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objcode/ConcurrencyHelpers.kt

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Helpers to control concurrency for one shot requests using Kotlin coroutines.
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ConcurrencyHelpers.kt
/* Copyright 2019 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import kotlinx.coroutines.CoroutineStart.LAZY
import kotlinx.coroutines.Deferred
import kotlinx.coroutines.async
import kotlinx.coroutines.cancelAndJoin
import kotlinx.coroutines.coroutineScope
import kotlinx.coroutines.sync.Mutex
import kotlinx.coroutines.sync.withLock
import kotlinx.coroutines.yield
import java.util.concurrent.atomic.AtomicReference
import kotlin.DeprecationLevel.ERROR
/**
* A helper class to execute tasks sequentially in coroutines.
*
* Calling [afterPrevious] will always ensure that all previously requested work completes prior to
* calling the block passed. Any future calls to [afterPrevious] while the current block is running
* will wait for the current block to complete before starting.
*/
class SingleRunner {
/**
* A coroutine mutex implements a lock that may only be taken by one coroutine at a time.
*/
private val mutex = Mutex()
/**
* Ensure that the block will only be executed after all previous work has completed.
*
* When several coroutines call afterPrevious at the same time, they will queue up in the order
* that they call afterPrevious. Then, one coroutine will enter the block at a time.
*
* In the following example, only one save operation (user or song) will be executing at a time.
*
* ```
* class UserAndSongSaver {
* val singleRunner = SingleRunner()
*
* fun saveUser(user: User) {
* singleRunner.afterPrevious { api.post(user) }
* }
*
* fun saveSong(song: Song) {
* singleRunner.afterPrevious { api.post(song) }
* }
* }
* ```
*
* @param block the code to run after previous work is complete.
*/
suspend fun <T> afterPrevious(block: suspend () -> T): T {
// Before running the block, ensure that no other blocks are running by taking a lock on the
// mutex.
// The mutex will be released automatically when we return.
// If any other block were already running when we get here, it will wait for it to complete
// before entering the `withLock` block.
mutex.withLock {
return block()
}
}
}
/**
* A controlled runner decides what to do when new tasks are run.
*
* Note: This implementation is for example only. It will not work in the presence of
* multi-threading and is not safe to call from Dispatchers.IO or Dispatchers.Default. In
* real code use the thread-safe implementation of [ControlledRunner] code listed below.
*
* By calling [joinPreviousOrRun], the new task will be discarded and the result of the previous task
* will be returned. This is useful when you want to ensure that a network request to the same
* resource does not flood.
*
* By calling [cancelPreviousThenRun], the old task will *always* be cancelled and then the new task will
* be run. This is useful in situations where a new event implies that the previous work is no
* longer relevant such as sorting or filtering a list.
*/
@Deprecated("This code is not thread-safe and should not be used. Use " +
"the ControlledRunner implementation below instead.", level = ERROR)
class ControlledRunnerExampleImplementation<T> {
private var activeTask: Deferred<T>? = null
/**
* Cancel all previous tasks before calling block.
*
* When several coroutines call cancelPreviousThenRun at the same time, only one will run and
* the others will be cancelled.
*/
@Deprecated("This code is not thread-safe. Use ControlledRunner below instead.",
level = ERROR)
suspend fun cancelPreviousThenRun(block: suspend () -> T): T {
// If there is an activeTask, cancel it because it's result is no longer needed
//
// By waiting for the cancellation to complete with `cancelAndJoin` we know that activeTask
// has stopped executing before continuing.
activeTask?.cancelAndJoin()
// use a coroutineScope builder to safely start a new coroutine in a suspend function
return coroutineScope {
// create a new task to call the block
val newTask = async {
block()
}
// when the new task completes, reset activeTask to null
// this will be called by cancellation as well as normal completion
newTask.invokeOnCompletion {
activeTask = null
}
// save the newTask to activeTask, then wait for it to complete and return the result
activeTask = newTask
newTask.await()
}
}
/**
* Don't run the new block if a previous block is running, instead wait for the previous block
* and return it's result.
*
* When several coroutines call joinPreviousOrRun at the same time, only one will run and
* the others will return the result from the winner.
*/
@Deprecated("This code is not thread-safe. Use ControlledRunner below instead.",
level = ERROR)
suspend fun joinPreviousOrRun(block: suspend () -> T): T {
// if there is an activeTask, return it's result and don't run the block
activeTask?.let {
return it.await()
}
// use a coroutineScope builder to safely start a new coroutine in a suspend function
return coroutineScope {
// create a new task to call the block
val newTask = async {
block()
}
// when the task completes, reset activeTask to null
newTask.invokeOnCompletion {
activeTask = null
}
// save newTask to activeTask, then wait for it to complete and return the result
activeTask = newTask
newTask.await()
}
}
}
/**
* A controlled runner decides what to do when new tasks are run.
*
* By calling [joinPreviousOrRun], the new task will be discarded and the result of the previous task
* will be returned. This is useful when you want to ensure that a network request to the same
* resource does not flood.
*
* By calling [cancelPreviousThenRun], the old task will *always* be cancelled and then the new task will
* be run. This is useful in situations where a new event implies that the previous work is no
* longer relevant such as sorting or filtering a list.
*/
class ControlledRunner<T> {
/**
* The currently active task.
*
* This uses an atomic reference to ensure that it's safe to update activeTask on both
* Dispatchers.Default and Dispatchers.Main which will execute coroutines on multiple threads at
* the same time.
*/
private val activeTask = AtomicReference<Deferred<T>?>(null)
/**
* Cancel all previous tasks before calling block.
*
* When several coroutines call cancelPreviousThenRun at the same time, only one will run and
* the others will be cancelled.
*
* In the following example, only one sort operation will execute and any previous sorts will be
* cancelled.
*
* ```
* class Products {
* val controlledRunner = ControlledRunner<Product>()
*
* fun sortAscending(): List<Product> {
* return controlledRunner.cancelPreviousThenRun { dao.loadSortedAscending() }
* }
*
* fun sortDescending(): List<Product> {
* return controlledRunner.cancelPreviousThenRun { dao.loadSortedDescending() }
* }
* }
* ```
*
* @param block the code to run after previous work is cancelled.
* @return the result of block, if this call was not cancelled prior to returning.
*/
suspend fun cancelPreviousThenRun(block: suspend() -> T): T {
// fast path: if we already know about an active task, just cancel it right away.
activeTask.get()?.cancelAndJoin()
return coroutineScope {
// Create a new coroutine, but don't start it until it's decided that this block should
// execute. In the code below, calling await() on newTask will cause this coroutine to
// start.
val newTask = async(start = LAZY) {
block()
}
// When newTask completes, ensure that it resets activeTask to null (if it was the
// current activeTask).
newTask.invokeOnCompletion {
activeTask.compareAndSet(newTask, null)
}
// Kotlin ensures that we only set result once since it's a val, even though it's set
// inside the while(true) loop.
val result: T
// Loop until we are sure that newTask is ready to execute (all previous tasks are
// cancelled)
while(true) {
if (!activeTask.compareAndSet(null, newTask)) {
// some other task started before newTask got set to activeTask, so see if it's
// still running when we call get() here. If so, we can cancel it.
// we will always start the loop again to see if we can set activeTask before
// starting newTask.
activeTask.get()?.cancelAndJoin()
// yield here to avoid a possible tight loop on a single threaded dispatcher
yield()
} else {
// happy path - we set activeTask so we are ready to run newTask
result = newTask.await()
break
}
}
// Kotlin ensures that the above loop always sets result exactly once, so we can return
// it here!
result
}
}
/**
* Don't run the new block if a previous block is running, instead wait for the previous block
* and return it's result.
*
* When several coroutines call jonPreviousOrRun at the same time, only one will run and
* the others will return the result from the winner.
*
* In the following example, only one network operation will execute at a time and any other
* requests will return the result from the "in flight" request.
*
* ```
* class Products {
* val controlledRunner = ControlledRunner<Product>()
*
* fun fetchProducts(): List<Product> {
* return controlledRunner.joinPreviousOrRun {
* val results = api.fetchProducts()
* dao.insert(results)
* results
* }
* }
* }
* ```
*
* @param block the code to run if and only if no other task is currently running
* @return the result of block, or if another task was running the result of that task instead.
*/
suspend fun joinPreviousOrRun(block: suspend () -> T): T {
// fast path: if there's already an active task, just wait for it and return the result
activeTask.get()?.let {
return it.await()
}
return coroutineScope {
// Create a new coroutine, but don't start it until it's decided that this block should
// execute. In the code below, calling await() on newTask will cause this coroutine to
// start.
val newTask = async(start = LAZY) {
block()
}
newTask.invokeOnCompletion {
activeTask.compareAndSet(newTask, null)
}
// Kotlin ensures that we only set result once since it's a val, even though it's set
// inside the while(true) loop.
val result: T
// Loop until we figure out if we need to run newTask, or if there is a task that's
// already running we can join.
while(true) {
if (!activeTask.compareAndSet(null, newTask)) {
// some other task started before newTask got set to activeTask, so see if it's
// still running when we call get() here. There is a chance that it's already
// been completed before the call to get, in which case we need to start the
// loop over and try again.
val currentTask = activeTask.get()
if (currentTask != null) {
// happy path - we found the other task so use that one instead of newTask
newTask.cancel()
result = currentTask.await()
break
} else {
// retry path - the other task completed before we could get it, loop to try
// setting activeTask again.
// call yield here in case we're executing on a single threaded dispatcher
// like Dispatchers.Main to allow other work to happen.
yield()
}
} else {
// happy path - we were able to set activeTask, so start newTask and return its
// result
result = newTask.await()
break
}
}
// Kotlin ensures that the above loop always sets result exactly once, so we can return
// it here!
result
}
}
}
@objcode
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objcode commented Sep 27, 2021

@objcode, could you please add a license file to the gist?

Thanks for ping!

Done.

@dissident76
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dissident76 commented Dec 31, 2021 •
edited
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Thank you for the great articles about coroutines, clearest explanation on the topic I have found.

I'm using ControlledRunner in my app, but I'm struggling to understand where to call cancelPreviousThenRun {}
From your article, I understood it was not a good idea to start coroutines from the repository:

"Since a repository doesn’t have a natural lifecycle — it’s just an object — it would have no way to cleanup work. As a result, any coroutines started in the repository will leak by default."

What coroutineContext is the coroutineScope { } block inheriting if we call cancelPreviousThenRun {} from the repository? Will it inherit from viewModelScope if we call the repository suspend function from a coroutine launched from the viewModel with viewModelScope as scope? Why not move the cancelPreviousThenRun {} call to the viewModel then? Will the coroutine started by cancelPreviousThenRun {} leak if we call the repository suspend function from a CoroutineScope without a natural life cycle?

Sorry, lots of questions, hopefully they're all related :)

Thank you!

@objcode
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objcode commented Jan 4, 2022

Thanks!

All of these follow structured concurrency as they're suspend functions - and can be used in a structured way.

Also note, on that advice, while it's a good idea to be intentional with your scopes there are some operations that are application scoped. It's OK to make a coroutine that you intend to run as long as the process if that's the right behavior.

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