStream(CHANNEL_NAME));

// localhost:7999/api/events/test (listens on 'test' because {id} is replaced by route parameter)
app.get('/api/events/:id', connectEventStream('{id}'));

2. (advanced) Call connectEventStream as a function, and pass in a function that takes a request object and returns a string or an array of strings. These strings will be used as the names of the channels to listen to.

Publish Events

See Publishing Events section below

To use in Next.js:

Next.js's development server continuously monitors and rebuilds files. Each time this happens, your singleton instance of ConnectEventStream will be recreated and previous instances will become unreachable.

To keep the singleton accessible, use the getConnectEventStreamSingleton function exported from this package. This function takes an object as an argument, and this is the same object that you would pass to the constructor of ConnectEventStream.

Add Routes

Add API routes to your to Next.js application in the standard way, to handle requests to serve event streams. From these API routes, call connectEventStream in the same way as in Express and then export them as the default export from your route files.

1. Call connectEventStream and pass in a string or array of strings.

/lib/eventStream.js

import { getConnectEventStreamSingleton } from "@fanoutio/connect-eventstream";
export const CHANNEL_NAME = 'test';
export const connectEventStream = getConnectEventStreamSingleton({grip: process.env.GRIP_URL});

/api/events.js

import { connectEventStream, CHANNEL_NAME } from "../../lib/eventStream";
// localhost:7999/api/events (listens on 'test' because it is literal string passed in)
export default connectEventStream(CHANNEL_NAME);

/api/events/[id].js

import { connectEventStream } from "../../lib/eventStream";
// localhost:7999/api/events/test (listens on 'test' because {id} is replaced by route parameter)
export default connectEventStream('{id}');

2. (advanced) Call connectEventStream and pass in a function that returns a string or an array of strings.

Publish Events

See Publishing Events section below

Publishing Events

To publish, call connectEventStream.publishEvent(channel, { event, data }). event is the string name and data is a JavaScript object that represents the Server Sent Event. This is an async function, so you may await it if you wish to block until the event has sent. Notably, if GRIP is being used, this will block until GRIP publish has completed.

await connectEventStream.publishEvent(CHANNEL_NAME, { event: 'message', 'data': { name: 'John' } });

Alternatively, if you will be sending many events to the same channel, you can get a ChannelPublisher by calling connectEventStream.getChannelPublisher(channel). Then you can call publishEvent({ event, data }) on the returned object.

const publisher = connectEventStream.getChannelPublisher('test');
await publisher.publishEvent({ event: 'message', 'data': { name: 'Alice' } });
await publisher.publishEvent({ event: 'message', 'data': { name: 'Bob' } });

If you wish to pipe a stream, you can call connectEventStream.createChannelWritable(channel) and pass the name of a channel. This will return a stream.Writeable object whose write() method can be used to emit objects to clients listening to the appropriate channels from the event streams endpoints created above.

const writable = connectEventStream.createChannelWritable('test'); // or publisher.createWritable()
writable.write({ event: 'message', 'data': { baz: [ 'hi', 'ho', 'hello', ] } });
writable.end();

There will be appropriate backpressure on this Writeable so that writing goes only as fast as events can be dispatched, which is especially important when publishing through GRIP.

Advanced Usage

Direct invocation

If you wish to run connect-eventstream's functionality directly, for example in a conditional way, you may call connectEventStream.run(req, res, channels).

app.get('/', async (req, res, next) => {
    // Only do connectEventStream if header 'foo' has value 'bar' 
    if (req.headers['foo'] === 'bar') {
        try {
            await connectEventStream.run(req, res, ['test']);
        } catch(ex) {
            next(ex instanceof Error ? ex : new Error(ex));
        }
    } else {
        next();
    }
});

Multiple singletons

getConnectEventStreamSingleton takes an optional second parameter. There may be advanced scenarios where you need more than one instance of ConnectEventStream. In such a case you can use this second parameter to identify each instance.

import { getConnectEventStreamSingleton } from "@fanoutio/connect-eventstream";
const connectEventStream1 = new getConnectEventStreamSingleton({grip:process.env.GRIP_URI_1});
const connectEventStream2 = new getConnectEventStreamSingleton({grip:process.env.GRIP_URI_2});