1. Introduction

One of the key features of coroutines is the ability to switch between threads or thread pools to execute tasks. Coroutine dispatchers are responsible for determining which thread or thread pool a coroutine will execute on. There are several built-in dispatchers available in Kotlin, each designed for specific use cases.

In this tutorial, we’ll briefly explore what coroutine dispatchers are, how they work, and the differences between two of the most commonly used dispatchers in Kotlin: the IO dispatcher and the Default dispatcher.

We’ll also look at examples of when to use each dispatcher to help us make the right choice for our particular use case. By the end of this tutorial, we’ll have a better understanding of how to use coroutine dispatchers effectively in our Kotlin projects.

2. Understanding CoroutineDispatchers

Before we dive into the differences between the IO and Default dispatchers, let’s review what a CoroutineDispatcher is. Simply put, a CoroutineDispatcher is responsible for managing the execution of coroutines on a thread or a set of threads.

By using a CoroutineDispatcher, we can specify the thread or thread pool that should be used to execute a coroutine. This allows for fine-grained control over concurrency and can help prevent running into issues like deadlocks and race conditions. Let’s take an example:

suspend fun switchDispatcher(dispatcher: CoroutineDispatcher) {
    println("Started execution on ${Thread.currentThread().name}")

    withContext(dispatcher) {
        // This code block will execute on the given dispatcher's thread
        println("Now executing on ${Thread.currentThread().name}")

In the code above, we created a function that takes a CoroutineDispatcher as a parameter. First, we print the name of the thread on which the function originally starts its execution. We then use the withContext() method to switch the execution to the given CouroutineDispatcher. Finally, we print out the name of the new thread the function is executing on. This demonstrates that the coroutine is executing on a different thread from the one that originally started the execution of the function.

The Kotlin language offers several predefined CoroutineDispatcher implementations to fit a variety of use cases. These dispatchers are specifically designed and optimized to perform particular tasks efficiently. It’s worth mentioning, though, that Kotlin doesn’t enforce a specific way to use these dispatchers. This leaves it up to developers to leverage them in a way that fits their needs.

Now, let’s look at how the IO and Default dispatchers work and when we should use them.

3. What Is the IO Dispatcher?

The IO dispatcher is a CoroutineDispatcher that is designed to handle system input/output (I/O) operations in our code.

This dispatcher uses a pool of threads that is separate from the thread pool used by the Default dispatcher. This means that coroutines running on the IO dispatcher won’t block the main thread or other coroutines running on the Default dispatcher.

By default, the number of threads in the IO dispatcher thread pool is set to either 64 or to the number of CPU cores available to the system, whichever is higher. However, it’s possible to modify the number of threads in the pool by modifying the value of the system property kotlinx.coroutines.io.parallelism.

3.1. When to Use the IO Dispatcher

The IO dispatcher has a large number of threads in its thread pool, allowing for many parallel blocking tasks to run on this dispatcher. This makes it suitable for IO-intensive tasks that involve blocking operations such as reading and writing files, performing database queries, or making network requests.

In general, I/O operations do not consume a lot of CPU resources but may take a long time to finish. Therefore, having access to the IO dispatcher’s large pool of threads can boost our application’s ability to finish I/O operations faster.

Let’s look at an example of how to use the IO dispatcher to perform long-running I/O operations:

suspend fun fetchData(): String {
    return withContext(Dispatchers.IO) {
        // perform network request to fetch data
        val response = networkRequest()

        // parse response into a String
        val result = parseResponse(response)

        return@withContext result

In this example, we define an IO dispatcher using the Dispatchers.IO factory method. We then define a fetchData() function that uses the withContext() function to execute the network request on the IO dispatcher. By doing so, we ensure that the network request doesn’t block the main thread or other coroutines running on the Default dispatcher.

4. What Is the Default Dispatcher?

The Kotlin Coroutines framework uses the Default dispatcher as the default CoroutineDispatcher. This means that Kotlin will use the Default dispatcher if we don’t specify one explicitly:

fun unspecifiedDispatcher() {
    GlobalScope.launch {
        // this coroutine will run on Dispatchers.Default
        println("Running in ${Thread.currentThread().name}")

In this example, we create a coroutine using the GlobalScope.launch() builder and print a message inside the coroutine. Since we didn’t specify a dispatcher, Kotlin will use the Dispatchers.Default dispatcher to run the coroutine.

The Default dispatcher is backed by a shared pool of threads. The maximum number of threads used by this dispatcher is equal to the number of CPU cores available to the system but is always at least two.

4.1. When to Use the Default Dispatcher

As we know, the Default dispatcher is limited by the number of CPU cores. This means that it can only run a certain number of tasks in parallel. This makes it suitable for CPU-bound tasks that require a lot of computation and benefit from parallelism. Short-running calculations and small-scale data processing tasks are the ideal candidates for running on the Default dispatcher. Let’s take a look at an example:

suspend fun calculateSum(a: Int, b: Int): Int {
    return withContext(Dispatchers.Default) {
        // perform CPU-bound calculation
        val sum = a + b

        return@withContext sum

In this example, we define a Default dispatcher using the Dispatchers.Default factory method. We then define a calculateSum() function that uses the withContext() function to execute the CPU-bound calculation on the Default dispatcher. Since the calculation is CPU-bound and doesn’t involve blocking operations such as I/O or waiting for events, it’s a good candidate to run on the Default dispatcher.

5. Conclusion

The IO and Default dispatchers are two important tools we can use for managing concurrency in Kotlin code. We should use the IO dispatcher for I/O-bound operations, while the Default dispatcher is a good choice for CPU-bound operations.

We also learned that, contrary to what its name may suggest, the Default dispatcher isn’t suitable for all use cases, and it’s always important to choose the appropriate dispatcher based on the task that the coroutine will perform. By using these dispatchers appropriately, developers can write efficient and responsive Kotlin code.

As always, we can find the full code samples for this article over on GitHub.

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