I'm learning Coroutines of Kotlin.
The following content is from the artical https://developer.android.com/kotlin/coroutines.
Important: Using suspend doesn't tell Kotlin to run a function on a background thread. It's normal for suspend functions to operate on the main thread. It's also common to launch coroutines on the main thread. You should always use withContext() inside a suspend function when you need main-safety, such as when reading from or writing to disk, performing network operations, or running CPU-intensive operations.
Normally it's spend long time when I pull data from a remote server, so I need to place "the pull data function" in background thread in order not to freeze main UI.
Should I always add withContext(Dispatchers.IO) in suspend when I use suspend to pull data from remote server?
BTW,
The Code A is from the project https://github.com/googlecodelabs/kotlin-coroutines, you can see it .
But I can't find the keyword withContext() in the project, why?
Code A
fun refreshTitle() = launchDataLoad {
repository.refreshTitle()
}
private fun launchDataLoad(block: suspend () -> Unit): Unit {
viewModelScope.launch {
try {
_spinner.value = true
block()
} catch (error: TitleRefreshError) {
_snackBar.value = error.message
} finally {
_spinner.value = false
}
}
}
Should I always add withContext(Dispatchers.IO) in suspend when I use suspend to pull data from remote server?
It depends. If the you use a library like Retrofit 2.6.0 that has native support for suspend, the dispatcher is already Dispatchers.IO (or whatever the library deems more appropriate).
If the call to pull data from a remote server is blocking, you need to make sure to run it on Dispatcher.IO yourself with withContext(Dispatchers.IO) to not block the main thread.
I can't find the keyword withContext() in the project, why?
Because the project uses Retrofit, so the switch to Dispatchers.IO happens under the hood:
https://github.com/googlecodelabs/kotlin-coroutines/blob/master/coroutines-codelab/finished_code/src/main/java/com/example/android/kotlincoroutines/main/MainNetwork.kt
Related
I have this piece of code:
// this method is used to evaluate the input string, and it returns evaluation result in string format
fun process(input: String): String {
val timeoutMillis = 5000L
val page = browser.newPage()
try {
val result = runBlocking {
withTimeout(timeoutMillis) {
val result = page.evaluate(input).toString()
return#withTimeout result
}
}
return result
} catch (playwrightException: PlaywrightException) {
return "Could not parse template! '${playwrightException.localizedMessage}'"
} catch (timeoutException: TimeoutCancellationException) {
return "Could not parse template! (timeout)"
} finally {
page.close()
}
}
It should throw exception after 5 seconds if the method is taking too long to execute (example: input potentially contains infinite loop) but it doesent (becomes deadlock I assume) coz coroutines should be cooperative. But the method I am calling is from another library and I have no control over its computation (for sticking yield() or smth like it).
So the question is: is it even possible to timeout such coroutine? if yes, then how?
Should I use java thread insted and just kill it after some time?
But the method I am calling is from another library and I have no control over its computation (for sticking yield() or smth like it).
If that is the case, I see mainly 2 situations here:
the library is aware that this is a long-running operation and supports thread interrupts to cancel it. This is the case for Thread.sleep and some I/O operations.
the library function really does block the calling thread for the whole time of the operation, and wasn't designed to handle thread interrupts
Situation 1: the library function is interruptible
If you are lucky enough to be in situation 1, then simply wrap the library's call into a runInterruptible block, and the coroutines library will translate cancellation into thread interruptions:
fun main() {
runBlocking {
val elapsed = measureTimeMillis {
withTimeoutOrNull(100.milliseconds) {
runInterruptible {
interruptibleBlockingCall()
}
}
}
println("Done in ${elapsed}ms")
}
}
private fun interruptibleBlockingCall() {
Thread.sleep(3000)
}
Situation 2: the library function is NOT interruptible
In the more likely situation 2, you're kind of out of luck.
Should I use java thread insted and just kill it after some time?
There is no such thing as "killing a thread" in Java. See Why is Thread.stop deprecated?, or How do you kill a Thread in Java?.
In short, in that case you do not have a choice but to block some thread.
I do not know a solution to this problem that doesn't leak resources. Using an ExecutorService would not help if the task doesn't support thread interrupts - the threads will not die even with shutdownNow() (which uses interrupts).
Of course, the blocked thread doesn't have to be your thread. You can technically launch a separate coroutine on another thread (using another dispatcher if yours is single-threaded), to wrap the libary function call, and then join() the job inside a withTimeout to avoid waiting for it forever. That is however probably bad, because you're basically deferring the problem to whichever scope you use to launch the uncancellable task (this is actually why we can't use a simple withContext here).
If you use GlobalScope or another long-running scope, you effectively leak the hanging coroutine (without knowing for how long).
If you use a more local parent scope, you defer the problem to that scope. This is for instance the case if you use the scope of an enclosing runBlocking (like in your example), which makes this solution pointless:
fun main() {
val elapsed = measureTimeMillis {
doStuff()
}
println("Completely done in ${elapsed}ms")
}
private fun doStuff() {
runBlocking {
val nonCancellableJob = launch(Dispatchers.IO) {
uncancellableBlockingCall()
}
val elapsed = measureTimeMillis {
withTimeoutOrNull(100.milliseconds) {
nonCancellableJob.join()
}
}
println("Done waiting in ${elapsed}ms")
} // /!\ runBlocking will still wait here for the uncancellable child coroutine
}
// Thread.sleep is in fact interruptible but let's assume it's not for the sake of the example
private fun uncancellableBlockingCall() {
Thread.sleep(3000)
}
Outputs something like:
Done waiting in 122ms
Completely done in 3055ms
So the bottom line is either live with this long thing potentially hanging, or ask the developers of that library to handle interruption or make the task cancellable.
I have a Kotlin Backend/server API using Ktor, and inside a certain endpoint's service logic I need to concurrently get details for a list of ids and then return it all to the client with the 200 response.
The way I wanted to do it is by using async{} and awaitAll()
However, I can't understand whether I should use runBlocking or GlobalScope.
What is really the difference here?
fun getDetails(): List<Detail> {
val fetched: MutableList<Details> = mutableListOf()
GlobalScope.launch { --> Option 1
runBlocking { ---> Option 2
Dispatchers.IO --> Option 3 (or any other dispatcher ..)
myIds.map { id ->
async {
val providerDetails = getDetails(id)
fetched += providerDetails
}
}.awaitAll()
}
return fetched
}
launch starts a coroutine that runs in parallel with your current code, so fetched would still be empty by the time your getDetails() function returns. The coroutine will continue running and mutating the List that you have passed out of the function while the code that retrieved the list already has the reference back and will be using it, so there's a pretty good chance of triggering a ConcurrentModificationException. Basically, this is not a viable solution at all.
runBlocking runs a coroutine while blocking the thread that called it. The coroutine will be completely finished before the return fetched line, so this will work if you are OK with blocking the calling thread.
Specifying a Dispatcher isn't an alternative to launch or runBlocking. It is an argument that you can add to either to determine the thread pool used for the coroutine and its children. Since you are doing IO and parallel work, you should probably be using runBlocking(Dispatchers.IO).
Your code can be simplified to avoid the extra, unnecessary mutable list:
fun getDetails(): List<Detail> = runBlocking(Dispatchers.IO) {
myIds.map { id ->
async {
getDetails(id)
}
}.awaitAll()
}
Note that this function will rethrow any exceptions thrown by getDetails().
If your project uses coroutines more generally, you probably have higher level coroutines running, in which case this should probably be a suspend function (non-blocking) instead:
suspend fun getDetails(): List<Detail> = withContext(Dispatchers.IO) {
myIds.map { id ->
async {
getDetails(id)
}
}.awaitAll()
}
In Vert.x, suppose I have functions like this:
fun caller() {
runBlocking {
val job = GlobalScope.launch(vertx.dispatcher()) {
val r = suspendPart()
println(r) // never execute
}
println(1) // printed
job.join()
println(2) // never execute
}
}
suspend fun asyncPart(): Future<Int> {
val promise: Promise<Int> = Promise.promise()
delay(500)
promise.complete(0)
return promise.future()
}
suspend fun suspendPart(): Int {
return asyncPart().await()
}
r(which is 0) and 2 will never be printed, only 1 is printed. How should I fix it?
My intention is to wait for asyncPart completes (I have a AsyncResult inside actually).
Presumably your caller() method is called by vert.x and this means you're breaking one of the pivotal rules of vert.x:
Don’t block me!
Vert.x is mostly based on very fast single-threaded work, what this means is that when you block the thread in caller, it is unable to execute the coroutine scheduled with launch leading to a deadlock.
The proper way to solve this is to remove your blocking code through the integration vert.x provides for kotlin coroutines.
Alternatively using a different dispatcher for launch would also work since the other thread would unblock the vert.x dispatcher. But this would not solve the primary issue of blocking calls in the vert.x dispatcher.
suspend funtions run on a seperate thread ?
If not, then what is the performance benefit ?
suspend fun requestToken():Token {..} // takes 2 sec to complete
suspend fun createPost (token:Token){..} // takes 3 sec to complete
suspend fun postItem() {
val token = requestToken()
val post =createPost(token)
processPost(post)
}
So, when we reach at processPost(post) and if suspend function do not run on a seperate thread then we have to wait for requestToken() and createPost(token) method
to complete (i.e 2+3= 5 seconds). As per the author, suspend is asyncronous,but if we are not spawning any new thread then how are we achieving asychronous behaviour ?
suspend is asynchronous
suspend funs execute synchronously with their caller. What you actually meant to say is "non-blocking" and that's a completely different story.
but if we are not spawning any new thread then how are we achieving asynchronous behaviour?
You are making the tacit assumption that all I/O must be blocking at some level. This is wrong. Non-blocking I/O works by pushing data to a send buffer and receiving notifications when there's data in the receive buffer. A suspend fun hooks into this mechanism by suspending itself after pushing the data to a send buffer and installing a callback that will resume it when response data is ready in the receive buffer.
Suspension-points can only be used within a coroutine context, for instance:
fun main() {
delay(1000)
}
Would not work because delay is a suspending function and the compiler wouldn't know how to handle that without a coroutine. When you do have a coroutine it can use something called a dispatcher to control thread ownership. Suspending means that the thread is no longer used to execute that part of your program but its doing something else or going idle. The way it works is that you can have several coroutines working at the same time without having a thread for each, that thread can then execute parts of each coroutine up to a suspension point. Generally the idea is that you can view suspending functions as "generators" which have stages for producing a result.
suspend fun hello() {
println("Hello")
delay(1000) // suspend here, let someone else use the thread while we wait
println("World")
delay(1000) // suspend again, we can still use the thread while waiting
println("done")
}
Everytime this is suspended it uses the thread to work on another function until that suspends and the delay expires, at that point this function will eventually resume execution up to the next suspension point or finish execution entirely.
This is different to blocking code as it does not waste the thread by putting it into wait state but rather borrows it to another function. That way no other threads need to be created to have concurrency as you can still work on multiple functions without true parallelism, instead it uses concurrent execution of parts of the functions.
Coroutines don't necessarily protect you from blocking, if you call Thread.sleep(1000) its still gonna be a blocking call. It is the responsibility of the programmer to use suspending equivalents to blocking functions in order to maximize effectiveness of this concept.
For more details, please read the documentation as its very detailed and helpful.
Background single thread or multiple background threads of a thread pool can be explicitly declared and then used, for example by passing it as a parameter, let's call this parameter "scheduler". The really cool thing about it is that initially started from the main thread it's automatically switched to the scheduler thread to execute a particular task on it and virtual machine kind of suspended or interrupted at this place and the thing which is even cooler the main thread gets unblocked and can execute something else while there is the task in background.
As soon as the task is finished, virtual machine sort of gets back to the point where it was suspended or interrupted before and then it resumes its execution from that point but now by also having the result returned from the background thread of the scheduler, below is the code snippet:
private val backgroundThread = ThreadPoolExecutor(1, 1, 15L, TimeUnit.SECONDS, LinkedBlockingQueue())
GlobalScope.launch(Dispatchers.Main, CoroutineStart.DEFAULT) {
postItem(backgroundThread))
}
suspend fun CoroutineScope.postItem(scheduler: ThreadPoolExecutor): Boolean {
val token = requestToken(scheduler)
val post = createPost(token, scheduler)
return processPost(post, scheduler)
}
private suspend fun CoroutineScope.requestToken(scheduler: ThreadPoolExecutor): String {
val def: Deferred<String?> = async(scheduler.asCoroutineDispatcher(), CoroutineStart.DEFAULT) {
val token = networkApi.requestToken()
}
return def.await() ?: ""
}
private suspend fun CoroutineScope.createPost(token: String, scheduler: ThreadPoolExecutor): String {
val def: Deferred<String?> = async(scheduler.asCoroutineDispatcher(), CoroutineStart.DEFAULT) {
val post = networkApi.createPost(token)
}
return def.await() ?: ""
}
private suspend fun CoroutineScope.processPost(post: String, scheduler: ThreadPoolExecutor): Boolean {
val def: Deferred<Boolean?> = async(scheduler.asCoroutineDispatcher(), CoroutineStart.DEFAULT) {
val result = networkApi.processPost(post)
}
return def.await() ?: false
}
I'm currently writing a test-function which should run a block or (when a certain timeout is reached) throws an exception.
I was trying this with Coroutines in Kotlin but ended up with a mixture of Coroutines and CompletableFuture:
fun <T> runBlockWithTimeout(maxTimeout: Long, block: () -> T ): T {
val future = CompletableFuture<T>()
// runs the coroutine
launch { block() }
return future.get(maxTimeout, TimeUnit.MILLISECONDS)
}
This works, but I'm not sure if this is the intended way to solve that problem in kotlin.
I also tried other approaches:
runBlocking {
withTimeout(maxTimeout) {
block()
}
}
But this seems not to work as soon as the block calls e.g. Thread.sleep(...)
So is the CompletableFuture approach the way to go or is there a better one?
update 1
What I want to achieve:
Async Integration-Test code (like receiving data from RabbitMq) should be tested somehow like this:
var rabbitResults: List = ... // are filled async via RabbitListeners
...
waitMax(1000).toSucceed {
assertThat(rabbitResults).hasSize(1)
}
waitMax(1000).toSucceed {
assertThat(nextQueue).hasSize(3)
}
...
withTimeout { ... } is designed to cancel the ongoing operation on timeout, which is only possible if the operation in question is cancellable.
The reason it works with future.get(timeout, unit) is because it only waits with timeout. It does not actually cancel or abort in any way your background operation which still continues to execute after timeout had elapsed.
If you want to mimick similar behavior with coroutines, then you should wait with timeout, like this:
val d = async { block() } // run the block code in background
withTimeout(timeout, unit) { d.await() } // wait with timeout
It works properly because await is a cancellable function which you can verify by reading its API documentation.
However, if you want to actually cancel the ongoing operation on timeout, then then you should implement your code in asyncronous and cancellable way. Cancellation is cooperative, so, to start, the underlying library that you are using in your code has to provide asynchronous API that supports cancellation of ongoing operation.
You can read more about cancellation and timeouts in the corresponding section of the coroutines guide and watch the KotlinConf's Deep Dive into Coroutines on how to integrate coroutines with asynchronous libraries.