Design a user journey stage engine
PremiumMachine codingMedium
- Rules engine
- Event matching
- State machine
Quick answer
This machine coding interview question asks you to design a user journey stage 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 user journey stage engine: an in-memory library that tracks where each user is in journeys such as onboarding, like the engines inside lifecycle marketing tools. Each journey has ordered stages, and a user advances to the next stage when an incoming event meets that stage's criteria. The hard part is fast, correct matching across many journeys while events arrive on many threads.
Write working code in any language. Journeys are registered at start-up and events arrive through processEvent(event).
What must a user journey stage engine do?
- Process events: processEvent(event) takes eventId, userId, name and attributes, applies the rules and returns the transitions it caused as (journeyId, fromStage, toStage).
- Define journeys: registerJourney(journey) accepts an id and 1 to 10 ordered stages, and rejects bad definitions (duplicate id, no stages, unknown operator).
How well must it do it?
- Correct under concurrency: Two events for one user on different threads never skip a stage, advance twice for one event, or lose a count.
- Fast matching: Work per event grows with the stages that listen for its name, not with the number of journeys.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Rules engine
- Event matching
- State machine
- Inverted index
- Lock striping
- Idempotency
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 user journey stage engine 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.
Related questions
Updated · Written by the SevyDevy team