Design a per-key ordered task executor
PremiumMachine codingHard
- Per key ordering
- Concurrency
- Thread pool
Quick answer
This machine coding interview question asks you to design a per-key ordered task executor. 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 a per-key ordered task executor: a library where callers submit tasks with a key, tasks with the same key run in order one at a time, and tasks with different keys run in parallel. It is the pattern behind per-account or per-document job processing. The hard part is keeping per-key order under a global concurrency limit without starving keys.
Write working code in any language with threads or an async runtime. In an async runtime, a running task is a started promise that has not settled yet.
What must a per-key ordered task executor do?
- Ordered per key: submit(key, task) returns a future. Tasks with the same key run one at a time, in submit order.
- Parallel across keys: Tasks with different keys may run at the same time, but never more than maxConcurrency tasks in total.
How well must it do it?
- Correct order: Two tasks with the same key never overlap, and they finish in submit order, under any timing.
- Cheap scheduling: submit and task completion cost O(1) scheduler work, with no busy waiting.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Per key ordering
- Concurrency
- Thread pool
- FIFO queues
- Graceful shutdown
- Futures and promises
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 a per-key ordered task executor and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Hard. It suits engineers preparing for senior interviews, where you are expected to drive the discussion and go deep on trade-offs.
- 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.
Related questions
Updated · Written by the SevyDevy team