Charm metadata

The only file that must be present in a charm is metadata.yaml, in the root directory. It must be a valid YAML dictionary.

This page covers the various fields that can be included. Some are required and many are optional.

Required fields

Every charm should have these fields declared:

  • name: The charm name, which is used to form the charm URL.
    • The following criteria are applied:
      • Contains only characters a-z, 0-9, and - (hyphen)
      • Must start with a-z
      • Must not end with a - (hyphen)
    • May only end with digits if the digits are not directly preceded by a space.
    • Simple examples: 'foo' and 'foo-bar-baz'.
  • summary: A one-line description of the charm.
  • description: A longer description of the charm and its features. It will appear in the Juju GUI.

Here's a minimal, but 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

Charm Store fields

Charms destined for the Charm Store should set the below three fields:

  • maintainer: 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 firstname lastname <author@email>.

  • maintainers: A list of people who maintain the charm. Use the YAML sequence format if there are multiple people.

  • tags: A list of descriptive tags used for organisation purposes in the Charm Store. Choose from among 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

Miscellaneous fields

  • series: A list of series that the charm supports.
    • Supports Ubuntu code names (e.g. 'trusty', 'xenial') or CentOS release names (e.g. 'centos7').
    • The top-most entry acts as the default series.
  • terms: A list of the terms the user must agree to before using the charm.
  • min-juju-version: The minimum Juju version running on the controller (machine agent) that this charm is compatible with.
  • provides, requires, and peers: Define the charm's relations.
  • subordinate: Indicates a subordinate charm (set to 'true'). Such a charm must contain at least one requires relation with container scope.

Storage field

The storage field is used to declare information related to storage.

    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

See developer Storage documentation for more information.

Resources field

The resources field allows one to add blobs that a charm can make use of.

    type: file # "file" is the only type supported currently
    filename: example.tar.gz
    description: example resource

Payloads field

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 via the payloads field by assigning a class and type. A class defines 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':

        type: docker
        type: kvm
        type: lxd

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

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

See the Hook tools documentation for further details on these payload commands.

Extra-bindings field

The extra-bindings field is associated with an extra bindable endpoint that is not used with relations. These are useful when you want to have Juju provide distinct addresses for an application on one or more spaces. For example, adding this field to a YAML file for an application called "foo":


Will permit you to deploy the charm using --bind to deploy on units that have access to the "admin-api", "public-api", and "internal-api" spaces with a different network interface and address for each binding, using this:

juju deploy ~/path/to/charm/foo --bind "cluster=admin-api public=public-api internal-api"

And running network-get cluster --primary-address will return only the address coming from the "admin-api" space.

Endpoint names are strings and must not match existing relation names from the Provides, Requires, or Peers metadata sections. The values beside each endpoint name must be left out (i.e. "foo": <anything> is invalid).

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