Kotlin Multiplatform backup into the user's own cloud: iCloud on iOS (CloudKit or iCloud Drive), the Google Drive app-data folder on Android. No server, no account on your side, no OAuth client setup on iOS.

• Why · Features · BackupKit 101 · A more advanced example
• Support matrix · Requirements · Samples · Testing
• Who's using it · Communication · Limits · Compared with

Every app with a wordbook or a journal eventually needs "a new phone should not lose my stuff". A backend for that means accounts, hosting and a privacy policy that says you hold user data. The user already pays for a cloud. BackupKit puts the files there, invisible to them, readable only by your app, with an engine that keeps the mirror correct across kills, retries and account switches.

• The user's cloud, not yours. App-private iCloud container or Drive's hidden appDataFolder (the one WhatsApp uses).
• Zero code on iOS, one tap on Android. The iCloud entitlement is the iOS setup; Android shows Google's dialog once.
• Complete-or-absent sync. Diffs, uploads the marker last, saves state after every step, resumes after a kill, refuses to merge two accounts, and refuses to delete most of a healthy backup without the user's say-so.
• Restore that survives a kill. Typed probe, write hold, resumable download with a commit boundary and per-file attempts.
• Typed errors. Seven CloudError values, nothing platform-specific leaks out.
• Small. About 1,200 lines. No DI framework, no Compose, no Firebase; iOS links no HTTP client.

// libs.versions.toml            backupkit = { module = "com.vocabloot:backupkit", version = "0.3.0" }
// build.gradle.kts (shared)     commonMain.dependencies { implementation(libs.backupkit) }

One entitlement on iOS, one OAuth client on Android (setup), then:

// iOS (iosMain)                                  // Android (androidMain)
val storage: CloudStorage = CloudKitStorage(       val storage: CloudStorage = GoogleDriveStorage(context)
    checkpointPath = "$dir/ck-checkpoint.json",
    cacheDirectory = "$dir/ck-cache",
)                                                  // or ICloudStorage() for files the user may open in Files

// common
val engine = SyncEngine(
    storage = storage,
    stateStore = FileSyncStateStore("$dir/backupkit-state.json"),
    policy = SyncPolicy(markerPath = "backup.json"),
)
val notes: ByteArray = notesJson()
val header: ByteArray = headerJson()               // uploaded last: its presence means "complete"
val outcome = engine.sync(
    SyncSnapshot(
        listOf(
            SyncEntry("notes.json", SyncSource.Bytes(notes), notes.size.toLong(), hash = sha256Hex(notes)),
            SyncEntry("backup.json", SyncSource.Bytes(header), header.size.toLong(), hash = sha256Hex(header)),
        ),
        isEmpty = notes.isEmpty(),                 // a fresh install never overwrites a real backup
    ),
)
when (outcome) {
    is SyncOutcome.Synced -> showUpToDate()
    is SyncOutcome.Unavailable -> showWhy(outcome.reason)   // NoAccount, NeedsConsent, RestorePending
    is SyncOutcome.Failed -> showStuck(outcome.error)       // Offline, StorageFull, AuthRevoked, Transport, ...
}

Photos and other write-once files go in as SyncSource.LocalFile(path) with hash = null: compared by size, uploaded first, never re-uploaded.

Offer a restore on first launch, then pull the files down with a commit boundary and resume after a kill:

when (val probe = engine.probe()) {
    is RemoteProbe.Found -> if (askUser(parseHeader(probe.marker))) restore(probe)
    RemoteProbe.NotReady -> showStillUploading()
    else -> Unit
}

suspend fun restore(found: RemoteProbe.Found) {
    engine.setHold(WriteHold.RestoreRunning)
    val restore = RestoreEngine(engine, storage, FileRestoreRecordStore("$dir/restore.json"), placement = { group, files ->
        importIntoMyModel(group, files); PlacementResult.Placed      // your schema, your rules
    })
    val outcome = restore.start(RestorePlan(found.source, files = listOf(
        RestoreFile("notes.json", toLocalPath = "$dir/notes.json", required = true, group = "meta"),
    ))) { p -> show("${'$'}{p.groupsDone} of ${'$'}{p.groupsTotal}") }
    if (outcome !is RestoreOutcome.Failed) engine.setHold(WriteHold.None)
}

Full walkthrough, holds and resume(): Restore on first launch.

CloudKit for app data the user never opens as files; iCloud Drive when the files should show in the Files app. Every row, and the differences between the transports: support matrix.

Targets: android, iosArm64, iosSimulatorArm64, iosX64. Versioning and the experimental API policy: stability.

com.vocabloot:backupkit-test (same version) ships the fakes the library's own tests run on, so your sync and restore code is unit-testable with no cloud:

val storage = FakeCloudStorage(readLocal = files::read, writeLocal = files::write)
val engine = SyncEngine(storage, MemorySyncStateStore(), SyncPolicy(markerPath = "backup.json"))
storage.failPutsContaining = "photos/"              // then assert the outcome and storage.putLog

• Vocabloot (App Store, Google Play): wordbook, photos and doodles mirror through this exact code. BackupKit is that code, extracted; Vocabloot 1.2 is the first store build that carries it.

Works with anything that gives you bytes or a file path: SQLDelight, Room, Okio, kotlinx-serialization, your own Ktor client on Android. Using BackupKit? Open a PR and add yourself.

• Questions and ideas: Discussions.
• Bugs: Issues, with the CloudError and platform.
• Security: SECURITY.md, privately.
• Contributing: CONTRIBUTING.md. Conduct: CODE_OF_CONDUCT.md.

• Drive: files above 5 MB use Drive's resumable protocol in 8 MiB chunks, streamed from disk; the multipart path stays for small files.
• Drive app-data counts against the user's Drive quota (Android Auto Backup does not).
• iCloud on the simulator needs an iCloud login on the simulator.
• RestoreEngine is @ExperimentalRestoreApi: its shape may still change in a minor release. Nothing Vocabloot-specific lives in it: files carry an opaque group and the app supplies a RestorePlacement.
• The sample app exercises ICloudStorage and GoogleDriveStorage; USE_CLOUDKIT switches it to CloudKit.
• Not included: scheduling, encryption, restore-into-your-model, any UI, Dropbox/OneDrive (see CloudBridge for those).

Open items with workarounds: known issues.

Docs site: setup, the SyncEngine contract, restore, consent, errors, scheduling, recipes, FAQ, known issues, stability. API reference (Dokka). Design notes and publishing steps are in docs/ for maintainers.

Inspired by react-native-cloud-storage (the Layer 1 verbs) and by IceCream and Apple's CKSyncEngine (the engine owns the state).

Apache 2.0. Made by Vaazh Studios.