Internals overview

Dependency graph

The Socket.IO codebase is split accross several repositories:

The following diagram displays the relationships between each project:

Each project brings its own set of features:

engine.io-parser

This is the JavaScript parser for the engine.io protocol encoding, shared by both engine.io-client and engine.io.

The specification for the protocol can be found here: https://github.com/socketio/engine.io-protocol

engine.io

Engine.IO is the implementation of transport-based cross-browser/cross-device bi-directional communication layer for Socket.IO.

Its main feature is the ability to swap transports on the fly. A connection (initiated by an engine.io-client counterpart) starts with XHR polling, but can then switch to WebSocket if possible.

It uses the engine.io-parser to encode/decode packets.

engine.io-client

This is the client for Engine.IO, the implementation of transport-based cross-browser/cross-device bi-directional communication layer for Socket.IO.

It runs in both the browser (including HTML5 WebWorker) and Node.js.

It uses the engine.io-parser to encode/decode packets.

socket.io-adapter

This is the default Socket.IO in-memory adapter class.

This module is not intended for end-user usage, but can be used as an interface to inherit from from other adapters you might want to build, like socket.io-redis.

socket.io-redis

This is the adapter using the Redis Pub/Sub mechanism to broadcast messages between multiple nodes.

socket.io-parser

A socket.io encoder and decoder written in JavaScript complying with version 3 of socket.io-protocol. Used by socket.io and socket.io-client.

socket.io

Socket.IO brings some syntactic sugar over the Engine.IO “raw” API. It also brings two new concepts, Rooms and Namespaces, which introduce a separation of concern between communication channels. Please see the associated documentation there.

By default, it exposes a browser build of the client at /socket.io/socket.io.js.

It uses the socket.io-parser to encode/decode packets.

socket.io-client

This is the client for Socket.IO. It relies on engine.io-client, which manages the transport swapping and the disconnection detection.

It handles reconnection automatically, in case the underlying connection is severed.

It uses the socket.io-parser to encode/decode packets.

Under the hood

Connection

const client = io('https://myhost.com');

The following steps take place:

on the client-side, a new engine.io-client instance is created
the engine.io-client instance tries to establish a polling transport

GET https://myhost.com/socket.io/?EIO=3&transport=polling&t=ML4jUwU&b64=1with:  "EIO=3"               # the current version of the Engine.IO protocol  "transport=polling"   # the transport being established  "t=ML4jUwU&b64=1"     # a hashed timestamp for cache-busting

the engine.io server responds with:

{  "type": "open",  "data": {    "sid": "36Yib8-rSutGQYLfAAAD",  // the unique session id    "upgrades": ["websocket"],      // the list of possible transport upgrades    "pingInterval": 25000,          // the 1st parameter for the heartbeat mechanism    "pingTimeout": 5000             // the 2nd parameter for the heartbeat mechanism  }}

the content is encoded by the engine.io-parser as:

'96:0{"sid":"hLOEJXN07AE0GQCNAAAB","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000}2:40'with:  "96"  # the length of the first message  ":"   # a separator between length and content  "0"   # the "open" message type  '{"sid":"hLOEJXN07AE0GQCNAAAB","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000}' # the JSON-encoded handshake data  "2"   # the length of the second message  ":"   # a separator between length and content  "4"   # the "message" message type  "0"   # the "open" message type in Socket.IO protocol

the content is then decoded by the engine.io-parser on the client-side
an open event is emitted at the engine.io-client level
a connect event is emitted at the socket.io-client level

Upgrade

Once all the buffers of the existing transport (XHR polling) are flushed, an upgrade gets tested on the side by sending a probe.

GET wss://myhost.com/socket.io/?EIO=3&transport=websocket&sid=36Yib8-rSutGQYLfAAADwith:  "EIO=3"                     # again, the current version of the Engine.IO protocol  "transport=websocket"       # the new transport being probed  "sid=36Yib8-rSutGQYLfAAAD"  # the unique session id

a “ping” packet is sent by the client in a WebSocket frame, encoded as 2probe by the engine.io-parser, with 2 being the “ping” message type.
the server responds with a “pong” packet, encoded as 3probe, with 3 being the “pong” message type.
upon receiving the “pong” packet, the upgrade is considered complete and all following messages go through the new transport.