139 lines
5.2 KiB
Python
139 lines
5.2 KiB
Python
# vim: tabstop=4 shiftwidth=4 softtabstop=4
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# Copyright 2012 Red Hat, Inc.
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may
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# not use this file except in compliance with the License. You may obtain
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# a copy 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
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# under the License.
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"""
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Code for rpc message dispatching.
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Messages that come in have a version number associated with them. RPC API
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version numbers are in the form:
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Major.Minor
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For a given message with version X.Y, the receiver must be marked as able to
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handle messages of version A.B, where:
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A = X
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B >= Y
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The Major version number would be incremented for an almost completely new API.
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The Minor version number would be incremented for backwards compatible changes
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to an existing API. A backwards compatible change could be something like
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adding a new method, adding an argument to an existing method (but not
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requiring it), or changing the type for an existing argument (but still
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handling the old type as well).
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The conversion over to a versioned API must be done on both the client side and
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server side of the API at the same time. However, as the code stands today,
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there can be both versioned and unversioned APIs implemented in the same code
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base.
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EXAMPLES
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========
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Nova was the first project to use versioned rpc APIs. Consider the compute rpc
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API as an example. The client side is in nova/compute/rpcapi.py and the server
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side is in nova/compute/manager.py.
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Example 1) Adding a new method.
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-------------------------------
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Adding a new method is a backwards compatible change. It should be added to
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nova/compute/manager.py, and RPC_API_VERSION should be bumped from X.Y to
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X.Y+1. On the client side, the new method in nova/compute/rpcapi.py should
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have a specific version specified to indicate the minimum API version that must
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be implemented for the method to be supported. For example::
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def get_host_uptime(self, ctxt, host):
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topic = _compute_topic(self.topic, ctxt, host, None)
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return self.call(ctxt, self.make_msg('get_host_uptime'), topic,
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version='1.1')
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In this case, version '1.1' is the first version that supported the
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get_host_uptime() method.
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Example 2) Adding a new parameter.
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----------------------------------
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Adding a new parameter to an rpc method can be made backwards compatible. The
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RPC_API_VERSION on the server side (nova/compute/manager.py) should be bumped.
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The implementation of the method must not expect the parameter to be present.::
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def some_remote_method(self, arg1, arg2, newarg=None):
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# The code needs to deal with newarg=None for cases
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# where an older client sends a message without it.
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pass
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On the client side, the same changes should be made as in example 1. The
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minimum version that supports the new parameter should be specified.
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"""
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from quantum.openstack.common.rpc import common as rpc_common
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class RpcDispatcher(object):
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"""Dispatch rpc messages according to the requested API version.
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This class can be used as the top level 'manager' for a service. It
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contains a list of underlying managers that have an API_VERSION attribute.
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"""
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def __init__(self, callbacks):
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"""Initialize the rpc dispatcher.
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:param callbacks: List of proxy objects that are an instance
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of a class with rpc methods exposed. Each proxy
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object should have an RPC_API_VERSION attribute.
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"""
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self.callbacks = callbacks
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super(RpcDispatcher, self).__init__()
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def dispatch(self, ctxt, version, method, **kwargs):
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"""Dispatch a message based on a requested version.
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:param ctxt: The request context
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:param version: The requested API version from the incoming message
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:param method: The method requested to be called by the incoming
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message.
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:param kwargs: A dict of keyword arguments to be passed to the method.
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:returns: Whatever is returned by the underlying method that gets
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called.
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"""
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if not version:
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version = '1.0'
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had_compatible = False
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for proxyobj in self.callbacks:
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if hasattr(proxyobj, 'RPC_API_VERSION'):
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rpc_api_version = proxyobj.RPC_API_VERSION
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else:
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rpc_api_version = '1.0'
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is_compatible = rpc_common.version_is_compatible(rpc_api_version,
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version)
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had_compatible = had_compatible or is_compatible
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if not hasattr(proxyobj, method):
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continue
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if is_compatible:
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return getattr(proxyobj, method)(ctxt, **kwargs)
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if had_compatible:
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raise AttributeError("No such RPC function '%s'" % method)
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else:
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raise rpc_common.UnsupportedRpcVersion(version=version)
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