InfraDrill · for Staff backend engineers

Know exactly what you're missing before your Staff backend interview

A Staff backend loop tests 10 competencies across four or five rounds. Below: what each round tests, what is here to practise it, and a check that tells you which of the 10 you are short on — relative to Staff, not as one number.

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The readiness check is part of every pass and takes 10–15 minutes. It never predicts an offer; it says which competencies are Ready, Borderline or a Gap for this level.

What a Staff backend loop tests

The competencies the readiness check measures for this target, in the order the report lists them. “Weighted” means a gap there counts more in the highest-risk verdict, because loops weight it more.

1.System design & trade-offsCoreweighted 1.5× in the check
The senior interview is a conversation about trade-offs; every architecture is right for some constraints and wrong for others.
168 medium and hard questions
2.Distributed systemsCore
Every service a senior designs is several machines pretending to be one; consensus, replication and consistency decide what that pretence costs.
163 medium and hard questions · 1 coding exercises
3.Data modelling & storageCore
Choosing a store, a schema and an isolation level is the decision that is hardest to reverse; it deserves the most reasoning.
256 medium and hard questions
4.Reliability & failure modesCore
Systems fail partially and at the worst moment; timeouts, retries, shedding and isolation decide whether a fault stays local.
379 medium and hard questions · 5 coding exercises
5.SecurityCore
Identity, least privilege and secrets are design inputs, not a review at the end; a senior knows which boundary each control actually enforces.
236 medium and hard questions
6.API design & contractsCore
An API is a promise other teams build on; versioning, idempotency and error semantics are what make it keepable.
129 medium and hard questions
7.ConcurrencyCore
Goroutines, locks and channels are where correct-looking code loses data; a senior reasons about interleavings, not lines.
127 medium and hard questions · 7 coding exercises
8.Debugging and incident reasoningCore
Senior loops hand the candidate a symptom — a latency spike, a stalled consumer, a pod that will not start — and grade whether they reach the mechanism before reaching for a fix; the round that decides most hires is this one, not recall.
316 medium and hard questions · 11 coding exercises
9.Technical judgment: decisions and trade-offsSeniorityweighted 1.5× in the check
Every ladder above Senior names judgment — Dropbox, GitLab and Monzo all grade whether a candidate can weigh two workable options and say what each costs; a question that asks which design fits, and why the others do not, measures that directly.
374 medium and hard questions
10.Collaboration & leadershipSeniority
Staff and principal work is mostly deciding with other people: reviews, incidents, disagreements and what gets written down.
61 medium and hard questions

Round by round

Loops in 2026 are graded per round, not per topic. Each round here is a view over the same bank and exercises; the competencies in bold are the ones the Staff check measures.

Coding

Write and fix code under time: correctness, edge cases, concurrency, tests. Practised with the runnable exercises.

Concurrency, Testing & verification, Code quality & design, Algorithms & data structures, Go language & runtime, AI-assisted engineering

24 runnable Go exercises with real tests, in the browser · 537 questions

The coding round is practised in Go. A TypeScript runner is planned; it is not here yet.

System design

Design a system to requirements and defend the trade-offs: storage, messaging, caching, capacity and cost. The round weighted most in 2026 loops.

Distributed systems, Data modelling & storage, Caching, Messaging & async integration, API design & contracts, Capacity & scaling, Cost, Cloud architecture, System design & trade-offs, AI agents & tooling

1020 medium and hard questions · 2 exercises

Technical deep dive

How things actually work and why they fail: networking, concurrency, performance, security, and diagnosing a symptom to its mechanism.

Concurrency, Data modelling & storage, Networking, Security, Performance, Kubernetes & containers, Algorithms & data structures, AI agents & tooling, Go language & runtime, Debugging and incident reasoning

1355 medium and hard questions · 24 exercises

Operations and reliability

Running it in production: reliability, observability, Kubernetes, infrastructure as code, incidents and cost — graded explicitly since 2026.

Reliability & failure modes, Observability, Capacity & scaling, Cost, Infrastructure as code, Kubernetes & containers, Cloud architecture, Operations & SRE, Debugging and incident reasoning

1187 medium and hard questions · 15 exercises

Behavioral and judgment

Decisions with facts on the table: trade-offs, influence, incidents and ownership. Not what you would say — what follows from the situation.

Collaboration & leadership, Technical judgment: decisions and trade-offs

412 medium and hard questions

Two questions from the free demo

Real questions from this target's first axes, not written for this page. A demo quiz opens with these two; the answer, the explanation and the source come when you submit — no account needed.

System design & trade-offsmedium

A team removes a deprecated int32 legacy_id = 4; field from a protobuf message and ships the new schema. Six months later someone adds string region = 4;. Old clients still exist in the field. What goes wrong, and what should have been done when the field was removed?

Distributed systemsmedium

An engineer proposes replacing the team's retry logic with a circuit breaker, describing them as two solutions to the same problem. Why is that framing wrong?

Answer them on the demo

Then practise the gaps

A pass starts with the check and its report, and unlocks practising what it finds: the question bank (2040 questions, every answer citing its source), practice by round, fixed packs you can repeat, spaced repetition and the coding exercises. Passes are one-time — an Interview Pass for one cycle, an Extended Pass for a longer search. Nothing renews.

See the passes Try the free demo