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Nova contains a config variable osapi_compute_unique_server_name_scope.
Its help text describes it pretty well:

When set, compute API will consider duplicate hostnames invalid within
the specified scope, regardless of case. Should be empty, "project" or
"global".

So, by default hostnames are not unique, but depending on this setting
they could be unique either globally or in the scope of a project.

Ideally a unique constraint would be enforced by the database but,
presumably because this is a config variable, that isn't the case here.
Instead it is enforced in code, but the code which does this predictably
races. My first attempt to fix this using the obvious SQL solution
appeared to work, but actually fails in MySQL as it doesn't support that
query structure[1][2]. SQLite and PostgreSQL do support it, but they
don't support the query structure which MySQL supports. Note that this
isn't just a syntactic thing. It looks like it's still possible to do
this if we compound the workaround with a second workaround, but I'm
starting to wonder if we'd be better fixing the design.

First off, do we need this config variable? Is anybody actually using
it? I suspect the answer's going to be yes, but it would be extremely
convenient if it's not.

Assuming this configurability is required, is there any way we can
instead use it to control a unique constraint in the db at service
startup? This would be something akin to a db migration. How do we
manage those?

Related to the above, I'm not 100% clear on which services run this
code. Is it possible for different services to have a different
configuration of this variable, and does that make sense? If so, that
would preclude a unique constraint in the db.

Thanks,

Matt

[1] Which has prompted me to get the test_db_api tests running on MySQL.
See this series if you're interested:
https://review.openstack.org/#/c/156299/

[2] For specifics, see my ramblings here:
https://review.openstack.org/#/c/141115/7/nova/db/sqlalchemy/api.py,cm
line 2547
-- 
Matthew Booth
Red Hat Engineering, Virtualisation Team

Phone: +442070094448 (UK)
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