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Version: 4.x

Handling disconnections

Now, let's highlight two really important properties of Socket.IO:

  1. a Socket.IO client is not always connected
  2. a Socket.IO server does not store any event

Even over a stable network, it is not possible to maintain a connection alive forever.

Which means that your application needs to be able to synchronize the local state of the client with the global state on the server after a temporary disconnection.


The Socket.IO client will automatically try to reconnect after a small delay. However, any missed event during the disconnection period will effectively be lost for this client.

In the context of our chat application, this implies that a disconnected client might miss some messages:

The disconnected client does not receive the 'chat message' eventThe disconnected client does not receive the 'chat message' event

We will see in the next steps how we can improve this.