openstack/nova
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[placement] Add a bit about extraction plans to placement_dev

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While we've not made any immediate plans to do the extraction we
should avoid adding complexity that would make it harder later, so
this new section discusses the plan to eventually extract, and the
structures that either help or hinder this.

Change-Id: Ia3e95e4b85aa768b8f94d4a99963c7ec719b8a13
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Chris Dent 2017-01-04 17:20:31 +00:00 committed by Matt Riedemann
parent 88a456b189
commit 9dd48cf4c8
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doc/source/placement_dev.rst
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@ -318,7 +318,40 @@ example in devstack. See `gabbi-run`_ to get started.
Futures
=======
.. TODO(cdent) extraction to own thing plans
Since before it was created there has been a long term goal for the placement
service to be extracted to its own repository and operate as its own
independent service. There are many reasons for this, but two main ones are:
* Multiple projects, not just nova, will eventually need to manage resource
providers using the placement API.
* A separate service helps to maintain and preserve a strong contract between
the placement service and the consumers of the service.
To lessen the pain of the eventual extraction of placement the service has been
developed in a way to limit dependency on the rest of the nova codebase and be
self-contained:
* Most code is in `nova/api/openstack/placement` except for oslo versioned
object code in `nova/objects/resource_provider.py`.
* Database query code is kept within the objects.
* The methods on the objects are not remotable, as the only intended caller is
the placement API code.
There are some exceptions to the self-contained rule (which will have to be
addressed if the extraction ever happens):
* Exceptions unique to the placement API are still within the `nova.exceptions`
package.
* Code related to a resource class cache is within the `nova.db` package.
* Database models, migrations and tables use the nova api database.
* The nova `FaultWrapper` middleware is being used.
* `nova.i18n` package provides the ``_`` and related functions.
* `nova.conf` is used for configuration.
* Unit and functional tests depend on fixtures and other functionality in base
classes provided by nova.
When creating new code for the placement service, please be aware of the plan
for an eventual extraction and avoid creating unnecessary interdependencies.
.. _WSGI: https://www.python.org/dev/peps/pep-3333/
.. _versioned objects: http://docs.openstack.org/developer/oslo.versionedobjects/