websocket-realtime-expert

license	Apache-2.0
name	websocket-realtime-expert
description	WebSockets, SSE, and real-time communication with Socket.io and native APIs. Activate on: WebSocket, real-time, SSE, Socket.io, live updates, push notifications, bidirectional, presence. NOT for: message queue infrastructure (use event-driven-architecture-expert), API gateway routing (use api-gateway-reverse-proxy-expert).
allowed-tools	Read,Write,Edit,Bash(npm:*,npx:*)
category	Backend & Infrastructure
tags	["websocket","realtime","sse","socket-io","push"]
pairs-with	[{"skill":"event-driven-architecture-expert","reason":"Backend events feed real-time channels"},{"skill":"cache-strategy-invalidation-expert","reason":"Cache invalidation triggers real-time updates"},{"skill":"api-gateway-reverse-proxy-expert","reason":"WebSocket upgrade handling at the gateway"}]

WebSocket & Real-Time Expert

Build reliable real-time communication systems using WebSockets, Server-Sent Events, and managed real-time services.

Activation Triggers

Activate on: "WebSocket", "real-time", "SSE", "Socket.io", "live updates", "push notifications", "bidirectional", "presence", "live cursors", "collaborative editing"

NOT for: Message queue setup → event-driven-architecture-expert | Gateway WebSocket routing → api-gateway-reverse-proxy-expert | Streaming data pipelines → streaming-pipeline-architect

Quick Start

1. Choose protocol — WebSocket for bidirectional, SSE for server-push, WebTransport for low-latency
2. Plan reconnection — exponential backoff with jitter, resume from last event ID
3. Design message format — typed JSON with type discriminator and monotonic sequence IDs
4. Scale horizontally — use Redis Pub/Sub or NATS as a broadcast backplane
5. Handle presence — heartbeat-based with configurable timeout (30s default)

Core Capabilities

Domain	Technologies
WebSocket	ws (Node), Socket.io 4.8+, uWebSockets.js
SSE	Native EventSource, @microsoft/fetch-event-source
Managed	Supabase Realtime, Ably, Pusher, PartyKit
Scaling	Redis Pub/Sub, NATS, @socket.io/redis-adapter
Protocols	WebSocket (RFC 6455), SSE, WebTransport (HTTP/3)

Architecture Patterns

Scaled WebSocket with Redis Backplane

Client A ──ws──→ Server 1 ←──redis pub/sub──→ Server 2 ←──ws── Client B
                     │                             │
                     └─────── Redis Cluster ───────┘

Each server subscribes to channels. When Server 1 receives a message
for a room, it publishes to Redis. Server 2 picks it up and forwards
to its connected clients.

SSE with Last-Event-ID Resume

// Server: SSE endpoint with resume support
app.get('/events',  {
  res.(, {
    : ,
    : ,
    : ,
  });

   lastId = (req.[] || );
  
   missed = eventStore.(lastId);
  missed.( {
    res.();
  });

  
   unsub = eventBus.( {
    res.();
  });
  req.(, unsub);
});