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Version: 4.x


Here is a list of common questions about Socket.IO:

Something does not work properly, please help?

Please check the Troubleshooting guide.

How does it work under the hood?

The Socket.IO connection can be established with different low-level transports:

Socket.IO will automatically pick the best available option, depending on:

  • the capabilities of the browser (see here and here)
  • the network (some networks block WebSocket and/or WebTransport connections)

You can find more detail about that in the "How it works" section.

What are the features provided by Socket.IO over plain WebSocket?

WebSockets are awesome! No, really. They provide an efficient way for transferring data between a client and a server. Among the advantages:

  • you don't need to rely on periodic polling to fetch data from the server
  • you don't need to repeatedly send all the HTTP headers when sending data to the server

Which make them perfect for low-latency and data-intensive applications like games, chats, collaborative solutions...

That being said, WebSockets are also pretty low-level and developing a realtime applications with WebSockets often requires an additional layer over them:

  • fallback to HTTP long-polling, in case the WebSocket connection can't be established
  • automatic reconnection, in case the WebSocket connection gets closed
  • acknowledgements, to send some data and expect a response from the other side
  • broadcast to all or to a subset of connected clients
  • scale up to multiple instances of the server
  • connection recovery, for short periods of disconnection

As you might have guessed, this additional layer is implemented by the Socket.IO library.

What is WebTransport?

In short, WebTransport is an alternative to WebSocket which fixes several performance issues that plague WebSockets like head-of-line blocking.

If you want more information about this new web API (which was included in Chrome in January 2022 and in Firefox in June 2023), please check those links:


Support for WebTransport is not enabled by default in Socket.IO, as it requires a secure context (HTTPS). Please check the dedicated tutorial if you want to play with WebTransport.

Does Socket.IO store the messages?

The Socket.IO server does not store any message.

It is the duty of your application to persist those messages somewhere for the clients that are not currently connected.


That being said, Socket.IO will store the messages for a brief period of time if you enable the Connection state recovery feature.

What are the delivery guarantees of Socket.IO?

Socket.IO does guarantee message ordering, no matter which low-level transport is used (even when switching between two transports).

Moreover, by default Socket.IO provides an at most once guarantee of delivery (also known as "fire and forget"), which means that under certain circumstances a message might get lost and no retry will be attempted.

More information about this here.

How to identify a given user?

There is no concept of user in Socket.IO.

It is the duty of your application to link a given Socket.IO connection to a user account.

For Node.js applications, you can for example:

Where can I find the changelog?

Please see here.