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Interview guide

System Design Interview Questions (2026)

Senior system design rounds rarely start with 'design Twitter' cold — they probe specific fundamentals. Here's what actually gets asked.

Last updated: August 2026

System design interviews look unstructured from the outside — an open-ended prompt and a whiteboard — but interviewers are scoring against specific fundamentals underneath. Knowing what those fundamentals are is more useful prep than memorizing a template for "design X."

Estimation and scale

  • "How would you estimate the read and write QPS for this system?"
  • "At what scale does a single database stop being enough?"

Interviewers want a defensible back-of-envelope number, not precision — and they want to see you use that number to justify later design choices, not just state it and move on.

Caching

  • "Where would you add a cache, and why there specifically?"
  • "How do you invalidate this cache when the underlying data changes?"
  • "What happens on a cache stampede?"

Consistency and CAP

  • "Does this system need strong consistency, or is eventual consistency acceptable?"
  • "What happens to writes during a network partition?"

The strongest answers tie the choice to a concrete user-facing consequence — a stale balance, a duplicate order — rather than reciting the CAP theorem abstractly.

Replication and partitioning

  • "How would you shard this dataset, and what's your partition key?"
  • "What happens when a replica falls behind?"

Queues and async messaging

  • "Why introduce a queue here instead of a synchronous call?"
  • "How do you handle a poison message that keeps failing?"

Failures, timeouts, and retries

  • "What happens when this downstream service times out?"
  • "How do you prevent a retry storm from making an outage worse?"

Observability

  • "How would you know this system is degraded before customers report it?"
  • "What would you alert on, and why not everything?"

How to prepare, fundamental by fundamental

Cramming full mock interviews is inefficient if you're shaky on the underlying pieces — caching, consistency, queues, replication, observability, idempotency — that every design question draws from. Senior Loop's Deep Dives break system design down into exactly these fundamentals, each with the tradeoffs interviewers actually probe. A free set is available with no card required.

Part of the Senior Backend Engineer Interview Prep guide.