Why slapping a message queue between microservices doesn’t fix your architecture.
In almost every system design interview, there is a moment where a candidate realizes two microservices are too tightly coupled. Their immediate reflex? Draw a box in the middle and confidently declare,
“We’ll just put Kafka here to make it asynchronous.”
They sit back, thinking they just achieved perfect decoupling. The interviewer secretly writes down a red flag.
In big tech, an event stream isn’t a magic wand, it is a massive operational liability. If you want to clear a senior or staff loop, you have to stop treating message queues like a black box and start designing for asynchronous failure.
Here are 3 ways to show true depth when introducing a message queue:
- The Poison Pill Problem: What happens when your consumer crashes every time it tries to process a corrupted data payload? If you don’t explicitly design a Dead Letter Queue (DLQ), that single bad message will endlessly retry, blocking the entire partition and bringing your downstream service to a grinding halt.
- The Ordering Illusion: Candidates naturally assume messages are processed in the exact order they are sent. In a distributed queue with multiple consumer instances, race conditions are guaranteed. Tell the interviewer exactly how you will handle out-of-order execution — like using deterministic routing keys to bind specific data (like a user_id) to a single, strict partition.
- Surviving Consumer Lag: When your downstream database goes offline for 5 minutes, your queue backs up with millions of pending events. When the database finally comes back online, your consumers will aggressively try to process that massive backlog, effectively DDoS-ing your own database. Stand out by introducing backpressure and rate-limiting at the consumer level.
Moving Beyond the Whiteboard
Adding a tool to an architecture diagram doesn’t magically remove complexity; it just shifts the friction somewhere else.
If you are ready to stop dropping buzzwords and start mastering the real-world mechanics of system design, let’s build together at Levelop.dev.
