115 lines
3.9 KiB
Python
115 lines
3.9 KiB
Python
# Copyright (c) 2013 Red Hat, Inc.
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may not
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# use this file except in compliance with the License. You may obtain a copy
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# of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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# License for the specific language governing permissions and limitations under
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# the License.
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"""This module implements a common Pipeline object.
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The pipeline can be used to enhance the storage layer with filtering, routing,
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multiplexing and the like. For example:
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>>> stages = [MessageFilter(), EncryptionFilter(), QueueController()]
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>>> pipeline = Pipeline(stages)
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Every stage has to implement the method it wants to hook into. This method
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will be called when the pipeline consumption gets to that point - stage
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ordering matters - and will continue unless the method call returns a value
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that is not None.
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At least one of the stages has to implement the calling method. If none of
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them do, an AttributeError exception will be raised.
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"""
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import contextlib
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from oslo_log import log as logging
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from zaqar.common import decorators
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from zaqar.i18n import _
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LOG = logging.getLogger(__name__)
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class Pipeline(object):
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def __init__(self, pipeline=None):
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self._pipeline = pipeline and list(pipeline) or []
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@decorators.memoized_getattr
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def __getattr__(self, name):
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with self.consumer_for(name) as consumer:
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return consumer
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@contextlib.contextmanager
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def consumer_for(self, method):
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"""Creates a closure for `method`
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This method creates a closure to consume the pipeline
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for `method`.
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:params method: The method name to call on each stage
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:type method: `str`
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:returns: A callable to consume the pipeline.
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"""
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def consumer(*args, **kwargs):
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"""Consumes the pipeline for `method`
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This function walks through the pipeline and calls
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`method` for each of the items in the pipeline. A
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warning will be logged for each stage not implementing
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`method` and an Attribute error will be raised if
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none of the stages do.
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:param args: Positional arguments to pass to the call.
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:param kwargs: Keyword arguments to pass to the call.
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:raises AttributeError: if none of the stages implement `method`
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"""
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# NOTE(flaper87): Used as a way to verify
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# the requested method exists in at least
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# one of the stages, otherwise AttributeError
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# will be raised.
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target = None
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result = None
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for stage in self._pipeline:
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try:
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target = getattr(stage, method)
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except AttributeError:
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sstage = str(stage)
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msgtmpl = _(u"Stage %(stage)s does not "
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"implement %(method)s")
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LOG.debug(msgtmpl, {'stage': sstage, 'method': method})
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continue
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tmp = target(*args, **kwargs)
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# NOTE(flaper87): preserve the last, not None, result
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if tmp is not None:
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result = tmp
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# NOTE(flaper87): Will keep going forward
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# through the stageline unless the call returns
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# something.
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if result is not None:
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return result
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if target is None:
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msg = _(u'Method %s not found in any of '
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'the registered stages') % method
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LOG.error(msg)
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raise AttributeError(msg)
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yield consumer
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