Elixir Findings: Asynchronous Task Streams

The other day I was solving an Exercism.io exercise that involved calculating the frequency of letters in multiple texts in parallel using only a specific number of “workers”. This same exercise helped me stumble into Task.async_stream/3 and Task.async_stream/5 .

For those unfamiliar with the Task module here's the description you can find on Elixir's documentation:

Conveniences for spawning and awaiting tasks.

Tasks are processes meant to execute one particular action throughout their lifetime, often with little or no communication with other processes. The most common use case for tasks is to convert sequential code into concurrent code by computing a value asynchronously.

Let’s say we want to increase a list of numbers by 1 in parallel, up until now I’d mainly use the Task module in the following way to accomplish this task:

Using Enum.map together with Task.async and Task.await to increment a list of numbers

This is a little cumbersome because you basically have to define a nested anonymous function, the one inside the first call to Enum.map , and you have to basically make two function calls to start the task and fetch the result. First, we create the task with Task.async , and then we fetch the result using Task.await.


Luckily, Task.async_stream let us do this in a more simple, cleaner way.

Using Task.async_stream will allow you to create a stream of asynchronous tasks, where each task will run a specific function on each element of the provided enumerable, in this case, the list of numbers.

Here’s how you’d implement the same example shown above but using the Task.async_stream/3 function:

Using Task.async_stream to increment a list of numbers

Here’s what I’d say is the first main advantage you get by using Task.async_stream , you can see that the code looks way cleaner, and is simpler to reason about, at least for this specific example.

Furthermore, if you wish you can pass the reference to a given function in Task.async_stream instead of providing an anonymous function, or example:

Using Task.async_stream with a function reference

Lazy Evaluation

If you run the previous example on iex you'll get output similar to this:

#Function<1.111840141/2 in Task.build_stream/3>

This is because Task.async_stream, contrary toTask.await , uses lazy evaluation, which means that the tasks won't be executed until you actually need their results. However, you won't need to call Task.await this time around.

In order to get the result of running the tasks, you can use something like Enum.to_list/1 :

Calling only Task.async_stream won't run the tasks, use something like Enum.to_list to fetch the results

Running this on iex will return the following output:

[ok: 2, ok: 3, ok: 4, ok: 5, ok: 6, ok: 7, ok: 8, ok: 9, ok: 10, ok: 11]

You might be wondering why the final result is a Keyword list, however, the result is actually a list of tuples. Every task returns a tuple where the first element is either :ok or :error and the second element is the result of the function call, or :timeout as we'll see in a future section.

With this in mind if you want to retrieve only the result of the function calls you can instead run:

Retrieving only the function call result from the list of tuples

This will return the following list:

[2, 3, 4, 5, 6, 7, 8, 9, 10, 11]


Another feature that Task.async_stream provides is to define how many tasks you want to run in parallel at any given time. You can define this by specifying a value for:max_concurrency on the function options, which defaults to System.schedulers_online/0.

Here’s an example that will let us confirm this behaviour, allowing only 2 tasks to be run in parallel at any given time:

With :max_concurrency set to 2 only a maximum of two tasks are running in parallel at any given time

In the example above, on Processor.process/1 we're returning a tuple where the first element is the argument that was provided to the function and the second one is the current time in milliseconds.

Using a :max_concurrency of 2 should let us see that, for example, the time differences between number 1 and number 3 should be of roughly 1000 milliseconds, the same goes for between 2 and 4, 6 and 8, etc.

If you run this example in iex you'll get output similar to this:

ok: {1, 1549319667100},
ok: {2, 1549319667100},
ok: {3, 1549319668102},
ok: {4, 1549319668102},
ok: {5, 1549319669102},
ok: {6, 1549319669102},
ok: {7, 1549319670103},
ok: {8, 1549319670103},
ok: {9, 1549319671104},
ok: {10, 1549319671104}

We can confirm that the result is what we were expecting. In short, seeing as we set :max_concurrency to 2, the tasks are processed sort of like in "bundles", 1 is processed at the same time as 2, 3 is processed at the same time as 4, 5 is processed at the same time as 6, etc.

Notice that this only happens because the time to execute Processor.process/1 is always the same, the tasks are not dependent on one another, so no task waits for another to finish.


You can limit the runtime of the tasks that are created with Task.async_stream , the default value is of 5000 milliseconds. I'm going to user :timer.sleep with a value greater than 5000 (the default timeout value) to force a task to timeout and see what's the behaviour when that happens:

Task.async_stream default timeout value is of 5000 milliseconds so the stream won't finish processing with this code.

If you run this example on iex you can confirm that the process that spawned the tasks exits with an output like:

** (exit) exited in: Task.Supervised.stream(5000)
** (EXIT) time out
(elixir) lib/task/supervised.ex:276: Task.Supervised.stream_reduce/7
(elixir) lib/enum.ex:3015: Enum.reverse/1
(elixir) lib/enum.ex:2649: Enum.to_list/1

In this case, it doesn’t matter which task times out, if one of the task times out the process exits, even if a number of tasks completed successfully. If we don’t want the task to timeout we can specify the :timeout value in the Task.async_stream/3 function options like so:

By specifying the :timeout value on Task.async_stream the stream of tasks finishes processing

However, Task.async_stream/3 allows us to specify different behaviours for when tasks time out by changing the value of :on_timeout on the function options.

Here’s the list of values that can be used on :on_timeout:

  • :exit - This is the default value. In this situation the process that spawned the task exits
  • :kill_task - In this case, only the task that timed out is killed. If the first task in the stream fails the others might be able to still finish. The value emitted for this task is the tuple {:exit, :timeout}

Let’s check how using :kill_task only terminates the task that times out while letting the other tasks finish:

Testing different timeout behaviours with :on_timeout

In the example above we see that for each number we make the following call :timer.sleep(10000 - (number * 1000)). Provided that our list of numbers is from 1 to 10 here's what the value passed into :timer.sleep is for each number:

1 - 9000 milliseconds
2 - 8000 milliseconds
3 - 7000 milliseconds
4 - 6000 milliseconds
5 - 5000 milliseconds
6 - 4000 milliseconds
7 - 3000 milliseconds
8 - 2000 milliseconds
9 - 1000 milliseconds
10 - 0 milliseconds

Taking into account that the default timeout value is of 5000 milliseconds than we should see that only the tasks for 6 or greater are completed (5 shouldn’t be completed because of the overhead of setting up the task).

Running the example on iex yields the following result:

exit: :timeout,
exit: :timeout,
exit: :timeout,
exit: :timeout,
exit: :timeout,
ok: 6,
ok: 7,
ok: 8,
ok: 9,
ok: 10

This is what we expected, only the tasks with a number greater than or equal to 6 were terminated while the other ones were killed because they timed out. You can confirm this because the result for the tasks that were started with a number less than or equal to 5 all returned the tuple {:exit, :timeout}.

With this in mind use on_timeout: :kill_task whenever you want to allow tasks to finish independently of other tasks timing out.


In conclusion, I think Task.async_stream allows you to apply a certain function call, in parallel, to an enumerable without having to make calls to Task.async and Task.await which, in my opinion, leads to cleaner code.

On the other hand, you also get different timeout behaviours, which is not possible using Task.await , although Task.await also allows you to specify timeouts.

Finally, you also get the possibility of specifying the maximum number of tasks you want to run in parallel at any given time. I think this can be super useful, for example, in a situation where you’re making multiple requests to a given service and you want to control the load you’re putting into it, without compromising its availability.

If you want to check the full documentation for both Task.async_stream/3 and Task.async_stream/5 check the Elixir Docs.

Follow me on Twitter → https://twitter.com/joaofcosta_
Check out my personal website → http://dino.codes/