Design a cron expression parser
PremiumMachine codingMedium
- Cron syntax
- Tokenizer
- Field expansion
Quick answer
This machine coding interview question asks you to design a cron expression parser. 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 cron expression parser, the piece inside job schedulers that reads a line such as /15 0 1,15 1-5 and decides when a job runs. It expands each field into exact values and finds the next matching time. The hard parts are strict validation and a next-run search that never loops forever.
What must a cron expression parser do?
- Parse: parse(expr) reads five space-separated fields (minute, hour, day of month, month, day of week) and an optional command, and returns a schedule or throws an error naming the field.
- Field syntax: Each field accepts `, a value, a list 1,15, a range 1-5, a step /15 or 10-50/20, and mixes such as 1-5,10`.
How well must it do it?
- Strict validation: Reject out-of-range values, reversed ranges (5-1), zero steps (*/0), empty list items and the wrong number of fields, with a clear message.
- Fast next run: next skips whole months, days and hours that cannot match instead of testing every minute.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Cron syntax
- Tokenizer
- Field expansion
- Next run time
- Input validation
- Bitsets
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 cron expression parser and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- 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