WebSocket & Real-Time Architecture#

When your app needs data the instant it changes — chat messages, stock prices, live scores, multiplayer game state — you need real-time architecture.

HTTP request-response doesn't cut it. This guide covers the protocols, patterns, and scaling strategies for building real-time systems.

Real-Time Protocols#

WebSocket#

Full-duplex, persistent connection. Both client and server can send messages anytime.

Client ← → Server (persistent TCP connection)

Best for: Chat, gaming, collaborative editing, live dashboards Latency: < 50ms Scaling: Requires sticky sessions or pub/sub fan-out

Server-Sent Events (SSE)#

Server pushes events to client. One-directional (server → client only).

Client ← Server (HTTP stream, auto-reconnect)

Best for: Notifications, live feeds, stock tickers Latency: < 100ms Scaling: Easier than WebSocket (HTTP-compatible, works with CDNs)

Long Polling#

Client makes HTTP request, server holds it until data is available.

Client → Server (request) ... waits 30s ... ← response
Client → Server (immediately re-request) ... waits ...

Best for: Fallback when WebSocket isn't available Latency: 0-30s (variable) Scaling: Simplest, but highest server resource usage

Comparison#

Architecture Patterns#

1. Chat Application#

Client A → WebSocket Gateway → Message Router
Client B → WebSocket Gateway → Message Router
                                    ↓
                              Redis Pub/Sub
                              ↓         ↓
                        WS Gateway 1  WS Gateway 2
                        (Client A)    (Client B)
                                    ↓
                              Message Store (PostgreSQL)
                              Presence Service (Redis)

Key decisions:

• Message ordering: Per-channel sequence numbers
• Delivery guarantee: At-least-once (client dedup by message ID)
• Presence: Track online users in Redis with TTL heartbeats

2. Live Dashboard#

Data Source → Kafka → Stream Processor → Redis (latest values)
                                              ↓
                                    SSE/WebSocket Gateway
                                    ↓         ↓
                                 Dashboard  Dashboard
                                 (User 1)  (User 2)

Key decisions:

• SSE is simpler and sufficient (server → client only)
• Throttle updates to 1/second for UI (even if data changes faster)
• Cache latest state in Redis for new connections

3. Multiplayer Game#

Player A → WebSocket → Game Server (authoritative state)
Player B → WebSocket → Game Server
Player C → WebSocket → Game Server
                           ↓
                    State broadcast every 50ms (20 ticks/sec)
                    Input processing + physics
                    Anti-cheat validation

Key decisions:

• Server authoritative: Server owns game state, clients send inputs
• Client prediction: Client predicts locally, server corrects
• Tick rate: 20-60Hz depending on game type
• Interpolation: Smooth movement between server updates

4. Collaborative Editing (Google Docs)#

User A types → Local CRDT → Broadcast delta via WebSocket
User B types → Local CRDT → Broadcast delta via WebSocket
                                    ↓
                              Sync Server (merge CRDTs)
                                    ↓
                              Document Store

Key decisions:

• CRDT (Conflict-free Replicated Data Type) for automatic merge
• OT (Operational Transform) as alternative (Google Docs uses this)
• Cursor positions broadcast separately (high frequency, no persistence)

5. Notifications#

Event occurs → Notification Service → check preferences
                                    → Redis Pub/Sub
                                    → SSE to connected clients
                                    → Push notification (FCM/APNs) for offline
                                    → Email for important notifications

Key decisions:

• SSE for in-app (simpler than WebSocket for one-way)
• Fallback to push/email for offline users
• User preferences control which channels

Scaling WebSocket#

The hard part: WebSocket connections are stateful. A user connected to Server A can't receive messages routed to Server B.

Pattern 1: Pub/Sub Fan-Out (Redis)#

Server A (1000 connections) ←→ Redis Pub/Sub ←→ Server B (1000 connections)
                                              ←→ Server C (1000 connections)

Every message published to Redis. All servers subscribe and forward to their connected clients.

Pattern 2: Sticky Sessions#

Load balancer routes same user to same server:

User ABC → hash("ABC") → always Server A
User DEF → hash("DEF") → always Server B

Simpler but limits horizontal scaling. Server failure disconnects all its users.

Pattern 3: Dedicated Connection Servers#

Separate WebSocket servers from business logic:

Client → WS Connection Server (stateless routing)
              → publishes to Kafka/Redis
              → subscribes to user's channels

Business Logic Servers → publish events to Kafka
                       → WS servers fan out to clients

Connection servers handle millions of idle connections. Business logic scales independently.

Connection Management#

Heartbeats#

Detect dead connections:

Server → ping every 30s → Client responds pong
No pong in 60s → close connection → client reconnects

Reconnection#

Clients must handle disconnections gracefully:

const connect = () => {
  const ws = new WebSocket(url);
  ws.onclose = () => {
    // Exponential backoff: 1s, 2s, 4s, 8s, max 30s
    setTimeout(connect, Math.min(30000, 1000 * 2 ** retries++));
  };
  ws.onopen = () => { retries = 0; };
};

Message Queuing#

Buffer messages during disconnection:

Client disconnects → server queues messages in Redis (TTL: 5min)
Client reconnects → server sends queued messages → resume real-time

Tools & Libraries#

When NOT to Use WebSocket#

Generate a real-time architecture →

Summary#

1. WebSocket for bidirectional, low-latency (chat, gaming)
2. SSE for server-push only (notifications, feeds) — simpler to scale
3. Scale with pub/sub — Redis or Kafka for fan-out across servers
4. Always handle reconnection — exponential backoff + message queuing
5. Separate connection from logic — WS servers scale differently than business logic
6. Don't use WebSocket for everything — SSE and polling have their place

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Design real-time architectures at codelit.io — generate interactive diagrams with WebSocket, Kafka, and Redis patterns.