The Task struct.

It contains these fields:

• :pid - the PID of the task process; nil if the task does not use a task process

• :ref - the task monitor reference

• :owner - the PID of the process that started the task

Starts a task that must be awaited on.

This function spawns a process that is linked to and monitored by the caller process. A Task struct is returned containing the relevant information.

Read the Task module documentation for more info on general usage of async/1 and async/3.

See also async/3.

Starts a task that must be awaited on.

A Task struct is returned containing the relevant information. Developers must eventually call Task.await/2 or Task.yield/2 followed by Task.shutdown/2 on the returned task.

Read the Task module documentation for more info on general usage of async/1 and async/3.

Linking

This function spawns a process that is linked to and monitored by the caller process. The linking part is important because it aborts the task if the parent process dies. It also guarantees the code before async/await has the same properties after you add the async call. For example, imagine you have this:

x = heavy_fun()
y = some_fun()
x + y

Now you want to make the heavy_fun() async:

x = Task.async(&heavy_fun/0)
y = some_fun()
Task.await(x) + y

As before, if heavy_fun/0 fails, the whole computation will fail, including the parent process. If you don’t want the task to fail then you must change the heavy_fun/0 code in the same way you would achieve it if you didn’t have the async call. For example, to either return {:ok, val} | :error results or, in more extreme cases, by using try/rescue. In other words, an asynchronous task should be thought of as an extension of a process rather than a mechanism to isolate it from all errors.

If you don’t want to link the caller to the task, then you must use a supervised task with Task.Supervisor and call Task.Supervisor.async_nolink/2.

In any case, avoid any of the following:

• Setting :trap_exit to true - trapping exits should be used only in special circumstances as it would make your process immune to not only exits from the task but from any other processes.

  Moreover, even when trapping exits, calling await will still exit if the task has terminated without sending its result back.

• Unlinking the task process started with async/await. If you unlink the processes and the task does not belong to any supervisor, you may leave dangling tasks in case the parent dies.

Message format

The reply sent by the task will be in the format {ref, result}, where ref is the monitor reference held by the task struct and result is the return value of the task function.

Returns a stream that runs the given function fun concurrently on each item in enumerable.

Each enumerable item is passed as argument to the given function fun and processed by its own task. The tasks will be linked to the current process, similarly to async/1.

Example

Count the codepoints in each string asynchronously, then add the counts together using reduce.

iex> strings = ["long string", "longer string", "there are many of these"]
iex> stream = Task.async_stream(strings, fn text -> text |> String.codepoints |> Enum.count end)
iex> Enum.reduce(stream, 0, fn {:ok, num}, acc -> num + acc end)
47

See async_stream/5 for discussion, options, and more examples.

Returns a stream that runs the given module, function, and args concurrently on each item in enumerable.

Each item will be prepended to the given args and processed by its own task. The tasks will be linked to an intermediate process that is then linked to the current process. This means a failure in a task terminates the current process and a failure in the current process terminates all tasks.

When streamed, each task will emit {:ok, value} upon successful completion or {:exit, reason, element} if the caller is trapping exits, where element is the stream element. Results are emitted in the same order as the original enumerable.

The level of concurrency can be controlled via the :max_concurrency option and defaults to System.schedulers_online/0. A timeout can also be given as an option representing the maximum amount of time to wait without a task reply.

Finally, consider using Task.Supervisor.async_stream/6 to start tasks under a supervisor. If you find yourself trapping exits to handle exits inside the async stream, consider using Task.Supervisor.async_stream_nolink/6 to start tasks that are not linked to the current process.

Options

• :max_concurrency - sets the maximum number of tasks to run at the same time. Defaults to System.schedulers_online/0.
• :ordered - whether the results should be returned in the same order as the input stream. This option is useful when you have large streams and don’t want to buffer results before they are delivered. Defaults to true.
• :timeout - the maximum amount of time (in milliseconds) each task is allowed to execute for. Defaults to 5000.

• :on_timeout - what do to when a task times out. The possible values are:

  • :exit (default) - the process that spawned the tasks exits.
  • :kill_task - the task that timed out is killed. The value emitted for that task is {:exit, :timeout, element}, where element is the element it timed out on.

Example

Let’s build a stream and then enumerate it:

stream = Task.async_stream(collection, Mod, :expensive_fun, [])
Enum.to_list(stream)

The concurrency can be increased or decreased using the :max_concurrency option. For example, if the tasks are IO heavy, the value can be increased:

max_concurrency = System.schedulers_online * 2
stream = Task.async_stream(collection, Mod, :expensive_fun, [], max_concurrency: max_concurrency)
Enum.to_list(stream)

Awaits a task reply and returns it.

A timeout, in milliseconds, can be given with default value of 5000. In case the task process dies, this function will exit with the same reason as the task.

If the timeout is exceeded, await will exit; however, the task will continue to run. When the calling process exits, its exit signal will terminate the task if it is not trapping exits.

This function assumes the task’s monitor is still active or the monitor’s :DOWN message is in the message queue. If it has been demonitored, or the message already received, this function will wait for the duration of the timeout awaiting the message.

This function can only be called once for any given task. If you want to be able to check multiple times if a long-running task has finished its computation, use yield/2 instead.

Compatibility with OTP behaviours

It is not recommended to await a long-running task inside an OTP behaviour such as GenServer. Instead, you should match on the message coming from a task inside your GenServer.handle_info/2 callback.

Examples

iex> task = Task.async(fn -> 1 + 1 end)
iex> Task.await(task)
2

Unlinks and shuts down the task, and then checks for a reply.

Returns {:ok, reply} if the reply is received while shutting down the task, {:exit, reason} if the task died, otherwise nil.

The shutdown method is either a timeout or :brutal_kill. In case of a timeout, a :shutdown exit signal is sent to the task process and if it does not exit within the timeout, it is killed. With :brutal_kill the task is killed straight away. In case the task terminates abnormally (possibly killed by another process), this function will exit with the same reason.

It is not required to call this function when terminating the caller, unless exiting with reason :normal or if the task is trapping exits. If the caller is exiting with a reason other than :normal and the task is not trapping exits, the caller’s exit signal will stop the task. The caller can exit with reason :shutdown to shutdown all of its linked processes, including tasks, that are not trapping exits without generating any log messages.

If a task’s monitor has already been demonitored or received and there is not a response waiting in the message queue this function will return {:exit, :noproc} as the result or exit reason can not be determined.

Starts a task.

This is only used when the task is used for side-effects (i.e. no interest in the returned result) and it should not be linked to the current process.

Starts a task.

This is only used when the task is used for side-effects (i.e. no interest in the returned result) and it should not be linked to the current process.

Starts a process linked to the current process.

This is often used to start the process as part of a supervision tree.

Starts a task as part of a supervision tree.

Temporarily blocks the current process waiting for a task reply.

Returns {:ok, reply} if the reply is received, nil if no reply has arrived, or {:exit, reason} if the task has already exited. Keep in mind that normally a task failure also causes the process owning the task to exit. Therefore this function can return {:exit, reason} only if

• the task process exited with the reason :normal
• it isn’t linked to the caller
• the caller is trapping exits

A timeout, in milliseconds, can be given with default value of 5000. If the time runs out before a message from the task is received, this function will return nil and the monitor will remain active. Therefore yield/2 can be called multiple times on the same task.

This function assumes the task’s monitor is still active or the monitor’s :DOWN message is in the message queue. If it has been demonitored or the message already received, this function will wait for the duration of the timeout awaiting the message.

If you intend to shut the task down if it has not responded within timeout milliseconds, you should chain this together with shutdown/1, like so:

case Task.yield(task, timeout) || Task.shutdown(task) do
  {:ok, result} ->
    result
  nil ->
    Logger.warn "Failed to get a result in #{timeout}ms"
    nil
end

That ensures that if the task completes after the timeout but before shutdown/1 has been called, you will still get the result, since shutdown/1 is designed to handle this case and return the result.

Yields to multiple tasks in the given time interval.

This function receives a list of tasks and waits for their replies in the given time interval. It returns a list of tuples of two elements, with the task as the first element and the yielded result as the second.

Similarly to yield/2, each task’s result will be

• {:ok, term} if the task has successfully reported its result back in the given time interval
• {:exit, reason} if the task has died
• nil if the task keeps running past the timeout

Check yield/2 for more information.

Example

Task.yield_many/2 allows developers to spawn multiple tasks and retrieve the results received in a given timeframe. If we combine it with Task.shutdown/2, it allows us to gather those results and cancel the tasks that have not replied in time.

Let’s see an example.

tasks =
  for i <- 1..10 do
    Task.async(fn ->
      Process.sleep(i * 1000)
      i
    end)
  end

tasks_with_results = Task.yield_many(tasks, 5000)

results = Enum.map(tasks_with_results, fn {task, res} ->
  # Shutdown the tasks that did not reply nor exit
  res || Task.shutdown(task, :brutal_kill)
end)

# Here we are matching only on {:ok, value} and
# ignoring {:exit, _} (crashed tasks) and `nil` (no replies)
for {:ok, value} <- results do
  IO.inspect value
end

In the example above, we create tasks that sleep from 1 up to 10 seconds and return the amount of seconds they slept. If you execute the code all at once, you should see 1 up to 5 printed, as those were the tasks that have replied in the given time. All other tasks will have been shut down using the Task.shutdown/2 call.