Mid · Backend

Interview questions for a mid-level backend engineer.

At mid-level, you're hiring for judgment on specific decisions plus capacity to grow. These ten calibrate for both.

10 questions · rubric-backed · Built for small-team hiring panels
The rubric these map to
45%
Technical depth
Workload-first reasoning · trade-offs articulated · senior-level nuance unprompted.
15%
Experience relevance
Resume-anchored probes. Specifics + numbers beat adjectives.
15%
Attitude & ownership
Incident response, escalation judgment, what they'd institutionalize afterward.
10%
Communication clarity
Technical trade-offs explained without jargon creep.
15%
Culture fit
Behavior-based, scored against your explicit values.

The questions.

  1. 01
    technical medium · caching

    You're adding Redis caching to speed up a slow endpoint. What could go wrong?

    Strong answer
    Mentions cache invalidation (the hard problem), stampede on cold cache, TTL reasoning, consistency issues between cache and DB.
    Weak answer
    Treats Redis as magic speed. Doesn't mention invalidation or staleness.
  2. 02
    communication baseline · writing

    How do you write a good PR description?

    Strong answer
    States context + change + how tested. Mentions screenshots for UI. Links related issues. Keeps it skimmable. Reviews their own diff first.
    Weak answer
    'I just describe what I changed.' No mention of context or testing.
  3. 03
    technical medium · debugging

    An API endpoint is returning 500s for 2% of requests. How do you debug?

    Strong answer
    Looks at logs correlated by request ID. Checks if there's a pattern (specific users, payloads, times). Reproduces locally if possible. Checks recent deploys.
    Weak answer
    Suggests restarting the service or 'adding more logging' as a first step.
  4. 04
    culture baseline · learning

    Tell me about the last thing you didn't know how to do, and how you figured it out.

    Strong answer
    Specific topic. Specific resources. Iteration visible ('tried X, didn't work, then Y'). Names the person they asked for help.
    Weak answer
    Vague 'I googled it.' No specifics. No mention of asking others.
  5. 05
    technical baseline · http

    Walk me through what happens when a browser makes an HTTPS request to an API.

    Strong answer
    DNS → TCP handshake → TLS handshake → HTTP request → server routing → response. Bonus: mentions HTTP/2 multiplexing or keepalive.
    Weak answer
    Stops at 'it makes a request to the server.' Doesn't understand TLS or connection reuse.
  6. 06
    technical baseline · postgres

    How do database indexes work, and when would you NOT add one?

    Strong answer
    Understands B-tree structure. Knows the cost of indexes on writes. Cites use cases where a table scan is actually cheaper (small tables, low-cardinality columns).
    Weak answer
    'Always add indexes.' Doesn't mention write amplification or cardinality.
  7. 07
    technical baseline · testing

    What's your testing strategy for a CRUD service?

    Strong answer
    Unit tests for business logic, integration tests for DB layer, a thin layer of e2e. Knows the trade-offs. Mocks externals, doesn't mock own code.
    Weak answer
    Says '100% code coverage' without discussing trade-offs. Mocks DB in unit tests that should be integration tests.
  8. 08
    technical medium · rest-vs-graphql

    When would you pick REST vs GraphQL for a new API?

    Strong answer
    Discusses client flexibility, caching, over-fetching, schema evolution — picks based on actual constraints not fashion.
    Weak answer
    Picks GraphQL because 'it's newer' or REST because 'it's standard.'
  9. 09
    situational medium · oncall

    It's 2am, pager goes off — users can't log in. Walk me through your first 15 minutes.

    Strong answer
    Acknowledges the page. Checks status dashboards first. Identifies scope (all users? some?). Rolls back recent deploys before deep-diving.
    Weak answer
    Jumps into code diffs first without checking status. Spends 10 min reading logs with no hypothesis.
  10. 10
    experience baseline · resume-probe

    Your resume mentions building X. What specifically did you own vs what was the team's?

    Strong answer
    Clear boundary. Doesn't inflate role. Credits teammates specifically. Knows why trade-offs were made even for decisions they didn't own.
    Weak answer
    Claims credit for team output. Vague about their individual contribution.
Why stop at ten?

Lucida runs round one for you, and books the shortlist on your calendar.

Adaptive interviews tuned to the role, rubric-backed scoring, and a founder-ready briefing for every candidate who clears the bar. You do the human call.

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