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Actions for the charm author

Actions are executables associated with a charm that may be invoked by the user. For example, the charm author might include a snapshot action on a database charm. See Juju Actions on how to use actions.

The user may pass arguments when invoking the action. The charm uses an actions.yaml file to specify the parameter type for the arguments. In the Juju GUI, the invocation of an action will be automatically built based on actions.yaml.

Action tools may be used by the author to define how actions interact with Juju. Actions can retrieve parameters passed by the user, set responses in a map, or set a failure status with a message.

Implementing actions

Every implemented action must include:

  • its executable (script, binary, etc.) in the charm's /actions directory.
  • the executable's name as a top-level key in a YAML map in the actions.yaml file.

Note: action names must start and end with lowercase alphanumeric characters, and only contain lowercase alphanumeric characters, the hyphen "-" or full stop"." characters.

Here is a sample partial layout of a charm's root directory that shows actions called 'pause', 'resume', and 'snapshot':

├── actions
│   ├── pause
│   ├── resume
│   └── snapshot
├── actions.yaml
...

Below is an actions.yaml file that shows the three executables as "top-level keys":

pause:
resume:
snapshot:

That is a "barebones" configuration. As the next section will make clear, there is a lot more that can, and should, be placed in this file.

Options and format: actions.yaml

Here is a list of options that can be used in actions.yaml along with their respective format.

  • Each action is defined as a top-level key of a YAML map, with the same name as its corresponding executable.
  • The value of each action should include a description key and may include a params key. If no description is given, an empty description will be used.
  • The value of the params key is also a YAML map where each key is a string whose value is a 'JSON Schema' transformed to a YAML map. The 'JSON Schema' itself, may, in turn, be nested.
  • The 'JSON Schema' keys required and additionalProperties can be used under the action key (parallel to description and params) but they can also be used anywhere within a nested schema.
  • Use additionalProperties: false under the action key to reject additional parameters passed by the user.
  • At this time, the $schema and $ref keys are not supported by Juju, as they may trigger resolution of remote objects and other issues.

Detailed example

Using the above information the previous example can be extended with the description key:

pause:
  description: Pause the database.
resume:
  description: Resume a paused database.
snapshot:
  description: Take a snapshot of the database.

The snapshot action can be enriched with the params key:

...
snapshot:
  description: Take a snapshot of the database.
  params:
    outfile:
      type: string
      description: The filename to write to.
...

A sample complete actions.yaml file after extending the snapshot action and adding the required and additionalProperties keys:

pause:
  description: Pause the database.
resume:
  description: Resume a paused database.
snapshot:
  description: Take a snapshot of the database.
  params:
    filename:
      type: string
      description: The name of the snapshot file.
    compression:
      type: object
      description: The type of compression to use.
      properties:
        kind:
          type: string
          enum: [gzip, bzip2, xz]
        quality:
          description: Compression quality
          type: integer
          minimum: 0
          maximum: 9
  required: [filename]
  additionalProperties: false

This action would support a call such as:

juju run-action mysql/0 snapshot filename=out.tar.gz compression.type=gzip

Action tools

Three tools are provided to the action author for the action to interact with Juju:

  • action-get retrieves the params passed when invoking the action.
  • action-set sets values in a map to be returned after the action finishes.
  • action-fail sets the action status to failed when it finishes, with a message to be returned to the user. The results set by action-set are preserved.

Use juju help-tool <tool name> to see more detail on each tool.

action-get

action-get prints the value of the parameter at the given key, serialized according to the --format option. If multiple keys are passed, action-get will recurse into the parameter map as needed.

For example, if an action named snapshot was defined on a MySQL charm, and was invoked by the user as follows:

juju run-action mysql/0 snapshot outfile="foo"

then the snapshot could use action-get to retrieve the filename:

#!/bin/bash
# An action named "snapshot"

action-get outfile
# "foo" will be printed

action-set

action-set permits the action to set results in a map to be returned at the completion of the action. Here is an example:

#!/bin/bash
# An action named "report"

action-set result-map.time-completed="$(date)" result-map.message="Hello world!"
action-set outcome="success"

Using the above action ('report'), the command

juju action fetch <ID>

will produce output similar to:

# ...
message: "" # No error message.
results:
  result-map:
    time-completed: <some date>
    message: Hello world!
  outcome: success
status: completed

action-fail

action-fail causes the action to finish as failed upon completion. For instance, this might be used to indicate a full disk if a database dump was attempted, or to indicate that a remote service was unable to be resolved. The results set by action-set before or after failure are retained, and an action fail status cannot be unset.

Example:

#!/bin/bash
# An action named "sayhello"

command=$(action-get command)

action-set received.value="$command"

if [ "$command" != "hello" ];
  then
    action-fail "I only know one command."
    action-set received.known="no"
  else
    action-set received.known="yes"
fi

action-set timestamp="$(date)"

Using the above action ('sayhello'), the command

juju run-action <unit> sayhello command="greetme"
# ...

will produce output similar to:

message: I only know one command.
results:
  received:
    known: no
    value: greetme
  timestamp: Thu Jan 15 13:28:25 EST 2015
status: failed

Params transformation

This section covers the transformation from params to JSON-Schema. This is meant as an aid to creating more complex schemas and should not be necessary for most actions.

The transformation to JSON-Schema is as follows:

# actions.yaml
<string A>:
  [description: <string AB>]
  [params:
    <string PAA>:
      <YAML map YAA>
    <string PAB>:
      <YAML map YAB>]
  [string C]: X
  [string D]: Y
  [string ...]: Z
<string B>:
  ...

The above, which defines an action for <A> (and <B> , <C> , and so on), becomes the following in JSON-Schema format:

{
  "description": "<AB>",
  "title": "<A>",
  "type": "object",
  "properties": {
    "<PAA>": {YAA as JSON},
    "<PAB>": {YAB as JSON}
  },
  "required": ["filename"],
  "<C>": X,
  "<D>": Y,
  ...
}

For example, a simple YAML file such as:

# actions.yaml
snapshot:
  description: Take a snapshot of the database.
  params:
    outfile:
      type: string

will be transformed to this:

{
  "description": "Take a snapshot of the database.",
  "title": "snapshot",
  "type": "object",
  "properties": {
    "outfile": {
      "type": "string"
    }
  }
}

A complex YAML transform example

Here we transform the example given at the top of this page:

# actions.yaml
pause:
  description: Pause the database.
resume:
  description: Resume a paused database.
snapshot:
  description: Take a snapshot of the database.
  params:
    filename:
      type: string
      description: The name of the snapshot file.
    compression:
      type: object
      description: The type of compression to use.
      properties:
        kind:
          type: string
          enum: [gzip, bzip2, xz]
        quality:
          description: Compression quality
          type: integer
          minimum: 0
          maximum: 9
  required: [filename]
  additionalProperties: false

It becomes three actions, with respective JSON-Schema:

pause:

{
  "description": "Pause the database.",
  "title": "pause",
  "type": "object",
  "properties": {}
}

resume:

{
  "description": "Resume a paused database.",
  "title": "resume",
  "type": "object",
  "properties": {}
}

snapshot:

{
  "description": "Take a snapshot of the database.",
  "title": "snapshot",
  "type": "object",
  "properties": {
    "filename": {
      "type": "string",
      "description": "The name of the snapshot file."
    },
    "compression": {
      "type": "object",
      "description": "The type of compression to use.",
      "properties": {
        "kind": {
          "type": "string",
          "enum": ["gzip", "bzip2", "xz"]
        },
        "description": {
          "type": "integer",
          "description": "Compression quality",
          "minimum": 0,
          "maximum": 9
        }
      }
    }
  },
  "required": ["filename"],
  "additionalProperties": "false"
}