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Hello,

I wanted to start discussion about versioned objects backporting for conductor-less projects.
In Vancouver we discussed compatibility mode, which works like that:

1. We define one version for every object we use, this means adding base object, for example:

Class HeatObject:
	VERSION=1.5

All objects inherit from this one, each time we change it, we bump up this variable.

2. We introduce new config option in heat.conf 

Combatibility_mode = 1.4

3. Implement mechanism which will automatically backport each outgoing message to 1.4 as long as we have this var.

Upgrade flow looks like that:
1. We have all nodes using 1.4 version
2. We incrementally run new version with 1.4 compatibility mode
3. When all nodes are already up-to-date, we incrementally turn off compatibility mode

This solution has one rather big disadvantage: 2 restarts.
This can be mitigated by adding another call to reread config without restart. Oslo.config has this capability, but we need to add call to run it.

Thoughts?

Regards,
Michał Jastrzębski