Use oslo-versioned-objects to help with dealing with upgrades.

Include the URL of your launchpad blueprint:

https://blueprints.launchpad.net/heat/+spec/versioned-objects

Problem description

We are looking to improve the way we deal with versioning (of all sorts db/rpc/rest/templates/plugins). Nova has come up with the idea of versioned objects, that Ironic has also now used. This has now been proposed as an oslo library: https://review.openstack.org/#/c/127532/

https://etherpad.openstack.org/p/kilo-crossproject-upgrades-and-versioning

Versioned-objects will help us deal with DB schema being at a different version than the code expects. This will allow Heat to be operated safely during upgrades.

Looking forward as we pass more and more data over RPC we can make use of versioned-objects to ensure upgrades happen without spreading the version dependant code across the code base.

Proposed change

Since it will take some time before versioned-objects goes into the oslo library, the plan is to get an early version of it for Heat and transition to oslo-versioned-objects when it is ready.

Create a directory heat/objects/ that will contain wrapper objects that are a layer above the db objects. This allows the remainder of Heat to not having to worry about dealing with older DB objects.

Once the objects are in place the rest of the code will be changed to use the versioned objects instead of the db_api directly. This can be done object-by-object to avoid overly large changes.

Alternatives

Data model impact

None. The objects being introduced are not stored in the database. Instead, these objects are a replacement for sqlalchemy objects that is being used to represent stack, resource, etc throughout Heat internals.

Developer impact

It will take some time to convert heat internals over to the object model, so the existing convention of direct database calls should be accepted until all object models are in place.

Implementation

Assignee(s)

Primary assignee:
Angus Salkeld <asalkeld@mirantis.com> <others are welcome to help out>

Milestones

Target Milestone for completion:
Kilo-2

Work Items

  • (If needed) Obtain an early version of oslo.versionedobjects.
  • Implement the objects for each DB object type we have.
  • Update code that uses the DB to use versioned-objects instead.
  • Write some developer docs on how to deal with older schema.
  • Transition to oslo-versioned-objects as soon as it is available.

Dependencies

  • oslo-versioned-objects