sse-events

SSE event source polyfill wrapped in Node.js like EventEmitter with performance fixes and custom api. Compatible with React/React-Native.

Motivation

• Improved retry algorithm to avoid loops and memory leaks
• Automatic close of connection upon server error codes (4xx, 5xx, ...)
• EventEmitter api
• Singletron pattern
• Listeners persisting across retries and/or connection close events
• Dynamic change of headers, path and parameters
• Random retry interval to reduce server request concurrency
• Dynamic progressive fallback interval for long lasting connection retries
• React/React-Native compatible

Implementation based on:

• EventSource remy/polyfills
• jordanbyron/react-native-event-source
• EventSource/eventsource.

Installing

Run the following command in your project's directory to grab the latest published version of this code:

$ npm install sse-events --save

or

$ yarn add sse-events

Using in your project

import EventSource from 'sse-events';

Create an instance, providing the configuration options (optional)

const sse = new EventSource({
  url: SSE_BASE_URL,
  path: '/some/path',
  options: {
    headers: {
      'User-Agent': USER_AGENT,
      Authorization: '...'
    }
  },
  reconnectInterval: 500,
  retryOnNetworkError: false
});

Bind few event listeners:

 
const { ON_ERROR, ON_OPEN } = EventSource.types;
 
sse.addEventListener('init', message => debug('SSE Init: ', message));
sse.addEventListener(ON_OPEN, () => {
  sse.retrying = false;
});
 
sse.addEventListener(ON_ERROR, (error: ESError) => {
  // refresh login
  if (
    error &&
    !sse.retrying &&
    error.data && error.data.code &&
    (
      (error.data.code >= 200 && error.data.code <= 202) ||
      error.data.code === 204
    )
  ) {
    delete sse.options.headers.Authorization;
    sse.retrying = true;
    store.dispatch(refreshLogin())
      .then(res => (res.data && sse.open()))
      .catch(() => null);
    return;
  }
 
  sse.retrying = false;
});

Now call the open method to establish the connection. No matter how may times this method is called, just one AND ONLY ONE connection is open a time. Some errors can interrupt the SSE retry algorithm and thus further call to this method is required to re-establish the connection. Listeners DON'T CLOSE if this method is called.

sse.open();

SSE connection can be suspended calling the close method. Instance headers, params or other options don't apply to the on-going connection unless manually closed and open again.

sse.close();

To close the connection and remove all listeners call the destroy method. Following calls to open WILL NOT restore previous event listeners.

sse.destroy();

Headers params can be set/updated calling this method.

sse.setHeaders({
  Authorization: '...'
});

Url params can be set/updated calling this method.

sse.setParams({
  scope: 'all'
});

Connection url and path can be set/updated directly from instance properties.

sse.path = '/path/to/sse/resource';
sse.url = 'example.com'

Connection States

// States
const {
  CONNECTING = 0, // trying to connect. No events.
  OPEN = 1, // connection established and running. Auto-retry enabled and events
  CLOSED = 2 // connection is closed. Auto-retry disabled and no events
} = sse;

Event Types

const {
  ON_OPEN = 'open', // readyState goes from CONNECTING -> OPEN
  ON_ERROR = 'error', // SSE, network or server response errors
  ON_CLOSE = 'close', // readyState goes to CLOSED
  ON_STATE = 'state', // readyState has changed
  ON_TIMEOUT = 'timeout' // server closed connection
} = EventSource.types;

Configuration Options

/**
 * Reference time interval between retry attempts in ms. Actual interval is randomly selected
 * between 100ms and this value to reduce server request blocking. For retry attempts, after
 * network errors, actual interval is randomly selected using a fallback progressive.
 * Default value 1000 ms.
 * @type {Number} 
 */
sse.reconnectInterval || 1000;
 
/**
 * Max fallback retry interval in ms.
 * Default value 15000 ms
 * @type {Number} 
 */
sse.maxInterval || 15000;
 
/**
 * Min fallback retry interval in ms.
 * Default value 1000 ms
 * @type {Number} 
 */
sse.minInterval || 1000;
 
/**
 * Max number of retry attempts before closing the connection. Negative value means unlimited
 * attempts.
 * Default value -1 (no limit).
 * @type {Number} 
 */
sse.maxAttempts || -1;
 
/**
 * Retry to connect on network or other internal errors.
 * Default value true.
 * @type {Boolean} 
 */
sse.retryOnNetworkError || true;
 
/**
 * Retry to connect on server error response.
 * Default value false.
 * @type {Boolean} 
 */
sse.retryOnServerError || false;
 
/**
 * Server error status codes to retry connection.
 * Default value [502, 503, 504]
 * @type {Array} 
 */
sse.serverErrorCodes  || [502, 503, 504];
 
/**
 * Connection URL
 * @type {String} 
 */
sse.url
 
/**
 * SSE Connection Options
 * @type {Object} 
 */
sse.options
 
/**
 * Connection URL params
 * @type {Object} 
 */
sse.params
 
/**
 * Connection URL path
 * @type {Object} 
 */
sse.path
 
/**
 * SSE Connection State
 * @type {CONNECTING|OPENED|CLOSED} 
 */
sse.readyState

License

This project is licensed under the MIT License