NAME¶
Padre::Task - Padre Task API 3.0
SYNOPSIS¶
# Fire a task that will communicate back to an owner object
My::Task->new(
owner => $padre_role_task_object,
on_run => 'owner_run_method',
on_status => 'owner_status_method',
on_message => 'owner_message_method',
on_finish => 'owner_finish_method',
my_param1 => 123,
my_param2 => 'abc',
)->schedule;
package My::Task;
sub new {
my $class = shift;
my $self = $class->SUPER::new(@_);
# Check params and validate the task
return $self;
}
sub prepare {
my $self = shift;
# Run after scheduling immediately before serialised to a worker
return 0 if $self->my_last_second_abort_check;
return 1; # Continue and run
}
sub run {
my $self = shift;
# Called in child, do the work here
return 1;
}
sub finish {
my $self = shift;
# Called in parent after successful completion
return 1;
}
1;
DESCRIPTION¶
The Padre Task API implements support for background and parallel execution of
code in the Padre IDE, and is based on the CPAN Process API.
A
Task Class is a class that completely encapsulates a single unit of
work, describing not only the work to be done, but also how the unit of work
is created, how is serialised for transport, and any initialisation or cleanup
work needs to be done.
A
Task is a single self-contained unit of work, and is implemented as a
single instance of a particular Task Class.
The lifecycle of a Task object¶
From the perspective of a task author, the execution of a task will occur in
four distinct phases.
1. Construction
The creation of a task is always done completely independantly of its execution.
Typically this is done via the "new" method, or something that calls
it.
This separate construction step allows validation of parameters in advance, as
well as allowing bulk task pre-generation and advanced task management
functionality such as prioritisation, queueing, throttling and load-balancing
of tasks.
2. Preparation
Once a task has been constructed, an arbitrarily long time may pass before the
code is actually run (if it is ever run at all).
If the actual execution of the task will result in certain work being done in
the parent thread, this work cannot be done in the constructor. And once
created as an object, no futher task code will be called until the task is
ready for execution.
To give the author a chance to allow for any problems that may occur as a result
of this delay, the Task API provides a preparation phase for the task via the
"prepare" method.
This preparation code is run in the parent thread once the task has been
prioritised, has a worker allocated to it, and has been encapsulated in its
Padre::TaskHandle, but before the object is serialised for transport into the
thread.
A task can use this preparation phase to detach from non-serialisable resources
in the object such as database handles, to copy any interesting parent state
late rather than early, or decide on a last-second self-abort.
Once the preparation phase is completed the task will be serialised, transported
into assigned worker thread and then executed
immediately.
Because it will execute in the parent thead, the rest of the Padre instance is
available for use if needed, but the preparation code should run quickly and
must not block.
3. Execution
The main phase of the task is where the CPU-intensive or blocking code can be
safely run. It is run inside a worker thread in the background, without
impacting on the performance of the parent thread.
However, the task execution phase must be entirely self-contained.
The worker threads not only do not have access to the Padre IDE variable
structure, but most Padre classes (including heavily used modules such as
Padre::Current) will not be loaded at all in the worker thread.
Any output that needs to be transported back to the parent should be stored in
the object somewhere. When the cleanup phase is run, these values will be
available automatically in the parent.
4. Cleanup
When the execution phase of the task is completed, the task object will be
serialised for transport back up to the parent thread.
On arrival, the instance of the task in the parent will be gutted and its
contents replaced with the contents of the version arriving from the child
thread.
Once this is complete, the task object will fire a "finish" handler
allowing it to take action in the parent thread based on the work done in the
child.
This can include having the task contact any "owner" object that had
commissioned the task in the first place.
METHODS¶
new¶
My::Task->new(
owner => $padre_role_task_object,
on_run => 'owner_run_method',
on_status => 'owner_status_method',
on_message => 'owner_message_method',
on_finish => 'owner_finish_method',
my_param1 => 123,
my_param2 => 'abc',
);
The "new" method creates a new "task", a self-contained
object that represents a unit of work to be done in the background (although
not required to be done in the background).
In addition to defining a set of method for you to provide as the task
implementer, the base class also provides implements a "task
ownership" system in the base class that you may use for nearly no cost
in terms of code.
This task owner system will consume three parameters.
The optional "owner" parameter should be an object that inherits from
the role Padre::Role::Task. Message and finish events for this task will be
forwarded on to handlers on the owner, if they are defined.
The optional "on_run" parameter should be the name of a method that
can be called on the owner object, to be called once the task has started
running and control of the worker message queue has been handed over to the
task.
The optional "on_message" parameter should be the name of a method
that can be called on the owner object, to be called when a message arrives
from the child object during its execution.
The required (if "owner" was provided) "on_finish" parameter
should be the name of a method that can be called on the owner object, to be
called when the task has completed and returns to the parent from the child
object.
When implementing your own task, you should always call the
"SUPER::new" method first, to ensure that integration with the task
owner system is done.
You can then check any other parameters, capture additional information from the
IDE, and validate that the task is correctly requested and should go ahead.
The creation of a task object does NOT imply that it will be executed, merely
that the require for work to be done is validly formed. A task object may
never execute, or may only execute significantly later than it was created.
Anything that the task needs to do once it is certain that the task will be run
should be done in the "prepare" method (see below).
Returns a new task object if the request is valid, or throws an exception if the
request is invalid.
on_run¶
The "on_run" accessor returns the name of the owner's "run"
notification handler method, if one was defined.
on_status¶
The "on_status" accessor returns the name of the owner's status
handler method, if one was defined.
on_message¶
The "on_message" accessor returns the name of the owner's message
handler method, if one was defined.
on_finish¶
The "on_finish" accessor returns the name of the owner's finish
handler method, if one was defined.
as_string¶
The "as_string" method is used to serialise the task into a string for
transmission between the parent and the child (in both directions).
By default your task will be serialised using Storable's "nfreeze"
method, which is suitable for transmission between threads or processes
running the same instance of Perl with the same module search path.
This should be sufficient in most situations.
from_string¶
The "from_string" method is used to deserialise the task from a string
after transmission between the parent and the child (in both directions).
By default your task will be deserialised using Storable's "thaw"
method, which is suitable for transmission between threads or processes
running the same instance of Perl with the same module search path.
This should be sufficient in most situations.
locks¶
The "locks" method returns a list of locks that the task needs to
reserve in order to execute safely.
The meaning, usage, and available quantity of the required locks are tracked by
the task manager. Enforcement of resource limits may be strict, or may only
serve as hints to the scheduler.
Returns a list of strings, or the null list if the task is light with trivial or
no resource consumption.
schedule¶
$task->schedule;
The "schedule" method is used to trigger the sending of the task to a
worker for processing at whatever time the Task Manager deems it appropriate.
This could be immediately, with the task sent before the call returns, or it may
be delayed indefinately or never run at all.
Returns true if the task was dispatched immediately.
Returns false if the task was queued for later dispatch.
prepare¶
The optional "prepare" method will be called by the task manager on
your task object while still in the parent thread, immediately before being
serialised to pass to the worker thread.
This method should be used to compensate for the potential time difference
between when "new" is oridinally called and when the task will
actually be run.
For example, a GUI element may indicate the need to run a background task on the
visible document but does not care that it is the literally
"current" document at the time the task was spawned.
By capturing the contents of the current document during "prepare"
rather than "new" the task object is able to apply the task to the
most up to date information at the time we are able to do the work, rather
than at the time we know we need to do the work.
The "prepare" method can take a relatively heavy parameter such as a
reference to a Wx element, and flatten it to the widget ID or contents of the
widget instead.
The "prepare" method also gives your task object a chance to determine
whether or not it is still necessary. In some situations the delay between
"new" and "prepare" may be long enough that the task is no
longer relevant, and so by the use of "prepare" you can indicate
execution should be aborted.
Returns true if the task is stil valid, and so the task should be executed.
Returns false if the task is no longer valid, and the task should be aborted.
run¶
The "run" method is called on the object in the worker thread
immediately after deserialisation. It is where the actual computations and
work for the task occurs.
In many situations the implementation of run is simple and procedural, doing
work based on input parameters stored on the object, blocking if necessary,
and storing the results of the computation on the object for transmission back
to the parent thread.
In more complex scenarios, you may wish to do a series of tasks or a recursive
set of tasks in a loop with a check on the "cancelled" method
periodically to allow the aborting of the task if requested by the parent.
In even more advanced situations, you may embed and launch an entire event loop
such as POE or AnyEvent inside the "run" method so that long running
or complex functionality can be run in the background.
Once inside of "run" your task is in complete control and the task
manager cannot interupt the execution of your code short of killing the thread
entirely. The standard "cancelled" method to check for a request
from the parent to abort your task is cooperative and entirely voluntary.
Returns true if the computation was completed successfully.
Returns false if the computation was not completed successfully, and so the
parent should not run any post-task logic.
finish¶
The "finish" method is called on the object in the parent thread once
it has been passed back up to the parent, if "run" completed
successfully.
It is responsible for cleaning up the task and taking any actions based on the
result of the computation.
If your task is fire-and-forget or void and you don't care about when the task
completes, you do not need to implement this method.
The default implementation of "finish" implements redirection to the
"on_finish" handler of the task owner object, if one has been
defined.
is_parent¶
The "is_parent" method returns true if the task object is in the
parent thread, or false if it is in the child thread.
is_child¶
The "is_child" method returns true if the task object is in the child
thread, or false if it is in the parent thread.
cancelled¶
sub run {
my $self = shift;
# Abort a long task if we are no longer wanted
foreach my $thing ( @{$self->{lots_of_stuff}} ) {
return if $self->cancelled;
# Do something expensive
}
return 1;
}
The "cancelled" method should be called in the child worker, and
allows the task to be cooperatively aborted before it has completed.
The abort mechanism is cooperative. Tasks that do not periodically check the
"cancelled" method will continue until they are complete regardless
of the desires of the task manager.
tell_status¶
# Indicate we are waiting, but only while we are waiting
$task->tell_status('Waiting...');
sleep 5
$task->tell_status;
The "tell_status" method allows a task to trickle informative status
messages up to the parent thread. These messages serve a dual purpose.
Firstly, the messages will (or at least
may) be displayed to the user to
indicate progress through a long asynchronous background task. For example, a
filesystem search task might send a status message for each directory that it
examines, so that the user can monitor the task speed and level of completion.
Secondly, the regular flow of messages from the task indicates to the
Padre::TaskManager that the task is running correctly, making progress through
its assigned workload, and has probably not crashed or hung.
While the task manager does not currently kill hanging threads, it will almost
certainly do so in the future. And so it may even be worth sending a periodic
null status message every few seconds just to assure the task manager that
your long-running task is still alive.
SEE ALSO¶
Padre, Process
COPYRIGHT¶
Copyright 2008-2013 The Padre development team as listed in Padre.pm.
This program is free software; you can redistribute it and/or modify it under
the same terms as Perl 5 itself.
The full text of the license can be found in the LICENSE file included with this
module.