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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

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

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