See also:

Many applications require access to some persistent storage. Juju and Ops provide a facility for working with and defining charms that require storage.


Defining storage

Charm storage is defined in the storage key in charmcraft.yaml.

The storage map definition:

Field Type Default Description
type string required Type of storage requested. Supported values are block or filesystem.

The filesystem type yields a directory in which the charm may store files. The block type yields a raw block device, typically disks or logical volumes.

If the charm specifies a filesystem-type store, and the storage provider supports provisioning only disks, then a disk will be created, attached, partitioned, and a filesystem created on top. The filesystem will be presented to the charm as normal.
description string nil Description of the storage requested
multiple map
(see table below)
nil By default, stores are singletons; a charm will have exactly one of the specified stores. The multiple field specifies the number of storage instances to be requested.

Unless a number is explicitly specified during deployment, units of the application will be allocated the minimum number of storage instances specified in the charm metadata. It is then possible to add instances (up to the maximum) by using the juju storage add command.
minimum-size string 1GiB Size in the forms: 1.0G, 1GiB, 1.0GB. Supported size multipliers are M, G, T, P, E, Z, Y. Not specifying a multiplier implies M.
location string nil Specifies the mount location for filesystem stores. For multi-stores, the location acts as the parent directory for each mounted store.
properties string[] nil List of properties for the storage. Currently only transient is supported
shared bool false True indicates that all units of the application share the storage.

The multiple map definition:

Field Type Default Description
range string/int nil Value can be an int for a precise number, or a string in the forms: m-n, m+, m-, where m and n are of type int.

Examples: range: 2 or range: 0-10.

An example of a storage definition inside metadata.yaml:

# ...
  # Name of this storage is 'data'
    type: filesystem
    description: junk storage
    minimum-size: 100M
    location: /srv/data
# ...

Storage on Kubernetes

In addition to the above, there is some additional data required to define storage for Kubernetes charms. You will still need to define the top-level storage map (as above), but also specify which containers you would like the storage mounted into. Consider the following metadata.yaml snippet:

# ...
  # define a container named "important-app"
    # use the "app-image" oci resource
    resource: app-image
    # mount our 'logs' store at /var/log/important-app
    # in the workload container
      - storage: logs
        location: /var/log/important-app
  # This is another container with no storage
    resource: supporting-app-image

    type: filesystem
    # specifying location on the charm container is optional
    # when unspecified, defaults to /var/lib/juju/storage/<name>/<num>
# ...

The above snippet will ensure that both the important-app container and charm container inside each Pod has the logs store mounted. Under the hood, the storage map is translated into a series of PersistentVolumes, mounted into Pods with PersistentVolumeClaims.

The location attribute must be specified when mounting a storage into a workload container as shown above - this will dictate the mount point for the specific container.

Optionally, developers can specify the location attribute on the storage itself, which will specify the mount point in the charm container. If left unset, the charm container will have the storage volume mounted at a predictable path at /var/lib/juju/storage/<name>/<num>, where <num> is the index of the storage. This defaults to 0.

For the above metadata.yaml, the charm container would have the storage available at: /var/lib/juju/storage/logs/0.

Storage events

There are two key events associated with storage:

Event name Event Type Description
<name>_storage_attached StorageAttachedEvents This event is triggered when new storage is available for the charm to use. Callback methods bound to this event allow the charm to run code when storage has been added.

Such methods will be run before the install event fires, so that the installation routine may use the storage. The name prefix of this hook will depend on the storage key defined in the metadata.yaml file.
<name>_storage_detaching StorageDetachingEvent Callback methods bound to this event allow the charm to run code before storage is removed.

Such methods will be run before storage is detached, and always before the stop event fires, thereby allowing the charm to gracefully release resources before they are removed and before the unit terminates.

The name prefix of the hook will depend on the storage key defined in the metadata.yaml file.

Charm and Container Accessing to Storage

When you use storage mounts with juju, it will be automatically mounted into the charm container at either:

  • the specified location based on the storage section of metadata.yaml or

  • the default location /var/lib/juju/storage/<storage-name>/<num> where num is zero for “normal”/singular storages or integer id for storages that support multiple attachments.

The operator framework provides the Model.storages dict-like member that maps storage names to a list of storages mounted under that name. It is a list in order to handle the case of storage configured for multiple instances. For the basic singular case, you will simply access the first/only element of this list.

Charm developers should not directly assume a location/path for mounted storage. To access mounted storage resources, retrieve the desired storage’s mount location from within your charm code - e.g.:

def _my_hook_function(self, event):
    storage = self.model.storages['my-storage'][0]
    root = storage.location

    fname = 'foo.txt'
    fpath = os.path.join(root, fname)
    with open(fpath, 'w') as f:
        f.write('super important config info')

This example utilizes the framework’s representation of juju storage - i.e. self.model.storages which returns a mapping of <storage_name> to Storage objects, which exposes the name, id and location of each storage to the charm developer, where id is the underlying storage provider ID.

If you have also mounted storage in a container, that storage will be located directly at the specified mount location. For example with the following content in your metadata.yaml:

    resource: foo-image
      - storage: data
        location: /foo-data

storage for the “foo” container will be mounted directly at /foo-data. There are no storage name or integer-indexed subdirectories. Juju does not currently support multiple storage instances for charms using “containers” functionality. If you are writing a container-based charm (e.g. for kubernetes clouds) it is best to have your charm code communicate the storage location to the workload rather than hard-coding the storage path in the container itself. This can be accomplished by various means. One method is passing the mount path via a file using the Container API:

def _on_mystorage_storage_attached(self, event):
    # get the mount path from the charm metadata
    container_meta = self.framework.meta.containers['my-container']
    storage_path = container_meta.mounts['my-storage'].location
    # push the path to the workload container
    c = self.model.unit.get_container('my-container')
    c.push('/my-app-config/storage-path.cfg', storage_path)

    ... # tell workload service to reload config/restart, etc.

Scaling Storage

While juju provides an add-storage command, this does not “grow” existing storage instances/mounts like you might expect. Rather it works by increasing the number of storage instances available/mounted for storages configured with the multiple parameter. For charm development, handling storage scaling (add/detach) amounts to handling <name>_storage_attached and <name_storage_detaching events. For example, with the following in your metadata.yaml file:

        type: filesystem
            range: 1-10

juju will deploy the application with the minimum of the range (1 storage instance in the example above). Storage with this type of multiple:... configuration will have each instance residing under an indexed subdirectory of that storage’s main directory - e.g. /var/lib/juju/storage/my-storage/1 by default in charm container. Running juju add-storage <unit> my-storage=32G,2 will add two additional instances to this storage - e.g.: /var/lib/juju/storage/my-storage/2 and /var/lib/juju/storage/my-storage/3. “Adding” storage does not modify or affect existing storage mounts. This would generate two separate storage-attached events that should be handled.

In addition to juju client requests for adding storage, the StorageMapping returned by self.model.storages also exposes a request method (e.g. self.model.storages.request()) which provides an expedient method for the developer to invoke the underlying storage-add hook tool in the charm to request additional storage. On success, this will fire a <storage_name>-storage-attached event.

Contributors: @jnsgruk, @lengau, @tmihoc

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