Mock system design interview at Datadog

A live 45-minute round on a question reported in Datadog interviews, run by an AI interviewer that asks one thing at a time, probes what you say, looks at your whiteboard, and weighs what that round is reported to weigh. It ends with a debrief: a score, what was strong, what was missing.

your question

Design a Rate Limiter

A distributed service that decides, in under a millisecond, whether a request from a client is allowed under its quota.

You will not see the requirements, the numbers or the design before you start. That is the point.

Walk through it first

What the Datadog round is reported to look like

45–60 minutes in a drawing tool of your choice, often on high-volume data close to what the company runs: metrics or log ingestion, time-series storage, alerting. Some candidates report a general product prompt instead.

What it weighs

Write-heavy ingestion: sustained throughput and what happens when traffic spikes
Storage efficiency and query speed for time-series data
Backpressure and failure handling along the pipeline

The common mistake

Reaching for a generic web-app template. On the data-shaped prompts, throughput, storage and query speed need to be in the design from the start, not added once the boxes are drawn.

From public interview guides and candidate reports, and Datadog’s own interview guidance where it publishes any. Not endorsed by Datadog. The interviewer is an AI running in that style; it is practice, not a prediction.

Start the round

Timed · 45 minutesInterview conditions. The AI interviewer owns the clock; the AI Mentor is off.With the AI Mentor · untimedNo clock, and an AI Mentor beside you to ask when you are stuck.

The interviewer’s opening question is free. Answering it is part of Pro. See an example round and its debrief →

The 6 questions this round draws from

Each is reported in Datadog interviews. Pick one to sit, or read its reference design and walk through a strong answer first.

Design a Rate LimiterA distributed service that decides, in under a millisecond, whether a request from a client is allowed under its quota.
drawn
Design an Ad Click AggregatorCount a million ad clicks a second, show advertisers a dashboard that is seconds behind, stop a campaign the moment its budget runs out, and be exactly right when the invoice is cut.
Design a Distributed Job SchedulerRun millions of cron jobs and one-off tasks on time across a fleet, exactly once each, and keep going when a worker dies halfway through one.
Design a Metrics and Monitoring SystemIngest 10 million samples a second from 100,000 hosts, keep them queryable for a year at shrinking resolution, draw dashboards in under a second, and page the right person within a minute.
Design AWS Step FunctionsA managed workflow engine that runs 200 M state-machine executions a day, some lasting a year, applying every transition exactly once.
Design a Distributed Logging SystemCollect 10 million log lines a second from 100,000 hosts, make them searchable within seconds, stream them live, and keep a year affordably.

Other companies: Anthropic · Google · Meta · Amazon · OpenAI · Microsoft · Uber · DoorDash