base_handler.py

#!/usr/bin/env python
# Copyright 2010 Google Inc. All Rights Reserved.
#
# 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.

"""Base handler class for all mapreduce handlers."""



# pylint: disable=protected-access
# pylint: disable=g-bad-name
# pylint: disable=g-import-not-at-top

import httplib
import logging

try:
  import json
except ImportError:
  import simplejson as json

try:
  from mapreduce import pipeline_base
except ImportError:
  pipeline_base = None
try:
  # Check if the full cloudstorage package exists. The stub part is in runtime.
  import cloudstorage
  if hasattr(cloudstorage, "_STUB"):
    cloudstorage = None
except ImportError:
  cloudstorage = None

from google.appengine.ext import webapp
from mapreduce import errors
from mapreduce import json_util
from mapreduce import model
from mapreduce import parameters


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


class BadRequestPathError(Error):
  """The request path for the handler is invalid."""


class TaskQueueHandler(webapp.RequestHandler):
  """Base class for handlers intended to be run only from the task queue.

  Sub-classes should implement
  1. the 'handle' method for all POST request.
  2. '_preprocess' method for decoding or validations before handle.
  3. '_drop_gracefully' method if _preprocess fails and the task has to
     be dropped.

  In Python27 runtime, webapp2 will automatically replace webapp.
  """

  _DEFAULT_USER_AGENT = "App Engine Python MR"

  def __init__(self, *args, **kwargs):
    # webapp framework invokes initialize after __init__.
    # webapp2 framework invokes initialize within __init__.
    # Python27 runtime swap webapp with webapp2 underneath us.
    # Since initialize will conditionally change this field,
    # it needs to be set before calling super's __init__.
    self._preprocess_success = False
    super(TaskQueueHandler, self).__init__(*args, **kwargs)
    if cloudstorage:
      cloudstorage.set_default_retry_params(
          cloudstorage.RetryParams(
              min_retries=5,
              max_retries=10,
              urlfetch_timeout=parameters._GCS_URLFETCH_TIMEOUT_SEC,
              save_access_token=True,
              _user_agent=self._DEFAULT_USER_AGENT))

  def initialize(self, request, response):
    """Initialize.

    1. call webapp init.
    2. check request is indeed from taskqueue.
    3. check the task has not been retried too many times.
    4. run handler specific processing logic.
    5. run error handling logic if precessing failed.

    Args:
      request: a webapp.Request instance.
      response: a webapp.Response instance.
    """
    super(TaskQueueHandler, self).initialize(request, response)

    # Check request is from taskqueue.
    if "X-AppEngine-QueueName" not in self.request.headers:
      logging.error(self.request.headers)
      logging.error("Task queue handler received non-task queue request")
      self.response.set_status(
          403, message="Task queue handler received non-task queue request")
      return

    # Check task has not been retried too many times.
    if self.task_retry_count() + 1 > parameters.config.TASK_MAX_ATTEMPTS:
      logging.error(
          "Task %s has been attempted %s times. Dropping it permanently.",
          self.request.headers["X-AppEngine-TaskName"],
          self.task_retry_count() + 1)
      self._drop_gracefully()
      return

    try:
      self._preprocess()
      self._preprocess_success = True
    # pylint: disable=bare-except
    except:
      self._preprocess_success = False
      logging.error(
          "Preprocess task %s failed. Dropping it permanently.",
          self.request.headers["X-AppEngine-TaskName"])
      self._drop_gracefully()

  def post(self):
    if self._preprocess_success:
      self.handle()

  def handle(self):
    """To be implemented by subclasses."""
    raise NotImplementedError()

  def _preprocess(self):
    """Preprocess.

    This method is called after webapp initialization code has been run
    successfully. It can thus access self.request, self.response and so on.
    """
    pass

  def _drop_gracefully(self):
    """Drop task gracefully.

    When preprocess failed, this method is called before the task is dropped.
    """
    pass

  def task_retry_count(self):
    """Number of times this task has been retried."""
    return int(self.request.headers.get("X-AppEngine-TaskExecutionCount", 0))

  def retry_task(self):
    """Ask taskqueue to retry this task.

    Even though raising an exception can cause a task retry, it
    will flood logs with highly visible ERROR logs. Handlers should uses
    this method to perform controlled task retries. Only raise exceptions
    for those deserve ERROR log entries.
    """
    self.response.set_status(httplib.SERVICE_UNAVAILABLE, "Retry task")
    self.response.clear()


class JsonHandler(webapp.RequestHandler):
  """Base class for JSON handlers for user interface.

  Sub-classes should implement the 'handle' method. They should put their
  response data in the 'self.json_response' dictionary. Any exceptions raised
  by the sub-class implementation will be sent in a JSON response with the
  name of the error_class and the error_message.
  """

  def __init__(self, *args):
    """Initializer."""
    super(JsonHandler, self).__init__(*args)
    self.json_response = {}

  def base_path(self):
    """Base path for all mapreduce-related urls.

    JSON handlers are mapped to /base_path/command/command_name thus they
    require special treatment.

    Raises:
      BadRequestPathError: if the path does not end with "/command".

    Returns:
      The base path.
    """
    path = self.request.path
    base_path = path[:path.rfind("/")]
    if not base_path.endswith("/command"):
      raise BadRequestPathError(
          "Json handlers should have /command path prefix")
    return base_path[:base_path.rfind("/")]

  def _handle_wrapper(self):
    """The helper method for handling JSON Post and Get requests."""
    if self.request.headers.get("X-Requested-With") != "XMLHttpRequest":
      logging.error("Got JSON request with no X-Requested-With header")
      self.response.set_status(
          403, message="Got JSON request with no X-Requested-With header")
      return

    self.json_response.clear()
    try:
      self.handle()
    except errors.MissingYamlError:
      logging.debug("Could not find 'mapreduce.yaml' file.")
      self.json_response.clear()
      self.json_response["error_class"] = "Notice"
      self.json_response["error_message"] = "Could not find 'mapreduce.yaml'"
    except Exception, e:
      logging.exception("Error in JsonHandler, returning exception.")
      # TODO(user): Include full traceback here for the end-user.
      self.json_response.clear()
      self.json_response["error_class"] = e.__class__.__name__
      self.json_response["error_message"] = str(e)

    self.response.headers["Content-Type"] = "text/javascript"
    try:
      output = json.dumps(self.json_response, cls=json_util.JsonEncoder)
    # pylint: disable=broad-except
    except Exception, e:
      logging.exception("Could not serialize to JSON")
      self.response.set_status(500, message="Could not serialize to JSON")
      return
    else:
      self.response.out.write(output)

  def handle(self):
    """To be implemented by sub-classes."""
    raise NotImplementedError()


class PostJsonHandler(JsonHandler):
  """JSON handler that accepts POST requests."""

  def post(self):
    self._handle_wrapper()


class GetJsonHandler(JsonHandler):
  """JSON handler that accepts GET posts."""

  def get(self):
    self._handle_wrapper()


class HugeTaskHandler(TaskQueueHandler):
  """Base handler for processing HugeTasks."""

  class _RequestWrapper(object):
    """Container of a request and associated parameters."""

    def __init__(self, request):
      self._request = request
      self._params = model.HugeTask.decode_payload(request)

    def get(self, name, default=""):
      return self._params.get(name, default)

    def set(self, name, value):
      self._params[name] = value

    def __getattr__(self, name):
      return getattr(self._request, name)

  def __init__(self, *args, **kwargs):
    super(HugeTaskHandler, self).__init__(*args, **kwargs)

  def _preprocess(self):
    self.request = self._RequestWrapper(self.request)


if pipeline_base:
  # For backward compatiblity.
  PipelineBase = pipeline_base.PipelineBase
else:
  PipelineBase = None