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.
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.
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
- End with a letter or a number
Examples: ‘foo’ and ‘foo-bar-baz’
A one-line summary of the charm.
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:
Displayed in the Charm Store and other user-facing areas.
A list of series that the charm supports. The first element of the list acts as the default series.
|Operating system*||Supported series|
* Not all clouds support all operating systems. For best support, use
kubernetes or an Ubuntu series.
A list of the terms the user must agree to before using the charm before a deployment will proceed.
The minimum Juju version running on the controller (machine agent) that this charm is compatible with.
These fields define the charm’s relations.
Describes the relations that this charm provides for others.
Describes the relations that this charm consumes from other charms. Typically
requires relations are essential for the correct functioning of the application.
Allows information to be exchanged between units of the same application.
Indicates a subordinate charm (when set to ‘true’). Subordinate applications must contain at least one
requires relation with container scope.
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 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 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:
See the Hook tools documentation for further details on these payload commands.
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"
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).