Charm metadata

The only file that must be present in a charm is metadata.yaml, in the root directory. A metadata file must be a valid yaml dictionary, containing at least the following fields:

  • name is the charm name, which is used to form the charm URL.
    • It can only contain a-z, 0-9, and -; must start with a-z; must not end with a -; and may only end with digits if the digits are not directly preceded by a space. Stick with names like foo and foo-bar-baz and you needn't pay further attention to the restrictions.
  • summary is a one-line description of the charm.
  • description is a long-form description of the charm and its features. It will also appear in the juju GUI.
  • tags is a descriptive tag that is used to sort the charm in the store.

Here's a valid metadata file:

    name: mongodb
    summary: An open-source document database, and the leading NoSQL database
    description: |
      MongoDB is a high-performance, open source, schema-free document- oriented
      data store that's easy to deploy, manage and use. It's network accessible,
      written in C++ and offers the following features:  
      - Collection oriented storage
      - easy storage of object-style data
      - Full index support, including on inner objects
      - Query profiling
      - Replication and fail-over support
      - Efficient storage of binary data including large objects (e.g. videos)
      - Auto-sharding for cloud-level scalability (Q209) High performance,
      scalability, and reasonable depth of functionality are the goals for the
      project.

With only those fields, a metadata file is valid, but not very useful. Charms for use in the Charm Store should always set the following fields as well, for categorization and display in the GUI:

  • maintainer is the name and email address for the main point of contact for the development and maintenance of the charm. The maintainer field should be in the format Charm Author Name <author@email>.

  • maintainers is a list of people who maintain the charm. Use the yaml sequence format when there are more than one person maintaining the project.

  • tags is a list containing one or more of the following:

    • analytics
    • big_data
    • ecommerce
    • openstack
    • cloudfoundry
    • cms
    • social
    • streaming
    • wiki
    • ops
    • backup
    • identity
    • monitoring
    • performance
    • audits
    • security
    • network
    • storage
    • database
    • cache-proxy
    • application_development
    • web_server

In almost all cases, only one tag will be appropriate. The categories help keep the Charm Store organised.

Juju Charm Store metadata Listing

Storage can also be declared in a charm's metadata, as such:

storage:
  data:
    type: filesystem
    description: junk storage
    shared: false # not yet supported, see description below
    read-only: false # not yet supported, see description below
    minimum-size: 100M
    location: /srv/data

A metadata file defines the charm's relations, and whether it's designed for deployment as a subordinate service.

  • subordinate should be set to true if the charm is a subordinate. If omitted, the charm will be presumed not to be subordinate.
  • provides, requires, and peers define the various relations the charm will participate in.
  • if the charm is subordinate, it must contain at least one requires relation with container scope.

Other available fields are:

  • series is a list of series that the charm supports.
    • It can include code names of Ubuntu releases such as 'trusty' or 'xenial'.
    • It can also include code names for non-Ubuntu series such as 'centos7'.

Other field names should be considered to be reserved; please don't use any not listed above to avoid issues with future versions of Juju.

Payloads

Payloads provide a means for the charm author to get information from a deployed charm. This is especially useful in large and complex deployments. For instance, the author may want to check the status of some element of the deployment such as a Docker container.

Payloads are defined in the payloads section of metadata.yaml by assigning a class and type. Classes refer to simply the name of the payload and the type describes the nature of the payload. Both are author-defined and are not validated by Juju.

The most common types of payload are based on Docker, KVM, and LXD.

As an example, below the following class/type pairs are defined: 'monitoring/docker', 'kvm- guest/kvm', and 'lxd-container/lxd':

payloads:
    monitoring:
        type: docker
    kvm-guest:
        type: kvm
    lxd-container:
        type: lxd

Payloads can be viewed using [juju list-payloads][list-payloads] and managed from the charm hook using the following commands:

  • payload-register
  • payload-unregister
  • payload-status-set

See the [Hook tools documentation][hook-payloads] for further details on these payload commands.