Design an offline sync engine for mobile
PremiumMobile systemHard
- Offline first
- Mutation outbox
- Conflict resolution
Quick answer
This mobile system interview question asks you to design an offline sync engine for mobile. 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
Design an offline sync engine for mobile apps: a reusable library that lets people read and edit data with no network for days, then syncs both ways when they reconnect. The hard parts are conflicts between devices, ordering, deletes and upgrades while edits are still queued.
The first app is a field inspection tool for technicians who work in basements and plants. A team shares jobs, so two people can edit the same job offline at the same time.
What must an offline sync engine for mobile do?
- Offline read and write: Every screen works with no network for up to 5 days, including create, edit and delete.
- Two-way sync: Local edits reach the server and others' edits reach the device, automatically and in the background.
How well must it do it?
- No lost edits: An edit is never lost, even if the app is killed mid-sync or upgraded with edits queued.
- Exactly-once effect: Retried pushes never apply a mutation twice.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Offline first
- Mutation outbox
- Conflict resolution
- Hybrid logical clocks
- Delta sync cursors
- Tombstones
How you’ll practise
- Requirements
- App architecture
- Data and offline sync
- Networking and push
- Performance, battery and releases
- 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 mobile system 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 mobile system question. You design an offline sync engine for mobile 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