System Design Expectations (Senior Level)

1// System Design: Offline-First Sync Architecture (High-Level Code)
2// This is the kind of architectural code Google expects you to sketch
3
4// 1. Data Model with sync metadata
5data class Note(
6    val id: String = UUID.randomUUID().toString(),
7    val title: String,
8    val content: String,
9    val updatedAt: Long = System.currentTimeMillis(),
10    val syncStatus: SyncStatus = SyncStatus.PENDING,
11    val version: Int = 1
12)
13
14enum class SyncStatus { SYNCED, PENDING, CONFLICT }
15
16// 2. Repository Pattern — Single source of truth
17class NoteRepository(
18    private val localDb: NoteDao,
19    private val remoteApi: NoteApi,
20    private val syncEngine: SyncEngine
21) {
22    // Local-first: always read from local DB
23    fun getNotes(): Flow<List<Note>> = localDb.getAllNotes()
24
25    // Write locally first, then sync
26    suspend fun saveNote(note: Note) {
27        val updated = note.copy(
28            syncStatus = SyncStatus.PENDING,
29            updatedAt = System.currentTimeMillis(),
30            version = note.version + 1
31        )
32        localDb.upsert(updated)
33        syncEngine.schedulePush(updated.id)
34    }
35
36    // Sync engine handles push/pull
37    suspend fun sync() {
38        syncEngine.pushPendingChanges(localDb, remoteApi)
39        syncEngine.pullRemoteChanges(localDb, remoteApi)
40    }
41}
42
43// 3. Sync Engine — Conflict resolution strategy
44class SyncEngine {
45    suspend fun pushPendingChanges(local: NoteDao, remote: NoteApi) {
46        val pending = local.getNotesByStatus(SyncStatus.PENDING)
47        for (note in pending) {
48            try {
49                val serverNote = remote.pushNote(note)
50                local.upsert(note.copy(
51                    syncStatus = SyncStatus.SYNCED,
52                    version = serverNote.version
53                ))
54            } catch (e: ConflictException) {
55                // Last-write-wins or manual conflict resolution
56                local.upsert(note.copy(syncStatus = SyncStatus.CONFLICT))
57            }
58        }
59    }
60
61    suspend fun pullRemoteChanges(local: NoteDao, remote: NoteApi) {
62        val lastSync = local.getLastSyncTimestamp()
63        val remoteChanges = remote.getChangesSince(lastSync)
64        for (change in remoteChanges) {
65            val localNote = local.getById(change.id)
66            if (localNote == null || localNote.syncStatus == SyncStatus.SYNCED) {
67                local.upsert(change.copy(syncStatus = SyncStatus.SYNCED))
68            }
69            // If local has pending changes, mark as conflict
70        }
71    }
72
73    fun schedulePush(noteId: String) {
74        // Use WorkManager for reliable background sync
75        // Exponential backoff, network constraint
76    }
77}