1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
|
..
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.
Convention for heading levels in Neutron devref:
======= Heading 0 (reserved for the title in a document)
------- Heading 1
~~~~~~~ Heading 2
+++++++ Heading 3
''''''' Heading 4
(Avoid deeper levels because they do not render well.)
Neutron RPC API Layer
=====================
Neutron uses the oslo.messaging library to provide an internal communication
channel between Neutron services. This communication is typically done via
AMQP, but those details are mostly hidden by the use of oslo.messaging and it
could be some other protocol in the future.
RPC APIs are defined in Neutron in two parts: client side and server side.
Client Side
-----------
Here is an example of an rpc client definition:
::
import oslo_messaging
from neutron_lib import rpc as n_rpc
class ClientAPI(object):
"""Client side RPC interface definition.
API version history:
1.0 - Initial version
1.1 - Added my_remote_method_2
"""
def __init__(self, topic):
target = oslo_messaging.Target(topic=topic, version='1.0')
self.client = n_rpc.get_client(target)
def my_remote_method(self, context, arg1, arg2):
cctxt = self.client.prepare()
return cctxt.call(context, 'my_remote_method', arg1=arg1, arg2=arg2)
def my_remote_method_2(self, context, arg1):
cctxt = self.client.prepare(version='1.1')
return cctxt.call(context, 'my_remote_method_2', arg1=arg1)
This class defines the client side interface for an rpc API. The interface has
2 methods. The first method existed in version 1.0 of the interface. The
second method was added in version 1.1. When the newer method is called, it
specifies that the remote side must implement at least version 1.1 to handle
this request.
Server Side
-----------
The server side of an rpc interface looks like this:
::
import oslo_messaging
class ServerAPI(object):
target = oslo_messaging.Target(version='1.1')
def my_remote_method(self, context, arg1, arg2):
return 'foo'
def my_remote_method_2(self, context, arg1):
return 'bar'
This class implements the server side of the interface. The
oslo_messaging.Target() defined says that this class currently implements
version 1.1 of the interface.
.. _rpc_versioning:
Versioning
----------
Note that changes to rpc interfaces must always be done in a backwards
compatible way. The server side should always be able to handle older clients
(within the same major version series, such as 1.X).
It is possible to bump the major version number and drop some code only needed
for backwards compatibility. For more information about how to do that, see
https://wiki.openstack.org/wiki/RpcMajorVersionUpdates.
Example Change
~~~~~~~~~~~~~~
As an example minor API change, let's assume we want to add a new parameter to
my_remote_method_2. First, we add the argument on the server side. To be
backwards compatible, the new argument must have a default value set so that the
interface will still work even if the argument is not supplied. Also, the
interface's minor version number must be incremented. So, the new server side
code would look like this:
::
import oslo_messaging
class ServerAPI(object):
target = oslo_messaging.Target(version='1.2')
def my_remote_method(self, context, arg1, arg2):
return 'foo'
def my_remote_method_2(self, context, arg1, arg2=None):
if not arg2:
# Deal with the fact that arg2 was not specified if needed.
return 'bar'
We can now update the client side to pass the new argument. The client must
also specify that version '1.2' is required for this method call to be
successful. The updated client side would look like this:
::
import oslo_messaging
from neutron.common import rpc as n_rpc
class ClientAPI(object):
"""Client side RPC interface definition.
API version history:
1.0 - Initial version
1.1 - Added my_remote_method_2
1.2 - Added arg2 to my_remote_method_2
"""
def __init__(self, topic):
target = oslo_messaging.Target(topic=topic, version='1.0')
self.client = n_rpc.get_client(target)
def my_remote_method(self, context, arg1, arg2):
cctxt = self.client.prepare()
return cctxt.call(context, 'my_remote_method', arg1=arg1, arg2=arg2)
def my_remote_method_2(self, context, arg1, arg2):
cctxt = self.client.prepare(version='1.2')
return cctxt.call(context, 'my_remote_method_2',
arg1=arg1, arg2=arg2)
More Info
---------
For more information, see the oslo.messaging documentation:
https://docs.openstack.org/oslo.messaging/latest/.
|
