Metadata Persistence

Metadata Persistence

Cinder drivers are not stateless, and the interface between the Cinder core code and the drivers allows them to return data that can be stored in the database. Some drivers, that have not been updated, are even accessing the database directly.

Because cinderlib uses the Cinder drivers as they are, it cannot be stateless either.

Originally cinderlib stored all the required metadata in RAM, and passed the responsibility of persisting this information to the user of the library.

Library users would create or modify resources using cinderlib, and then serialize the resources and manage the storage of this information themselves. This allowed referencing those resources after exiting the application and in case of a crash.

This solution would result in code duplication across projects, as many library users would end up using the same storage types for the serialized data. That’s when the metadata persistence plugin was introduced in the code.

With the metadata plugin mechanism we can have plugins for different storages and they can be shared between different projects.

Cinderlib includes 2 types of plugins providing 3 different persistence solutions:

  • Memory (the default)

  • Database

  • Database in memory

Using the memory mechanisms users can still use the JSON serialization mechanism to store the medatada.

Currently we have memory and database plugins. Users can store the data wherever they want using the JSON serialization mechanism or with a custom metadata plugin.

Persistence mechanism must be configured before initializing any Backend using the persistence_config parameter in the setup or global_setup methods.

Note

When deserializing data using the load method on memory based storage we will not be making this data available using the Backend unless we pass save=True on the load call.

Memory plugin

The memory plugin is the fastest one, but it’s has its drawbacks. It doesn’t provide persistence across application restarts and it’s more likely to have issues than the database plugin.

Even though it’s more likely to present issues with some untested drivers, it is still the default plugin, because it’s the plugin that exposes the raw plugin mechanism and will expose any incompatibility issues with external plugins in Cinder drivers.

This plugin is identified with the name memory, and here we can see a simple example of how to save everything to the database:

import cinderlib as cl

cl.setup(persistence_config={'storage': 'memory'})

lvm = cl.Backend(volume_driver='cinder.volume.drivers.lvm.LVMVolumeDriver',
                 volume_group='cinder-volumes',
                 target_protocol='iscsi',
                 target_helper='lioadm',
                 volume_backend_name='lvm_iscsi')
vol = lvm.create_volume(1)

with open('lvm.txt', 'w') as f:
    f.write(lvm.dumps)

And how to load it back:

import cinderlib as cl

cl.setup(persistence_config={'storage': 'memory'})

lvm = cl.Backend(volume_driver='cinder.volume.drivers.lvm.LVMVolumeDriver',
                 volume_group='cinder-volumes',
                 target_protocol='iscsi',
                 target_helper='lioadm',
                 volume_backend_name='lvm_iscsi')

with open('cinderlib.txt', 'r') as f:
    data = f.read()
backends = cl.load(data, save=True)
print backends[0].volumes

Database plugin

This metadata plugin is the most likely to be compatible with any Cinder driver, as its built on top of Cinder’s actual database layer.

This plugin includes 2 storage options: memory and real database. They are identified with the storage identifiers memory_db and db respectively.

The memory option will store the data as an in memory SQLite database. This option helps debugging issues on untested drivers. If a driver works with the memory database plugin, but doesn’t with the memory one, then the issue is most likely caused by the driver accessing the database. Accessing the database could be happening directly importing the database layer, or indirectly using versioned objects.

The memory database doesn’t require any additional configuration, but when using a real database we must pass the connection information using SQLAlchemy database URLs format as the value of the connection key.

import cinderlib as cl

persistence_config = {'storage': 'db', 'connection': 'sqlite:///cl.sqlite'}
cl.setup(persistence_config=persistence_config)

lvm = cl.Backend(volume_driver='cinder.volume.drivers.lvm.LVMVolumeDriver',
                 volume_group='cinder-volumes',
                 target_protocol='iscsi',
                 target_helper='lioadm',
                 volume_backend_name='lvm_iscsi')
vol = lvm.create_volume(1)

Using it later is exactly the same:

import cinderlib as cl

persistence_config = {'storage': 'db', 'connection': 'sqlite:///cl.sqlite'}
cl.setup(persistence_config=persistence_config)

lvm = cl.Backend(volume_driver='cinder.volume.drivers.lvm.LVMVolumeDriver',
                 volume_group='cinder-volumes',
                 target_protocol='iscsi',
                 target_helper='lioadm',
                 volume_backend_name='lvm_iscsi')

print lvm.volumes

Custom plugins

The plugin mechanism uses Python entrypoints to identify plugins present in the system. So any module exposing the cinderlib.persistence.storage entrypoint will be recognized as a cinderlib metadata persistence plugin.

As an example, the definition in setup.py of the entrypoints for the plugins included in cinderlib is:

entry_points={
    'cinderlib.persistence.storage': [
        'memory = cinderlib.persistence.memory:MemoryPersistence',
        'db = cinderlib.persistence.dbms:DBPersistence',
        'memory_db = cinderlib.persistence.dbms:MemoryDBPersistence',
    ],
},

But there may be cases were we don’t want to create entry points available system wide, and we want an application only plugin mechanism. For this purpose cinderlib supports passing a plugin instance or class as the value of the storage key in the persistence_config parameters.

The instance and class must inherit from the PersistenceDriverBase in cinderlib/persistence/base.py and implement all the following methods:

  • db

  • get_volumes

  • get_snapshots

  • get_connections

  • get_key_values

  • set_volume

  • set_snapshot

  • set_connection

  • set_key_value

  • delete_volume

  • delete_snapshot

  • delete_connection

  • delete_key_value

And the __init__ method is usually needed as well, and it will receive as keyword arguments the parameters provided in the persistence_config. The storage key-value pair is not included as part of the keyword parameters.

The invocation with a class plugin would look something like this:

import cinderlib as cl
from cinderlib.persistence import base

class MyPlugin(base.PersistenceDriverBase):
    def __init__(self, location, user, password):
        ...

persistence_config = {'storage': MyPlugin, 'location': '127.0.0.1',
                      'user': 'admin', 'password': 'nomoresecrets'}
cl.setup(persistence_config=persistence_config)

lvm = cl.Backend(volume_driver='cinder.volume.drivers.lvm.LVMVolumeDriver',
                 volume_group='cinder-volumes',
                 target_protocol='iscsi',
                 target_helper='lioadm',
                 volume_backend_name='lvm_iscsi')

Migrating storage

Metadata is crucial for the proper operation of cinderlib, as the Cinder drivers cannot retrieve this information from the storage backend.

There may be cases where we want to stop using a metadata plugin and start using another one, but we have metadata on the old plugin, so we need to migrate this information from one backend to another.

To achieve a metadata migration we can use methods refresh, dump, load, and set_persistence.

An example code of how to migrate from SQLite to MySQL could look like this:

import cinderlib as cl

# Setup the source persistence plugin
persistence_config = {'storage': 'db',
                      'connection': 'sqlite:///cinderlib.sqlite'}
cl.setup(persistence_config=persistence_config)

# Setup backends we want to migrate
lvm = cl.Backend(volume_driver='cinder.volume.drivers.lvm.LVMVolumeDriver',
                 volume_group='cinder-volumes',
                 target_protocol='iscsi',
                 target_helper='lioadm',
                 volume_backend_name='lvm_iscsi')

# Get all the data into memory
data = cl.dump()

# Setup new persistence plugin
new_config = {
    'storage': 'db',
    'connection': 'mysql+pymysql://user:password@IP/cinder?charset=utf8'
}
cl.Backend.set_persistence(new_config)

# Load and save the data into the new plugin
backends = cl.load(data, save=True)
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