ReconnectingEventSource

Fork Note: this DisruptOps fork adds only support for an async function to retrieve header values. It is used here to add an authorization header containing a bearer token, since we need to secure access to the event source: https://github.com/disruptops/neo/blob/master/src/apps/public/src/services/sse.ts#L40

This is a small wrapper library around the JavaScript EventSource API to ensure it maintains a connection to the server. Normally, EventSource will reconnect on its own, however there are some cases where it may not. This library ensures a reconnect always happens.

To use it, just replace:

var es = new EventSource(url);

with:

var es = new ReconnectingEventSource(url);

Adding to your project

npm install reconnecting-eventsource

In a browser environment, bring this in using:

<script src="/node_modules/reconnecting-eventsource/dist/ReconnectingEventSource[.min].js"></script>

(You are free to copy this file out of node_modules if you wish)

For node/browserify/webpack/etc, use:

import ReconnectingEventSource from "reconnecting-eventsource";

Note: This project assumes you have a working EventSource available. If you are targeting a browser that doesn't support it, such as IE or Edge, you'll need to use a polyfill for EventSource.

Configuration

Like the EventSource, the constructor takes an optional configuration object: new ReconnectingEventSource(url, configuration). The configuration object is passed through to the underlying EventSource and can optionally include the following configuration:

{
    // indicating if CORS should be set to include credentials, default `false`
    withCredentials: false,

    // the maximum time to wait before attempting to reconnect in ms, default `3000`
    // note: wait time is randomised to prevent all clients from attempting to reconnect simultaneously
    max_retry_time: 3000,
}

Building from source

If you wish to build this project, check out this repository and modify the source files in src/. Then, run the following command:

npm run build

The resulting files are in lib/ for the node build and dist/ for the browser build.

When does the normal EventSource not reconnect?

Typically, the normal EventSource only reconnects if it is unable to reach the server at all. However, if the server is reached and it responds with an error (e.g. 500 status), then EventSource will stop reconnecting.

Why reconnect if the server responds with an error?

Some errors, such as 502, are probably temporary, and ideally wouldn't cause client apps to break and require user intervention to get them started again.

So this thing reconnects forever. How do I make it stop?

The client has to explicitly stop by calling es.close(). If you want to control this from the server, have the server send some kind of close instruction for the client to act on.