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Nova Stable REST API
This document describes both the current state of the Nova REST API -- as of the Kilo release -- and also attempts to describe how the Nova team intends to evolve the REST API's implementation over time and remove some of the cruft that has crept in over the years.
Background
Nova currently includes two distinct frameworks for exposing REST API functionality. Older code is called the "V2 API" and exists in the /nova/api/openstack/compute/contrib/ directory. Newer code is called the "v2.1 API" and exists in the /nova/api/openstack/compute/plugins directory.
The V2 API is the old Nova REST API. It will be replaced by V2.1 API totally. The code tree of V2 API will be removed in the future also.
The V2.1 API is the new Nova REST API with a set of improvements which includes Microversion and standardized validation of inputs using JSON-Schema. Also the V2.1 API is totally backwards compatible with the V2 API (That is the reason we call it as V2.1 API).
Stable API
In the V2 API, there is a concept called 'extension'. An operator can use it to enable/disable part of Nova REST API based on requirements. An end user may query the '/extensions' API to discover what API functionality is supported by the Nova deployment.
Unfortunately, because V2 API extensions could be enabled or disabled from one deployment to another -- as well as custom API extensions added to one deployment and not another -- it was impossible for an end user to know what the OpenStack Compute API actually included. No two OpenStack deployments were consistent, which made cloud interoperability impossible.
API extensions, while not (yet) removed from the V2.1 API, are no longer needed to evolve the REST API, and no new API functionality should use the API extension classes to implement new functionality. Instead, new API functionality should use the microversioning decorators to add or change the REST API.
The extension is considered as two things in the Nova V2.1 API:
The '/extensions' API
In the V2 API the user can query it to determine what APIs are supported by the current Nova deployment.
In V2.1 API, microversions enable us to add new features in backwards-compatible ways. And microversions not only enable us to add new futures by backwards-compatible method, also can be added by appropriate backwards-incompatible method.
The '/extensions' API is frozen in Nova V2.1 API and will be deprecated in the future.
The plugin framework
One of the improvements in the V2.1 API was using stevedore to load Nova REST API extensions instead of old V2 handcrafted extension load mechanism.
There was an argument that the plugin framework supported extensibility in the Nova API to allow deployers to publish custom API resources.
We will keep the existing plugin mechanisms in place within Nova but only to enable modularity in the codebase, not to allow extending of the Nova REST API.
As the extension will be removed from Nove V2.1 REST API. So the concept of core API and extension API is eliminated also. There is no difference between Nova V2.1 REST API, all of them are part of Nova stable REST API.