Backend Scenarios interview questions

Production situations to diagnose out loud: latency spikes, memory growth, cascading failures, bad deploys.

8 questions in Backend Engineering.

Backend EngineeringA 28-minute read on how the topic fits together, before the questions.Backend engineering fundamentals56 short answers on one page, for revising rather than studying.

medium2

mediumScenarioCoding

You are dropped into an unfamiliar repository with one failing business rule and an AI assistant that only gives hints. What do you do in the first ten minutes?

Debugging an unfamiliar codebase starts by running the failing test, then reading it as the rule and tracing its call path to the code that computes the wrong value. Let the size of the error narrow the cause, ask the assistant to locate and explain rather than patch, and prove the fix with a test that failed before.

5 minmid, senior

mediumScenarioDesign

Users in three countries book appointments, and one of those countries has just changed when its clocks move. What do you store, and what do you refuse to store?

For a future appointment, store the local wall-clock time plus an IANA time zone ID such as Europe/Lisbon, and treat any UTC instant as a cache you recompute when time zone rules change. Refuse fixed offsets, abbreviations like IST, and server-local times. Past events are stored as UTC instants.

5 minmid, senior

hard6

hardScenarioDesign

How do you resolve a complex priority inversion scenario involving chained mutexes and interrupt service routines in a hard real-time system?

Assess the candidate's understanding of advanced priority inversion scenarios in RTOS environments, focusing on chained resources, priority inheritance protocols, and interrupt synchronisation.

3 minsenior, staff

hardScenario

Consumer lag on your orders topic climbs every morning and never clears before the next peak. Work through it.

Lag is the integral of arrival rate minus service rate, so establish which of the two moved and whether the consumer is slow or stalled. Check lag per partition before per group, rule out a rebalance loop, and remember that adding consumers beyond the partition count does nothing.

6 minmid, senior, staff

hardScenarioDesign

How do you optimise UDP packet loss for a high-tickrate competitive shooter?

Analyse approaches to mitigating packet loss in a fast-paced UDP environment, focusing on forward error correction and redundant data.

3 minsenior, staff

hardScenario

Your p99 latency jumped tenfold after a deploy while p50 is unchanged. Diagnose it out loud.

A flat p50 rules out a uniform slowdown, so the tail belongs either to a subset of requests or to a queueing effect. Partition the latency histogram by instance, endpoint and tenant before naming a cause, then confirm with traces, GC logs and pool-wait metrics.

6 minmid, senior, staff

hardScenarioDesign

How do you optimise an FIR filter implementation on a resource-constrained DSP for ultra-low-latency active noise cancellation?

Evaluate the candidate's proficiency in implementing and optimising digital signal processing algorithms on hardware, focusing on FIR/IIR trade-offs, fixed-point arithmetic, and instruction-level parallelism.

2 minsenior, staff

hardScenario

Your database fails over and is accepting connections again within thirty seconds, but your service returns errors for ten minutes. Where is that time going?

The database recovered; your clients did not. Sockets to the old primary hang rather than fail, a cached DNS answer keeps pointing at it, the promoted node refuses writes while it finishes, and then the whole fleet reconnects at once. Attribute the ten minutes to those stages before tuning anything.

6 minmid, senior, staff