When you start an application, the operating system will create a process, attach a thread to it, and then start the execution of that thread – commonly known as the main thread. In this section, we will detail the relationship between processes, threads, and coroutines. This is necessary in order to be able to understand and implement concurrency.

A process is an instance of an application that is being executed. Each time an application is started, a process is started for it. A process has a state; things such as handles to open resources, a process ID, data, network connections, and so on, are part of the state of a process and can be accessed by the threads inside that process.

An application can be composed of many processes, a common practice for example for internet browsers. But implementing a multi-process application brings challenges that are out of the scope of this book. For this book, we will cover the implementation of applications that run in more than one thread, but still in a single process.

A thread of execution encompasses a set of instructions for a processor to execute. So a process will contain at least one thread, which is created to execute the point of entry of the application; usually this entry point is the main() function of the application. This thread is called the main thread, and the life cycle of the process will be tied to it; if this thread ends, the process will end as well, regardless of any other threads in the process. For example:

fun main(args: Array<String>) {
    doWork()
}

When this basic application is executed, a main thread is created containing the set of instructions of the main() function. doWork() will be executed in the main thread, so whenever doWork() ends, the execution of the application will end with it.

Each thread can access and modify the resources contained in the process it's attached to, but it also has its own local storage, called thread-local storage.

Only one of the instructions in a thread can be executed at a given time. So if a thread is blocked, the execution of any other instruction in that same thread will not be possible until the blocking ends. Nevertheless, many threads can be created for the same process, and they can communicate with each other. So it is expected that an application will never block a thread that can affect negatively the experience of the user; instead, the blocking operations should be assigned to threads that are dedicated to them.

In Graphic User Interface (GUI) applications, there is a thread called a UI thread; its function is to update the User Interface and listen to user interactions with the application. Blocking this thread, obstructs the application from updating its UI and from receiving interactions from the user. Because of this, GUI applications are expected to never block the UI thread, in order to keep the application responsive at all times.

Android 3.0 and above, for example, will crash an application if a networking operation is made in the UI thread, in order to discourage developers from doing it, given that networking operations are thread-blocking.

Throughout the book, we will refer to the main thread of a GUI application both as a UI thread and as a main thread (because in Android, by default, the main thread is also the UI thread), while for command-line applications we will refer to it only as a main thread. Any thread different from those two will be called a background thread, unless a distinction between background threads is required, in which case each background thread will receive a unique identifier for clarity.

Given the way that the Kotlin has implemented concurrency, you will find that it's not necessary for you to manually start or stop a thread. The interactions that you will have with threads will commonly be limited to tell Kotlin to create or use a specific thread or pool of threads to run a coroutine – usually with one or two lines of code. The rest of the handling of threads will be done by the framework.

Kotlin's documentation often refers to coroutines as lightweight threads. This is mostly because, like threads, coroutines define the execution of a set of instructions for a processor to execute. Also, coroutines have a similar life cycle to that of threads.

A coroutine is executed inside a thread. One thread can have many coroutines inside it, but as already mentioned, only one instruction can be executed in a thread at a given time. This means that if you have ten coroutines in the same thread, only one of them will be running at a given point in time.

The biggest difference between threads and coroutines, though, is that coroutines are fast and cheap to create. Spawning thousands of coroutines can be easily done, it is faster and requires fewer resources than spawning thousands of threads.

Take this code as an example. Don't worry about the parts of the code you don't understand yet:

suspend fun createCoroutines(amount: Int) {
    val jobs = ArrayList<Job>()
    for (i in 1..amount) {
        jobs += launch {
            delay(1000)           
        }
    }
    jobs.forEach {
        it.join()
    }
}

This function creates as many coroutines as specified in the parameter amount, delays each one for a second, and waits for all of them to end before returning. This function can be called, for example, with 10,000 as the amount of coroutines:

fun main(args: Array<String>) = runBlocking {
    val time = measureTimeMillis {
        createCoroutines(10_000)
    }

    println("Took $time ms")
}

In a test environment, running it with an amount of 10,000 took around 1,160 ms, while running it with 100,000 took 1,649 ms. The increase in execution time is so small because Kotlin will use a pool of threads with a fixed size, and distribute the coroutines among those threads – so adding thousands of coroutines will have little impact. And while a coroutine is suspended – in this case because of the call to delay() – the thread it was running in will be used to execute another coroutine, one that is ready to be started or resumed.

How many threads are active can be determined by calling the activeCount() method of the Thread class. For example, let's update the main() function to do so:

fun main(args: Array<String>) = runBlocking {
    println("${Thread.activeCount()} threads active at the start")
    val time = measureTimeMillis {
        createCoroutines(10_000)
    }
    println("${Thread.activeCount()} threads active at the end")
    println("Took $time ms")
}

In the same test environment as before, it was found that in order to create 10,000 coroutines, only four threads needed to be created:

But once the value of the amount being sent to createCoroutines() is lowered to one, for example, only two threads are created:

It's important to understand that even though a coroutine is executed inside a thread, it's not bound to it. As a matter of fact, it's possible to execute part of a coroutine in a thread, suspend the execution, and later continue in a different thread. In our previous example this is happening already, because Kotlin will move coroutines to threads that are available to execute them. For example, by passing 3 as the amount to createCoroutines(), and updating the content of the launch() block so that it prints the current thread, we can see this in action:

suspend fun createCoroutines(amount: Int) {
    val jobs = ArrayList<Job>()
    for (i in 1..amount) {
        jobs += launch {
            println("Started $i in ${Thread.currentThread().name}")
            delay(1000)
            println("Finished $i in ${Thread.currentThread().name}")
        }
    }
    jobs.forEach {
         it.join()
    }
}

You will find that in many cases they are being resumed in a different thread:

A thread can only execute one coroutine at a time, so the framework is in charge of moving coroutines between threads as necessary. As will be explained in detail later, Kotlin is flexible enough to allow the developer to specify which thread to execute a coroutine on, and whether or not to confine the coroutine to that thread.

So far, we have learned that an application is composed of one or more processes and that each process has one or more threads. We have also learned that blocking a thread means halting the execution of the code in that thread, and for that reason, a thread that interacts with a user is expected to never be blocked. We also know that a coroutine is basically a lightweight thread that resides in a thread but is not tied to one. The following diagram encapsulates the content of this section so far. Notice how each coroutine is started in one thread but at some point is resumed in a different one:

In a nutshell, concurrency occurs when an application runs in more than one thread at the same time. So in order for concurrency to happen, two or more threads need to be created, and both communication and synchronization of those threads will most likely be needed for correct functioning of the application.