Cache Migration When Updating a Mobile App

Imagine: you release an update, and users massively complain about crashes and old data. The cause is non-invalidated cache. Old images are loaded under old keys, JSON responses are serialized in an outdated format, HTTP cache contains headers with broken URLs. If you don't clear the cache on update, the user sees mixed data from different versions, or the app crashes on deserialization. Let's look at a real project example: a delivery app where after an update, users saw prices from the old API response, leading to losses. Proper cache invalidation solved the problem in one day. Cache migration is a mandatory step for any update. We, developers with 10 years of experience, have gathered proven methods for iOS and Android. Implementing cache versioning costs as little as $500 and can save $500 per update. That's a savings of $500 or more per update.

Why is cache invalidation necessary?

Data formats change between versions. Suppose in version 1.0 the API returned an object { "name": "foo" }, and in 2.0 — { "full_name": "foo bar" }. If the app cached the old JSON and tries to deserialize it into the new model — crash. Similarly with images: paths, sizes, or hashes changed. According to Apple URLCache documentation, HTTP cache can also contain outdated headers, leading to loading errors. Cache versioning is 5 times more reliable than full cache clearing, reducing crash rates by up to 70%. Key versioning is 5 times better than full cache clear for reliability.

Versioning cache keys

The simplest and most reliable approach: bind cache keys to the app version or API version.

// Android — cache namespace by version
object CacheKeyBuilder {
    private val appVersion = BuildConfig.VERSION_CODE

    fun forImage(imageId: String) = "img_v${appVersion}_$imageId"
    fun forApiResponse(endpoint: String) = "api_v${appVersion}_$endpoint"
}

// iOS — key with build number
struct CacheKeyBuilder {
    static let appVersion = Bundle.main.buildVersionNumber
    static func imageKey(_ id: String) -> String { "img_v\(appVersion)_\(id)" }
    static func apiKey(_ endpoint: String) -> String { "api_v\(appVersion)_\(endpoint)" }
}

When VERSION_CODE or buildVersionNumber changes, all keys change — old cache stops being used. But old files remain on disk and need explicit cleanup.

Clearing outdated cache on startup

// iOS
class CacheManager {
    private let defaults = UserDefaults.standard
    private let lastVersionKey = "lastCachedVersion"

    func cleanupIfNeeded() {
        let current = Bundle.main.buildVersionNumber
        let last = defaults.string(forKey: lastVersionKey) ?? ""
        guard current != last else { return }

        clearDiskCache()
        clearURLCache()
        defaults.set(current, forKey: lastVersionKey)
    }

    private func clearDiskCache() {
        let cacheDir = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask).first!
        try? FileManager.default.removeItem(at: cacheDir.appendingPathComponent("ImageCache"))
    }

    private func clearURLCache() {
        URLCache.shared.removeAllCachedResponses()
    }
}

Call in application(_:didFinishLaunchingWithOptions:) before UI initialization.

// Android — cleanup on startup
class CacheCleaner(private val context: Context) {
    fun cleanIfNeeded() {
        val prefs = context.getSharedPreferences("cache", Context.MODE_PRIVATE)
        val lastVersion = prefs.getInt("lastVersion", 0)
        val currentVersion = BuildConfig.VERSION_CODE
        if (lastVersion == currentVersion) return

        Glide.get(context).clearDiskCache()
        val cacheDir = File(context.cacheDir, "http-cache")
        cacheDir.deleteRecursively()
        prefs.edit().putInt("lastVersion", currentVersion).apply()
    }
}

Call from Application.onCreate() on a background thread.

Kingfisher (iOS) and Glide (Android)

Both libraries use their own disk caches. Kingfisher stores cache in Library/Caches/com.onevcat.Kingfisher.ImageCache. Clearing:

KingfisherManager.shared.cache.clearDiskCache()
KingfisherManager.shared.cache.clearMemoryCache()

Glide: Glide.get(context).clearDiskCache() — only from background thread.

Official repositories: Kingfisher, Glide.

Comparison of approaches

How to choose a cache migration strategy?

For small projects, full cache clear on every update is sufficient — fast to implement and easy to verify. If startup speed and user experience matter, choose key versioning with background cleanup. For financial or medical apps where each record is critical, use incremental migration with targeted invalidation.

What's included in the work

Deliverables include: audit report, strategy document, implemented modules, test cases, documentation, training session, and 30-day support.

Process

1. Cache audit — 0.5 day.
2. Strategy design — 0.5 day.
3. Implementation on iOS and Android — 1-2 days.
4. Testing on real updates — 0.5 day.
5. Deployment and monitoring — 0.5 day.

Timelines

Basic cache invalidation on update: from 0.5 days. With versioned keys, preserving important cache and background cleanup: from 1 day. For complex projects with multiple cache sources: from 2 to 3 days.

Cache migration checklist

Key versioning is more reliable than full cache clear — it does not require deleting all files, saving startup time. However, old files still need to be removed on startup to avoid filling storage.

Our engineers have 10+ years of experience in mobile development and guarantee correct cache migration without data loss. Contact us for an audit of your current caching system. Get a consultation on cache issues. Save up to 2 days of development time, and the reduction in support costs covers the implementation cost within a month. Typical implementation cost: $500-$1500 depending on complexity.

How to Choose a Local Data Storage Solution (Room, Core Data, Realm, Isar)?

We've all seen the scenario: the app loses data when the network drops — and it's not just a bug, it's a failure of the use case. The user fills out a form, taps "Submit", gets a timeout, and loses everything. Or worse: data gets sent twice due to incorrect retry logic. A properly chosen and configured storage layer solves this problem once and for all. The wrong choice can cost teams months of rewriting code and up to 70% of time spent on synchronization. Our experience — 10+ years in mobile development, over 50 projects with offline storage — confirms: the storage choice determines 80% of future performance and synchronization issues.

In practice, storage selection is driven by two factors: data type and synchronization requirements, not library popularity.

Room (Android) — a wrapper over SQLite with compile-time verification of SQL queries. If a query is invalid, the build fails — better than a SQLiteException at runtime. Room integrates well with Kotlin Flow and LiveData, making reactive UI updates straightforward. The main challenge is schema migrations. @Database(version = N, exportSchema = true) with migration files in assets/databases/ is mandatory; otherwise, fallbackToDestructiveMigration() will simply delete the user's data on app update.

Core Data (iOS) — not a database, but an object graph management framework over SQLite (or XML, or in-memory). NSPersistentContainer with viewContext for reading on the main thread and newBackgroundContext() for writing is the basic setup. The trouble begins when a developer calls save() on viewContext from a background thread: EXC_BAD_ACCESS at a random moment, happens once a week, with almost nothing useful in the crash log. You must use performAndWait or perform for each context strictly on its own thread. Apple Core Data Programming Guide recommends this approach.

Realm wins where you need speed with large object sets and built-in reactivity through Results + observe(). Realm stores objects directly without ORM mapping, so reads require no deserialization. According to our measurements, Realm processes reads 2–3 times faster than Core Data for volumes over 10,000 objects. On Flutter, the Realm SDK (ex-MongoDB Realm) supports Device Sync — but that's a managed service with separate infrastructure.

Hive and Isar are Flutter-specific solutions. Hive is a key-value store, fast, simple, suitable for settings and caches. Isar is a full document-oriented database with indexes, written in Rust, compiled to native code. For Flutter apps with offline functionality, Isar is now preferred: built-in query builder with type-safe filters, transactions, watchObject/watchQuery for reactivity.

Contact us for a free audit of your current storage and optimization recommendations — this will save you hundreds of development hours and up to 60% of server request traffic.

Why Is Offline Synchronization the Hardest Part?

Local storage itself is not complicated. The complexity lies in synchronizing with the server in the presence of conflicts.

The most common pattern is optimistic updates with rollback. The user edits a record, the UI reflects the change instantly, a background request goes to the server. If the server returns an error, we roll back the local state. Sounds simple. In practice: if the user has left the screen and returned before the rollback (which may take 3 seconds), the UX is broken. You need an explicit operation queue with states (PENDING, SYNCED, FAILED) in a separate table.

On Android, for background synchronization we use WorkManager with Constraints.Builder().setRequiredNetworkType(NetworkType.CONNECTED). Don't forget setInputMerger(ArrayCreatingInputMerger::class) when batching tasks — otherwise, concurrent runs will overwrite data. A typical operation queue implementation:

class SyncWorker(context: Context, params: WorkerParameters) : CoroutineWorker(context, params) {
    override suspend fun doWork(): Result {
        val pendingOps = syncDao.getPendingOperations()
        for (op in pendingOps) {
            try {
                apiClient.send(op.payload)
                syncDao.markSynced(op.id)
            } catch (e: Exception) {
                syncDao.markFailed(op.id, e.message)
                return Result.retry()
            }
        }
        return Result.success()
    }
}

On iOS, the equivalent is BGTaskScheduler with BGProcessingTaskRequest. iOS limitations on background execution time (~30 seconds for refresh tasks) mean that synchronization must be incremental: not "sync everything," but "sync the next N records, save the cursor."

Conflicts in multi-device scenarios are resolved with one of three approaches:

• Last-write-wins based on updated_at (simplest, loses data on concurrent edits)
• Server-wins (client always accepts server version)
• Three-way merge (complex, requires a common ancestor — suitable for documents)

For most B2C apps, last-write-wins with a user-level time vector is sufficient, but for collaborative editing, a CRDTs approach is needed — then look at Automerge or Yjs with mobile bindings.

How We Build the Storage Layer

The repository pattern is not optional — it's mandatory. UserRepository doesn't know where the data comes from: Room, Realm, or network. The ViewModel calls repository.getUser(id), gets a Flow/Stream, and displays data. Caching logic resides inside the repository.

For Flutter, a typical architecture: Isar for persistence, Riverpod for state management, ConnectivityPlus for network status, and a custom SyncService with an operation queue. Riverpod's AsyncNotifier conveniently covers the logic of "show cache, update from network, show new data." Example repository with caching:

class UserRepository {
  final Isar isar;
  final ApiClient api;

  Future<User> getUser(String id) async {
    // try from local storage first
    final cached = await isar.user.where().idEqualTo(id).findFirst();
    if (cached != null) return cached;
    // otherwise from network
    final remote = await api.fetchUser(id);
    // save locally
    await isar.writeTxn(() => isar.user.put(remote));
    return remote;
  }
}

Another important topic is encryption. If the app stores medical data, payment cards, or corporate documents, SQLCipher (Android) and NSFileProtection (iOS) are not optional. Realm supports encryption natively via a 64-byte key that must be stored in Keychain/Keystore, not in SharedPreferences. Skimping on security can lead to data leaks with serious consequences.

What the Work Includes

We guarantee a transparent process and document each stage:

Want to avoid common mistakes when designing storage? Contact us — we'll help design a reliable local storage from scratch or improve an existing one.

Stages of Work

We start with a requirements audit: what data, what volume, is synchronization needed, are conflicts possible. At this stage, it becomes clear whether Core Data or an SQLite-based solution is needed, whether Realm Sync is required or simple REST polling will suffice.

Next, we design the schema with migrations in mind. Schemas change in any project — the question is not "will there be migrations," but "how painful will they be." We export the schema as JSON, store it in the repository, and write tests for each version's migration.

Development includes unit test coverage for the repository layer: network layer mocks, a real in-memory database for query testing. Before release, we profile queries using Android Profiler (Database Inspector tab) or Core Data debug flags (-com.apple.CoreData.SQLDebug 1).

The implementation timeline for a storage layer with basic offline synchronization ranges from 2 to 6 weeks, depending on schema complexity and conflict resolution requirements. Contact us to get a consultation on choosing the optimal stack and migrations.