path: root/google_appengine/google/appengine/ext/deferred/deferred.py

#!/usr/bin/env python
#
# Copyright 2007 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

"""A module that handles deferred execution of callables via the task queue.

Tasks consist of a callable and arguments to pass to it. The callable and its
arguments are serialized and put on the task queue, which deserializes and
executes them. The following callables can be used as tasks:

1) Functions defined in the top level of a module
2) Classes defined in the top level of a module
3) Instances of classes in (2) that implement __call__
4) Instance methods of objects of classes in (2)
5) Class methods of classes in (2)
6) Built-in functions
7) Built-in methods

The following callables can NOT be used as tasks:
1) Nested functions or closures
2) Nested classes or objects of them
3) Lambda functions
4) Static methods

The arguments to the callable, and the object (in the case of method or object
calls) must all be pickleable.

If you want your tasks to execute reliably, don't use mutable global variables;
they are not serialized with the task and may not be the same when your task
executes as they were when it was enqueued (in fact, they will almost certainly
be different).

If your app relies on manipulating the import path, make sure that the function
you are deferring is defined in a module that can be found without import path
manipulation. Alternately, you can include deferred.TaskHandler in your own
webapp application instead of using the easy-install method detailed below.

When you create a deferred task using deferred.defer, the task is serialized,
and an attempt is made to add it directly to the task queue. If the task is too
big (larger than about 10 kilobytes when serialized), a datastore entry will be
created for the task, and a new task will be enqueued, which will fetch the
original task from the datastore and execute it. This is much less efficient
than the direct execution model, so it's a good idea to minimize the size of
your tasks when possible.

In order for tasks to be processed, you need to set up the handler. Add the
following to your app.yaml handlers section:

handlers:
- url: /_ah/queue/deferred
  script: $PYTHON_LIB/google/appengine/ext/deferred/__init__.py
  login: admin

By default, the deferred module uses the URL above, and the default queue.

Example usage:

  def do_something_later(key, amount):
    entity = MyModel.get(key)
    entity.total += amount
    entity.put()

  # Use default URL and queue name, no task name, execute ASAP.
  deferred.defer(do_something_later, 20)

  # Providing non-default task queue arguments
  deferred.defer(do_something_later, 20, _queue="foo", countdown=60)
"""





import logging
import pickle
import types

from google.appengine.api.labs import taskqueue
from google.appengine.ext import db
from google.appengine.ext import webapp
from google.appengine.ext.webapp.util import run_wsgi_app


_TASKQUEUE_HEADERS = {"Content-Type": "application/octet-stream"}
_DEFAULT_URL = "/_ah/queue/deferred"
_DEFAULT_QUEUE = "default"


class Error(Exception):
  """Base class for exceptions in this module."""


class PermanentTaskFailure(Error):
  """Indicates that a task failed, and will never succeed."""


def run(data):
  """Unpickles and executes a task.

  Args:
    data: A pickled tuple of (function, args, kwargs) to execute.
  Returns:
    The return value of the function invocation.
  """
  try:
    func, args, kwds = pickle.loads(data)
  except Exception, e:
    raise PermanentTaskFailure(e)
  else:
    return func(*args, **kwds)


class _DeferredTaskEntity(db.Model):
  """Datastore representation of a deferred task.

  This is used in cases when the deferred task is too big to be included as
  payload with the task queue entry.
  """
  data = db.BlobProperty(required=True)


def run_from_datastore(key):
  """Retrieves a task from the datastore and executes it.

  Args:
    key: The datastore key of a _DeferredTaskEntity storing the task.
  Returns:
    The return value of the function invocation.
  """
  entity = _DeferredTaskEntity.get(key)
  if not entity:
    raise PermanentTaskFailure()
  try:
    ret = run(entity.data)
    entity.delete()
  except PermanentTaskFailure:
    entity.delete()
    raise


def invoke_member(obj, membername, *args, **kwargs):
  """Retrieves a member of an object, then calls it with the provided arguments.

  Args:
    obj: The object to operate on.
    membername: The name of the member to retrieve from ojb.
    args: Positional arguments to pass to the method.
    kwargs: Keyword arguments to pass to the method.
  Returns:
    The return value of the method invocation.
  """
  return getattr(obj, membername)(*args, **kwargs)


def _curry_callable(obj, *args, **kwargs):
  """Takes a callable and arguments and returns a task queue tuple.

  The returned tuple consists of (callable, args, kwargs), and can be pickled
  and unpickled safely.

  Args:
    obj: The callable to curry. See the module docstring for restrictions.
    args: Positional arguments to call the callable with.
    kwargs: Keyword arguments to call the callable with.
  Returns:
    A tuple consisting of (callable, args, kwargs) that can be evaluated by
    run() with equivalent effect of executing the function directly.
  Raises:
    ValueError: If the passed in object is not of a valid callable type.
  """
  if isinstance(obj, types.MethodType):
    return (invoke_member, (obj.im_self, obj.im_func.__name__) + args, kwargs)
  elif isinstance(obj, types.BuiltinMethodType):
    if not obj.__self__:
      return (obj, args, kwargs)
    else:
      return (invoke_member, (obj.__self__, obj.__name__) + args, kwargs)
  elif isinstance(obj, types.ObjectType) and hasattr(obj, "__call__"):
    return (obj, args, kwargs)
  elif isinstance(obj, (types.FunctionType, types.BuiltinFunctionType,
                        types.ClassType, types.UnboundMethodType)):
    return (obj, args, kwargs)
  else:
    raise ValueError("obj must be callable")


def serialize(obj, *args, **kwargs):
  """Serializes a callable into a format recognized by the deferred executor.

  Args:
    obj: The callable to serialize. See module docstring for restrictions.
    args: Positional arguments to call the callable with.
    kwargs: Keyword arguments to call the callable with.
  Returns:
    A serialized representation of the callable.
  """
  curried = _curry_callable(obj, *args, **kwargs)
  return pickle.dumps(curried, protocol=pickle.HIGHEST_PROTOCOL)


def defer(obj, *args, **kwargs):
  """Defers a callable for execution later.

  The default deferred URL of /_ah/queue/deferred will be used unless an
  alternate URL is explicitly specified. If you want to use the default URL for
  a queue, specify _url=None. If you specify a different URL, you will need to
  install the handler on that URL (see the module docstring for details).

  Args:
    obj: The callable to execute. See module docstring for restrictions.
    _countdown, _eta, _name, _url, _queue: Passed through to the task queue -
      see the task queue documentation for details.
    args: Positional arguments to call the callable with.
    kwargs: Any other keyword arguments are passed through to the callable.
  """
  taskargs = dict((x, kwargs.pop(("_%s" % x), None))
                  for x in ("countdown", "eta", "name"))
  taskargs["url"] = kwargs.pop("_url", _DEFAULT_URL)
  taskargs["headers"] = _TASKQUEUE_HEADERS
  queue = kwargs.pop("_queue", _DEFAULT_QUEUE)
  pickled = serialize(obj, *args, **kwargs)
  try:
    task = taskqueue.Task(payload=pickled, **taskargs)
    task.add(queue)
  except taskqueue.TaskTooLargeError:
    key = _DeferredTaskEntity(data=pickled).put()
    pickled = serialize(run_from_datastore, str(key))
    task = taskqueue.Task(payload=pickled, **taskargs)
    task.add(queue)


class TaskHandler(webapp.RequestHandler):
  """A webapp handler class that processes deferred invocations."""

  def post(self):
    headers = ["%s:%s" % (k, v) for k, v in self.request.headers.items()
               if k.lower().startswith("x-appengine-")]
    logging.info(", ".join(headers))

    try:
      run(self.request.body)
    except PermanentTaskFailure, e:
      logging.exception("Permanent failure attempting to execute task")


application = webapp.WSGIApplication([(".*", TaskHandler)])


def main():
  run_wsgi_app(application)


if __name__ == "__main__":
  main()