centrifuge

Centrifuge and Centrifugo bidirectional SDK for NodeJS, React-Native and browser

This SDK provides a client to connect to Centrifugo or any Centrifuge-based server using pure WebSocket or one of the fallback transports from web browser, ReactNative, or NodeJS environments.

The client behaves according to a common Centrifigo SDK spec. It's recommended to read that before starting to work with this SDK as the spec covers common SDK behavior - describes client and subscription state transitions, main options and methods. Then proceed with this readme for more specifics about centrifuge-js.

The features implemented by this SDK can be found in SDK feature matrix.

• Install
• Quick start
• WebSocket transport
• Using fallbacks
  • Bidirectional emulation
  • SockJS
• WebTransport (experimental)
• Client API
  • Client methods and events
  • Connection token
• Subscription API
  • Subscription methods and events
  • Subscription token
• Message batching
• Server-side subscriptions
• Configuration options
• Protobuf support
• Using with NodeJS
• Custom WebSocket constructor

Install

Using CDN (replace 3.0.0 to a concrete version number):

<script src="https://unpkg.com/centrifuge@3.0.0/dist/centrifuge.js"></script>

It's also available on cdnjs.

You can install this SDK via npm:

npm install centrifuge

And then:

import { Centrifuge } from 'centrifuge';

By default, library works with JSON only, see Protobuf support section to see how to import client with Protobuf support.

Quick start

The basic usage example may look like this:

// Use WebSocket transport endpoint.
const centrifuge = new Centrifuge('ws://centrifuge.example.com/connection/websocket');

// Allocate Subscription to a channel.
const sub = centrifuge.newSubscription('news');

// React on `news` channel real-time publications.
sub.on('publication', function(ctx) {
    console.log(ctx.data);
});

// Trigger subscribe process.
sub.subscribe();

// Trigger actual connection establishement.
centrifuge.connect();

Note, that we explicitly call .connect() method to initiate connection establishement with a server and .subscribe() method to move Subscription to subsribing state (which should transform into subscribed state soon after connection with a server is established). The order of .connect() and .subscribe calls does not actually matter here.

Centrifuge object and Subscription object are both instances of EventEmitter. Below we will describe events that can be exposed in detail.

Websocket transport

WebSocket is the main protocol used by centrifuge-js to communicate with a server. In browser environment it's available globally, but if you want to connect from NodeJS env – then you need to provide WebSocket constructor to centrifuge-js explicitly. See below more information about this.

Using fallbacks

In the quick start example above we used WebSocket endpoint to configure Centrifuge. WebSocket is the main transport – it's bidirectional out of the box.

In some cases though, WebSocket connection may not be established (for example, due to corporate firewalls and proxies). For such situations centrifuge-js offers several WebSocket fallback options.

Bidirectional emulation

SockJS is robust and stable product, but it's an extra dependency, it's pretty old, comes with some overhead and sticky sessions requirement for a distributed backend case. In most scenarios these days clients are fine to use WebSocket protocol for messaging. There are rare connection issues though which are caused by corporate firewall and proxy software. To deal with users behind such proxies Centrifuge SDK offers its own bidirectional emulation layer. This layer uses two HTTP-based transports:

• HTTP-streaming based on ReadableStream API
• SSE (EventSource).

Bidirectional emulation must be first enabled on a server-side. For example, see Centrifugo docs to find out how. Then in Javascript you can slightly change client initialization and point it to a list of endpoints and transports you want to use:

const transports = [
    {
        transport: 'websocket',
        endpoint: 'ws://example.com/connection/websocket'
    },
    {
        transport: 'http_stream',
        endpoint: 'http://example.com/connection/http_stream'
    },
    {
        transport: 'sse',
        endpoint: 'http://example.com/connection/sse'
    }
];
const centrifuge = new Centrifuge(transports);
centrifuge.connect()

In this case, client will try transports in order, one by one, during the initial handshake. Until success. Then will only use a successful transport during reconnects.

Supported transports are:

• websocket
• http_stream
• sse
• sockjs (yes, SockJS can also be used as a fallback in the bidirectional emulation layer, but sticky session must be used on the backend in distributed case, SockJS is currently in DEPRECATED status in Centrifugal ecosystem).
• webtransport (experimental, see details below)

If you want to use sticky sessions on a load balancer level as an optimimization for Centrifugal bidirectional emulation layer keep in mind that we currently use same-origin credentials policy for emulation requests in http_stream and sse transport cases. So cookies will only be passed in same-origin case. Please open an issue in case you need to configure more relaxed credentials. Though in most cases stickyness based on client's IP may be sufficient enough.

Using SockJS

SockJS usage is DEPRECATED in the Centrifugal ecosystem

If you want to use SockJS you must also import SockJS client before centrifuge.js

<script src="https://cdn.jsdelivr.net/npm/sockjs-client@1/dist/sockjs.min.js" type="text/javascript"></script>
<script src="https://unpkg.com/centrifuge@3.0.0/dist/centrifuge.js" type="text/javascript"></script>

Or provide it explicitly as a dependency:

import { Centrifuge } from 'centrifuge'
import SockJS from 'sockjs-client'

const transports = [{
    transport: "sockjs",
    endpoint: "http://localhost:8000/connection/sockjs"
}];

const centrifuge = new Centrifuge(transports, {
  sockjs: SockJS
})

Note, that in SockJS case endpoint starts with http://, not with ws:// as we used above when connecting to a pure WebSocket endpoint.

WebTransport (experimental)

WebTransport is experimental and is only supported by Centrifugo at the moment (i.e. it's not available in Centrifuge library for Go out of the box).

See information about it in Centrifugo WebTransport docs.

Client API

Let's look at top-level API of Centrifuge client.

Client methods and events

connect method

As we already showed above, we must call connect() method to make an actual connection request to Centrifugo server:

const centrifuge = new Centrifuge('ws://centrifuge.example.com/connection/websocket');
centrifuge.connect();

connect() triggers an actual connection request to server.

connected event

As soon as connection is established and client successfully authenticated – connected event on Centrifuge object instance will be called.

It's possible to listen to this event by setting event listener function on connected event:

centrifuge.on('connected', function(ctx) {
    // now client connected to Centrifugo and authenticated.
});

connecting event

connecting event fired when Centrifuge object goes to connecting state. This may be called during initial connect, or after being connected due to temporary connection loss.

centrifuge.on('connecting', function(ctx) {
    // do whatever you need in case of connecting to a server
});

disconnected event

disconnected event fired on Centrifuge object every time client disconnects for some reason. This can be terminal disconnect due to advice from a server or disconnect initiated by client-side.

centrifuge.on('disconnected', function(ctx) {
    // do whatever you need in case of disconnect from server
});

disconnect method

In some cases you may need to disconnect your client from server, use .disconnect() method to do this:

centrifuge.disconnect();

After calling this client will not try to reestablish connection periodically. You must call .connect() method manually again.

publish method

Sometimes you need to publish into channel without actually being subscribed to it. In this case you can use publish method:

centrifuge.publish("channel", {"input": "hello"}).then(function(res) {
    console.log('successfully published');
}, function(err) {
    console.log('publish error', err);
});

send method

This is only valid for Centrifuge library and does not work for Centrifugo server at the moment. send method allows sending asynchronous message from a client to a server.

centrifuge.send({"input": "hello"}).then(function(res) {
    console.log('successfully sent');
}, function(err) {
    console.log('send error', err);
});

rpc method

rpc method allows to send rpc request from client to server and wait for data response.

centrifuge.rpc("my.method.name", {"input": "hello"}).then(function(res) {
    console.log('rpc result', res);
}, function(err) {
    console.log('rpc error', err);
});

history method

Allows to get history from a server. This is a top-level analogue of Subscription.history method. But accepts a channel as first argument.

centrifuge.history("channel", {since: {offset: 0, epoch: "xyz"}, limit: 10}).then(function(resp) {
    console.log(resp);
}, function(err) {
    console.log('history error', err);
});

presence method

Allows to get presence info from a server. This is a top-level analogue of Subscription.presence method. But accepts a channel as first argument.

centrifuge.presence("channel").then(function(resp) {
    console.log(resp);
}, function(err) {
    console.log('presence error', err);
});

presenceStats method

Allows to get presence stats from a server. This is a top-level analogue of Subscription.presenceStats method. But accepts a channel as first argument.

centrifuge.presenceStats("channel").then(function(resp) {
    console.log(resp);
}, function(err) {
    console.log('presence stats error', err);
});

ready method

Returns a Promise which will be resolved upon connection establishement (i.e. when Client goes to connected state).

error event

To listen asynchronous error happening internally while Centrifuge client works you can set an error handler:

const centrifuge = new Centrifuge('ws://centrifuge.example.com/connection/websocket');

centrifuge.on('error', function(ctx) {
    console.log(ctx);
});

This can help you to log failed connection attempts, or token refresh errors, etc.

Connection Token

Depending on authentication scheme used by a server you may also want to provide connection token:

const centrifuge = new Centrifuge('ws://centrifuge.example.com/connection/websocket', {
    token: '<CONNECTION_TOKEN>'
});

In case of Centrifugo on a server side this may be a JSON Web Token - see authentication documentation for details on how to generate it on your backend side.

Connection token must come to the frontend from application backend - i.e. must be generated on the backend side. The way to deliver token to the application frontend is up to the developer. Usually you can pass it in template rendering context or issue a separate call to request a connection token from the backend.

If the token sets connection expiration then the client SDK will keep the token refreshed. It does this by calling a special callback function. This callback must return a new token. If a new token with updated connection expiration is returned from callback then it's sent to Centrifugo. If your callback returns an empty string – this means the user has no permission to connect to Centrifugo and the Client will move to a disconnected state. In case of error returned by your callback SDK will retry the operation after some jittered time.

An example of possible getToken function implementation:

function getToken(url, ctx) {
    return new Promise((resolve, reject) => {
        fetch(url, {
            method: 'POST',
            headers: new Headers({ 'Content-Type': 'application/json' }),
            body: JSON.stringify(ctx)
        })
        .then(res => {
            if (!res.ok) {
                throw new Error(`Unexpected status code ${res.status}`);
            }
            return res.json();
        })
        .then(data => {
            resolve(data.token);
        })
        .catch(err => {
            reject(err);
        });
    });
}

const client = new Centrifuge(
    'ws://localhost:8000/connection/websocket',
    {
        token: 'JWT-GENERATED-ON-BACKEND-SIDE',
        getToken: function (ctx) {
            return getToken('/centrifuge/connection_token', ctx);
        }
    }
);

:::tip

If initial token is not provided, but getToken is specified – then SDK assumes that developer wants to use token authentication. In this case SDK attempts to get a connection token before establishing an initial connection.

:::

Subscription API

What we usually want from Centrifugo is to receive new messages published into channels. To do this we must create Subscription object.

Subscription methods and events

Subscribe to a channel

The simplest usage that allow to subscribe on channel and listen to new messages is:

const sub = centrifuge.newSubscription('example');

sub.on('publication', function(ctx) {
    // handle new Publication data coming from channel "news".
    console.log(ctx.data);
});

sub.subscribe();

Subscription events

Some events which can be listened on Subscription object are:

• publication – called when new publication received from a Subscription channel
• join – called when someone joined channel
• leave – called when someone left channel
• subscribing - called when Subscription goes to subscribing state (initial subscribe and re-subscribes)
• subscribed – called when Subscription goes to subscribed state
• unsubscribed – called when Subscription goes to unsubscribed state
• error – called when subscription on channel failed with error. It can be called several times during lifetime as browser client automatically resubscribes on channels after successful reconnect (caused by temporary network disconnect for example or Centrifugo server restart)

Don't be frightened by amount of events available. In most cases you only need some of them until you need full control to what happens with your subscriptions.

Subscription objects are instances of EventEmitter.

presence method of Subscription

presence allows to get information about clients which are subscribed on channel at this moment. Note that this information is only available if presence option enabled in Centrifugo configuration for all channels or for channel namespace.

const sub = centrifuge.newSubscription("news");
sub.subscribe()

sub.presence().then(function(ctx) {
    console.log(ctx.clients);
}, function(err) {
    // presence call failed with error
});

presence is internally a promise that will be waiting for subscription subscribe success if required.

As you can see presence data is a map where keys are client IDs and values are objects with client information.

Format of err in error callback:

{
    "code": 108,
    "message": "not available"
}

• code - error code (number)
• message – error description (string)

Note, that in order presence to work corresponding options must be enabled in server channel configuration (on top level or for channel namespace)

presenceStats method of subscription

presenceStats allows to get two counters from a server: number of total clients currently subscribed and number of unique users currently subscribed. Note that this information is only available if presence option enabled in server configuration for a channel.

sub.presenceStats().then(function(ctx) {
    console.log(ctx.numClients);
}, function(err) {
    // presence stats call failed with error
});

history method of subscription

history method allows to get last messages published into channel. Note that history for channel must be configured in Centrifugo to be available for history calls from client.

sub.history({limit: 100}).then(function(ctx) {
    console.log(ctx.publications);
}, function(err) {
    // history call failed with error
});

Note, that in order history to work corresponding options must be enabled in server channel configuration (on top level or for channel namespace)

Some history options available:

• limit (number)
• since (StreamPosition)
• reverse (boolean)

resp = await subscription.history({'since': {'offset': 2, 'epoch': 'xcf4w'}, limit: 100});

If server can't fulfill a query for history (due to stream retention - size or expiration, or malformed offset, or stream already has another epoch) then an Unrecoverable Position Error will be returned (code 112).

To only call for current offset and epoch use:

resp = await subscription.history({limit: 0});

I.e. not providing since and using zero limit.

publish method of subscription

publish method of Subscription object allows publishing data into channel directly from a client.

Using client-side publish is not an idiomatic Centrifugo usage in many cases. Centrifugo is standalone server and when publishing from a client you won't get the message on the backend side (except using publish proxy feature of Centrifugo). In most real-life apps you need to send new data to your application backend first (using the convenient way, for example AJAX request in web app) and then publish data to Centrifugo over Centrifugo API.

Just like presence and history publish must be allowed in Centrifugo configuration for all channels or for channel namespace.

sub.publish({"input": "hello world"}).then(function() {
        // success ack from Centrifugo received
    }, function(err) {
        // publish call failed with error
    });
});

Note, that in order publish to work in Centrifugo corresponding option must be enabled in server channel configuration or client should have capability to publish.

unsubscribe method of subscription

You can call unsubscribe method to unsubscribe from a channel:

sub.unsubscribe();

Important thing to know is that unsubscribing from subscription does not remove event handlers you already set to that Subscription object. This allows to simply subscribe to channel again later calling .subscribe() method of subscription (see below). But there are cases when your code structured in a way that you need to remove event handlers after unsubscribe to prevent them be executed twice in the future. To do this remove event listeners explicitly after calling unsubscribe():

sub.unsubscribe();
sub.removeAllListeners();

ready method of subscription

Returns a Promise which will be resolved upon subscription success (i.e. when Subscription goes to subscribed state).

Subscription token

You may want to provide subscription token:

const sub = centrifuge.newSubscription("news", {
    token: '<SUBSCRIPTION_TOKEN>'
});

In case of Centrifugo on a server side this may be a JSON Web Token - see channel token auth documentation for details on how to generate it on your backend side.

Subscription token must come to the frontend from application backend - i.e. must be generated on the backend side. The way to deliver token to the application frontend is up to the developer. Usually you can pass it in template rendering context or issue a separate call to request a connection token from the backend.

If token sets subscription expiration client SDK will keep token refreshed. It does this by calling special callback function. This callback must return a new token. If new token with updated subscription expiration returned from a calbback then it's sent to Centrifugo. If your callback returns an empty string – this means user has no permission to subscribe to a channel anymore and subscription will be unsubscribed. In case of error returned by your callback SDK will retry operation after some jittered time.

An example:

function getToken(url, ctx) {
    return new Promise((resolve, reject) => {
        fetch(url, {
            method: 'POST',
            headers: new Headers({ 'Content-Type': 'application/json' }),
            body: JSON.stringify(ctx)
        })
        .then(res => {
            if (!res.ok) {
                throw new Error(`Unexpected status code ${res.status}`);
            }
            return res.json();
        })
        .then(data => {
            resolve(data.token);
        })
        .catch(err => {
            reject(err);
        });
    });
}

const client = new Centrifuge('ws://localhost:8000/connection/websocket', {});

const sub = centrifuge.newSubscription(channel, {
    token: 'JWT-GENERATED-ON-BACKEND-SIDE',
    getToken: function (ctx) {
        // ctx has channel in the Subscription token case.
        return getToken('/centrifuge/subscription_token', ctx);
    },
});
sub.subscribe();

:::tip

If initial token is not provided, but getToken is specified – then SDK assumes that developer wants to use token authorization for a channel subscription. In this case SDK attempts to get a subscription token before initial subscribe.

:::

Message batching

There is also a command batching support. It allows to send several commands to a server in one request - may be especially useful when connection established via one of HTTP-based transports.

You can start collecting commands by calling startBatching() method:

centrifuge.startBatching();

Finally if you don't want batching anymore call stopBatching() method:

centrifuge.stopBatching();

This call will flush all collected commands to a network.

Server-side subscriptions

TODO.

Configuration options

You can check out all available options with description in source code.

Let's look at available configuration parameters when initializing Centrifuge object instance.

debug

debug is a boolean option which is false by default. When enabled lots of various debug messages will be logged into javascript console. Mostly useful for development or troubleshooting.

minReconnectDelay

When client disconnected from a server it will automatically try to reconnect using a backoff algorithm with jitter. minReconnectDelay option sets minimal interval value in milliseconds before first reconnect attempt. Default is 500 milliseconds.

maxReconnectDelay

maxReconnectDelay sets an upper reconnect delay value. Default is 20000 milliseconds - i.e. clients won't have delays between reconnect attempts which are larger than 20 seconds.

maxServerPingDelay

maxServerPingDelay sets the maximum delay of server pings after which connection is considered broken and client reconnects. In milliseconds. Default is 10000.

protocol

By default, client works using json protocol. If you want to use binary transfer with Protobuf-based protocol this option must be set to protobuf. See more details about Protobuf communication in a special chapter.

token

Set initial connection token.

getToken

Set function for getting connection token. This may be used for initial token loading and token refresh mechanism (when initial token is going to expire).

data

Set custom data to send to a server withing every connect command.

name

Set custom client name. By default, it's set to js. This is useful for analitycs and semantically must identify an environment from which client establishes a connection.

version

Version of your application - useful for analitycs.

timeout

Timeout for operations in milliseconds.

websocket

websocket option allows to explicitly provide custom WebSocket client to use. By default centrifuge-js will try to use global WebSocket object, so if you are in web browser – it will just use native WebSocket implementation. See notes about using centrifuge-js with NodeJS below.

sockjs

sockjs option allows to explicitly provide SockJS client object to Centrifuge client.

Protobuf support

To import client with Protobuf protocol support:

<script src="https://unpkg.com/centrifuge@3.0.0/dist/centrifuge.protobuf.js"></script>

Or if you are developing with npm:

import Centrifuge from 'centrifuge/build/protobuf';

This client uses protobuf.js under the hood.

Centrifuge client with Protobuf support also works with JSON. To enable binary websocket add protocol: "protobuf" option to Centrifuge configuration options:

const centrifuge = new Centrifuge('ws://centrifuge.example.com/connection/websocket", {
    protocol: 'protobuf'
});

Using with NodeJS

NodeJS does not have native WebSocket library in std lib. To use centrifuge-js on Node you need to explicitly provide WebSocket constructor to the library.

First, install WebSocket dependency:

npm install ws

At this point you have 2 options. Explicitly pass WebSocket object to Centrifuge.

const Centrifuge = require('centrifuge');
const WebSocket = require('ws');

var centrifuge = new Centrifuge('ws://localhost:8000/connection/websocket', {
    websocket: WebSocket
})

Or define it globally:

const Centrifuge = require('centrifuge');
global.WebSocket = require('ws'); 

const centrifuge = new Centrifuge('ws://localhost:8000/connection/websocket');

Custom WebSocket constructor

If you are building a client for a non-browser environment and want to pass custom headers then you can use the following approach to wrap a WebSocket constructor and let custom options to be used on connection initialization:

var Centrifuge = require("centrifuge");
const WebSocket = require('ws');

const myWs = function (options) {
    return class wsClass extends WebSocket {
        constructor(...args) {
            super(...[...args, ...[options]])
        }
    }
}

It should be now possible to use pass your custom WebSocket constructor to centrifuge-js and so custom headers will be used when connecting to a server:

var centrifuge = new Centrifuge('ws://localhost:8000/connection/websocket', {
    websocket: myWs({ headers: { Authorization: '<token or key>' } }),
});