oslo.messaging: remove ending message for rpc reply


We are going to send a single message for RPC reply.

Problem description

Currently, when we wait for a RPC reply, for each msg_id we receive two AMQP messages - first one with the payload, a second one to ensure the other have finish to send the payload. This was made because a long time ago ‘reply’ allowed generator as payload to send multiple messages on one ‘rpc.call’ - see [1].

Oslo.messaging do not support providing a generator as the payload - it send reply with data and then reply with ending - so it becomes useless to double RPC messages for each call. Based on this suggestions, so we are going to remove the second AMQP message sending.

This change will be not backward compatible, so we have to choice how we handle this backward compatibility and the deprecation. This spec is the proposed change about that.

Proposed change

This change is not backward compatible, so we need to make sure that there is some way to run mixed versions of services, where version N can always talk to N+1 and the reverse, so that we can do rolling upgrades.

Plan for the rolling upgrade:

In Kilo:

The old behavior, fully compatible with Liberty and older versions.

RPC-server sends reply in two messages, can reply to clients, versions <= L+1. RPC-client expects replies in two messages, can receive replies from servers with versions <= L.

In Liberty:

We are going to implement a new behavior, but keep the old one by default. Fully compatible with Kilo and older versions.

We change the ReplyWaiters to handle reply in one message and two messages, like it does [2].

We create a boolean configuration option, defaulted to False. If this one is True, we sent the reply in one message otherwise we keep the current behavior of two messages. This config option will allow us to test the both - old and new - behavior into our tests suite and allow to enable this new behavior earlier. So this is only dedicated for early adopter and testing. This options is not need in normal upgrade workflow and must be removed as soon as we enable the new behavior by default to be sure that deployed cloud with L+1 will not break when they are upgraded to L+2 because cloud operator enforced the old behavior.

RPC-server sends reply in two messages, can reply to <= L+1 RPC-client expects replies in two messages or one message, can receive replies from all versions

In M release cycle:

We are going to enable the new behavior by default, remove the config option and support only the reply in one message. This will break backward compatibility with oslo.messaging <= kilo and oslo-incubator RPC legacy code.

rpc-server sends reply in one messages, can reply to >= L rpc-client expects replies in two messages or one messages from all versions

In N release cycle:

We are going to remove legacy code that allow to receive replies from <= L.

rpc-server sends reply in one messages, can reply to >= L+1 rpc-client expects replies in one messages, can receive replies from >= L+1


Using the oslo.messaging payload version. But this have been designed for the content of the message itself. Not really for this purpose. And the deserialization occurs in the lower layer of oslo.messaging. When we handle the reply the version fields have already been removed.

This breaks backward compatibility too.

We can already track the old and the new format because the old format has the attribute “result” OR “ending” the new one will have “result” AND “ending”.

Note that issue is in the RPC call replies code. In case of rolling upgrade, we have to think about the fact that the client will wait message that can come from same or upper version of oslo.messaging. We already do not support lower versions from the application PoV.

Or from the server point of view, we will send reply that must be understandable by a wide panel of versions.

This is the first time (I guess), we encounter this kind of issue, some other bugs need to break the backward compatibility to be fixed, too. (Because we need to change RabbitMQ queue attributes or move a queue to another exchange)

The main goal is to choose the backward compatible versions count and use the same kind of deprecation in other changes like this one.

Impact on Existing APIs


Security impact


Performance Impact

This will reduce by 2 the number of reply messages that will transit on a RabbitMQ/QPID cluster. Local performance tests shows, that this change brings nearly 30 percent increase in the number of RPC call messages per second.

Configuration Impact

A hidden configuration option will allow to switch to the future behavior for early adopter and for testing purpose.

Developer Impact


Testing Impact

We must test that the new ReplyWaiters code can handle reply that come from a oslo.messaging version that sent reply in one message and from the one that sent the reply into two messages. This is driver specific change, so we should implement this in unittests.



Primary assignee:
Other contributors:


Target Milestone for completion:
Liberty for the step 1 M or N for the step 2

Work Items

  1. Change the ReplyWaiters to handle reply in one message and two messages.
  2. Add a config option and change the _send_reply() method to allow sent reply and ending in a single message, based on config option.
  3. Remove the config option and enable the new behavior by default.
  4. Remove the ending parameter procession from the ReplyWaiters.



Documentation Impact

Inform deployer about future incompatibility with a too old oslo.messaging version