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Version: 4.x

Connection state recovery

First, let's handle disconnections by pretending that there was no disconnection: this feature is called "Connection state recovery".

This feature will temporarily store all the events that are sent by the server and will try to restore the state of a client when it reconnects:

  • restore its rooms
  • send any missed events

It must be enabled on the server side:

const io = new Server(server, {
connectionStateRecovery: {}

Let's see it in action:

As you can see in the video above, the "realtime" message is eventually delivered when the connection is reestablished.


The "Disconnect" button was added for demonstration purposes.

<form id="form" action="">
<input id="input" autocomplete="off" /><button>Send</button>
<button id="toggle-btn">Disconnect</button>

const toggleButton = document.getElementById('toggle-btn');

toggleButton.addEventListener('click', (e) => {
if (socket.connected) {
toggleButton.innerText = 'Connect';
} else {
toggleButton.innerText = 'Disconnect';

Great! Now, you may ask:

But this is an awesome feature, why isn't this enabled by default?

There are several reasons for this:

  • it doesn't always work, for example if the server abruptly crashes or gets restarted, then the client state might not be saved
  • it is not always possible to enable this feature when scaling up

That being said, it is indeed a great feature since you don't have to synchronize the state of the client after a temporary disconnection (for example, when the user switches from WiFi to 4G).

We will explore a more general solution in the next step.


You can run this example directly in your browser on: