Design Typing Indicator - System Design | WinJob

Design Typing Indicator — product and realtime context

Design a real-time typing indicator for a messaging product (WhatsApp / Slack / Discord scale): when user A composes in conversation C, eligible participants see “A is typing…” within a tight latency budget, without durable storage or chat-history pollution.

Why interviewers ask this

Ephemeral fan-out — high churn, tiny payloads, loss-tolerant semantics unlike message delivery.
Privacy & product rules — blocked users, disabled receipts, invisible mode, enterprise policies.
Thundering herds — every keystroke is not a server write; debounce and coalesce are mandatory.

Scale anchors (stated assumptions)

Signal	Assumption
DAU	500M
Active conversations	80M concurrent
Typing signals (debounced)	~8 per active minute per typer
Peak typing events	~2.2M events/s global (3× average)
Propagation SLA	p99 < 300 ms same region

Architecture headline

Clients emit typing.start / typing.stop over an existing WebSocket session. A regional Typing Gateway validates membership, applies debounce + TTL, and fan-outs to online recipients via per-user delivery channels (Redis pub/sub → gateway push). No durable DB for typing state — Redis with 5–8 s TTL is enough.

javaOne Dark Pro

1 public record TypingSignal(String conversationId, String userId, boolean isTyping, long epochMs) {}

pythonOne Dark Pro

1 def typing_key(conversation_id: str, user_id: str) -> str:
2     return f"typing:{conversation_id}:{user_id}"

typescriptOne Dark Pro

1 export interface TypingEvent {
2   conversationId: string;
3   userId: string;
4   isTyping: boolean;
5   serverTime: number;
6 }

Why interviewers care

Typing Indicator interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

Interview checkpoint

Name one failure story for Problem Statement & Context that proves you understand real outages, not happy-path diagrams.

Key Highlights

•Problem Statement & Context: key interview decision
•Debounce client and server before fan-out
•TTL clears ghost typing — no durable store

1	def typing_key(conversation_id: str, user_id: str) -> str:
2	return f"typing:{conversation_id}:{user_id}"

1	export interface TypingEvent {
2	conversationId: string;
3	userId: string;
4	isTyping: boolean;
5	serverTime: number;
6	}