## Composing Suspending Functions

This section covers various approaches to composition of suspending functions.

### Sequential by default

Assume that we have two suspending functions defined elsewhere that do something useful like some kind of remote service call or computation. We just pretend they are useful, but actually each one just delays for a second for the purpose of this example:

```suspend fun doSomethingUsefulOne(): Int {
delay(1000L) // pretend we are doing something useful here
return 13
}

suspend fun doSomethingUsefulTwo(): Int {
delay(1000L) // pretend we are doing something useful here, too
return 29
}
```

What do we do if we need them to be invoked sequentially — first `doSomethingUsefulOne` and then `doSomethingUsefulTwo`, and compute the sum of their results? In practice we do this if we use the result of the first function to make a decision on whether we need to invoke the second one or to decide on how to invoke it.

We use a normal sequential invocation, because the code in the coroutine, just like in the regular code, is sequential by default. The following example demonstrates it by measuring the total time it takes to execute both suspending functions:

```import kotlinx.coroutines.*
import kotlin.system.*

fun main() = runBlocking<Unit> {
//sampleStart
val time = measureTimeMillis {
val one = doSomethingUsefulOne()
val two = doSomethingUsefulTwo()
println("The answer is \${one + two}")
}
println("Completed in \$time ms")
//sampleEnd
}

suspend fun doSomethingUsefulOne(): Int {
delay(1000L) // pretend we are doing something useful here
return 13
}

suspend fun doSomethingUsefulTwo(): Int {
delay(1000L) // pretend we are doing something useful here, too
return 29
}
```

You can get full code here.

It produces something like this:

```The answer is 42
Completed in 2017 ms
```

### Concurrent using async

What if there are no dependencies between invocations of `doSomethingUsefulOne` and `doSomethingUsefulTwo` and we want to get the answer faster, by doing both concurrently? This is where async comes to help.

Conceptually, async is just like launch. It starts a separate coroutine which is a light-weight thread that works concurrently with all the other coroutines. The difference is that `launch` returns a Job and does not carry any resulting value, while `async` returns a Deferred — a light-weight non-blocking future that represents a promise to provide a result later. You can use `.await()` on a deferred value to get its eventual result, but `Deferred` is also a `Job`, so you can cancel it if needed.

```import kotlinx.coroutines.*
import kotlin.system.*

fun main() = runBlocking<Unit> {
//sampleStart
val time = measureTimeMillis {
val one = async { doSomethingUsefulOne() }
val two = async { doSomethingUsefulTwo() }
println("The answer is \${one.await() + two.await()}")
}
println("Completed in \$time ms")
//sampleEnd
}

suspend fun doSomethingUsefulOne(): Int {
delay(1000L) // pretend we are doing something useful here
return 13
}

suspend fun doSomethingUsefulTwo(): Int {
delay(1000L) // pretend we are doing something useful here, too
return 29
}
```

You can get full code here.

It produces something like this:

```The answer is 42
Completed in 1017 ms
```

This is twice as fast, because the two coroutines execute concurrently. Note that concurrency with coroutines is always explicit.

### Lazily started async

Optionally, async can be made lazy by setting its `start` parameter to CoroutineStart.LAZY. In this mode it only starts the coroutine when its result is required by await, or if its `Job`'s start function is invoked. Run the following example:

```import kotlinx.coroutines.*
import kotlin.system.*

fun main() = runBlocking<Unit> {
//sampleStart
val time = measureTimeMillis {
val one = async(start = CoroutineStart.LAZY) { doSomethingUsefulOne() }
val two = async(start = CoroutineStart.LAZY) { doSomethingUsefulTwo() }
// some computation
one.start() // start the first one
two.start() // start the second one
println("The answer is \${one.await() + two.await()}")
}
println("Completed in \$time ms")
//sampleEnd
}

suspend fun doSomethingUsefulOne(): Int {
delay(1000L) // pretend we are doing something useful here
return 13
}

suspend fun doSomethingUsefulTwo(): Int {
delay(1000L) // pretend we are doing something useful here, too
return 29
}
```

You can get full code here.

It produces something like this:

```The answer is 42
Completed in 1017 ms
```

So, here the two coroutines are defined but not executed as in the previous example, but the control is given to the programmer on when exactly to start the execution by calling start. We first start `one`, then start `two`, and then await for the individual coroutines to finish.

Note that if we just call await in `println` without first calling start on individual coroutines, this will lead to sequential behavior, since await starts the coroutine execution and waits for its finish, which is not the intended use-case for laziness. The use-case for `async(start = CoroutineStart.LAZY)` is a replacement for the standard `lazy` function in cases when computation of the value involves suspending functions.

### Async-style functions

We can define async-style functions that invoke `doSomethingUsefulOne` and `doSomethingUsefulTwo` asynchronously using the async coroutine builder with an explicit GlobalScope reference. We name such functions with the “...Async” suffix to highlight the fact that they only start asynchronous computation and one needs to use the resulting deferred value to get the result.

```// The result type of somethingUsefulOneAsync is Deferred<Int>
fun somethingUsefulOneAsync() = GlobalScope.async {
doSomethingUsefulOne()
}

// The result type of somethingUsefulTwoAsync is Deferred<Int>
fun somethingUsefulTwoAsync() = GlobalScope.async {
doSomethingUsefulTwo()
}
```

Note that these `xxxAsync` functions are not suspending functions. They can be used from anywhere. However, their use always implies asynchronous (here meaning concurrent) execution of their action with the invoking code.

The following example shows their use outside of coroutine:

```import kotlinx.coroutines.*
import kotlin.system.*

//sampleStart
// note that we don't have `runBlocking` to the right of `main` in this example
fun main() {
val time = measureTimeMillis {
// we can initiate async actions outside of a coroutine
val one = somethingUsefulOneAsync()
val two = somethingUsefulTwoAsync()
// but waiting for a result must involve either suspending or blocking.
// here we use `runBlocking { ... }` to block the main thread while waiting for the result
runBlocking {
println("The answer is \${one.await() + two.await()}")
}
}
println("Completed in \$time ms")
}
//sampleEnd

fun somethingUsefulOneAsync() = GlobalScope.async {
doSomethingUsefulOne()
}

fun somethingUsefulTwoAsync() = GlobalScope.async {
doSomethingUsefulTwo()
}

suspend fun doSomethingUsefulOne(): Int {
delay(1000L) // pretend we are doing something useful here
return 13
}

suspend fun doSomethingUsefulTwo(): Int {
delay(1000L) // pretend we are doing something useful here, too
return 29
}
```

You can get full code here.

This programming style with async functions is provided here only for illustration, because it is a popular style in other programming languages. Using this style with Kotlin coroutines is strongly discouraged for the reasons explained below.

Consider what happens if between the `val one = somethingUsefulOneAsync()` line and `one.await()` expression there is some logic error in the code and the program throws an exception and the operation that was being performed by the program aborts. Normally, a global error-handler could catch this exception, log and report the error for developers, but the program could otherwise continue doing other operations. But here we have `somethingUsefulOneAsync` still running in the background, even though the operation that initiated it was aborted. This problem does not happen with structured concurrency, as shown in the section below.

### Structured concurrency with async

Let us take the Concurrent using async example and extract a function that concurrently performs `doSomethingUsefulOne` and `doSomethingUsefulTwo` and returns the sum of their results. Because the async coroutine builder is defined as an extension on CoroutineScope, we need to have it in the scope and that is what the coroutineScope function provides:

```suspend fun concurrentSum(): Int = coroutineScope {
val one = async { doSomethingUsefulOne() }
val two = async { doSomethingUsefulTwo() }
one.await() + two.await()
}
```

This way, if something goes wrong inside the code of the `concurrentSum` function and it throws an exception, all the coroutines that were launched in its scope will be cancelled.

```import kotlinx.coroutines.*
import kotlin.system.*

fun main() = runBlocking<Unit> {
//sampleStart
val time = measureTimeMillis {
}
println("Completed in \$time ms")
//sampleEnd
}

suspend fun concurrentSum(): Int = coroutineScope {
val one = async { doSomethingUsefulOne() }
val two = async { doSomethingUsefulTwo() }
one.await() + two.await()
}

suspend fun doSomethingUsefulOne(): Int {
delay(1000L) // pretend we are doing something useful here
return 13
}

suspend fun doSomethingUsefulTwo(): Int {
delay(1000L) // pretend we are doing something useful here, too
return 29
}
```

You can get full code here.

We still have concurrent execution of both operations, as evident from the output of the above `main` function:

```The answer is 42
Completed in 1017 ms
```

Cancellation is always propagated through coroutines hierarchy:

```import kotlinx.coroutines.*

fun main() = runBlocking<Unit> {
try {
failedConcurrentSum()
} catch(e: ArithmeticException) {
println("Computation failed with ArithmeticException")
}
}

suspend fun failedConcurrentSum(): Int = coroutineScope {
val one = async<Int> {
try {
delay(Long.MAX_VALUE) // Emulates very long computation
42
} finally {
println("First child was cancelled")
}
}
val two = async<Int> {
println("Second child throws an exception")
throw ArithmeticException()
}
one.await() + two.await()
}
```

You can get full code here.

Note how both the first `async` and the awaiting parent are cancelled on failure of one of the children (namely, `two`):

```Second child throws an exception
First child was cancelled
Computation failed with ArithmeticException
```