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Placement Container Playground 2

This is a brief followup to placement container playground, wherein I've been using sticking placement into a container with the smallest footprint as a way of exploring the issues involved with extracting placement from nova.

I have two series that experiment with changes that could reduce the footprint:

But it turns out that neither of these are enough when integrating them into the container.

A newly revealed challenge is that when importing nova.conf (to access the global oslo_config) all the configuration is imported, leading to lots of things needing to import their own requirements. So, for example, conf/ imports castellan.

Placement doesn't need that.

Then, importing the db_api imports the cells_rpcapi, which eventually leads to nova/objects/ being imported, which wants cursive.

cells_rpcapi also imports the network_model which eventually leads to nova.utils, which wants os_service_types.

Placement doesn't need that either.

Placement only uses the db_api module to create a context manager that provides a database session. It's possible to isolate this code. A WIP now exists to Isolate the placement database config.

With that in place we hit the next roadblock: The database model/schema files are in the nova.db package and in there does this: from nova.db.api import *. Which means the cells_rpcapi stuff mentioned still gets imported. Argh!

It might be an option to move the model files into a different place. Changing the nova/db/ would lead to changes in many other files, but might be worth it as importing any code in nova/db will trigger the code in

There will probably be a -3 followup to this, with more steps forwards and more steps back.

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