The oslo.messaging library takes the existing RPC code from oslo and
wraps it in a sane API with well defined semantics around which we can
make a commitment to retain compatibility in future.
The patch is large, but the changes can be summarized as:
* oslo.messaging>=1.3.0a4 is required; a proper 1.3.0 release will be
pushed before the icehouse release candidates.
* The new rpc module has init() and cleanup() methods which manage the
global oslo.messaging transport state. The TRANSPORT and NOTIFIER
globals are conceptually similar to the current RPCIMPL global,
except we're free to create and use alternate Transport objects
in e.g. the cells code.
* The rpc.get_{client,server,notifier}() methods are just helpers
which wrap the global messaging state, specifiy serializers and
specify the use of the eventlet executor.
* In oslo.messaging, a request context is expected to be a dict so
we add a RequestContextSerializer which can serialize to and from
dicts using RequestContext.{to,from}_dict()
* The allowed_rpc_exception_modules configuration option is replaced
by an allowed_remote_exmods get_transport() parameter. This is not
something that users ever need to configure, but it is something
each project using oslo.messaging needs to be able to customize.
* The nova.rpcclient module is removed; it was only a helper class
to allow us split a lot of the more tedious changes out of this
patch.
* Finalizing the port from RpcProxy to RPCClient is straightforward.
We put the default topic, version and namespace into a Target and
contstruct the client using that.
* Porting endpoint classes (like ComputeManager) just involves setting
a target attribute on the class.
* The @client_exceptions() decorator has been renamed to
@expected_exceptions since it's used on the server side to designate
exceptions we expect the decorated method to raise.
* We maintain a global NOTIFIER object and create specializations of
it with specific publisher IDs in order to avoid notification driver
loading overhead.
* rpc.py contains transport aliases for backwards compatibility
purposes. setup.cfg also contains notification driver aliases for
backwards compat.
* The messaging options are moved about in nova.conf.sample because
the options are advertised via a oslo.config.opts entry point and
picked up by the generator.
* We use messaging.ConfFixture in tests to override oslo.messaging
config options, rather than making assumptions about the options
registered by the library.
The porting of cells code is particularly tricky:
* messaging.TransportURL parse() and str() replaces the
[un]parse_transport_url() methods. Note the complication that an
oslo.messaging transport URL can actually have multiple hosts in
order to support message broker clustering. Also the complication
of transport aliases in rpc.get_transport_url().
* proxy_rpc_to_manager() is fairly nasty. Right now, we're proxying
the on-the-wire message format over this call, but you can't supply
such messages to oslo.messaging's cast()/call() methods. Rather than
change the inter-cell RPC API to suit oslo.messaging, we instead
just unpack the topic, server, method and args from the message on
the remote side.
cells_api.RPCClientCellsProxy is a mock RPCClient implementation
which allows us to wrap up a RPC in the message format currently
used for inter-cell RPCs.
* Similarly, proxy_rpc_to_manager uses the on-the-wire format for
exception serialization, but this format is an implementation detail
of oslo.messaging's transport drivers. So, we need to duplicate the
exception serialization code in cells.messaging. We may find a way
to reconcile this in future - for example a ExceptionSerializer
class might work, but with the current format it might be difficult
for the deserializer to generically detect a serialized exception.
* CellsRPCDriver.start_servers() and InterCellRPCAPI._get_client()
need close review, but they're pretty straightforward ports of code
to listen on some specialized topics and connect to a remote cell
using its transport URL.
blueprint: oslo-messaging
Change-Id: Ib613e6300f2c215be90f924afbd223a3da053a69
1a91aacb85