Design a rules engine
PremiumMachine codingHard
- Condition tree
- Composite pattern
- Short circuit evaluation
Quick answer
This machine coding interview question asks you to design a rules engine. 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 rules engine, the kind of library behind loan approval, fraud checks and feature eligibility. Business rules are written as JSON condition trees, loaded once and evaluated against a facts object. The hard parts are nested all/any/not logic, missing facts, priority order and explaining why a rule fired.
Facts sit under applicant, so creditScore in a rule means applicant.creditScore. Tests load rule JSON and call engine.evaluate(facts, strategy) directly.
What must a rules engine do?
- Rule shape: A rule has an id, a priority, a condition tree and an event. A condition is all, any, not or a leaf {fact, operator, value}.
- Evaluate: evaluate(facts, strategy) returns events. firstMatch returns only the highest-priority matching rule; allMatches returns every match in priority order.
How well must it do it?
- Deterministic: The same rules and facts always give the same events and trace.
- Compile once: Fact paths and operators are resolved at load time, not on every evaluation.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Condition tree
- Composite pattern
- Short circuit evaluation
- Rule priority
- Rule validation
- Strategy pattern
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 rules engine 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.
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Updated · Written by the SevyDevy team