Mobile engineer system design interview questions

As asked

Design the data architecture for an offline-first mobile app that syncs with a server when connected. Cover conflict resolution and the user experience.

Sample answer outline

Local store (SQLite, Realm, WatermelonDB) as the source of truth for reads. Writes go to the local store first and queue for sync. Sync protocol: a sync log of operations or a CRDT, sent in batches with a server-assigned version. Conflict resolution: last-write-wins is simplest and sometimes wrong (for collaborative documents use a CRDT, for inventory use server-authoritative reconciliation). UX: show pending state on records that have not synced, give users a way to see and resolve conflicts, never silently overwrite. Discuss the cold-start sync for a new device and the partial-sync case.

Expect these follow-ups

• What if a user deletes a record offline while another user edits it online?
• How would you handle media (photos, video) in the same sync framework?
• When is offline-first too expensive to bother with?

As asked

Design the backend for sending push notifications to a 10M user mobile app. Cover device registration, fan-out, and delivery feedback.

Sample answer outline

Device registration: store device tokens (APNs, FCM) keyed by user and device. Tokens rotate, so handle the rotation flow and the unregister-on-uninstall case (APNs feedback service, FCM unregistered errors). Fan-out: when a notification is triggered, look up all devices for the user (or audience), batch by provider, send via APNs/FCM. Rate-limit per device. Delivery feedback: providers return per-token status; record bad tokens for cleanup. Schedule and dedupe to avoid sending the same notification twice on retries. Provide a templating layer and per-user notification preferences.

Expect these follow-ups

• How do you handle a token that returns 'not registered'?
• What is your strategy for a 10M user broadcast?
• How do you A/B test notification copy without spamming users?

As asked

Design offline support for a React Native field app where users create inspections without network access and sync them later. How do you handle conflicts and retries?

Sample answer outline

Use a local durable store such as SQLite or WatermelonDB for inspection records and an outbox table for pending mutations. Each mutation needs an idempotency key so retries after app restarts or flaky networks do not duplicate server state. Conflict handling depends on domain rules: last-write-wins is rarely acceptable for inspections, so prefer field-level merge, server validation, or explicit user resolution. Sync should be resumable, ordered where required, and observable through local status per record. Weak answers hide conflict resolution behind a generic queue and never define the server contract.

Expect these follow-ups

• How do you sync attached photos without exhausting device storage?
• What does the server need to store for idempotency?
• How do you test a week of offline edits?