This new quota driver, ``DbQuotaNoLockDriver``, does not create a lock
per (resource, project_id) but retrieves the instant (resource,
project_id) usage and the current (resource, project_id) reservations.
If the requested number of resources fit the available quota, a new
``Reservation`` register is created with the amount of units requested.
All those operations are done inside a DB transaction context. That
means the amount of resources and reservations is guaranteed inside
this transaction (depending on the DB backend isolation level defined)
and the new reservation created will not clash with other DB transation.
That will guarantee the number of resources and instant reservations
never exceed the quota limits defined for this (resource, project_id).
NOTES:
- This change tries to be as unobtrusive as possible. The new driver
uses the same ``DbQuotaDriver`` dabatase tables (except for
``QuotaUsage``) and the same Quota engine API, located in
``neutron.quota``. However, the Quota engine resources implements some
particular API actions like "dirty", that are not used in the new
driver.
- The Pecan Quota enforcement hooks,
``neutron.pecan_wgsi.hooks.quota_enforcement``, execute actions like
"resync", "mark_resources_dirty" or "set_resources_dirty", that has
no meaning in the new driver.
- The isolation between the Quota engine and the Pecan hook, and the
driver itself is not clearly defined. A refactor of the Quota engine,
Quota service, Quota drivers and a common API between the driver and
the engine is needed.
- If ``DbQuotaDriver`` is deprecated, ``CountableResource`` and
``TrackedResource`` will be joined in a single class. This resource
class will have a count method (countable) or a hard dependency on a
database table (tracked resource). The only difference will be the
"count" method implementation.
Closes-Bug: #1926787
Change-Id: I4f98c6fcd781459fd7150aff426d19c7fdfa98c1
e135a8221d