Design an in-memory key-value store with TTL
PremiumMachine codingEasy
- TTL
- Lazy expiration
- Hash map
Quick answer
This machine coding interview question asks you to design an in-memory key-value store with TTL. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Build an in-memory key-value store with TTL, a small version of the cache servers many apps put in front of a database. It keeps string keys and values in one process and forgets each key when its time-to-live runs out. The hard part is expiring keys on time without scanning every key.
The store is a library: other code calls its methods directly. Time comes from a clock the caller can replace, so tests can move time forward instantly instead of sleeping.
What must an in-memory key-value store with TTL do?
- Set: set(key, value, ttlMs?) stores or overwrites a value. Without ttlMs the key never expires, and any old TTL on that key is cleared.
- Get: get(key) returns the value, or null when the key is missing or expired.
How well must it do it?
- Correct expiry: A key must never be returned at or after its expiry time, even if cleanup has not run yet.
- Fast operations: get, set and delete run in O(1) average time; cleanup must not scan keys that are not due.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- TTL
- Lazy expiration
- Hash map
- Min heap
- Clock injection
- Nested transactions
How you’ll practise
- Requirements
- Entities and interfaces
- Core implementation
- Edge cases and tests
- Extensions
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any machine coding design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a machine coding question. You design an in-memory key-value store with TTL and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Easy, so it suits engineers who are new to machine coding interviews. It is also a good warm-up before harder questions.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team