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Hello,

After discussions, we've spotted possible gap in versioned objects: backporting of too-new versions in RPC.
Nova does that by conductor, but not every service has something like that. I want to propose another approach:

1. Milestone pinning - we need to make single reference to versions of various objects - for example heat in version 15.1 will mean stack in version 1.1 and resource in version 1.5.
2. Compatibility mode - this will add flag to service --compatibility=15.1, that will mean that every outgoing RPC communication will be backported before sending to object versions bound to this milestone.

With this 2 things landed we'll achieve rolling upgrade like that:
1. We have N nodes in version V
2. We take down 1 node and upgrade code to version V+1
3. Run code in ver V+1 with --compatibility=V
4. Repeat 2 and 3 until every node will have version V+1
5. Restart each service without compatibility flag

This approach has one big disadvantage - 2 restarts required, but should solve problem of backporting of too-new versions.
Any ideas? Alternatives?

Regards,
Michał