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.

Here’s a minimal valid, metadata.yaml file:

name: mongodb
summary: A document database
description: A database that stores JSON-like data. 

name

The charm’s name, which is used when juju deploy is called.

Charm names must:

  • Contain only letters (a-z), numbers (0-9), and hyphens (-)
  • Start with a-z
  • End with a letter or a number

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.

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

display-name

Displayed in the Charm Store and other user-facing areas.

series

A list of series that the charm supports. The first element of the list acts as the default series.

Operating system* Supported series
Kubernetes kubernetes
Ubuntu xenial, bionic, focal (and other series code names)
CentOS centos7
openSUSE opensuseleap
MS Windows win2012, win2012r2, win2012hv, win2012hvr2, win2016, win2016hv, win2016nano, win2019

* Not all clouds support all operating systems. For best support, use kubernetes or an Ubuntu series.

terms

A list of the terms the user must agree to before using the charm before a deployment will proceed.

min-juju-version

The minimum Juju version running on the controller (machine agent) that this charm is compatible with.

Relation fields

These fields define the charm’s relations.

provides

Describes the relations that this charm provides for others.

requires

Describes the relations that this charm consumes from other charms. Typically requires relations are essential for the correct functioning of the application.

peers

Allows information to be exchanged between units of the same application.

subordinate

Indicates a subordinate charm (when set to ‘true’). Subordinate applications must contain at least one requires relation with container scope.

Storage field

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

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

See developer Storage documentation for more information.

Resources field

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

resources:
  example:
    type: file
    filename: example.tar.gz
    description: example resource
  gitlab_image:
    type: oci-image
    description: |
      Image used for gitlab pod. Must use gitlab/gitlab-ce:12.0.12-ce.0 or older
      to use the mysql interface.

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’:

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

extra-bindings:
  cluster:
  public:

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).

I believe a display-name field has been added to the metadata file following this discussion: Proposal: display-name for charm metadata

Is it possible we can add this field to this page?