System Design interview questions and quick answers
Caching, scaling, databases and classic design problems for beginners.
Review 1,063 questions and short answers on System Design. Start with real examples below, then search, practise and follow links to full explanations.
System Design: questions worth revising first
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01What does an API gateway do in a microservice system?
An API gateway is the single entry point for clients. It routes requests to the right service and handles shared concerns such as authentication, rate limiting, TLS and logging, so each service does not repeat them.
Read the full explanation02How do you version a public API without breaking existing clients?
Put the version in the URL (/v1/) or a header, never change the meaning of existing fields, add new fields as optional, and announce deprecations with a long sunset period. Prefer additive changes so versions are rarely needed.
Read the full explanation03What does at-least-once delivery mean, and can you get exactly-once?
At-least-once means a message is never lost but may be delivered more than once. True exactly-once delivery is not practical across a network, so systems use at-least-once plus idempotent consumers to get exactly-once effects.
Read the full explanation04What is the difference between availability and reliability?
Availability is whether the system is up and responding when asked. Reliability is whether it keeps giving correct results over time without failing. A system can be available yet wrong, so it is available but not reliable.
Read the full explanation05What is backpressure and how do you handle a queue that keeps growing?
Backpressure means slowing or limiting producers when consumers cannot keep up. For a growing queue, add consumers, make processing faster, reject or throttle new work, and set limits so the queue cannot grow without bound.
Read the full explanation06How do Cache-Control and ETag headers work together?
Cache-Control says how long a response may be reused without asking. ETag is a fingerprint of the content; after expiry the browser sends it back, and the server replies 304 Not Modified if nothing changed, saving the download.
Read the full explanation07How would you explain "What does an API gateway do in a microservice system" in an interview?
"The gateway is the single front door: routing, auth, rate limiting, TLS and aggregation in one place. I run it redundantly and keep business rules out of it, so it stays a thin layer rather than a bottleneck."
Read the full explanation08How would you explain "What does an API gateway do in a microservice system" to a beginner?
Routing: /orders goes to the order service, /users to the user service.
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