Kotlin — Coroutines drills (extra practice)

▶ **Extra-practice drills that pair with the Kotlin Reactive course (Chapter 3: Coroutines & interop, Chapter 6: Flow).** Learn it there, drill it here. Build the mental model of coroutines from scratch — `async`/`await` aggregator, structured-concurrency cancellation cascade, `supervisorScope` sibling isolation, `withTimeout` deadlines, `Mutex` fair arbitration, async exception propagation, bounded `Channel` with back-pressure, cold `Flow` operators (`map`/`filter`/`take`/`distinct`/`combine`/`debounce`), `SharedFlow` vs `StateFlow` replay+conflation, and a Dispatcher worker pool. Run server-side via Piston.

40 challenges63 topics

How it works1Pick a challenge2Read the requirements3Code in a full IDE4Run the tests

40 challenges

easy25 min

Bug: launch returns no value (should be async)

async/await — Parallel Aggregator

Structured Concurrency — Cancellation Cascade

Bounded Channel with Back-pressure

Cold Flow Pipeline (map / filter / take / distinct)

Dispatcher Worker Pool — Fair Scheduling

supervisorScope — isolate sibling failures

withTimeout — deadline cancellation

Mutex — fair single-resource arbitration

Flow.combine — latest of each

Flow.debounce — drop bursts

async exception propagation

SharedFlow vs StateFlow — replay & conflation

Bug: launch returns no value (should be async)

Bug: sequential await() defeats parallelism

Bug: shared mutable state race (need a Mutex)

Bug: GlobalScope job leaks past parent cancel

Bug: try/catch swallows CancellationException

Channel — send & receive

Channel — close & iterate

produce + consumeEach

Pipeline — produce → transform → take

Prime sieve pipeline

Fan-out — distribute work

Fan-in — merge producers

Buffered channel — capacity

Channels are fair — ping/pong

Hedged Requests — select {} takes the first response

Actor Confinement — a counter with a mailbox, no locks

Pipeline Backpressure — suspension counts made visible

One Counter, Three Fixes — Mutex vs CAS vs Confinement

Cancellation & finally — NonCancellable cleanup

Fan-out Partial Failure — supervisorScope vs coroutineScope

Semaphore(K) — the FIFO acquisition timeline

Ticker Batching — event windows on a fixed clock

debounce vs throttle — who survives the burst

onUndeliveredElement — the leak the buffer hides

Mutex is NOT reentrant — the self-deadlock

shareIn(WhileSubscribed) — when the upstream really stops

CoroutineExceptionHandler — where it fires (and where it never will)

Rendezvous vs buffered — capacity, suspensions, makespan

async/await — Parallel Aggregator

Structured Concurrency — Cancellation Cascade

Bounded Channel with Back-pressure

Cold Flow Pipeline (map / filter / take / distinct)

Dispatcher Worker Pool — Fair Scheduling

supervisorScope — isolate sibling failures

withTimeout — deadline cancellation

Mutex — fair single-resource arbitration

Flow.combine — latest of each

Flow.debounce — drop bursts

async exception propagation

SharedFlow vs StateFlow — replay & conflation

Bug: launch returns no value (should be async)

Bug: sequential await() defeats parallelism

Bug: shared mutable state race (need a Mutex)

Bug: GlobalScope job leaks past parent cancel

Bug: try/catch swallows CancellationException

Channel — send & receive

Channel — close & iterate

produce + consumeEach

Pipeline — produce → transform → take

Prime sieve pipeline

Fan-out — distribute work

Fan-in — merge producers

Buffered channel — capacity

Channels are fair — ping/pong

Hedged Requests — select {} takes the first response

Actor Confinement — a counter with a mailbox, no locks

Pipeline Backpressure — suspension counts made visible

One Counter, Three Fixes — Mutex vs CAS vs Confinement

Cancellation & finally — NonCancellable cleanup

Fan-out Partial Failure — supervisorScope vs coroutineScope

Semaphore(K) — the FIFO acquisition timeline

Ticker Batching — event windows on a fixed clock

debounce vs throttle — who survives the burst

onUndeliveredElement — the leak the buffer hides

Mutex is NOT reentrant — the self-deadlock

shareIn(WhileSubscribed) — when the upstream really stops

CoroutineExceptionHandler — where it fires (and where it never will)

Rendezvous vs buffered — capacity, suspensions, makespan