Framework for executing actions and rollbacks
actionqueues is a lightweight way to queue up commands for execution and
rollback on failures.
The idea is that it provides a framework for safely executing sequences of action with side-effects, like database writes, that might need later rolling back if later actions fail. In addition, it provides a standardised way for actions to be retried.
For example, a user sign up process may write to several different systems.
If one system is down, then the other systems modified so far need cleaning
up before the failure is propagated back to the user. Using
with an action for each external system to be modified enables this pattern,
along with simple retry semantics for likely-transient failures such as network
pip install actionqueues
Using Action Queues
It's barebones, the main point is to provide a framework to work within for actions that have side effects. It's basically the Command pattern, with a tiny execution framework.
Action is the lowest-level building block. It's any object with
rollback methods. The
Action is what handles executing each step of the
overall workflow, and rolling back any changes made to external state. It's
a single object so it can save state for rollback -- for example, primary
keys for any created database rows so they can be deleted during rollback.
The main task of a user of
actionqueues is to create the
which implement the tasks their workflows require.
Action classes are written, they can be executed. An
Actions for execution and rollback. Add
Action objects to an
for execution. Call
ActionQueue#execute to start running each action's
execute method in the order the
Action objects were added to the
ActionQueue. Behaviour after this point is controlled by the
rollback methods on the
Action objects being executed by the queue.
The default case is that no exception is raised during an
execute and the
next action in the queue is has
execute called. This is shown below for a
Action objects within an
execute cause rollback
Action#execute raises an exception, the ActionQueue notes where it's
up to in the Actions queued up and then propagates the exception
back up to the caller.
It is then the caller's responsibility to catch the exception and then to call
ActionQueue#rollback. This is so the caller can know that the queue of
actions failed and is able to log the exception before calling
ActionQueue#rollback will execute the
rollback method on all
actions where the
execute method was called, including the one raising the
exception, in the reverse order to that which the
execute method was called.
Rollback will not be called on actions where
execute has not been called.
Again, the default case at this point is that
rollback methods succeed and
don't throw exceptions, leading to each being executed in succession.
In contrast to a raised exception from
execute, if an exception is raised
rollback method, the
silently swallow the exception and continue executing the
Action objects in the queue.
This is because, in the rollback scenario, it's most likely that all rollback
actions should happen so the library assumes this. Therefore
should do their own logging of exceptions before re-raising them.
Retrying failed operations
There is an exception to the above rules. If the
actionqueue.ActionRetryException then the
method will be called again. The
ActionRetryException init method takes an
ms_backoff argument to specify a time to sleep before trying the
method again, in milliseconds.
ActionQueue will retry as long as the action keeps raising
ActionRetryException, so the action must maintain a retry count
to avoid endless retries. See below for some
helper classes which cover common cases.
import random from actionqueues import actionqueue, action SUCCEED = 0 RETRY = 1 FAIL = 2 class MyAction(action.Action): def __init__(self, id): self._id = id self._value = 0 def execute(self): """Called if actions before it in the queue complete successfully. Raise any exception to indicate failure. """ action = random.choice([SUCCEED, RETRY, FAIL]) if action == RETRY: print self._id, "Throwing retry exception" raise actionqueue.ActionRetryException(ms_backoff=0) elif action == FAIL: print self._id, "Throwing failure exception" raise Exception() else: print self._id, "Executing success action" self._value = 1 def rollback(self): """Called in reverse order for all actions queued whose execute method was called when the ActionQueue's rollback method is called. """ print self._id, "Rolling back action" if self._value == 1: self._value = 0 q = actionqueue.ActionQueue() q.add(MyAction("a")) q.add(MyAction("b")) try: q.execute() except: q.rollback()
Retry exception helpers
It can be tedious to keep track of the backoff and retry count for an action.
actionqueues provides helpers for this called exception factories.
These are created when the
Action is initialised, and when an
method hits a retriable exception, it calls the factory's
method. In general, this will throw
ActionRetryException exceptions for a
given number of retries, then throw a generic exception, or one provided by
Using separate ExceptionFactory objects for
rollback is usually
The available exception factories are:
DoublingBackoffExceptionFactorywhich will throw a configurable number
ActionRetryExceptionexceptions, each doubling its backoff time.
In this example, the
ZeroDivisionError will cause 5 retries, at 100, 200,
400, 800 and 1600ms delays, by using a
from actionqueues import actionqueue, action from actionqueues.exceptionfactory import DoublingBackoffExceptionFactory class MyFailingAction(action.Action): def __init__(self): self._run = 1 self._execute_ex_factory = DoublingBackoffExceptionFactory( retries=5, ms_backoff_initial=100 ) self._rollback_ex_factory = DoublingBackoffExceptionFactory( retries=10, ms_backoff_initial=100 ) def execute(self): """Execute an always failing action, but have it retried 5 times.""" print "Executing action", self._run self._run += 1 try: 1 / 0 except ZeroDivisionError, ex: self._execute_ex_factory.raise_exception(original_exception=ex) def rollback(self): print "Rollback action", self._run self._run += 1 try: 1 / 0 except ZeroDivisionError, ex: self._rollback_ex_factory.raise_exception(original_exception=ex) q = actionqueue.ActionQueue() q.add(MyFailingAction()) try: q.execute() except: print "boom"
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