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Channels & Flows
Mohit Sarveiya
/heyitsmohit
twitter.com
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Channels & Flows
• Channels
• Actors
• Flows
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Coroutine A
Passing Data
Coroutine B
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Coroutine A
Passing Data
Coroutine B
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Passing Data
• Deferred
• Channels
• Flows
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Channels
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What is a Channel?
• Similar to BlockingQueue.
interface Channel {
suspend fun send(element: E)
suspend fun receive(): E
}
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Using Channels
runBlocking {
val channel = Channel()
launch {
channel.send(1)
}
val num = channel.receive()
}
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Using Channels
runBlocking {
val channel = Channel()
launch {
channel.send(1)
}
val num = channel.receive()
}
Parent Coroutine
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Using Channels
runBlocking {
val channel = Channel()
launch {
channel.send(1)
}
val num = channel.receive()
}
Child Coroutine
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Using Channels
runBlocking {
val channel = Channel()
launch {
channel.send(1)
}
val num = channel.receive()
}
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Using Channels
runBlocking {
val channel = Channel()
launch {
channel.send(1)
}
val num = channel.receive()
}
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Using Channels
runBlocking {
val channel = Channel()
launch {
channel.send(1)
}
val num = channel.receive()
}
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Channel Types
• Rendezvous
• Buffered
• Unlimited
• Conflated
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Creating Channel
fun Channel(capacity: Int = RENDEZVOUS) =
when (capacity) {
RENDEZVOUS !-> RendezvousChannel()
UNLIMITED !-> LinkedListChannel()
CONFLATED !-> ConflatedChannel()
BUFFERED !-> ArrayChannel(CHANNEL_DEFAULT_CAPACITY)
else !-> ArrayChannel(capacity)
}
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Rendezvous Channel
• No Buffer
• Waits for receiver to send data
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Rendezvous Channel
runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
}
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Rendezvous Channel
runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
}
SUSPEND
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Rendezvous Channel
runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
channel.receive()
}
Receiver
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runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
channel.receive()
}
Rendezvous Channel
RESUME
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runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
channel.receive()
}
Rendezvous Channel
SUSPEND
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Channel State
• Queue
• Buffer (Optional)
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Rendezvous Channel
val channel = Channel()
EmptyQueue
Queue
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Channel States
• Empty Queue
• Send Queued
• Send Buffered
• Closed
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Channel States
EmptyQueue
LockFreeLinkedList
SendBuffered SendQueued Closed
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Rendezvous Channel
val channel = Channel()
channel.send(1)
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Rendezvous Channel
class SendElement(
val data: Any?,
val cont: CancellableContinuation
) : LockFreeLinkedListNode()
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Rendezvous Channel
val channel = Channel()
channel.send(1)
SendElement(1)
Queue
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Rendezvous Channel
val channel = Channel()
channel.send(1)
channel.receive()
EmptyQueue
Queue
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class AbstractSendChannel {
fun toString() =
{$queueDebugStateString}$bufferDebugString”
}
Query Channel State
Current Channel State Buffer Size
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Query Channel State
• Get most recent state
• Size of buffer
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$queueDebugStateString
get() {
if (head !!=== queue) return "EmptyQueue"
}
Query Channel State
EmptyQueue
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$queueDebugStateString
get() {
when (head) {
is Closed!!<*> !-> head.toString()
is Receive!!<*> !-> "ReceiveQueued"
is Send !-> "SendQueued"
else !-> "UNEXPECTED:$head"
}
}
Query Channel State
SendElement
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$queueDebugStateString
get() {
if (tail !!!== head) {
result += ",queueSize=${countQueueSize()}"
}
}
Query Channel State
SendElement
Buffer
SendElement SendElement
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Query Channel State
val channel = Channel()
channel.send(1)
println(channel)
Output:
RendezvousChannel{SendQueued}
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Idiomatic Channel Creation
runBlocking {
val channel = Channel()
launch {
for (i in 1!..10) {
channel.send(i)
}
}
}
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Idiomatic Channel Creation
runBlocking {
val channel = Channel()
launch {
for (i in 1!..10) {
channel.send(i)
}
}
}
Producer
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Idiomatic Channel Creation
fun CoroutineScope.producer(): ReceiveChannel =
produce {
for (i in 1!..5) {
send(i)
}
}
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Idiomatic Channel Creation
runBlocking {
val channel: ReceiveChannel = producer()
}
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Producer Builder
• Creates Coroutine with Channel
• DSL for creating channels
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Produce Builder
fun CoroutineScope.produce(
context: CoroutineContext,
capacity: Int = 0,
block: suspend ProducerScope.() !-> Unit
): ReceiveChannel
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Produce Builder
fun CoroutineScope.produce(…) {
val channel = Channel(capacity)
val newContext = newCoroutineContext(context)
val coroutine = ProducerCoroutine(ctx, channel)
coroutine.start(…, coroutine, block)
return coroutine
}
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Idiomatic Channel Consumption
runBlocking {
val channel: ReceiveChannel = producer()
channel.close()
}
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Idiomatic Channel Consumption
runBlocking {
val channel: ReceiveChannel = producer()
channel.close()
for (num in channel) {
calculate(num)
}
}
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Channel Iterator
interface ChannelIterator {
suspend fun hasNextSuspend(): E
fun next(): E
}
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Coroutine A
Channel Iterator
Coroutine B
1 2 3 x
Closed token
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Coroutine A
Channel Iterator
Coroutine B
EmptyQueue
Channel Queue
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Coroutine A
Channel Iterator
Coroutine B
1
SendElement(1)
Channel Queue
hasNext()
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Coroutine A
Channel Iterator
Coroutine B
1
SendElement(1)
Channel Queue
next()
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Coroutine A
Channel Iterator
Coroutine B
1
SendElement(1)
Channel Queue
next()
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Coroutine A
Channel Iterator
Coroutine B
1
SendElement(3)
Channel Queue
next()
2 3
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Coroutine A
Channel Iterator
Coroutine B
1
Closed
Channel Queue
2 3 x
Closed token
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Coroutine A
Channel Iterator
Coroutine B
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Suspended
No Closed token
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Channel Iterator
fun hasNextSuspend() = suspendCancellable { cont !->
while (true) {
val result = channel.pollInternal()
this.result = result
if (result is Closed!!<*>) {
cont.resume(false)
} else {
cont.resume(true)
}
…
}
}
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Channel Types
• Rendezvous
• Buffered
• Unlimited
• Conflated
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Buffered Channel
• Buffer with specific capacity.
• Caches sent items in Array
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Buffered Channel
runBlocking {
val channel = Channel(capacity = 3)
}
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Buffered Channel
class ArrayChannel(val capacity: Int) {
val buffer = arrayOfNulls(capacity)
…
}
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runBlocking {
val channel = Channel(capacity = 3)
launch {
channel.send(1)
channel.send(2)
channel.send(3)
}
}
Buffered Channel
1 2 3
Buffer
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runBlocking {
val channel = Channel(capacity = 3)
launch {
channel.send(1)
channel.send(2)
channel.send(3)
channel.send(4)
}
}
Buffered Channel
SUSPEND
1 2 3
Buffer
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Unlimited Channel
• Buffer with unlimited capacity.
• LinkedList Buffer.
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Unlimited Channel
runBlocking {
val channel = Channel(Channel.UNLIMITED)
}
Empty Queue
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Unlimited Channel
runBlocking {
val channel = Channel(Channel.UNLIMITED)
launch {
channel.send(1)
channel.send(2)
channel.send(3)
}
} SendBuffered(1) SendBuffered(2) SendBuffered(3)
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Conflated Channel
• Get most recent sent element
• Similar to ReplaySubject (RxJava)
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Conflated Channel
val channel = Channel(Channel.CONFLATED)
channel.send(1)
channel.send(2) !// Conflate previously sent
println(channel.receive())
Output:
2
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Conflated Channel
val channel = Channel(Channel.CONFLATED)
channel.send(1)
SendBuffered(1)
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Conflated Channel
val channel = Channel(Channel.CONFLATED)
channel.send(1)
channel.send(2)
SendBuffered(2)
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Conflated Channel
val channel = Channel(Channel.CONFLATED)
channel.send(1)
channel.send(2)
SendBuffered(2)
Never Suspend
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Channel Types
• Rendezvous
• Buffered
• Unlimited
• Conflated
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Channels
Rendezvous
• Queue size = 1
• Buffer: None
Array Channel
• Queue size = 1
• Buffer: Array
Linked List Channel
• Queue size = Unlimited
• Buffer: Queue
Conflated
• Queue size = 1
• Buffer: Queue
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Tips for using Channels
• Query channel queue for debugging.
• Channel type determines back pressure strategy to use.
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Actors
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Problems they solve
• Shared mutable state
• Event conflation
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Problem
class MyViewModel(val interactor: Interactor) {
val scope = CoroutineScope(Dispatchers.Main)
fun refresh() {
scope.launch {
val response = interactor.getData()
handleResponse(response)
}
}
}
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Problem
class MyViewModel(val interactor: Interactor) {
val scope = CoroutineScope(Dispatchers.Main)
fun refresh() {
scope.launch {
val response = interactor.getData()
handleResponse(response)
}
}
}
Ensure 1 concurrent job runs
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Solution
• Check status of Job
• Use Actors
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Check Job State
class MyViewModel(val interactor: Interactor) {
val job: Job? = null
fun refresh() {
if (job!?.isCompleted !== false) return
val job = scope.launch {
val response = interactor.getData()
handleResponse(response)
}
}
}
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Actor
State
Actor
Channel
• Coroutine with mailbox (Channel)
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Actor
State
Actor
Channel
Coroutine
Event
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Actor
State
Actor
Channel
Coroutine
Event
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Events
Channel
Coroutine
LoadData
sealed class ActorEvents
object LoadData : ActorEvents()
…
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fun CoroutineScope.actor(
capacity: Int = 0,
block: suspend ActorScope.() !-> Unit
): SendChannel {
val channel = Channel(capacity)
val coroutine = ActorCoroutine(ctx, channel, true)
coroutine.start(start, coroutine, block)
return coroutine
}
Inside Actor
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fun CoroutineScope.modelActor() =
actor(capacity = Channel.CONFLATED) {
for (event in channel) {
when(event) {
is ActorEvents.LoadData !->
apiResponse = interactor.getData()
}
}
}
Using Actor
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Actor Use Cases
• UI Event Conflation
• Implement Redux-like architecture
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Flows
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What is a Flow?
• Cold stream
interface Flow {
suspend fun collect(collector: FlowCollector)
}
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What is a Flow?
interface Flow {
suspend fun collect(collector: FlowCollector)
}
Consume stream
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What is a Flow?
interface Flow {
suspend fun collect(collector: FlowCollector)
}
What to consume?
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What is a Flow?
interface FlowCollector {
suspend fun emit(value: T)
}
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Using Flows
runBlocking {
val flow: Flow = flowOf(1, 2, 3)
flow.collect {
println(it)
}
}
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Creating Flows
• flowOf(…)
• flow { … }
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flowOf Builder
flowOf(1, 2, 3)
fun flowOf(vararg elements: T): Flow
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flowOf Builder
fun flowOf(vararg elements: T): Flow {
object : Flow {
override suspend fun collect(collector: FlowCollector) {
for (element in elements) {
collector.emit(element)
}
}
}
}
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Flow Builder
runBlocking {
flow {
emit(1)
emit(2)
emit(3)
}
}
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Flow Builder
fun flow(
block: suspend FlowCollector.() !-> Unit
): Flow
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Terminal Operators
• Collect
• Single
• Reduce
• toList
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Single Operator
Collects only one value.
val flow = flowOf(1)
val num: Int = flow.single()
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Single Operator
Collects only one value.
val flow = flowOf(1,2,3)
val num: Int = flow.single()
java.lang.IllegalStateException: Expected only one element
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Flow Operators
• Map
• Filter
• Take
• Zip
• delayEach
• delayFlow
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DelayEach
val flow = flowOf(1,2,3)
flow.delayEach(1000)
.collect {
println(it)
}
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DelayEach
val flow = flowOf(1,2,3)
flow.delayEach(1000)
.collect {
println(it)
}
Operators are not suspending functions
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DelayEach
runBlocking {
val flow = getFlow()
flow.collect {
println(it)
}
}
fun getFlow() = flowOf(1,2,3).delayEach(1000)
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Flow Constraints
• Context Preservation
• Exception Transparency
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Flow Constraints
Emit from the same coroutine
val flow = flow {
launch(Dispatchers.IO) {
emit(1)
}
}
Flow invariant is violated: emission from another coroutine is detected
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Channel Flow Builder
val flow = channelFlow {
launch(Dispatchers.IO) {
send(1)
}
launch(Dispatchers.IO) {
send(2)
}
}
Flow
Channel
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Channel Flow Builder
class ChannelFlowBuilder(
val block: suspend ProducerScope.() !-> Unit,
capacity: Int = BUFFERED
) : ChannelFlow(context, capacity)
Uses Buffered Channel
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Flow Context
runBlocking {
val flow = flow {
emit(1)
emit(2)
emit(3)
}
flow
.map { it + 1 }
.collect { println(it) }
}
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Flow Context
runBlocking {
val flow = flow {
emit(1)
emit(2)
emit(3)
}
flow
.map { it + 1 }
.collect { println(it) }
}
Dispatchers.Main
Dispatchers.Main
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Flow Context
runBlocking {
val flow = flow {
emit(1)
emit(2)
emit(3)
}
flow
.map { it + 1 }
.flowOn(Dispatchers.IO)
.collect { println(it) }
}
Dispatchers.IO
Dispatchers.IO
Dispatchers.Main
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Exception Transparency
val flow = flow {
try {
makeRequest()
} catch (e: IOException) {
}
}
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Exception Transparency
fun Flow.catch(action:
suspend FlowCollector.(cause: Throwable) !-> Unit
): Flow
Emit exception in a wrapper
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Exception Transparency
sealed class ApiResponse {
data class Success(val data: T): ApiResponse()
data class Error(val t: Throwable): ApiResponse()
}
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Exception Transparency
val flow = flow> {
val data = makeRequest()
emit(ApiResponse.Success(data))
}.catch {
emit(ApiResponse.Error(it))
}
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Resources
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Debugging (JVM)
runBlocking {
DebugProbes.install()
val channel = Channel()
launch {
channel.send(2)
}
DebugProbes.dumpCoroutines()
}
Coroutine StandaloneCoroutine{Active}, state: SUSPENDED
Install Debugger
View Coroutines State
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Resources (Inside Channels)
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Thank you!
twitter.com/heyitsmohit