glance-specs/specs/kilo/store-capabilities.rst

Add capabilities to storage driver

https://blueprints.launchpad.net/glance/+spec/store-capabilities

This features will enable the static and dynamic ability of a storage driver instance based on it's implementation as well as the configuration. The runtime status of the backend storage will influence the capacity too.

Henceforth, glance_store library can perform proper operations on storage as requested by upper layer, e.g. Glance. For example, to enable or disable image adding function to glance based on current implementation. The store also recognized whether a driver can be reused for all requests it handles or if it has to be recreated for each request because it is not stateless.

Problem description

The capabilities of store driver can be affected by the following factors:

1. Status of a connected backend storage.
2. The deployment configuration.
3. State of the driver code.
4. Technically one of capabilities is static whereas the other is dynamic.

Currently glance_store and glance don't use these capabilities. The driver doesn't have a way to expose its capabilities. On the other hand in the store's backend handler or client, e.g. glance-{api,registry} there is no common method to check these generic operations of the store drivers.

Currently we observe at least two kind of problems:

• Some necessary applicability check on the operation related with store access are missing, which inhibits the usability of glance service and glance_store lib.
• Hard to implement such check logic neatly, including common check routine on these generic operations of store driver, and the check for particular feature.

Proposed change

1. Existing drivers need to be updated to expose proper capabilities.

   List of required store capabilities as needed currently.

   Capability Name	Description
   READ_ACCESS	Generic read access
   WRITE_ACCESS	Generic write access
   RW_ACCESS	READ_ACCESS and WRITE_ACCESS
   READ_OFFSET	Read all bits from a offset (Included READ_ACCESS)
   WRITE_OFFSET	Write all bits to a offset (Included WRITE_ACCESS)
   RW_OFFSET	READ_OFFSET and WRITE_OFFSET
   READ_CHUNK	Read required length of bits (Included READ_ACCESS)
   WRITE_CHUNK	Write required length of bits (Included WRITE_ACCESS)
   RW_CHUNK	READ_CHUNK and WRITE_CHUNK
   READ_RANDOM	READ_OFFSET and READ_CHUNK
   WRITE_RANDOM	WRITE_OFFSET and WRITE_CHUNK
   RW_RANDOM	RW_OFFSET and RW_CHUNK
   DRIVER_REUSABLE	driver is stateless and its instance can be reused safely

2. Add common check routine on these generic operations for the store drivers.

3. Refactoring existing drivers to leverage these capabilities.

4. Add the logic to recreate driver instance if the storage or driver isn't stateless.

Alternatives

None

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

There is no expectation for any obvious degradation in the performance.

The reason being that there is a simple addition of a checker function being triggered in the form of a hook for each store operation, with current inclusion of the ('get', 'delete', 'add') operations.

Other deployer impact

None.

Developer impact

Developers implementing new drivers to the glance_store library would need to be aware of this concept. The static and dynamic ability of the storage backend could influence their and their implementation design.

All changes maintain backward-compatibility.

Implementation

Assignee(s)

Primary assignee:

zhiyan (lzy-dev)

Reviewers

Core reviewer(s):

Nikhil Komawar (nikhil-komawar) Stuart McLaren (stuart-mcLaren)

Work Items

• glance_store change
• Glance change

Dependencies

None

Testing

Necessary unit and functional test cases, will be added into glance_store as well as glance.

Documentation Impact

None

References

Corresponding changes:

• glance_store change
• Glance change