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Design a real-time chat application like WhatsApp or Messenger that supports 1:1 and group messaging (up to 100 participants). Users should be able to send text and media messages, receive messages in real time when online, and retrieve undelivered messages when they come back online. The system must handle billions of users with low-latency delivery and guaranteed message durability.
Best once you can already structure a clean answer and want sharper trade-off pressure.
5 stages
45 min
Grade anytime
Workspace-first, hints visible, stage retry available. The cheap, repeatable loop — build the answer shape before you take it under pressure.
Solve once, compare against the checklist, then come back to the weak stage instead of starting over.
Strict timer, hints hidden, debrief deferred to the end. Use this once you can already structure a clean answer and want to pressure-test pacing and pushback.
Best after one structured rep · timed · focused on pacing and communication.
This is the framing pass. A strong answer quickly defines what the system must do, what quality bar it has to hit, and the numbers that will justify the rest of the design.
What must exist
What good looks like
Numbers to anchor the design
Each stage has a distinct job. Treat them like separate deliverables instead of one giant answer, and the round becomes much easier to navigate.
Define the contract clearly: the endpoints, auth boundary, error semantics, and the one or two decisions that matter most.
What you should produce
Now let's define the system interface.
Strong answers cover
Lay out the main components and trace the write path, read path, and any async path cleanly.
What you should walk through
Now walk me through the high-level architecture.
Strong answers cover
Pick the store, show the schema or key model, and explain why that storage choice fits the access pattern.
What you should lock in
Let's go deeper into the data model and storage layer.
Strong answers cover
Name the first bottleneck, failure modes, and the trade-offs that keep the system fast and reliable under pressure.
What you should pressure-test
Let's talk about scaling to 200M concurrent connections and making the system resilient.
Strong answers cover
Translate the prompt into concrete requirements, scale, and trade-offs before drawing architecture.
Give APIs in the API stage, data models in the storage stage, and failure modes in scaling. Don't blur them together.
Grade early, compare to the reference reasoning criteria, fix the biggest misses, and re-submit the weak stage instead of starting over.
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