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Design a feature-flag and rollout-control platform like LaunchDarkly. Applications ask "is this feature on for this user?" millions of times a second, so the answer has to be instant — which is the whole twist: evaluation happens locally inside the SDK, not as a network call. The hard system-design surface is the control plane: distributing rule changes to ~100M long-lived SDK connections within seconds, bucketing percentage rollouts deterministically so a user never flickers, and staying available for customer apps even when your own service is down.
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 define the API. There are two very different surfaces here: the dashboard/control-plane CRUD for editing flags, and the SDK contract that strea...
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
Walk me through the architecture.
Strong answers cover
Let's get concrete about data. What's the schema for flags, environments, segments, and rules? How do you version a ruleset so you can roll back, and crucially — how do you bucket a percentage rollout so it's deterministic and sticky?
Prompt you should be ready for
Let's get concrete about data.
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 deep dive. Where does this bottleneck first as you scale to 100M connections? Walk me through the connection fan-out, a reconnect storm, the...
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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