Difference between revisions of "TaskFlow/Task Arguments and Results"

Revision as of 19:08, 21 October 2013

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 flow usage pattern.

Set of names of task arguments is available as requires property of the task instance. When task is about to be executed values with this names are retrieved from storage and passed to execute method of the task as keyword arguments.

Set of names of task results (what task provides) is available as provides property of task instance. After task finishes successfully, it's result(s) (what task execute method returns) are available by these names from storage (there will be examples below).

Arguments Specification

There are different way to specify task argument 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 simplest way to specify task arguments. Optional arguments (with default values), and special arguments like self, *args and **kwargs are ignored on iferrence:

   >>> 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 value you want to pass to task is stored with name other then corresponding task argument. That's when rebind task constructor parameter comes handy. Using it flow author can instruct engine to fetch a value from storage by one name, but pass it to task's execute method with another.

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 paramters 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 task. The length of tuple or list should not be less then number of task required parameters. For example, you can achieve same effect as the previous example with:

   SpawnVMTask(rebind_args=('name', 'vm_image_id'))

which is equivalent to 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 stroage, and, 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=())