Kotlin Coroutines Structured Concurrency

Kotlin coroutines: structured concurrency

Core principle

A well-structured coroutine is a self-contained unit of asynchronous work — single entry, single exit, scoped to a lifecycle known at the call site.

Scopes should usually be tied to the caller's lifecycle, not stored as a property on the callee. A stored CoroutineScope is a strong review signal: the class must prove it owns cancellation, error reporting, restart behavior, and lifecycle. Most repositories, managers, use cases, and data sources cannot prove that, so they should expose suspend APIs instead.

The fix is almost always the same: make the API suspend and let the caller own the scope.

When to use this skill

You're writing or reviewing Kotlin code and you see any of these:

• A class with private val scope: CoroutineScope (constructor param stored as a property)
• An init { scope.launch { ... } } block
• A non-suspending public function whose body is scope.launch { ... }
• runBlocking { ... } in suspend-capable application code, or in tests where runTest should apply
• runCatching { suspendCall() } or a catch on Exception / Throwable around a suspend call without rethrowing CancellationException
• A catch (e: CancellationException) (or equivalent) around suspension that does not rethrow

The silent-cancellation bug

The reason an unowned CoroutineScope property is so dangerous: once a scope is cancelled, every future launch on it silently completes as cancelled — no exception, no log, nothing. The work just doesn't happen. This is one of the hardest coroutine bugs to diagnose, and it appears when a class holds a long-lived reference to a lifecycle it does not own.

If APIs are suspend, this can't happen: the caller's scope is either alive (work runs) or the call site cancels (the caller knows).

Anti-patterns and fixes

1. CoroutineScope stored as a property

// ❌ BAD
@Inject
class UserRepository(
    private val scope: CoroutineScope,
    private val api: UserApi,
) {
    fun refresh() {
        scope.launch { _state.value = api.fetchUser() }
    }
}

// ✅ GOOD
@Inject
class UserRepository(
    private val api: UserApi,
) {
    suspend fun refresh(): User = api.fetchUser()
}

The repository no longer needs to know about coroutines at all. The caller (a ViewModel, a use case) decides on what scope, with what error handling, with what cancellation semantics.

2. init-block launches

// ❌ BAD: construction-time side effect, unbounded work
class UserSession(private val scope: CoroutineScope, private val api: Api) {
    init { scope.launch { _user.value = api.load() } }
}

The constructor returns immediately. The caller can't await the load, can't see errors, can't cancel. The class is "alive" but its state is undefined.

// ✅ GOOD: explicit bootstrap, caller owns the suspension
class UserSession(private val api: Api) {
    private var _user: User? = null
    val user: User get() = checkNotNull(_user) { "Call init() first" }

    suspend fun init() { _user = api.load() }
}

3. Fire-and-forget from non-UI classes

A non-suspending public function on a non-UI class (repository, manager, use case, data source) that launches into a class-owned scope. The caller gets no result, no error, no cancellation, and no guarantee the work ever ran.

// ❌ BAD — repository with stored scope and fire-and-forget public API
class AnalyticsClient(private val scope: CoroutineScope, private val api: Api) {
    fun track(event: Event) {
        scope.launch { api.send(event) }      // caller has no idea what happens
    }
    fun signOut() {
        scope.launch { api.signOut() }        // silent failure if scope cancelled
    }
}

// ✅ GOOD
class AnalyticsClient(private val api: Api) {
    suspend fun track(event: Event) = api.send(event)
    suspend fun signOut() = api.signOut()
}

Carve-out: the UI ↔ state-holder boundary

UI frameworks are non-suspending. A Composable's onClick, a Fragment's onKeyEvent, an Activity's onNewIntent — none can suspend. The state holder (ViewModel, Decompose Component, feature model, etc. — anything whose role is to absorb UI events and hold UI state) is the boundary that translates one-shot UI events into asynchronous work bound to the UI lifecycle. That's its job.

// ✅ GOOD — state holder absorbs a non-suspending UI event onto its scope
class FavouritesViewModel(private val repo: FavouritesRepository) : ViewModel() {
    fun onToggleFavourite(item: Item) {
        viewModelScope.launch { repo.toggleFavourite(item) }
    }
}

// in Compose:
ListItem(onClick = { viewModel.onToggleFavourite(item) })

This is not the fire-and-forget anti-pattern. All three conditions must hold:

1. State holder for a UI surface — a ViewModel, Decompose Component, feature model, or equivalent UI state holder. Not a repository, manager, use case, or data source.
2. Lifecycle-bound scope — viewModelScope, a Component's coroutineScope that's cancelled on destroy, a Composable's rememberCoroutineScope(). Not AppScope, not an injected long-lived scope, not an ad-hoc CoroutineScope(...).
3. Caller really is a UI event — Composable callback, key handler, lifecycle hook. Not another business-logic class calling through the state holder.

The repository / use case / data source layers underneath still expose suspend APIs. The state holder is the only layer where the non-suspending → suspending translation belongs.

"It feels like a state holder" isn't enough. The question is "does the UI directly bind to this?" If no, the carve-out doesn't apply.

4. Stored scopes that aren't injected

The same anti-pattern, without an injected scope:

// ❌ BAD — same problem, scope is constructed in-class instead of injected
class FooManager {
    private val scope = MainScope()
    private val scope = CoroutineScope(Dispatchers.Default + SupervisorJob())
}

Lifecycle is now owned by nothing and lives forever. Replace with suspend APIs.

The same is true if the instantiation is nested inside a function body — fun foo() { CoroutineScope(...).launch { … } } is just a stored scope with extra steps. Each call leaks a new uncancellable scope; bundling it into a by lazy property doesn't fix the underlying issue (the scope shouldn't exist at all).

5. DI-bound singletons / initializers that launch

A specific pattern that is hard to spot: a DI-bound class (@SingleIn(AppScope), @Singleton, an Initializer.initialize()) launches a coroutine from its constructor / init block / initialize(). The launched work then has:

• A non-deterministic start time — whenever the graph realizes the binding. Cold-start ordering is invisible.
• No observable lifecycle. Nothing else in the codebase can see whether it's running or has crashed.
• No stop() / restart path. If upstream enters a bad state, the loop is uncancellable.
• No calling code to grep for. Readers can't find "who starts this and when".

§1 says scopes should be tied to the caller's lifecycle. The DI-bound variant violates this indirectly: the scope may be injected, but the launch is hidden inside construction — same effect, harder to see.

// ❌ BAD — singleton boots work as a side effect of being constructed
@SingleIn(AppScope::class)
@Inject
class TokenRefresher(
    @ForScope(AppScope::class) private val scope: CoroutineScope,
    private val auth: AuthService,
) {
    init {
        scope.launch {
            while (isActive) {
                delay(5.minutes)
                auth.refreshIfNeeded()
            }
        }
    }
}

// ❌ ALSO BAD — Initializer.initialize() that *launches*, not just registers
class TokenInvalidatorInitializer @Inject constructor(
    @ForScope(AppScope::class) private val scope: CoroutineScope,
    private val store: AuthStore,
    private val invalidator: TokenInvalidator,
) : Initializer {
    override fun initialize() {
        scope.launch { store.tokenChanges.collect { invalidator.invalidate() } }
    }
}

Both look like "application-scoped singletons", but the When NOT to apply carve-out is not permission to launch from init / initialize(). It's permission for a singleton to own a scope when its API is suspending.

First ask: does this background-loop class need to exist at all?

Most background-loop classes exist only because no one inverted the observation. Three answers, in order of preference:

Pattern 1 — invert into the consumer. The class observes state forever to react when it changes. But someone mutates the state — sign-out flow, profile switch, flag-update handler. That mutation site is already in a coroutine context and is the natural place to do the work directly.

// ✅ GOOD — no background loop, no scope, no class. The mutation site does the work.
class Authenticator(
    private val authStore: AuthStore,
    private val tokenInvalidator: TokenInvalidator,
) {
    suspend fun signOut() {
        authStore.clearTokens()
        tokenInvalidator.invalidate()   // direct call at the mutation site
    }
}

The background-loop class is deleted. The work happens where the state changes.

When this applies: the consumer of the state has a clear lifecycle (a use case, an Authenticator, a service handler) and can perform the reaction inline.

Pattern 2 — scheduled work. Genuinely periodic or deferred. Use WorkManager / BGTaskScheduler. The enqueue is one-shot; make it suspending and call it once from an orchestrator that already runs at startup.

Pattern 3 — explicit named launch site. Sometimes the consumer is a synchronous API with no observable lifecycle (e.g., OpenTelemetry's Sampler.shouldSample(...), an AIDL stub fanout, a broadcast receiver bridge). The observation has to live somewhere coroutine-aware, but it must live at an explicit named call site — not in the class's own init.

// ✅ GOOD — work is named; an explicit call site owns the launch
@SingleIn(AppScope::class)
class OtelConfigurableSampler(...) : Sampler {
    @Volatile private var delegate: Sampler = ...

    suspend fun observeRate(featureFlags: FeatureFlags) {
        featureFlags.observe(OTEL_SAMPLING_RATE).collect { rate ->
            delegate = Sampler.traceIdRatioBased(rate.coerceIn(0.0, 1.0))
        }
    }

    override fun shouldSample(...) = delegate.shouldSample(...)
}

// wired explicitly at the OTel SDK init module:
applicationScope.launch { otelSampler.observeRate(featureFlags) }

When this applies: the consumer is a synchronous API that calls into you with no observable lifecycle. The launch can't be invertible, but it must still be visible at a named call site.

Test for which pattern fits

"Is the consumer's lifecycle observable to me?"

• Yes, and they're already in a coroutine context → Pattern 1. Push the subscription into them; delete the background-loop class.
• The work is periodic / deferred → Pattern 2. Suspend enqueue called once.
• No, they're a synchronous API with no observable lifecycle → Pattern 3. Explicit launch site, not init.

If a fourth answer seems to fit — e.g., "I want a Bootable interface that launches everything for me" — that's the same anti-pattern with an extra layer of abstraction. The whole point is that launches be visible; auto-discovery by interface defeats it.

Initializers are still fine — if they only register

The Initializer pattern is correct when initialize() registers a listener or hook. The bug is when initialize() launches a coroutine.

// ✅ GOOD Initializer — registers a contributor, doesn't launch
class FavouritesContributorInitializer @Inject constructor(
    private val registry: ContributorRegistry,
    private val favouritesContributor: FavouritesContributor,
) : Initializer {
    override fun initialize() {
        registry.register(favouritesContributor)
    }
}

Initializer.initialize() must not launch a coroutine. If yours does, it's a Pattern 1/2/3 candidate.

Diagnostic for review

• Where is the start moment defined? If "wherever DI realizes me", bad.
• Who can observe whether the work is running? If "no one", bad.
• Who can stop or restart it? If "no one", bad.
• Can a reader grep for the launch site? If no, bad.

If the answers are "the consumer / the orchestrator / the named call site" — you're good.

6. Swallowing CancellationException

A catch clause around a suspend call that matches CancellationException — directly, or through Exception / Throwable — and doesn't rethrow usually turns cancellation into silent success. The parent coroutine thinks the child finished; the child keeps running (or its side effects do); the cancellation contract is broken.

Same failure shape as §1's stored-scope bug, viewed from the other end: §1 hides the work from the caller's lifecycle; this hides cancellation from the work.

// ❌ BAD — catches CancellationException, never rethrows
suspend fun fetch() {
    try {
        api.load()
    } catch (e: Exception) {           // matches CancellationException too
        logger.warn("load failed", e)
    }
}

// ❌ ALSO BAD — runCatching has the same problem
suspend fun fetch() {
    runCatching { api.load() }
        .onFailure { logger.warn("load failed", it) }
}

The acceptable shapes:

// ✅ Separate catch first
try { api.load() }
catch (e: CancellationException) { throw e }
catch (e: Exception) { logger.warn("load failed", e) }

// ✅ Conditional rethrow inside the broad catch
try { api.load() }
catch (e: Exception) {
    if (e is CancellationException) throw e
    logger.warn("load failed", e)
}

// ✅ ensureActive() — good when the catch handles ordinary failures and you only need
// to rethrow if the current coroutine is cancelled
try { api.load() }
catch (e: Exception) {
    currentCoroutineContext().ensureActive()
    logger.warn("load failed", e)
}

// ✅ runCatching with explicit guard
runCatching { api.load() }
    .onFailure {
        if (it is CancellationException) throw it
        logger.warn("load failed", it)
    }

// ✅ runCatching terminated with getOrThrow (cancellation flows back out)
runCatching { api.load() }.getOrThrow()

The trigger is "a suspend call inside the try", not "the enclosing function is declared suspend". This applies inside any suspending body — suspend fun, a launch { … } lambda, a Flow collect { … }, etc.

The common carve-out is an intentionally local timeout: catching TimeoutCancellationException from your own withTimeout and converting it to a domain result can be correct. Keep that catch narrow and close to the timeout. Do not use it as permission to swallow arbitrary cancellation.

Catching a non-cancellation subtype (IOException, your own exception types) is fine — they don't extend CancellationException.

7. runBlocking

runBlocking parks the current thread until the lambda finishes. Inside suspend-capable or lifecycle-scoped application paths it is wrong: a thread that meant to be async is now blocked, structured concurrency is broken, and any cancellation upstream has no effect. It is the "callee makes a structural decision for the caller" anti-pattern at its most direct.

// ❌ BAD — bridging to suspend by blocking the calling thread
fun saveUser(user: User) {
    runBlocking { repository.save(user) }
}

Three fixes, by context:

Suspend-capable application code — make the function suspend:

// ✅ GOOD
suspend fun saveUser(user: User) = repository.save(user)

If the immediate caller can't suspend either (a non-suspending UI callback, a BroadcastReceiver hook), use the existing lifecycle-bound scope at the boundary — see §3's UI ↔ state-holder carve-out. The fix is at the boundary, not inside saveUser.

Legitimate blocking boundaries exist: main in a CLI tool, Java interop APIs that must return synchronously, framework callbacks with no suspending alternative, and migration shims. Keep runBlocking at that outer boundary, keep the body small, and call suspending code immediately.

Tests — use runTest:

// ❌ BAD — real time, slow tests, no virtual delay
@Test fun loadsUser() = runBlocking {
    assertThat(repository.load().name).isEqualTo("Alice")
}

// ✅ GOOD
@Test fun loadsUser() = runTest {
    assertThat(repository.load().name).isEqualTo("Alice")
}

runTest gives you virtual time (delay() returns immediately), TestDispatcher integration, and proper coroutine cleanup. Real-time runBlocking in tests makes them slow and flaky.

ContentProvider carve-out — Android's ContentProvider methods (query, insert, update, delete, onCreate, call) are synchronous from outside the process. There is no way to suspend them. Inside member functions of a ContentProvider subclass (direct or indirect — not companion objects), runBlocking is the unavoidable bridge. Keep the body as short as possible and call into suspending code immediately:

// ✅ Acceptable in ContentProvider members only
class MyProvider : ContentProvider() {
    override fun query(...): Cursor? = runBlocking { dao.query(...) }
}

This carve-out is for android.content.ContentProvider subclasses only. "It's like a ContentProvider" doesn't apply, and a runBlocking in a ContentProvider's companion object is still a regular violation — the helper isn't part of the framework's synchronous surface.

Quick reference

Refactoring guidance

Removing an existing offender:

1. Start at the leaf. Pick the class farthest from any UI — usually a repository or data source. Its public surface should be the easiest to convert.
2. Convert public functions to suspend one at a time. The compiler will surface every caller.
3. At each caller, choose the scope deliberately: viewModelScope, lifecycleScope, coroutineScope { }, or an explicit job. This is the choice that was missing before.
4. Delete the CoroutineScope constructor parameter once nothing uses it. Remove the injection binding.

Don't try to fix every class in one MR. Removing an anti-pattern is incremental work.

When NOT to apply

• UI state holders absorbing UI events. A ViewModel/Component/feature model with fun onClick(...) { viewModelScope.launch { ... } } is correct — that's the boundary the framework needs. See §3 carve-out.
• Lifecycle owners with explicit cancellation and error policy. Actors/services, app infrastructure, or application-scoped singletons may own a scope when they expose clear close/cancel/restart behavior or otherwise map directly to an application lifecycle. Inject Application.applicationScope explicitly rather than creating one ad-hoc. This is not permission to launch from init / initialize() — see §5.
• Already-suspending APIs don't need any of this work.
• Tests sometimes use TestScope as a deliberate ambient scope — that's a different pattern with explicit virtual-time control.

Red flags during review

These thoughts mean the anti-pattern is back:

Related

• kotlin-flow-state-event-modeling — StateFlow, SharedFlow, Channel, stateIn, one-shot events, and related modeling.

Source: chrisbanes/skills — distributed by TomeVault.