you may confusing about the context

无论我们看openstack的哪个模块，在很多调用的函数参数中总能看到context的身影，context顾名思义是上下文，在调用时除了self，恐怕见到最多的是这个词，

很多时候并不知道context是什么，更不知道怎么用，从nova进去看看：

以/v2 rest 为例，

[composite:openstack_compute_api_v2]
use = call:nova.api.auth:pipeline_factory
noauth = compute_req_id faultwrap sizelimit noauth ratelimit osapi_compute_app_v2
keystone = compute_req_id faultwrap sizelimit authtoken keystonecontext ratelimit osapi_compute_app_v2
keystone_nolimit = compute_req_id faultwrap sizelimit authtoken keystonecontext osapi_compute_app_v2

这个keystonecontext作为一个middleware，定义在：

[filter:keystonecontext]
paste.filter_factory = nova.api.auth:NovaKeystoneContext.factory

作为一个middleware继承自wsgi.Middleware

class NovaKeystoneContext(wsgi.Middleware):

和其他middle一样，NovaKeystoneContext实现__call__函数：

        ctx = context.RequestContext(user_id,
                                     project_id,
                                     user_name=user_name,
                                     project_name=project_name,
                                     roles=roles,
                                     auth_token=auth_token,
                                     remote_address=remote_address,  #可以在request的header中指定，或者client时生成
                                     service_catalog=service_catalog,
                                     request_id=req_id)

        req.environ['nova.context'] = ctx
        return self.application

在controller中，执行具体相应时候，参数中有req，这时要使用时从req中取出即可

其他的模块都有类似的，比如keystone中：

[pipeline:admin_api]
# The last item in this pipeline must be admin_service or an equivalent
# application. It cannot be a filter.
pipeline = sizelimit url_normalize build_auth_context token_auth admin_token_auth xml_body_v2 json_body ec2_extension s3_extension crud_extension admin_service

[filter:build_auth_context]
paste.filter_factory = keystone.middleware:AuthContextMiddleware.factory

在process_request中：

request.environ[authorization.AUTH_CONTEXT_ENV] = auth_context

还有neutron中：

[composite:neutronapi_v2_0]
use = call:neutron.auth:pipeline_factory
noauth = request_id catch_errors extensions neutronapiapp_v2_0
keystone = request_id catch_errors authtoken keystonecontext extensions neutronapiapp_v2_0

[filter:keystonecontext]
paste.filter_factory = neutron.auth:NeutronKeystoneContext.factory

        ctx = context.Context(user_id, tenant_id, roles=roles,
                              user_name=user_name, tenant_name=tenant_name,
                              request_id=req_id, auth_token=auth_token)

        # Inject the context...
        req.environ['neutron.context'] = ctx

        return self.application

和nova一样，在controller中可以使用

知道context是什么，有什么，再看起来就更明白了