Within the PCP Execution Protocol layer, controllers invoke agent capabilities by requesting actions. This is done by sending request messages to one or more agents. Such messages specify the action name, module and input parameters.
Actions related to a given application are grouped together in modules; for
instance, the puppet module provides the run and status actions.
Action requests can be blocking or non-blocking; that changes the way the action outcome is referred and stored.
To inform the requester controller about the outcome of the requested action, the agent replies with a response message.
TODO(ale): refer to the transaction status action
TODO(ale): add note regarding non-blocking requests and notify_outcome