Difference between revisions of "TaskFlow/Task Arguments and Results"
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Revision as of 19:23, 21 October 2013
Contents
Overview
In TaskFlow, all flow & task state goes to storage (potentially persistent). That includes all the information that task/s in the flow needs when it is executed (task dependencies via arguments), and all the information task produces (serializable task results). A developer who implements tasks or flows can specify what arguments a task accepts and what result it returns in several ways. This document will help you understand what those ways are and how to use those ways to accomplish your desired taskflow usage pattern.
- Task arguments
- Set of names of task arguments available as the
requires
property of the task instance. When task is about to be executed values with these names are retrieved from storage and passed toexecute
method of the task as keyword arguments (ie, kwargs). - Task results
- Set of names of task results (what task provides) available as
provides
property of task instance. After task finishes successfully, it's result(s) (what the taskexecute
method returns) are available by these names from storage (see examples below).
Arguments Specification
There are different way to specify the task argument requires
set.
Arguments Inference
Task arguments can be inferred from arguments of execute
method of the task.
For example:
>>> class MyTask(task.Task): ... def execute(self, spam, eggs): ... return spam + eggs ... >>> MyTask().requires set(['eggs', 'spam'])
Inference from signature is the simplest way to specify task arguments. Optional arguments (with default values), and special arguments like self
, *args
and **kwargs
are ignored on inference (as these names have special meaning/usage in python).
For example:
>>> class MyTask(task.Task): ... def execute(self, spam, eggs=()): ... return spam + eggs ... >>> MyTask().requires set(['spam']) >>> >>> class UniTask(task.Task): ... def execute(self, *args, **kwargs): ... pass ... >>> UniTask().requires set([])
Rebind
There are cases when the value you want to pass to task is stored with a name other then the corresponding task arguments name. That's when the rebind
task constructor parameter comes handy. Using it the flow author can instruct the engine to fetch a value from storage by one name, but pass it to task's execute
method with another name.
There are two possible way of using it. First is to pass dictionary that maps task argument name to name of saved value.
For example:
If you have task
class SpawnVMTask(task.Task): def execute(self, vm_name, vm_image_id, **kwargs): pass # TODO(imelnikov): use parameters to spawn vm
and you saved vm name with 'name' key in storage, you can spawn vm with such name like this:
SpawnVMTask(rebind={'vm_name': 'name'})
Second, you can pass a tuple or list of argument names, and values with that names are passed to the task. The length of the tuple or list should not be less then number of task required parameters. For example, you can achieve the same effect as the previous example with:
SpawnVMTask(rebind_args=('name', 'vm_image_id'))
which is equivalent to a more elaborate:
SpawnVMTask(rebind=dict(vm_name='name', vm_image_id='vm_image_id'))
In both cases, if your task accepts arbitrary arguments with **kwargs
construct, you can specify extra arguments.
For example:
SpawnVMTask(rebind=('name', 'vm_image_id', 'admin_key_name'))
When such task is about to be executed, name
, vm_image_id
and admin_key_name
values are fetched from storage and
value from name
is passed to execute
method as
vm_name
, value from vm_image_id
is passed as
vm_image_id
, and value from admin_key_name
is passed
as admin_key_name
parameter in kwargs
.
Manually Specifying Requirements
TODO(imelnikov): describe requires
parameter, optional task
args and **kwargs
.
Results Specification
In python, function results are not named, so we can not infer what task
returns. Of course, complete task result (what execute
method
returns) is saved in storage, but it is not accessible by unless task
specifies names or values via provides
task constructor parameter.
Returning One Value
If task returns just one value, privodes
should be string -- the
name of the value:
class TheAnswerReturningTask(task.Task): def execute(self): return 42
TheAnswerReturningTask(provides='the_answer')
Returning Tuple
For task that returns several values, one option (as usual in python) is return a tuple:
class BitsAndPiecesTask(task.Task): def execute(self): return 'BITs', 'PIECEs'
Then, you can give the value individual names, by passing tuple or list as
provides
parameter:
BitsAndPiecesTask(provides=('bits', 'pieces'))
After such task executes, you (and engine, which is useful for other tasks) will be able to get elements from storage by name:
>>> storage.fetch('bits') 'BITs' >>> storage.fetch('pieces') 'PIECEs'
Provides argument can be shorter then actual tuple returned by task -- then
extra values are ignored (but, of course, saved and passed to
revert
).
Provides argument can be longer then actual tuple returned by task -- then extra
parameters are left undefined: a warning is printed to logs and if use of such
parameter is attempted NotFound
exception is raised.
Returning Dictionary
Other option to return several values is dictionary:
class BitsAndPiecesTask(task.Task): def execute(self): return { 'bits': 'BITs', 'pieces': 'PIECEs' }
TaskFlow expects that dict will be returened if provides
argument
is a set
:
BitsAndPiecesTask(provides=set(['bits', 'pieces']))
After such task executes, you (and engine, which is useful for other tasks) will be able to get elements from storage by name:
>>> storage.fetch('bits') 'BITs' >>> storage.fetch('pieces') 'PIECEs'
Some items from dict returned by task can be not present in provides arguments
-- then extra values are ignored (but, of course, saved and passed to
revert
).
Provides argument have some items not present in actual dict returned by task --
then extra parameters are left undefined: a warning is printed to logs and if
use of such parameter is attempted NotFound
exception is raised.
Default Provides
As mentioned above, by default task provides nothing, which means task results are not accessible by all the other tasks in the flow.
Task author can override this and specify default value for provides using
default_provides
class variable:
class BitsAndPiecesTask(task.Task): default_provides = ('bits', 'pieces') def execute(self): return 'BITs', 'PIECEs'
Of course, flow author can override this to change names:
BitsAndPiecesTask(provides=('b', 'p'))
or to change structure -- e.g. this instance will make whole tuple accessible to other tasks by name 'bnp':
BitsAndPiecesTask(provides='bnp')
or flow author may want to return default behavior and hide the results of the task from other tasks in the flow (e.g. to avoid naming conflicts):
BitsAndPiecesTask(provides=())