Kotlin Coroutines With Timeout

1. Introduction

Coroutines are a powerful tool for managing asynchronous code in Kotlin programming. Asynchronous code allows us to write non-blocking code that can perform multiple tasks concurrently. It can help to improve the overall performance of our programs. However, it’s important to have mechanisms in place to manage the behavior of our code when things don’t go as planned. One such mechanism is the use of timeouts.

In this tutorial, we’ll explore the concept of timeouts in Kotlin Coroutines. We’ll examine different approaches for implementing timeouts, including the withTimeout() and withTimeoutOrNull() functions. Also, we will discuss how to handle timeouts using exception handling and cancellation, and we’ll provide the best practices for using timeouts in Kotlin Coroutines to help us write more robust and reliable asynchronous code.

2. Implementing Timeouts in Kotlin Coroutines

When working with asynchronous code, it’s important to be able to set timeouts to ensure that our code does not block indefinitely. Kotlin Coroutines provide two main functions for implementing timeouts: withTimeout() and withTimeoutOrNull().

2.1. Using the withTimeout() Function

The withTimeout() function is a suspending function that takes a timeout duration and a block of code to run within a coroutine. If the block of code takes longer than the specified timeout duration, a TimeoutCancellationException will be thrown, which can be caught and handled as needed.

To execute the block of code, the function starts a new coroutine and suspends the current coroutine until the timeout duration elapses or the block of code completes. If the block of code completes before the timeout duration elapses, the function returns the result of the block of code. Otherwise, the exception is thrown:

suspend fun performTask(): String {
    delay(2000) // Simulate a long-running task
    return "Task completed successfully!"
}

fun withTimeoutDemo(): String = runBlocking {
    val result = withTimeout(1000) {
        performTask()
    }
    result
}

In this example, we have a performTask() function that simulates a long-running task by suspending it for two seconds. We then use the withTimeout() function to run this task with a timeout of one second. Since the task takes longer than the timeout duration, the function throws an exception. We can catch and handle it by returning a message indicating the task timed out.

2.2. Using the withTimeoutOrNull() Function

The withTimeoutOrNull() function is similar to the withTimeout() function, except that it does not throw a TimeoutCancellationException when the timeout duration elapses. Instead, it returns null. This can be useful if we want to handle the timeout condition differently than we would handle other exceptions.

The function suspends the coroutine in which it’s called and starts a new coroutine to execute the specified block of code. Then, it waits for the specified timeout duration to elapse. If the block of code completes before the timeout duration elapses, the function returns the result of the block of code. If the block of code does not complete before the timeout duration elapses, the function returns null.

Let’s see a quick example:

fun withTimeoutOrNullDemo(): String? = runBlocking {
    withTimeoutOrNull(1000) {
        performTask()
    }
}

Here, we use the withTimeoutOrNull() function to run the performTask() function with a timeout of one second. Since the task takes longer than the timeout duration, the function returns null, which we check for and handle by printing a message indicating that the task timed out.

3. Timeout Handling in Kotlin Coroutines

When a timeout occurs in a Kotlin Coroutine, we can handle it in two ways: using exception handling or using null handling. Each approach has its advantages and disadvantages, and the approach will depend on our specific use case.

3.1. Using Exception Handling

One way to handle a timeout in a Kotlin Coroutine is to use exception handling. When a timeout occurs, the withTimeout() function will throw a TimeoutCancellationException, which we can catch and handle in our code.

The advantage of using exception handling to handle timeouts is that it allows us to handle the timeout condition in a more fine-grained way. We can catch the TimeoutCancellationException and take specific actions based on the timeout, such as retrying the operation or providing an alternative implementation:

fun usingExceptionHandlingDemo(): String = runBlocking {
    try {
        val result = withTimeout(1000) {
            performTask()
        }
        result
    } catch (ex: TimeoutCancellationException) {
        println("Task timed out!")
        val result = withTimeout(2500) {
            performTask()
        }
        "Retrying... $result"
    }
}

In this example, we use the withTimeout() function to run the performTask() function with a timeout of one second. When the timeout occurs, we catch the TimeoutCancellationException and retry the operation by calling withTimeout() again with a longer timeout duration.

3.2. Using null Handling

Another way to handle a timeout in a Kotlin Coroutine is to use null handling. When a timeout occurs, the withTimeoutOrNull() function will return null, which we can handle in our code as appropriate:

fun usingNullHandlingDemo() = runBlocking {
    val result = withTimeoutOrNull(1000) {
        performTask()
    }
    if (result == null) {
        // Handle the timeout case here
        "Task timed out with null!"
    } else {
        // Handle the successful completion case here
        result
    }
}

4. Asynchronous Timeout and Resources

It’s important to note that the timeout event in withTimeout() is not synchronized with the code running in its block. It can occur anytime, even just before returning from inside the block. This means that if we open or acquire a resource inside the block that needs closing or releasing outside of the block, we need to take care to ensure that the resource is properly managed regardless of whether the timeout occurs.

Let’s say we want to simulate a closable resource, using the Resource class, that keeps track of the number of times it was created by incrementing the acquired counter and decrementing it in its release() function:

var acquired = 0
class Resource {
    fun acquire() {
        acquired++
    }
    fun release() {
        if (acquired > 0) {
            acquired--
        }
    }
}

To demonstrate a potential resource leak, we can create multiple coroutines where each coroutine creates a Resource at the end of the withTimeout() block and releases the resource outside the block. Adding a slight delay can increase the likelihood of the timeout event happening right after the withTimeout() block has finished, potentially resulting in a resource leak:

fun acquireAndReleaseWithLeak(): Int {
    runBlocking {
        launch {
            val resource = withTimeout(60) {
                // Acquire a resource right before timeout happens
                Resource()
                    .apply { acquire() }
                    .also { delay(59) }
            }
            resource.release() // Release the resource
        }
    }
    return acquired
}

Running the example above demonstrates that the expected value of acquired will not always equal zero. This means that the resource won’t be released every time. We can avoid the resource leak by storing a reference to the resource in a variable instead of returning it from the withTimeout() block:

fun acquireAndReleaseWithoutLeak(): Int {
    runBlocking {
        launch {
            val resource = Resource() // Not acquired yet
            try {
                withTimeout(60) {
                    delay(50)
                    resource.acquire()
                }
            } finally {
                resource.release()
            }
        }
    }
    return acquired
}

For Closeable resources, Kotlin also has some built-in helpers to ensure resources are cleaned up.

5. Conclusion

In this article, we explored how to use Kotlin Coroutines with a timeout. We learned how to use withTimeout() and withTimeoutOrNull() functions and discussed two ways of handling timeouts: using exception handling and using null handling.

The withTimeout() and withTimeoutOrNull() functions provide easy-to-use timeout capabilities in Kotlin coroutines. But, it’s important to use them carefully and handle timeouts correctly.

We learned that when working with resources, it’s important to be aware of the asynchronous nature of timeout events. It means that we have to ensure that all resources are properly released in case of timeouts.