I started with the message send/receive flow which felt natural, but I underestimated how much time the fanout piece would eat up.
Start by clarifying requirements and scale, then model conversations as groups with membership and message storage. Design the message flow with fanout-on-write for small groups and fanout-on-read for large groups, and address ordering, offline sync, presence, and read receipts as separate concerns.
Pro tip: Emphasize trade-offs: fanout-on-write gives low latency but high write amplification, while fanout-on-read is cheaper but adds read latency. Choose based on group size and activity patterns, and consider a hybrid approach.
Ask about user scale, group size distribution, message volume, latency expectations, and consistency needs. This shapes all subsequent design decisions.
Design schemas for users, groups, memberships, messages, and receipts. Choose storage technologies (e.g., SQL for metadata, NoSQL for messages) based on access patterns and scale.
Decide on fanout strategy: write (push to each member's inbox) vs. read (pull from group log). Discuss hybrid approaches and how to handle large groups.
Use per-group sequence numbers or timestamps for ordering. For offline sync, store messages in per-user inboxes and sync on reconnect. Track read receipts via per-user last-read pointers.
Design presence using heartbeats and a pub/sub system. Use WebSockets for real-time delivery and fallback to push notifications for offline users.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.