Many applications require binary resources to complete the install process. While it is possible for a charm to download this software from package repositories or other locations, some charms may be deployed in data centers with restrictive firewalls that do not allow access to all areas of the Internet. It may also be desirable to more strictly control what specific resources are deployed.
Starting with Juju 2.0, users can upload resources to the controller or the Juju Charm Store from where they are accessible to charms.
Use discrete files for resources. That is, if your charm requires a software package and a dependency, then use one resource for each. Juju optimizes distribution of resources. If resources are grouped into one file (resource) then they are larger and must be updated everywhere when they are updated.
Also see Juju resources in the User guide for an end-user perspective.
Developing a charm with resources
Charm developers can add a
resources key to the
metadata.yaml file to define one or more resources.
resources: software: type: file filename: software.zip description: "One line description that is useful when operators need to push it."
The ‘filename’ is what Juju will name the file locally when it is downloaded. Juju will check the extension on the file being uploaded and will prevent files with different extensions from being uploaded.
Resources can be uploaded to a local Juju controller, where only charms from that controller can access the resources, or to the Juju Charm Store where access is controlled by permissions assigned to the charms to which the resources are attached.
Users can list the resources that are currently available on the controller by using the
juju resources command. The command shows resources for a service or a unit.
juju resources resources-example
[Service] Resource Supplied by Revision software admin@local 0
juju resources resources-example/0
[Unit] Resource Revision software 0
Users can display the resources that are currently available in Charm Store for a charm or a specific revision number with the
charm list-resources command (from the Charm tools).
charm list-resources cs:~lazypower/etcd
Resource Revision etcd 0 etcdctl 0
juju attach-resource command uploads a file from local disk to the Juju controller to be used as a resource for an application. You must specify the charm name, the resource name, and the path to the file.
juju attach-resource charm-name resource-name=filepath
If you attach a resource to a running charm the
upgrade-charm hook is run. This gives charm authors the ability to handle new resources appropriately.
Resources may be uploaded to the Juju controller at deploy time by specifying the
--resource flag followed by a
<resource-name>=<filepath> pair. This flag may be repeated more than once to upload more than one resource.
juju deploy charm-name --resource foo=/some/file.tgz --resource bar=./docs/cfg.xml
Where “foo” and “bar” are the resource names in the
metadata.yaml file for the charm-name charm.
charm attach command (Charm tools) uploads a file to the Charm Store as a new resource for the charm. The default channel is the stable channel. You must specify the fully qualified charm name, including the version (e.g. ~you/mycharm-0 instead of just ~you/mycharm when using the stable channel).
charm attach ~mbruzek/trusty/consul-0 software=./consul_0.6.4_linux_amd64.zip
A revision number is not required when using another channel.
charm attach ~mbruzek/trusty/consul-0 software=./consul_0.6.4_linux_amd64.zip -c unpublished
Using resources in a charm
The hook tool
resource-get will fetch a resource from the Juju controller or the Charm Store. The command returns a local path to the file for a named resource.
resource-get has not been run for the named resource previously, then the resource is downloaded from the controller at the revision associated with the unit’s application. That file is stored in the unit’s local cache. If it has been run before then each subsequent run synchronizes the resource with the controller. This ensures that the revision of the unit-local copy of the resource matches the revision of the resource associated with the unit’s application.
The output (path) provided by
resource-get references the up-to-date file for the resource. Note that the resource may get updated on the controller for the service at any time, meaning the cached copy may be out of date at any time after the command is called. Consequently, the command should be run whenever it is critical for the resource to be up-to-date.