Offline sync and mobile system design

Mobile system design starts with constraints the server cannot fully see: network availability changes, process lifetime is uncertain, storage is finite, radios are expensive, and the user can leave halfway through any request. A senior answer makes those constraints explicit before proposing endpoints or libraries.

This lesson focuses on offline-capable product features and the design vocabulary used in mobile system design interviews.

Learning goals

• Separate local intent from remote acknowledgement.
• Design idempotent writes, outboxes, and conflict policy.
• Choose sync styles (fetch, push, periodic, push-notification triggered) deliberately.
• Define product truth the UI can render consistently.
• Structure a 30–40 minute system design answer for Android.

Separate local intent from remote acknowledgement

For an offline-capable write:

1. Persist the user’s change and an outbox operation in one local transaction.
2. UI reads local state immediately (optimistic or pending).
3. A worker drains the outbox under network constraints.
4. Server responses update local rows and clear/fail operations.

suspend fun updateNote(note: Note) = db.withTransaction {
    noteDao.upsert(note.copy(syncState = SyncState.PENDING))
    outboxDao.insert(
        OutboxOp(
            operationId = Uuid.random().toString(),
            type = OpType.UPDATE_NOTE,
            entityId = note.id,
            payload = note.toPayload(),
            createdAt = clock.now(),
        )
    )
}

Why the operation ID matters

Networks fail after the server applied a write but before the client saw the response. A client retry without idempotency creates duplicates. An operation ID (or idempotency key) lets the server recognize a retry and return the previous result.

POST /notes/123
Idempotency-Key: 6b1e0c2a-...

Define product truth

Before drawing boxes, write sentences like:

• “The note list shows last known local state, with pending badges on unsynced edits.”
• “If two devices edit the same note, last-writer-wins by server timestamp” (or CRDT, or manual merge - pick one).
• “Deletes are tombstones until sync completes.”
• “Auth expiry pauses the outbox and routes the user to re-login without losing queued ops.”

If product truth is vague, engineers invent three competing truths in UI, cache, and server.

Sync patterns

Pattern	Mechanism	Good for
Read-through cache	Show DB; refresh from network	Feeds, catalogs
Outbox writes	Queue mutations	Notes, drafts, likes
Delta sync	updatedSince / version tokens	Large collections
Full snapshot	Replace partition	Small rare datasets
Push-triggered	FCM → fetch	Chat, breaking updates
Periodic	WorkManager	Non-urgent reconciliation

Pagination and sync

Cursor/page tokens are server read concerns; local DB remains the UI source of truth. Avoid “in-memory only” pages if offline matters.

// Remote page → upsert into Room → PagingSource reads Room
suspend fun loadNext(pageToken: String?) {
    val response = api.list(pageToken)
    dao.upsertAll(response.items.map { it.toEntity() })
    remoteKeysDao.save(response.nextToken)
}

Conflicts

Conflict policy is a product decision with technical teeth:

Strategy	Meaning	UX impact
Last-writer-wins	Highest timestamp/version wins	Simple; silent data loss possible
Server authoritative	Client always rebases	Need clear “your edit was overwritten”
Field merge	Merge non-overlapping fields	Complex
CRDT / OT	Convergent structures	Heavy; great for collaborative text
Manual resolve	User picks	Honest; more UI

Interviewers listen for you naming the conflict class for the domain (banking ≠ notes ≠ collaborative docs).

Retries, backoff, and ordering

• Retry transient failures (timeouts, 502) with exponential backoff + jitter.
• Do not blindly retry non-idempotent POSTs without keys.
• Preserve per-entity ordering if operation order matters (create before update).
• Use WorkManager constraints: NetworkType.CONNECTED, battery, etc.

val request = OneTimeWorkRequestBuilder<OutboxWorker>()
    .setConstraints(
        Constraints.Builder()
            .setRequiredNetworkType(NetworkType.CONNECTED)
            .build()
    )
    .setBackoffCriteria(BackoffPolicy.EXPONENTIAL, 30, TimeUnit.SECONDS)
    .build()

Security and trust on device

Offline-first does not mean “trust the client forever.”

• Authenticate every sync call; refresh tokens carefully (single-flight refresh).
• Store secrets in Keystore-backed storage; not plain SharedPreferences.
• Validate server data before upsert (ignore impossible states).
• Consider encrypted DB if threat model requires it (performance cost).

Resource constraints unique to mobile

Call these out unprompted in designs:

• Battery & radios - batch transfers; avoid wakeups.
• Storage - bound caches; eviction policy; user-visible media sizes.
• Process death - outbox on disk, not memory.
• Bandwidth - delta sync, compression, image sizes.
• OS background limits - WorkManager vs forbidden background services.

A skeleton system-design answer (notes app)

1. Requirements - offline edit, multi-device, eventual consistency, conflict policy LWW, auth.
2. Client architecture - single Activity, feature module, Room source of truth, outbox, WorkManager, repository.
3. API - CRUD + idempotency keys + updatedSince feed; 401 handling.
4. Data model - notes, outbox, sync_metadata; tombstones.
5. Sync algorithm - push outbox; pull deltas; apply transactions; surface errors.
6. Edge cases - clock skew, partial failure, large attachments (upload session), logout with pending ops.
7. Observability - sync success rate, outbox age, conflict counts.
8. Trade-offs - LWW simplicity vs possible silent overwrite; why not full CRDT yet.

Caching layers

Each layer needs size limits and invalidation rules. “Cache everything forever” is not a strategy.

Common pitfalls

1. Optimistic UI without durable outbox - data vanishes on process death.
2. Duplicate creates from naive retries.
3. Using only in-memory state for multi-screen flows that must survive.
4. Ignoring auth expiry mid-sync.
5. Designing server-only truth while product promised airplane-mode editing.

How interviewers probe this

• “Design offline notes / queue / chat / photo backup.”
• “What if the response is lost after success?”
• “How do two devices conflict?”
• “What runs when the app is backgrounded?”
• “How do you show sync status without lying?”
• “Where is the source of truth?”

Practice checklist

1. Write an outbox schema and drain algorithm for a single entity type.
2. List ten failure modes for “edit note offline, open on second phone.”
3. Practice a 5-minute whiteboard of delta sync + pagination.
4. Define metrics you would alert on for sync health.

What you should be able to explain

• Local intent vs remote ack with idempotency.
• Outbox + worker pattern.
• Conflict policy as product + engineering.
• Mobile constraints in system design.
• Structured interview delivery.

Next: Reliability, observability, and technical decisions - shipping and living with the system you designed.