System · Design

System design interview questions.

System design interviews reward structured thinking, not trivia. Ten scenarios calibrated to elicit the structure.

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 baseline · postgres

    You have a Postgres table with 500M rows of loan applications. A query filtering by applicant_id (indexed) and status (not indexed) takes 3 seconds. Walk me through how you'd diagnose and fix this.

    Strong answer
    Asks about workload before jumping to solutions · proposes EXPLAIN ANALYZE · mentions partial or composite indexes, not just 'add an index' · seniors reference statistics, vacuum, or plan cache unprompted.
    Weak answer
    Immediately says 'add an index on status.' Doesn't ask about cardinality. Doesn't mention the query planner.
  2. 02
    communication baseline · explaining

    Explain why ACID transactions matter to a non-technical co-founder asking 'can't we just use a NoSQL database?'

    Strong answer
    Concrete examples (double-charging, lost orders). No jargon drop. Acknowledges valid NoSQL use cases. Seniors: helps them think, doesn't just explain.
    Weak answer
    Jumps to CAP theorem. Condescending tone. Doesn't engage with the question as asked.
  3. 03
    situational medium · incident-response

    It's Saturday 11pm. A loan-disbursal job is failing silently. The team that built it no longer exists. Walk me through what you do in the next hour and next 48 hours.

    Strong answer
    First hour: triage scope + comms to right people + restore service before root-causing. Next 48h: postmortem, takes ownership of the service, suggests durable change to prevent silent failures.
    Weak answer
    Stops at 'check the logs.' No comms plan. Next 48h is 'fix the bug.'
  4. 04
    situational hard · dates-vs-quality

    Your CEO commits you to a public launch date your team can't safely hit. Walk me through what you say to them.

    Strong answer
    Separates date concern from reputation concern. Proposes options (scope cut, phased rollout, extended beta). Doesn't just say no. Seniors: surfaces trade-offs so the CEO chooses.
    Weak answer
    Either agrees and burns out the team, or refuses outright without offering alternatives.
  5. 05
    experience baseline · resume-probe

    You mentioned leading a migration to microservices. Team size, hardest thing that broke, what you'd do differently today?

    Strong answer
    Concrete numbers. Specific non-generic failure. 'Differently' shows updated thinking (not just 'hired sooner').
    Weak answer
    Paraphrases the resume. Vague failure. 'Differently' is generic.
  6. 06
    technical hard · scaling

    Walk me through how you'd scale a write-heavy service from 100 to 10,000 writes/sec. What breaks first?

    Strong answer
    Starts with profiling current bottleneck, not with 'add Kafka.' Acknowledges write-amplification, index bloat, replication lag. Discusses shard keys + hot-partition mitigation.
    Weak answer
    Jumps to 'we'd add a queue and scale horizontally' without identifying where the current bottleneck is.
  7. 07
    technical hard · observability

    What does a useful logging + metrics + tracing stack look like for a service with 100M requests/day?

    Strong answer
    Distinguishes logs/metrics/traces by use case. Knows sampling strategies. Structured logs. Cardinality discipline. Mentions specific tools without being dogmatic about them.
    Weak answer
    Conflates logs and metrics. No sampling strategy. Suggests logging every request body (cardinality disaster).
  8. 08
    technical medium · api-design

    You're designing an API endpoint that returns a user's feed. Pagination strategy? Why?

    Strong answer
    Discusses cursor vs offset-based pagination · knows when each is appropriate · mentions stability under inserts · sets sensible default + max page size · considers ETag/If-None-Match for caching.
    Weak answer
    Picks offset pagination by default without considering stability. No max page size. No caching thought.
  9. 09
    technical medium · distributed-systems

    Describe an event-driven architecture you've designed or significantly contributed to. Delivery guarantees? Failure modes you had to plan for?

    Strong answer
    Names specific system + team + problem. Discusses trade-offs (throughput vs ordering, latency vs durability) explicitly. Cites a real failure mode they lived through. Seniors: how they'd evolve the design today.
    Weak answer
    Describes a textbook Kafka setup without team or context. Vague on delivery guarantees. No real failure mode.
  10. 10
    situational medium · teammate-conflict

    A teammate ships a change you think is wrong after the fact. Already in production. What do you say, what do you do?

    Strong answer
    Separates the product concern from the relationship concern. 1:1 first. Focuses on systemic change (PR review, tests) — not on blame.
    Weak answer
    Avoids the conversation or jumps to escalation. Assigns blame. No systemic improvement suggested.
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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