Microservices interview questions

Service boundaries, data ownership, saga and outbox patterns, and when a monolith is the right answer.

5 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.

hard5

hardConceptScenarioDesign

A teammate says a saga can just roll everything back if step four fails. What is wrong with that, and what would you tell them to build instead?

A saga has no rollback. Each step already committed in its own database and was visible to everyone, so you can only run new forward transactions that semantically offset it. Compensations are not atomic, some actions cannot be compensated at all, and compensations themselves fail - so a saga needs idempotent handlers and an ordering rule for irreversible steps.

5 minmid, senior, staff

hardDesignConcept

You need to add one field to an event and remove another, and five teams consume it. How does that roll out?

Additive changes with defaults are safe, removals are not, so split every removal into deprecate-then-delete with evidence that nobody reads the field. Register the schema, pick a compatibility mode deliberately, and note that backward compatibility requires consumers to deploy first.

6 minmid, senior, staff, lead

hardDesign

One screen needs data owned by three different services, and it has to render in 200 milliseconds with a filter and a sort applied. How does that request get served?

Fanning out in parallel is fine while every service can answer independently, but a filter or sort spanning owners cannot be composed at all. Either make one service the driver and hydrate its page of identifiers, or build a read model from events and pay for staleness deliberately.

6 minmid, senior, staff, lead

hardScenarioDesign

How do you safely rebuild a corrupted read model in an event-sourced architecture without incurring significant downtime?

Rebuild a corrupted CQRS read model by replaying the event store into a fresh projection, applying upcasters for old event versions, validating parity, and cutting over with a blue-green deployment.

2 minsenior, staff

hardDesignConcept

How do you decide where one service ends and the next begins, and when is a modular monolith the more honest answer?

Draw boundaries on data ownership so exactly one service writes each table, then accept that a workflow crossing them needs a saga of compensating steps and an outbox to make the write and the publish atomic. While the boundaries are still moving, a modular monolith is the honest answer.

7 minmid, senior, staff, lead