Design a messaging system like WhatsApp.

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Anonymous · Accepted Answer

I would design the system around persistent WebSocket connections for real-time
messaging. Clients connect through a load balancer to WebSocket gateway servers,
which handle connections and forward messages to a message service.

The message service would persist messages in a distributed database and publish
events to Kafka for asynchronous delivery. Kafka partitions could be based on
conversation ID to preserve message ordering within each conversation while
allowing different conversations to be processed in parallel.

For offline users, messages remain persisted and are delivered when the user
reconnects, with push notifications through APNs/FCM. I'd use Redis for
ephemeral presence information such as online status and gateway connection
mappings.

I'd also use client-generated message IDs for idempotency, allowing
at-least-once delivery without creating duplicate messages. For large groups,
I'd use different fan-out strategies depending on group size. Finally, I'd
design for failures at the gateway, Kafka, and database layers using
replication, retries, and monitoring.

Design a messaging system like WhatsApp.

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