Implement the v2.1 API on the V3 API codebase

Implement v2 compatible API based on v3 API infrastructure.

Problem description

On v3 API development, we have improved API infrastructure such as API plugin loading, input validation, policy check, etc. In addition, to fix inconsistent interfaces of v2 API, we have made a significant number of backwards incompatible changes of the Nova API (Change success status codes, API attribute names, and API URLs). There is a comprehensive description of the problems with the v2 API for users, operators and developers here:

However, there have been intensive discussions over the future of Nova and the maintenance overhead implications from having to support two APIs such as v2 and v3 simultaneously for a long period of time.

Proposed change

Through a lot of discussions, we have understood the advantages of v3 API infrastructure (API plugin loading, input validation, policy check, etc). However, their backwards incompatible interfaces seem a little premature at this time, because now we aren’t sure that current v3 API is the best. That means we cannot be sure that any more backwards incompatible changes are unnecessary even if switching to current v3 API.

This spec proposes the removal of backwards incompatible changes from v3 code. That means current v3 consistent interfaces would go back to v2 inconsistent ones like:

--- a/nova/api/openstack/compute/plugins/v3/
+++ b/nova/api/openstack/compute/plugins/v3/
@@ -752,7 +752,7 @@ class ServersController(wsgi.Controller):
The field image_ref is mandatory when no block devices have been
defined and must be a proper uuid when present.
- image_href = server_dict.get('image_ref')
+ image_href = server_dict.get('imageRef')

This proposal is painful for v3 API developers because they have worked hard to make consistent interfaces over a year and v3 interfaces are exactly better than v2 ones. However, through the discussions, we have known that backwards incompatible changes are very painful for users and we must pay attention to these changes.

On this spec, we would provide v2 compatible API with the other v3 advantages as the first step. After this spec, we will provide consistent interfaces by defining API rules step by step. These rules will prevent the same backwards incompatible changes and keep consistent interfaces even if adding a lot of new APIs in the future. However, that is out of scope from this spec now.

It is also agreed that we wont implement proxies for other OpenStack APIs such as glance, cinder or neutron as part of the initial v2.1 implementation. These will instead be added later, but before the removal of the original v2 code.


Through these discussions, we got an idea that we could support both v2 API and v3 API on the top of the v3 API codebase. On this idea, nova translates a v2 request to v3 request and passes it to v3 API method. After v3 API method operation, nova translates its v3 response to v2 response again and returns it to a client. However, there was an intensive discussion against this idea also because it would be difficult to debug API problems due to many translations when we have a lot of backwards incompatible changes in the long term.

Data model impact


REST API impact

The V2.1 REST API presented will be identical to the V2 API except as noted above.

Note however that V2.1 will not support the XML version of the V2 API, only the JSON one. However the XML version of the V2 API is currently marked as deprecated.

Security impact

Better up front input validation will reduce the ability for malicious user input to exploit security bugs.

Notifications impact


Other end user impact

Potentially it may be advantageous if python novaclient could talk to /v2.1 instead of /v2 but code changes may not be required to change this. It may be simpler just to do this through keystone configuration. The API itself remains identical.

Performance Impact

More stringent input validation also means more work that is needed to be done in the API layer but overall this is a good thing.

Other deployer impact

If the deployer wanted to export the API as /v2 rather than /v2.1 then they would need to modify the api-paste.ini file (a couple of line change to disable the original V2 API and use the APIRouterV21 as the /v2 API.

The long term goal would be to deprecate and eventually remove the original V2 API code when deployers and users are satisfied that v2.1 satisfies their requirements.

The process which we would use is:

  • V2.1 fully implemented with Tempest verification (including extra verification that is being added in terms of response data)
  • Verification from multiple sources (cloud providers, users etc) that V2.1 is compatible with V2
    • This could be done in various ways
      • Keystone changes so /v2.1 is advertised instead of /v2
      • Exporting the V2.1 as /v2
      • Combined with the possibility of putting V2.1 input validation into a log rather than reject mode.
  • V2.1 is in an openstack release for N versions
  • After widespread confirmation that the V2.1 API is compatible, V2 would be marked as deprecated

Developer impact

Long term advantages for developers are:

  • All the API implementations are on the new API framework
  • Reduction in maintenance overhead of supporting two major API versions
  • Have a better framework for handling future backwards incompatible changes.

In the short term while the old V2 API code exists there will still be a dual maintenance overhead.



Primary assignee:
Other contributors:
oomichi Alex Xu

Work Items

  • Change v3 success status codes to v2 ones.
  • Change v3 API routings to v2 ones.
    • Handle API URLs include a project id.
    • Change the API resource paths. (e.g: /keypairs(v3) -> /os-keypairs(v2))
    • Change action names. (e.g: migrate_live(v3) -> os-migrateLive(v2))
  • Change v3 API attribute names to v2 ones.
    • Change the API parsers of v3 code.
    • Change the API schemas of input validation.
  • Change v3 API behaviors to v2 ones. On some APIs, there are different behaviors. For example, v3 “create a private flavor” API adds a flavor access for its own project automatically, but v2 one doesn’t.

The following work item is not mandatory and it is one of wishlist.

  • Change v3 plugin code path. e.g:

    -> nova/api/openstack/compute/plugins/




Tempest has already contained a lot of v2 API tests, and that is a good test coverage now. For this v2.1 API, we need to run v2 API tests for both current v2 and v2.1 in parallel. As an idea, we will add v2.1 API tests by inheriting from the existing v2 API test classes and executing them against /v2.1. A spec for this idea has been already proposed:

Documentation Impact

The documentation for the v2 API will essentially remain the same as the API will not change except for improvements in input validation. There will need to be some updates on possible error status codes.

Longer term the improved infrastructure for input validation and the development of JSON schema for response validation will make it much easier to automate the generation of documentation for v2 rather relying on the current mostly manual process.