Case Study: ESP-Point — IoT Hardware + iOS Sync, Offline-First Mimari

# Case Study: ESP-Point — IoT Hardware + iOS Sync, Offline-First Mimari

Problem: ESP32 microcontroller (DIY IoT cihaz) ile iOS app arasında çift yönlü realtime sync: cihaz sensor verisini yükler, app cihaza komut + firmware update gönderir. Offline-first zorunlu — cihaz internetsiz lokasyonlarda kullanılıyor.

Sonuç: 50ms BLE handshake median, 3-gün offline survival (local cache + delayed sync), OTA firmware update %99.2 success rate, 12K device sold.

1. Architecture

1[ESP32 Device]
2    ↓ BLE (peripheral)
3[iOS App — central]
4    ↓ Wi-Fi when available
5[Firebase Realtime DB + Cloud Storage]
6    ↓ sync queue
7[Web Dashboard for fleet ops]

iOS app proxy / router rolünde: BLE \<-\> Cloud bridge.

2. BLE State Machine: Gerçek Konu

BLE'nin tek "easy demo"su olmaz. State machine fragmentation real production konu:

States:

• unpairedScanning
• pairing
• paired (connected)
• paired-disconnected (cached)
• firmwareUpdating
• error (her state'ten geçer)

Her transition için iOS CBCentralManager callback'lerinde 2-3 race condition çözdük:

1. didDisconnect + didFailToConnect overlap — UUID-based dedup
2. scanForPeripherals background restoration — CBCentralManagerOptionRestoreIdentifierKey set
3. Bluetooth state .poweredOff → .poweredOn transition'da scan otomatik resume etmiyor — explicit scanForPeripherals re-call

Result: 50ms handshake median. Connection drop rate %0.8 (industry standard %4-6).

3. Offline Storage: Realm + Sync Queue

3-day offline target. Her sensor reading (saat başı) device storage'a yazılır, sonra sync queue Realm'e push edilir:

1class SyncQueue: Object {
2    @Persisted(primaryKey: true) var id: ObjectId
3    @Persisted var payload: Data
4    @Persisted var createdAt: Date
5    @Persisted var retryCount: Int = 0
6    @Persisted var lastError: String?
7}

Sync worker:

1final class SyncWorker {
2    func runOnce() async {
3        let realm = try await Realm()
4        let pending = realm.objects(SyncQueue.self)
5            .where { $0.retryCount < 5 }
6            .sorted(by: \.createdAt)
7            .prefix(50)
8 
9        for item in pending {
10            do {
11                try await uploadToFirebase(item.payload)
12                try realm.write { realm.delete(item) }
13            } catch {
14                try realm.write {
15                    item.retryCount += 1
16                    item.lastError = error.localizedDescription
17                }
18            }
19        }
20    }
21}

Network reachability NWPathMonitor ile sync worker tetiklenir. App background'da BGAppRefreshTask ile day-1 retry.

4. OTA Firmware Update

8-12MB binary blob transfer over BLE:

• Bluetooth chunk size: 244 bytes (BLE 5 max safe)
• Sequence number + CRC32 her chunk
• Acknowledgment per 10 chunk batch
• Total transfer: 4-6 dakika (10MB firmware için)
• Resume on disconnect support

Success rate: %99.2. Failure modes:

• %0.5: connection drop son chunk öncesi
• %0.2: device flash corrupt (re-upload OK)
• %0.1: power loss critical moment (manuel recovery)

OTA UI önemli — progress bar + "do not close app" warning + battery indicator. App Store reviews'da +%18 satisfaction.

5. Test Strategy

• Unit tests:: Sync queue state machine, BLE manager state transitions (mocked CBCentralManager)
• Integration tests:: Realm migration, sync conflict resolution
• Device farm:: 6 fiziksel ESP32 + 4 iPhone (iOS 16-18) — Cl pipeline'da physical test
• Snapshot tests:: UI state for each BLE state

Coverage: Domain layer %92, BLE manager %78, UI %65. Total %78.

6. iOS App Architecture

• SwiftUI iOS 16+
• BLEManager: @MainActor actor (Bluetooth callbacks main thread)
• DataLayer: Realm primary + Firebase secondary (offline-first)
• Sync orchestrator: BG task + foreground refresh

Memory baseline: 105MB. Peak (during OTA): 220MB (firmware buffer).

7. Öğrenimler

1. BLE physical testing zorunlu. Simulator BLE simulate edemiyor. CI'da fiziksel cihaz şart.

2. `CBCentralManagerOptionRestoreIdentifierKey` background relaunch için kritik — yoksa app kill'den sonra connection kaybolur.

3. State machine documentation. 6-state machine production'da debug için her transition log'lanır.

4. Realm migration schema değişikliklerinde sync queue'yu boşaltır (test ile validate edilmedi → 200 user data loss yaşandı).

5. OTA "abort" button kullanıcıya verme — corrupt flash riski. Sadece pause/resume.

6. Firebase rules offline write throughput'ı kısıtlayabilir — requireAuth: false writeRule + custom token validation.

7. Battery profiling BLE constantly scan = 8 saat battery. Connection-on-demand pattern %35 tasarruf.

Sonuç: IoT + iOS sync, "happy path" gösterilenden 10x daha zor. State machine + offline storage + OTA = 3 ay teknik borç eğer baştan plan yapılmazsa.