Room Kmp Setup

Room in a multiplatform module

The database class, its entities and its DAOs live in commonMain. Only two things vary per platform: where the file goes and what the builder is handed. Both hide behind one expect:

// commonMain
@Database(entities = [...], version = <N>, exportSchema = true, autoMigrations = [...])
@TypeConverters(Converters::class)
abstract class MusicDatabase : RoomDatabase() {
    abstract fun getDatabaseDao(): DatabaseDao
}

expect fun getDatabaseBuilder(converters: Converters): RoomDatabase.Builder<MusicDatabase>

The driver and the query dispatcher are set once, at the shared injection site, so no platform can disagree about them:

// commonMain DI
single(createdAtStart = true) {
    getDatabaseBuilder(get<Converters>())
        .setDriver(BundledSQLiteDriver())
        .setQueryCoroutineContext(Dispatchers.IO)
        .build()
}

The actuals differ only in the overload they reach for. Desktop and iOS pass an absolute file path as name; Android passes a context, the class and a bare filename:

// jvmMain            Room.databaseBuilder<MusicDatabase>(name = absolutePathTo(DB_NAME))
// iosMain            Room.databaseBuilder<MusicDatabase>(name = documentsDir() + "/$DB_NAME")
// androidMain        Room.databaseBuilder(context, MusicDatabase::class.java, DB_NAME)

Traps

One annotation-processor configuration per declared target — a missing one is silent until link time. The processor generates the database implementation per compilation, so every target you declare needs its own entry:

dependencies {
    add("kspAndroid", libs.room.compiler)
    add("kspJvm", libs.room.compiler)
    add("kspIosArm64", libs.room.compiler)
    add("kspIosSimulatorArm64", libs.room.compiler)
}

Forget one and that target compiles your source fine, then fails with no generated implementation for the abstract database class (reproduced below, against every simulator variant too).

Anything registered on the builder lives in the platform actual, so it exists only on the platforms that register it. Migrations, addCallback, destructive-fallback policy and open-helper tweaks are all builder calls. In this codebase the hand-written migrations and the trigger-recreating onOpen callback are attached inside the Android actual only; the JVM and iOS actuals register nothing beyond the type converter. Nothing warns about it — those platforms simply run a database with no triggers and no hand-written migration steps. Put everything shared in a common extension the actuals call, and keep the actual down to the path.

The expect signature is the lowest common denominator, so a platform-only dependency has to be reached another way. getDatabaseBuilder(converters) cannot take an Android Context, so the Android actual pulls one from the service locator (getKoin().get<Context>()). That works, but it moves a compile-time error to a runtime one: the container must already hold a context by the time the database singleton is created. If your container builds eagerly, register the context first.

Export the schema directory or generated migrations cannot exist. Generated migrations are produced by diffing two exported schema files at build time:

room { schemaDirectory("$projectDir/schemas") }

Commit every <version>.json it writes. Deleting an old one does not break the build — it breaks the upgrade path for users still on that version, months later.

The bundled driver ships one native slice per CPU architecture, and one missing slice kills the whole target. Do not assume coverage from the fact that the artifact is "multiplatform". Open the JAR and look:

# pin the version you actually build with — read it from your version catalog first
unzip -l $(find ~/.gradle/caches -name "sqlite-bundled-jvm-<your-version>.jar" | head -1) \
  | grep -oE 'natives/[a-z0-9_]+/' | sort -u

A Gradle cache holds every version ever resolved, so an unpinned wildcard with head -1 can audit a version you no longer ship.

At one version we inspected, this printed linux_arm64, linux_x64, osx_arm64, osx_x64 and windows_x64 — Windows on ARM was the one combination with no slice, even though Linux and macOS both had theirs. A desktop build promising that target starts, paints its first screen, and then fails at the first database connection. Re-run the command against your version rather than trusting this list; slices get added over time (below, this repo's own pinned version drops a different one). The same audit applies to every native dependency in a desktop target: run it before you promise an architecture, because one gap drops the target.

Driver choice and native linking are configured in different files. The iOS framework here declares linkerOpts.add("-lsqlite3") — the flag a system-SQLite driver needs — while the shared DI module installs the bundled driver. Neither file mentions the other. Verify which driver the running app got, by logging it at the injection site or by breaking on the builder, rather than by reading either file alone.

Type converters are constructor-injected, so they are a dependency of the database, not a detail of it. addTypeConverter(converters) takes an instance; whatever that instance needs (a JSON format, a clock) must be constructible before the database. Registering it as its own eagerly-created singleton keeps that ordering explicit instead of accidental.

Verifying it

Run from the repository root.

1. Audit the bundled driver's native slices against your own pinned version — a recorded list goes stale in either direction:

   grep -n 'sqlite = "' gradle/libs.versions.toml
   unzip -l $(find ~/.gradle/caches -name "sqlite-bundled-jvm-2.7.0.jar" | head -1) | grep -oE 'natives/[a-z0-9_]+/' | sort -u
   

   Run here, at the version this repo actually pins (2.7.0): linux_arm64, linux_x64, osx_arm64, windows_x64 — osx_x64 (Intel Mac) is the slice missing, not the Windows-ARM one this file names above. That mismatch is the point: re-run against your version.

2. Migrations/callback and the ksp processor are each all-or-nothing across targets, never a silent subset:

   grep -n "addMigrations\|addCallback" core/data/src/androidMain/kotlin/com/maxrave/data/db/MusicDatabase.android.kt \
     core/data/src/jvmMain/kotlin/com/maxrave/data/db/MusicDatabase.jvm.kt core/data/src/iosMain/kotlin/com/maxrave/data/db/MusicDatabase.ios.kt
   grep -n 'add("ksp' core/data/build.gradle.kts
   grep -nE '^\s*(android|jvm) \{|ios[A-Za-z0-9]+\(\)' core/data/build.gradle.kts
   

   Pass condition: the first command hits only in the .android.kt file; the second and third lists match one-for-one — here, kspAndroid/kspJvm/kspIosArm64/kspIosSimulatorArm64 against android {/jvm {/iosArm64()/iosSimulatorArm64().

3. By hand: log which driver the running app actually got, at the injection site, on both the desktop and iOS builds. Correct: the log always names the bundled driver — the iOS linker flag alone would suggest otherwise, and neither file mentions the other.