Elixir v1.3.4 Task View Source
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:
task = Task.async(fn -> do_some_work() end)
res = do_some_other_work()
res + Task.await(task)
Tasks spawned with async
can be awaited on by their caller
process (and only their caller) as shown in the example above.
They are implemented by spawning a process that sends a message
to the caller once the given computation is performed.
Besides async/1
and await/2
, tasks can also be
started as part of a supervision tree and dynamically spawned
on remote nodes. We will explore all three scenarios next.
async and await
One of the common uses of tasks is to convert sequential code
into concurrent code with Task.async/1
while keeping its semantics.
When invoked, a new process will be created, linked and monitored
by the caller. Once the task action finishes, a message will be sent
to the caller with the result.
Task.await/2
is used to read the message sent by the task.
There are two important things to consider when using async
:
If you are using async tasks, you must await a reply as they are always sent. If you are not expecting a reply, consider using
Task.start_link/1
detailed below.async tasks link the caller and the spawned process. This means that, if the caller crashes, the task will crash too and vice-versa. This is on purpose: if the process meant to receive the result no longer exists, there is no purpose in completing the computation.
If this is not desired, use
Task.start/1
or consider starting the task under aTask.Supervisor
usingasync_nolink
orstart_child
.
Task.yield/2
is an alternative to await/2
where the caller will
temporarily block, waiting until the task replies or crashes. If the
result does not arrive within the timeout, it can be called again at a
later moment. This allows checking for the result of a task multiple
times. If a reply does not arrive within the desired time,
Task.shutdown/2
can be used to stop the task.
Supervised tasks
It is also possible to spawn a task under a supervisor
with start_link/1
and start_link/3
:
Task.start_link(fn -> IO.puts "ok" end)
Such tasks can be mounted in your supervision tree as:
import Supervisor.Spec
children = [
worker(Task, [fn -> IO.puts "ok" end])
]
Since these tasks are supervised and not directly linked to
the caller, they cannot be awaited on. Note start_link/1
,
unlike async/1
, returns {:ok, pid}
(which is
the result expected by supervision trees).
By default, most supervision strategies will try to restart
a worker after it exits regardless of the reason. If you design the
task to terminate normally (as in the example with IO.puts/2
above),
consider passing restart: :transient
in the options to worker/3
.
Dynamically supervised tasks
The Task.Supervisor
module allows developers to dynamically
create multiple supervised tasks.
A short example is:
{:ok, pid} = Task.Supervisor.start_link()
task = Task.Supervisor.async(pid, fn ->
# Do something
end)
Task.await(task)
However, in the majority of cases, you want to add the task supervisor to your supervision tree:
import Supervisor.Spec
children = [
supervisor(Task.Supervisor, [[name: MyApp.TaskSupervisor]])
]
Now you can dynamically start supervised tasks:
Task.Supervisor.start_child(MyApp.TaskSupervisor, fn ->
# Do something
end)
Or even use the async/await pattern:
Task.Supervisor.async(MyApp.TaskSupervisor, fn ->
# Do something
end) |> Task.await()
Finally, check Task.Supervisor
for other supported operations.
Distributed tasks
Since Elixir provides a Task supervisor, it is easy to use one to dynamically spawn tasks across nodes:
# On the remote node
Task.Supervisor.start_link(name: MyApp.DistSupervisor)
# On the client
Task.Supervisor.async({MyApp.DistSupervisor, :remote@local},
MyMod, :my_fun, [arg1, arg2, arg3])
Note that, when working with distributed tasks, one should use the async/4
function
that expects explicit module, function and arguments, instead of async/2
that
works with anonymous functions. That’s because anonymous functions expect
the same module version to exist on all involved nodes. Check the Agent
module
documentation for more information on distributed processes as the limitations
described there apply to the whole ecosystem.
Link to this section Summary
Functions
The Task struct
Starts a task that must be awaited on
Starts a task that must be awaited on
Awaits a task reply
Unlinks and shuts down the task, and then checks for a reply
Starts a task
Starts a task
Starts a task 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
Yields to multiple tasks in the given time interval
Link to this section Types
Link to this section Functions
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
totrue
- 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 exists, 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.
await(t, timeout) :: term | no_return
Awaits a task reply.
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 handle_info
callback.
shutdown(t, timeout | :brutal_kill) :: {:ok, term} | {:exit, term} | nil
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.
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 block forever awaiting the message.
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 task as part of a supervision tree.
start_link(module, atom, [term]) :: {:ok, pid}
Starts a task as part of a supervision tree.
yield(t, timeout) :: {:ok, term} | {:exit, term} | nil
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.
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 diednil
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 ->
:timer.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.