I started with the messaging flow and worked outward but spent way too long on the client-server handshake before even touching storage or delivery guarantees.
Start by clarifying requirements and scale (e.g., 2B users, 100B messages/day), then design a high-level architecture covering client, edge, and backend services. Deep dive into critical components like message delivery, storage, and real-time communication, explaining trade-offs for scale, performance, and reliability.
Pro tip: Emphasize the end-to-end message flow and how you ensure exactly-once delivery and ordering, as this is the core challenge. Show awareness of Meta's specific infrastructure (e.g., TAO, Memcached, Haystack) to demonstrate domain knowledge.
Clarify functional (1:1 chat, group chat, media, presence) and non-functional (latency, availability, consistency) requirements. Estimate scale: 2B users, 100B messages/day, peak QPS, storage needs.
Outline major components: clients (mobile/web), edge (CDN, load balancers), API gateways, chat servers, message queues, storage (message DB, media storage), and push notification services.
Focus on message delivery: how messages are routed, stored, and delivered in real-time. Discuss WebSocket connections, message queues (Kafka), and storage choices (e.g., HBase, Cassandra) for durability and low latency.
Explain scaling strategies: sharding by user ID, replication for fault tolerance, caching for presence and recent messages. Discuss performance optimizations (batching, compression) and reliability (retries, idempotency, exactly-once semantics).
Summarize key trade-offs (e.g., consistency vs. availability, latency vs. durability) and how you'd monitor and iterate. Mention potential bottlenecks and future improvements.
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