cinder-specs/specs/kilo/db-volume-filtering.rst

Generic filter support for volume queries

https://blueprints.launchpad.net/cinder/+spec/db-volume-filtering

The filtering support of the volume DB APIs is inconsistent. For example, filtering is supported when querying all volumes and when querying volumes by project. However, filtering is not supported when querying volumes by host or by group.

Problem description

DB functions exist to get all volumes, to get all volumes in a particular project, to get all volumes in a particular group, and to get all volumes hosted on a particular host. See the following functions in the DB API:

• volume_get_all
• volume_get_all_by_project
• volume_get_all_by_group
• volume_get_all_by_host

Only the queries that get all volumes and that get all volumes by project support additional filtering.

The purpose of this blueprint is to make the filtering support consistent across these APIs.

Use Cases

Proposed change

The current Volume filtering logic is already encapsulated in the common _generate_paginate_query function. The filtering needs to be moved into a common function (something like _process_volume_filters) that would update a model query object with the filter information.

Then, for example, the volume_get_all_by_host could utilize it as follows:

def volume_get_all_by_host(context, host, filters=None):
    """Retrieves all volumes hosted on a host."""
    if host and isinstance(host, basestring):
        session = get_session()
        with session.begin():
            host_attr = getattr(models.Volume, 'host')
            conditions = [host_attr == host,
                          host_attr.op('LIKE')(host + '#%')]
            query = _volume_get_query(context).filter(or_(*conditions))
            if filters:
                query = _process_volume_filters(query, filters)
                if not query:
                    return None
            return query.all()
    elif not host:
        return []

Alternatives

Instead of adding filter support to the other volume APIs, a caller could simply invoke the volume_get_all API with a filter that defines the host or the group information. The downside of this approach is that is requires the caller to understand how to form that query; this is especially problematic for the host API since the filter is actually an OR of:

• A exact string match for the given host
• A REGEX match where the host matches a value in the form of "<host>#<pool>"

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

Complicated DB filters could affect query performance; however, this already exists for the volume_get_all and the volume_get_all_by_project APIs.

Other deployer impact

None

Developer impact

None

Implementation

Assignee(s)

Primary assignee:

Steven Kaufer (kaufer)

Other contributors:

None

Work Items

• Refactor Volume filter logic in the sqlalchemy DB API into a common function and invoke it from the existing _generate_paginate_query function
• Update the functions to get volumes by host and by group to use the common filtering function

Dependencies

None

Testing

Since common filter processing will be used for all volume DB queries, the existing test coverage is sufficient.

Documentation Impact

None

References

None