Learning Objectives & Key Takeaways
0 / 4 Completed- Understand System Design core architecture and runtime execution model
- Audit real-world latency, throughput benchmarks, and memory footprint
- Implement zero-downtime deployment patterns with resilience guards
- Verify code snippets and architect blueprints against production SLOs
- An ordering key, not a global sequence, is what Pub/Sub uses to guarantee per-conversation order.
- Push subscriptions cap you at one outstanding message per key, so ordered chat needs Pull or StreamingPull.
- Acknowledging message three before message two still blocks delivery progress until two is acked.
- API Gateway WebSocket APIs are bidirectional: your backend can callback-push to connected clients, not just respond to them.
- Publishing the same conversation's messages from two regions breaks ordering even if each region is internally consistent.
The Night Replies Arrived Before Questions
A support team at a fintech startup built a group chat feature so agents could tag each other inside a customer ticket, backed by API Gateway WebSocket API for delivery and a fleet of Lambda workers for fan-out. The first week in production, agents started screenshotting a bizarre bug: someone's reply to a question showed up above the question itself.
The root cause was not the WebSocket layer. Connections were fine, message payloads were fine, the bug was that two different backend workers processed two messages from the same conversation concurrently and pushed them to Pub/Sub without any ordering guarantee between them. Whichever worker finished first won, and worker finishing order has nothing to do with message creation order.
- Why Fan-Out Naturally Breaks Order
- Enabling Ordered Delivery on the Subscription
- Wiring Ordered Delivery Into a WebSocket API
- Trade-Offs and Pitfalls Teams Hit in Production
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