Is an EventSource (SSE) supposed to try to reconnect indefinitely?

Question

I\'m working on a project utilizing Server-Sent-Events and have just run into something interesting: connection loss is handled differently between Chrome and Firefox.

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Answer 1

Server Side Events work differently in all of the browsers, but they all close the connection during certain circumstances. Chrome, for example, closes the connection on 502 errors while a server is restarted. So, it is best to use a keep-alive as others suggest or reconnect on every error. Keep-alive only reconnect at a specified interval that must be kept long enough to avoid overwhelming the server. Reconnecting on every error has the lowest possible delay. However, it is only possible if you take an approach that keeps server load to a minimum. Below, I demonstrate an approach that reconnects at a reasonable rate.

This code uses a debounce function along with reconnect interval doubling. It works well, connecting at 1 second, 4, 8, 16...up to a maximum of 64 seconds at which it keeps retrying at the same rate. I hope this helps some people.

function isFunction(functionToCheck) {
  return functionToCheck && {}.toString.call(functionToCheck) === '[object Function]';
}

function debounce(func, wait) {
    var timeout;
    var waitFunc;

    return function() {
        if (isFunction(wait)) {
            waitFunc = wait;
        }
        else {
            waitFunc = function() { return wait };
        }

        var context = this, args = arguments;
        var later = function() {
            timeout = null;
            func.apply(context, args);
        };
        clearTimeout(timeout);
        timeout = setTimeout(later, waitFunc());
    };
}

// reconnectFrequencySeconds doubles every retry
var reconnectFrequencySeconds = 1;
var evtSource;

var reconnectFunc = debounce(function() {
    setupEventSource();
    // Double every attempt to avoid overwhelming server
    reconnectFrequencySeconds *= 2;
    // Max out at ~1 minute as a compromise between user experience and server load
    if (reconnectFrequencySeconds >= 64) {
        reconnectFrequencySeconds = 64;
    }
}, function() { return reconnectFrequencySeconds * 1000 });

function setupEventSource() {
    evtSource = new EventSource(/* URL here */); 
    evtSource.onmessage = function(e) {
      // Handle even here
    };
    evtSource.onopen = function(e) {
      // Reset reconnect frequency upon successful connection
      reconnectFrequencySeconds = 1;
    };
    evtSource.onerror = function(e) {
      evtSource.close();
      reconnectFunc();
    };
}
setupEventSource();