What is Socket.IO?

Socket.IO is a cross-browser JavaScript library that provides bidirectional, event-driven communication capabilities for use in realtime applications. It consists of a client-side browser library and a server-side Node.js library, both of which use similar syntax to enable an easily scriptable interface for developers.

The primary function of Socket.IO is to act as an abstraction layer for existing realtime protocols. In particular, Socket.IO is able to connect clients with servers over a WebSocket connection, all the while handling the intricate details of the underlying protocol with a simple, understandable API. It also places focus on connection reliability, with in-built mechanisms for disconnection detection, auto-reconnect, and firewall / proxy bypassing.

Socket.IO opens a connection by using its own transport protocol negotiation method, which automatically attempts to open a WebSocket connection in the browser. If the client does not support WebSocket, a fallback is provided in the form of a long-polling connection. Either way, the Socket.IO interface remains the same. Socket.IO connections may also be multiplexed and broadcast across grouped connection channels from within the same connection.

A Socket.IO scheme can be implemented in any application that relies on event-driven, realtime data streams over WebSocket, Comet-style, or Peer-to-Peer connections. This includes, but is not limited to, instant messaging, multi-user collaboration, real-time analytics, file-sharing, and notifications.

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