Design a real-time chat application like Slack or WhatsApp
Reported in Adyen European engineering loops. Mid-to-senior design spanning WebSockets, message storage, and presence.
Interview scenario
Often asked in Adyen loops at European offices (London, Berlin, Amsterdam, Paris, Stockholm, Dublin, and remote EU). Prepare a clear spoken answer plus key trade-offs.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Adyen: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Functional: 1:1 and group messaging, delivery/read receipts, online presence, message history, media attachments.
High level: Clients connect via WebSocket to chat servers; servers publish to message bus; persistence layer stores messages; push notifications for offline users.
Storage: Messages partitioned by conversation ID (time-series friendly—Cassandra, Scylla, or sharded SQL). Index by user inbox for recent threads. Object storage (S3) for media with CDN URLs in message payload.
Scale: Fan-out on write for small groups; fan-out on read for celebrity/large channels. Connection maps (user → server) in Redis for routing. CRDTs or sequence IDs for ordering.
Discuss end-to-end encryption trade-offs, idempotent send API, and sync protocol for mobile offline queues.
Discussion
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