path: root/docs/topics/cancellation-and-timeouts.md

<!--- TEST_NAME CancellationGuideTest -->

[//]: # (title: Cancellation and timeouts)

This section covers coroutine cancellation and timeouts.

## Cancelling coroutine execution

In a long-running application you might need fine-grained control on your background coroutines.
For example, a user might have closed the page that launched a coroutine and now its result
is no longer needed and its operation can be cancelled. 
The [launch] function returns a [Job] that can be used to cancel the running coroutine:

```kotlin
import kotlinx.coroutines.*

fun main() = runBlocking {
//sampleStart
    val job = launch {
        repeat(1000) { i ->
            println("job: I'm sleeping $i ...")
            delay(500L)
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancel() // cancels the job
    job.join() // waits for job's completion 
    println("main: Now I can quit.")
//sampleEnd    
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-01.kt).
>
{type="note"}

It produces the following output:

```text
job: I'm sleeping 0 ...
job: I'm sleeping 1 ...
job: I'm sleeping 2 ...
main: I'm tired of waiting!
main: Now I can quit.
```

<!--- TEST -->

As soon as main invokes `job.cancel`, we don't see any output from the other coroutine because it was cancelled. 
There is also a [Job] extension function [cancelAndJoin] 
that combines [cancel][Job.cancel] and [join][Job.join] invocations.

## Cancellation is cooperative

Coroutine cancellation is _cooperative_. A coroutine code has to cooperate to be cancellable.
All the suspending functions in `kotlinx.coroutines` are _cancellable_. They check for cancellation of 
coroutine and throw [CancellationException] when cancelled. However, if a coroutine is working in 
a computation and does not check for cancellation, then it cannot be cancelled, like the following 
example shows:

```kotlin
import kotlinx.coroutines.*

fun main() = runBlocking {
//sampleStart
    val startTime = System.currentTimeMillis()
    val job = launch(Dispatchers.Default) {
        var nextPrintTime = startTime
        var i = 0
        while (i < 5) { // computation loop, just wastes CPU
            // print a message twice a second
            if (System.currentTimeMillis() >= nextPrintTime) {
                println("job: I'm sleeping ${i++} ...")
                nextPrintTime += 500L
            }
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
//sampleEnd    
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-02.kt).
>
{type="note"}

Run it to see that it continues to print "I'm sleeping" even after cancellation
until the job completes by itself after five iterations.

<!--- TEST 
job: I'm sleeping 0 ...
job: I'm sleeping 1 ...
job: I'm sleeping 2 ...
main: I'm tired of waiting!
job: I'm sleeping 3 ...
job: I'm sleeping 4 ...
main: Now I can quit.
-->

## Making computation code cancellable

There are two approaches to making computation code cancellable. The first one is to periodically 
invoke a suspending function that checks for cancellation. There is a [yield] function that is a good choice for that purpose.
The other one is to explicitly check the cancellation status. Let us try the latter approach. 

Replace `while (i < 5)` in the previous example with `while (isActive)` and rerun it. 

```kotlin
import kotlinx.coroutines.*

fun main() = runBlocking {
//sampleStart
    val startTime = System.currentTimeMillis()
    val job = launch(Dispatchers.Default) {
        var nextPrintTime = startTime
        var i = 0
        while (isActive) { // cancellable computation loop
            // print a message twice a second
            if (System.currentTimeMillis() >= nextPrintTime) {
                println("job: I'm sleeping ${i++} ...")
                nextPrintTime += 500L
            }
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
//sampleEnd    
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-03.kt).
>
{type="note"}

As you can see, now this loop is cancelled. [isActive] is an extension property 
available inside the coroutine via the [CoroutineScope] object.

<!--- TEST
job: I'm sleeping 0 ...
job: I'm sleeping 1 ...
job: I'm sleeping 2 ...
main: I'm tired of waiting!
main: Now I can quit.
-->

## Closing resources with `finally`

Cancellable suspending functions throw [CancellationException] on cancellation which can be handled in 
the usual way. For example, `try {...} finally {...}` expression and Kotlin `use` function execute their
finalization actions normally when a coroutine is cancelled:

```kotlin
import kotlinx.coroutines.*

fun main() = runBlocking {
//sampleStart
    val job = launch {
        try {
            repeat(1000) { i ->
                println("job: I'm sleeping $i ...")
                delay(500L)
            }
        } finally {
            println("job: I'm running finally")
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
//sampleEnd    
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-04.kt).
>
{type="note"}

Both [join][Job.join] and [cancelAndJoin] wait for all finalization actions to complete, 
so the example above produces the following output:

```text
job: I'm sleeping 0 ...
job: I'm sleeping 1 ...
job: I'm sleeping 2 ...
main: I'm tired of waiting!
job: I'm running finally
main: Now I can quit.
```

<!--- TEST -->

## Run non-cancellable block

Any attempt to use a suspending function in the `finally` block of the previous example causes
[CancellationException], because the coroutine running this code is cancelled. Usually, this is not a 
problem, since all well-behaving closing operations (closing a file, cancelling a job, or closing any kind of a 
communication channel) are usually non-blocking and do not involve any suspending functions. However, in the 
rare case when you need to suspend in a cancelled coroutine you can wrap the corresponding code in
`withContext(NonCancellable) {...}` using [withContext] function and [NonCancellable] context as the following example shows:

```kotlin
import kotlinx.coroutines.*

fun main() = runBlocking {
//sampleStart
    val job = launch {
        try {
            repeat(1000) { i ->
                println("job: I'm sleeping $i ...")
                delay(500L)
            }
        } finally {
            withContext(NonCancellable) {
                println("job: I'm running finally")
                delay(1000L)
                println("job: And I've just delayed for 1 sec because I'm non-cancellable")
            }
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
//sampleEnd    
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-05.kt).
>
{type="note"}

<!--- TEST
job: I'm sleeping 0 ...
job: I'm sleeping 1 ...
job: I'm sleeping 2 ...
main: I'm tired of waiting!
job: I'm running finally
job: And I've just delayed for 1 sec because I'm non-cancellable
main: Now I can quit.
-->

## Timeout

The most obvious practical reason to cancel execution of a coroutine 
is because its execution time has exceeded some timeout.
While you can manually track the reference to the corresponding [Job] and launch a separate coroutine to cancel 
the tracked one after delay, there is a ready to use [withTimeout] function that does it.
Look at the following example:

```kotlin
import kotlinx.coroutines.*

fun main() = runBlocking {
//sampleStart
    withTimeout(1300L) {
        repeat(1000) { i ->
            println("I'm sleeping $i ...")
            delay(500L)
        }
    }
//sampleEnd
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-06.kt).
>
{type="note"}

It produces the following output:

```text
I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...
Exception in thread "main" kotlinx.coroutines.TimeoutCancellationException: Timed out waiting for 1300 ms
```

<!--- TEST STARTS_WITH -->

The `TimeoutCancellationException` that is thrown by [withTimeout] is a subclass of [CancellationException].
We have not seen its stack trace printed on the console before. That is because
inside a cancelled coroutine `CancellationException` is considered to be a normal reason for coroutine completion. 
However, in this example we have used `withTimeout` right inside the `main` function. 

Since cancellation is just an exception, all resources are closed in the usual way. 
You can wrap the code with timeout in a `try {...} catch (e: TimeoutCancellationException) {...}` block if 
you need to do some additional action specifically on any kind of timeout or use the [withTimeoutOrNull] function
that is similar to [withTimeout] but returns `null` on timeout instead of throwing an exception:

```kotlin
import kotlinx.coroutines.*

fun main() = runBlocking {
//sampleStart
    val result = withTimeoutOrNull(1300L) {
        repeat(1000) { i ->
            println("I'm sleeping $i ...")
            delay(500L)
        }
        "Done" // will get cancelled before it produces this result
    }
    println("Result is $result")
//sampleEnd
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-07.kt).
>
{type="note"}

There is no longer an exception when running this code:

```text
I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...
Result is null
```

<!--- TEST -->

## Asynchronous timeout and resources

<!-- 
  NOTE: Don't change this section name. It is being referenced to from within KDoc of withTimeout functions.
-->

The timeout event in [withTimeout] is asynchronous with respect to the code running in its block and may happen at any time,
even right before the return from inside of the timeout block. Keep this in mind if you open or acquire some
resource inside the block that needs closing or release outside of the block. 

For example, here we imitate a closeable resource with the `Resource` class, that simply keeps track of how many times 
it was created by incrementing the `acquired` counter and decrementing this counter from its `close` function.
Let us run a lot of coroutines with the small timeout try acquire this resource from inside
of the `withTimeout` block after a bit of delay and release it from outside.

```kotlin
import kotlinx.coroutines.*

//sampleStart
var acquired = 0

class Resource {
    init { acquired++ } // Acquire the resource
    fun close() { acquired-- } // Release the resource
}

fun main() {
    runBlocking {
        repeat(100_000) { // Launch 100K coroutines
            launch { 
                val resource = withTimeout(60) { // Timeout of 60 ms
                    delay(50) // Delay for 50 ms
                    Resource() // Acquire a resource and return it from withTimeout block     
                }
                resource.close() // Release the resource
            }
        }
    }
    // Outside of runBlocking all coroutines have completed
    println(acquired) // Print the number of resources still acquired
}
//sampleEnd
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-08.kt).
>
{type="note"}

<!--- CLEAR -->

If you run the above code you'll see that it does not always print zero, though it may depend on the timings 
of your machine you may need to tweak timeouts in this example to actually see non-zero values. 

> Note, that incrementing and decrementing `acquired` counter here from 100K coroutines is completely safe,
> since it always happens from the same main thread. More on that will be explained in the next chapter
> on coroutine context.
> 
{type="note"}

To workaround this problem you can store a reference to the resource in the variable as opposed to returning it 
from the `withTimeout` block. 

```kotlin
import kotlinx.coroutines.*

var acquired = 0

class Resource {
    init { acquired++ } // Acquire the resource
    fun close() { acquired-- } // Release the resource
}

fun main() {
//sampleStart
    runBlocking {
        repeat(100_000) { // Launch 100K coroutines
            launch { 
                var resource: Resource? = null // Not acquired yet
                try {
                    withTimeout(60) { // Timeout of 60 ms
                        delay(50) // Delay for 50 ms
                        resource = Resource() // Store a resource to the variable if acquired      
                    }
                    // We can do something else with the resource here
                } finally {  
                    resource?.close() // Release the resource if it was acquired
                }
            }
        }
    }
    // Outside of runBlocking all coroutines have completed
    println(acquired) // Print the number of resources still acquired
//sampleEnd
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-cancel-09.kt).
>
{type="note"}

This example always prints zero. Resources do not leak.

<!--- TEST 
0
-->

<!--- MODULE kotlinx-coroutines-core -->
<!--- INDEX kotlinx.coroutines -->

[launch]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/launch.html
[Job]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html
[cancelAndJoin]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/cancel-and-join.html
[Job.cancel]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/cancel.html
[Job.join]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/join.html
[CancellationException]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-cancellation-exception/index.html
[yield]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/yield.html
[isActive]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/is-active.html
[CoroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/index.html
[withContext]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html
[NonCancellable]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-non-cancellable.html
[withTimeout]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-timeout.html
[withTimeoutOrNull]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-timeout-or-null.html

<!--- END -->