• service-worker.js

  • The code in oninstall and onactivate force the service worker to control the clients ASAP.

    self.oninstall = function(event) {
    self.onactivate = function(event) {
  • When fetching, distinguish if this is a resource fetch. If so, apply the server selection algorithm. Else, let the request reach the network. Could should be autoexplanatory.

    self.onfetch = function(event) {
      var request = event.request;
      if (isResource(request)) {
      } else {
  • A request is a resource request if it is a GET for something inside imgs.

    function isResource(request) {
      return request.url.match(/\/imgs\/.*$/) && request.method === 'GET';
  • Fetching from the best server consists of getting the server loads, selecting the server with lowest load, and compose a new request to find the resource in the selected server.

    function fetchFromBestServer(request) {
      var session = request.url.match(/\?session=([^&]*)/)[1];
      return getServerLoads(session)
        .then(function(serverUrl) {
  • Get the resource path and combine with serverUrl to get the resource URL but in the selected server.

          var resourcePath = request.url.match(/\/imgs\/[^?]*/)[0];
          var serverRequest = new Request(serverUrl + resourcePath);
          return fetch(serverRequest);
  • Query the back-end for servers loads.

    function getServerLoads(session) {
      return fetch('./server-loads?session=' + session).then(function(response) {
        return response.json();
  • Get the server with minimum load and return its URL. In a real scenario this could return servers in other domains, just remember to set the CORS headers properly.

    function selectServer(serverLoads) {
  • Not very efficient but super-clear way of finding the index of the server with minimum load.

      var min = Math.min.apply(Math, serverLoads);
      var serverIndex = serverLoads.indexOf(min);
  • Servers are 1, 2, 3…

      return './server-' + (serverIndex + 1);

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