How does suspension work in Kotlin coroutines?

As we mentioned already, coroutines are components that can be suspended and resumed. It might sound like threads, but this is far from that. Threads cannot be suspended, only blocked. When a thread is blocked, it still consumes resources and needs to be managed by the operating system. When a coroutine is suspended, the only thing that remains is an object, that keeps references to local variables and the place where this coroutine was suspended. Coroutines are lightweight abstractions that run on top of threads, and they are managed by the coroutine library, not by the operating system.

There is a metaphor I like using to explain the difference between threads and coroutines. Imagine a game. A thread is like this old-school game that you can't save or pause. If you want to take a break for a while, you need to leave the game running. A coroutine is like a modern game that you can save and pause at any time. That allows you to take a break from the game at any time, without wasting resources. But it also allows you to play multiple games concurrently, as at any time you can save one game and start another. Finally, this allows you to resume the game on a different computer. That is important because if you play games on internet cafés, you can take any available computer and resume your game from the point where you left off. Those are the main benefits of coroutines. Thanks to the suspension mechanism, they can be paused and resumed at any time, on any thread. So we can suspend them at minimal cost, we can run multiple coroutines concurrently on a single thread, and we can efficiently manage threads, because coroutines are not strictly bound to particular threads. We will see those benefits throughout the book, but first, let's see how suspension works.

When we transform an application from blocking threads to suspending coroutines, the single most important change is using suspend modifier in front of many functions. Such functions are called suspending functions, and they are the hallmark of Kotlin coroutines. Here is a simple example of a backend application using coroutines. Notice that the only difference between using coroutines and blocking threads is the suspend modifier.

So what are suspending functions? Do they start coroutines? No, they don't! Suspending functions are just functions that can suspend a coroutine. That means that a suspending function must be called on a coroutine (because it needs a coroutine to suspend). In practice, suspend functions must be called by other suspending functions or by coroutine builders that start coroutines. Of course, a suspending function can also call regular functions.

Suspending functions are not coroutines, but they require coroutines. That is why a framework like Spring reacts to the suspend modifier in controller mapping functions and calls such functions on a coroutine. On the other hand suspending functions allow suspension, so network libraries like Retrofit⁵ reacts to suspend modifier and suspend coroutines (instead of blocking threads) when they need to wait for a network response.

So let's see it in action. For this, we need a coroutine. The simplest way to start it is to use a suspending main function. Such a function is wrapped by the Kotlin compiler and started in a coroutine. However, if we call another suspending function from main, this function will be called on the same coroutine. You can say that suspend functions are synchronized, but the simplest explanation is that they are not coroutines themselves, they just can suspend coroutines.

What will happen if we suspend our main function in the middle? For that, we can use the suspendCancellableCoroutine.

If you call the above code, you will not see the "After", and the code will not stop running (as our main function never finished). The coroutine is suspended after "Before". Our game was stopped and never resumed. So, how can we resume?

Take a look again at the suspendCancellableCoroutine invocation and notice that it ends with a lambda expression ({ }). The function passed as an argument will be invoked before the suspension. This function has an argument of type Continuation. This is the object that we can use to resume the coroutine. We could use it to resume immediately²:

suspendCancellableCoroutine calls the lambda expression immediately, just like functions let, apply, or useLines. That is necessary to make it possible to use the continuation just before the suspension. After the suspendCancellableCoroutine call, it would be too late. So, the lambda expression passed as a parameter to the suspendCancellableCoroutine function is invoked just before the suspension. This lambda is used to store this continuation somewhere or to plan whether to resume it.

The continuation is an object that stores the state of the coroutine. It must store the local variables and the place where the coroutine was suspended. We will see how it actually works in the next chapter, but for now, let's discuss what it contains. Consider the following code:

What do you expect to be stored in the continuation? It must be the state of the coroutine, and coroutine is started by Kotlin to execute suspending main function. So this state must contain not only references to the local variables of the a function, but also references to the local variables of the main function. At least those that are used after the suspension. This is the only way to make it possible to resume the coroutine and continue from the point where it was suspended. So continuation must contain references to the values of list and a variables. It must also keep the place where the coroutine was suspended, what means label 1 for function a, and label 2 for function main (because delay takes label 1). Those are the essentials of explaining what is and how the continuation works. Details are presented in the next chapter.

How can we suspend a coroutine for some time? The naive approach would be to start a thread that will resume the continuation after a defined period. That is possible:

Of course, such an approach is not practical. Threads are expensive, and we should not create them just to suspend a coroutine for a while. However, it is an important observation that continuation can be passed around and resumed from another thread. We can even make a function that will resume our continuation after a defined period. In such a case, the continuation is captured by the lambda expression, as shown in the code snippet below.

Such a mechanism works, but it unnecessarily creates threads only to end them after just a second of inactivity. Threads are not cheap, so why waste them? A better way would be to set up an "alarm clock". In JVM, we can use ScheduledExecutorService for that. We can set it to call some continuation.resume(Unit) after a defined amount of time.

Suspending for a set amount of time seems like a useful feature. Let's extract it into a function. We will name it delay.

The executor still uses a thread, but it is one thread for all coroutines using the delay function. This is much better than blocking one thread every time we need to wait for some time.

This is exactly how delay from the Kotlin Coroutines library is implemented in older versions of this library. The current implementation is more complicated, mainly so as to support testing, but the essential idea remains the same.

One thing that might concern you is why we passed Unit to the resume function. You might also be wondering why we used Unit as a type argument for the suspendCancellableCoroutine. The fact that these two are the same is no coincidence. Unit is also returned from the function and is the generic type of the Continuation parameter.

When we call suspendCancellableCoroutine, we can specify which type will be returned in its continuation. The same type needs to be used when we call resume.

This does not fit well with the game analogy. I don't know of any game in which you can put something inside the game when resuming a save³ (unless you cheated and googled how to solve the next challenge). However, it makes perfect sense with coroutines. Often we are suspended because we are waiting for some data, such as a network response from an API. This is a common scenario. Your thread is running business logic until it reaches a point where it needs some data. So, it asks your network library to deliver it. Without coroutines, this thread would then need to sit and wait. This would be a huge waste as threads are expensive, especially if this is an important thread, like the Main Thread on Android. With coroutines, it just suspends and gives the library a continuation with the instruction "Once you’ve got this data, just send it to the resume function". Then the thread can go do other things. Once the data is there, the thread will be used to resume from the point where the coroutine was suspended.

To see this in action, let's see how we might suspend until we receive some data. In the example below, we use a callback function requestUser that is implemented externally.

Calling suspendCancellableCoroutine directly is not convenient. We would prefer to have a suspending function instead. We can extract it ourselves.

Currently, suspending functions are already supported by many popular libraries, such as Retrofit and Room. This is why we rarely need to use callback functions in suspending functions. However, it is good to know how to turn a callback function into a suspending function if you ever need to.

You might wonder what happens if the API gives us not data but some kind of problem. What if the service is dead or responds with an error? In such a case, we cannot return data; instead, we should throw an exception from the place where the coroutine was suspended. This is where we need to resume with an exception.

Every function we call might return some value or throw an exception. The same is true for suspendCancellableCoroutine. When resume is called, it returns data passed as an argument. When resumeWithException is called, the exception that is passed as an argument is conceptually thrown from the suspension point.

This mechanism is used, for instance, to signal network exceptions.

One thing that needs to be emphasized here is that we suspend a coroutine, not a function. Suspending functions are not coroutines, just functions that can suspend a coroutine⁴. Imagine that we store a function in some variable and try to resume it after the function call.

This makes no sense. It is equivalent to stopping a game and planning to resume it at a later point in the game. resume will never be called. You will only see "Before", and your program will never end unless we resume on another thread or another coroutine. To show this, we can set another coroutine to resume after a second.

I hope now you have a clear picture of how suspension works from the user’s point of view. It is important, as we will build on top of that throughout the book. You also learned some practical patterns, like how to turn callback functions into suspending functions. If you are like me and like to know exactly how things work, you are likely still wondering about how suspension is implemented. If you're curious about this, it will be covered in the next chapter. If you don't feel you need to know, just skip it. It is not very practical, it just reveals the magic of Kotlin coroutines.