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Engineering

Backend Engineer mock interview

Builds and runs the server-side systems behind applications: APIs, databases, background jobs and integrations. Day to day they design data models, write services, tune performance and keep production reliable. Interviewers usually probe how you actually do these things, so expect follow-up questions.

Ava, the AI interviewer, will ask you questions on api design, databases & data modelling, distributed systems & scalability and reliability & observability and 4 more skills, follow up on what you say, and finish with a report that quotes your own words back to you.

Start a backend engineer practice interview

Skills assessed in a backend engineer interview

  • API design

    Role skill

    Designs clear, consistent resources and error handling, thinks about versioning and backward compatibility, and considers the client's needs.

  • Databases & data modelling

    Role skill

    Chooses schemas and indexes for real query patterns, understands transactions, and describes safe migrations on live data.

  • Distributed systems & scalability

    Role skill

    Discusses queues, retries, idempotency, caching and failure modes, and reasons about when added complexity is justified.

  • Reliability & observability

    Role skill

    Uses logs, metrics and traces to investigate, sets meaningful alerts and service objectives, and learns from incidents.

  • Security fundamentals

    Role skill

    Mentions input validation, parameterised queries, authentication and authorisation, secret management and least privilege without prompting.

  • Performance & concurrency

    Role skill

    Explains race conditions, locking or optimistic concurrency, and profiles before optimising.

  • Ownership

    Behavioural

    Owns services end to end including on-call, follows incidents through to prevention, and improves things beyond the ticket.

  • Cross-team communication

    Behavioural

    Negotiates interfaces with other teams, documents contracts, and communicates changes and outages early and clearly.

Sample backend engineer interview questions

8 of the 13 questions Ava can ask for this role, one or more per skill. In a real session she adapts them to your level and follows up on your answers.

  1. Q1API designEntry level

    If you were building an API for a simple to-do app, what endpoints would you create, and how would you handle errors?

    A strong answer: Sensible resource-based routes, correct use of request methods and status codes, input validation, consistent error responses, and thinking about pagination.

  2. Q2Databases & data modellingEntry level

    What is a database index, and how would you decide whether a column needs one?

    A strong answer: Faster lookups at the cost of storage and slower writes, indexing columns used in frequent filters and joins, and checking query plans.

  3. Q3Security fundamentalsEntry level

    How do you protect a web application from someone sending malicious input, such as an SQL injection attack?

    A strong answer: Parameterised queries or a safe query builder, validating and sanitising input, escaping output, least-privilege database accounts, and not trusting the client.

  4. Q4Reliability & observabilityMid level

    An endpoint starts timing out in production at peak hours. How would you investigate, and what would you look at first?

    A strong answer: Checking dashboards, traces and logs, isolating the slow dependency, looking at database load, connection pools and recent changes, and mitigating before fixing.

  5. Q5Performance & concurrencyMid level

    How do you handle two requests trying to update the same record at the same time?

    A strong answer: Recognising the race condition, transactions, row locks or optimistic version checks, unique constraints, and clear handling for the losing request.

  6. Q6Distributed systems & scalabilitySenior

    How would you design a system that sends order confirmation emails reliably, even if the email provider is down for a while?

    A strong answer: Decoupling with a queue or outbox pattern, retries with backoff, idempotency to avoid duplicates, dead letter handling, and monitoring the backlog.

  7. Q7OwnershipAny level

    Tell me about an incident you were part of. What was your role, and what changed afterwards?

    A strong answer: Calm mitigation, clear communication, a blameless review, a real root cause, and follow-up actions they personally drove to completion.

  8. Q8Cross-team communicationAny level

    Tell me about a time you had to agree on an interface or contract with another team that had different priorities. How did you get there?

    A strong answer: Understanding the other team's constraints, written proposals, compromise, documenting the agreement, and keeping each other informed.

What a strong backend engineer answer looks like

  • API design

    Designs clear, consistent resources and error handling, thinks about versioning and backward compatibility, and considers the client's needs.

  • Databases & data modelling

    Chooses schemas and indexes for real query patterns, understands transactions, and describes safe migrations on live data.

  • Ownership

    Owns services end to end including on-call, follows incidents through to prevention, and improves things beyond the ticket.

How the practice works

  1. Step 1

    Choose your level

    Pick intern through lead. The backend engineer question bank is labelled for 3 entry-level, 5 mid-level and 3 senior questions (plus 2 for any level), and Ava leans toward the ones that fit.

  2. Step 2

    Answer out loud or by typing

    Ava asks 4 or 7 main questions, depending on the length you choose, such as the aPI design question above. You can also add a short coding exercise. Typing is always available if you can't speak.

  3. Step 3

    Get feedback you can act on

    Each skill is scored against the evidence you gave, with the specific points from your answers. In practice mode you also get a check against the situation, task, action and result structure, a stronger version of each answer built only from what you said, and what to practise next.

Try the backend engineer interview

Frequently asked questions

Which skills does the Backend Engineer mock interview cover?
API design, Databases & data modelling, Distributed systems & scalability, Reliability & observability, Security fundamentals, Performance & concurrency, Ownership and Cross-team communication. The first 6 are role skills; ownership and cross-team communication are the behavioural part.
What kind of questions will I get?
Questions come from a bank of 13 written for this role. An entry-level one is “If you were building an API for a simple to-do app, what endpoints would you create, and how would you handle errors?” and a senior one is “How would you design a system that sends order confirmation emails reliably, even if the email provider is down for a while?”.
How will my distributed systems & scalability answer be judged?
Against what strong evidence looks like for that skill: discusses queues, retries, idempotency, caching and failure modes, and reasons about when added complexity is justified. Anything you don't mention is not credited, and nothing is invented for you.
Is this useful for API engineer or BE engineer roles?
Yes. API engineer, BE engineer, Java developer and Node developer are treated as close to backend engineer, so the same skills and questions apply. You can also type any other role on the setup screen.
Is it free, and do I need an account?
Starting practice is free, and a free account is required before the interview begins. The feedback is guidance for your preparation, not a prediction of any hiring decision.

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