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# Websocket Notes

The WebSocket protocol is a modern chat messaging protocol.

It provides a persistent channel that sends text and binary data in both directions (full-duplex).

The data can even be sent simultaneously, if needed.

While originally designed to facilitate the communication between a server and browser, WebSocket can be (and widely are) used in mobile apps.

# WS vs HTTP

HTTP and WebSocket both are communication protocols used in client-server communication.

WS vs HTTP

Connection via WS Connection via HTTP
Bi-directional communication Unidirectional communication
Connection is kept alive untile terminated by client or server Connection is terminated after request/response
Real time data is received on a single communication channel & can be continuously updated HTTP data requests use simple RESTfull API. They send a ome-tine current state response for a query
Best used for applications in need of quick connections & realtime data Best used for applications that dont reqire quick, 2-way connections
E.g: crypto market cap, multiplayer game, collaborative platform, messaging app E.g: Browser search, email, social media updates, final game scores, ...

# WS vs SSE

Feature WS SSE
Communication Direction wo-way (full duplex) One-way (server to browser)
Complexity More complex Simpler
Data Format F lexible (text and binary) Primarily text-based

# WS vs XMPP

XMPP (Extensible Messaging and Presence Protocol) is an open standard communications protocol that allows for transmitting XML fragments between a server and client. A client, in this case, would be a browser or a mobile app. XMPP is one of the oldest and most widely supported instant messaging protocols. It has been in use for more than two decades.

Pros of XMPP Pros of WS
An open, decentralized standard that makes it extensible & flexible A persistent connection which makes it one of fastest transfer protocols
Great securtiy features such as authentication, authorization, and encryption Multiple users can be connected through a single connection - lowering latency
Two decades of use & widespread support can send any binary or text-based data type
Can stack with other protocols such as Websocket to remove some limitations Extensible & flexible - can connect multiple types of protocols together
Cons of XMPP Cons of WS
Not optimized for transfering binary data Fewer built-int security measures than XMPP
Decentralized architecture emans slower transmission speed Doesnt provide any messaging or presense functionality & requires an additional chat protocol to provide these functionalities

# Horizontal Scalable

# Issue #1: State

Start with the HAProxy

defaults
  mode http
  option http-server-close
  timeout connect     5s
  timeout client      30s
  timeout client-fin  30s
  timeout server      30s
  timeout tunnel      1h
  default-server inter 1s rise 2 fall 1 on-marked-down shutdown-sessions
  option forwardfor

frontend all
  bind 127.0.0.1:8080
  default_backend backends

backend backends
  server srv1 127.0.0.1:8081 check
  server srv2 127.0.0.1:8082 check

By default, HAProxy is using a round-robin strategy

Most REST APIs are stateless. It means that nothing related to a single user making a request is saved on an instance itself. The thing is, it is not the same case with WebSockets.

issue1

# Solution: Sticky sessions

Sticky sessions (sticky connection)

backend backends
  balance leastconn
  cookie serverid insert
  server srv1 127.0.0.1:8081 check cookie srv1
  server srv2 127.0.0.1:8082 check cookie srv2
  • Instead of using round-robin, go with leastconn => new user is connected to the instance with the lowest overall number of connections.
  • Sign every request from a single user with a cookie. It will contain the name of the backend to be used.

# Issue #2 Broadcasting

The WebSocket Server knows only about clients connected to this specific instance

# Solution: Pub/Sub

The easiest option is to introduce communication between different instances. All of them could be subscribed to a specific channel and handle upcoming messages.

There are many ready-to-go solutions, like Redis, Kafka, or Nats.

# Refs

← Logging / Audit notes →