MCollective RPC

MCollective RPC is a framework that first appeared in The Marionette Collective that Choria provides a compatability layer for.

It provides the following:

  • Conventions for writing agents and clients, favoring convention over custom design
  • Easy to write agents including input validation and a sensible feedback mechanism in case of error
  • Provide audit logs of all calls to agents
  • Provide the ability to do fine grain Authorization of calls to agents and actions.
  • Has a Data Definition Language used to describe agents and assist in giving hints to auto generating user interfaces.
  • The provided generic client tool - mco rpc - should be able to speak to most compliant agents
  • Should you need to you can still write your own clients, this should be very easy too
  • Return data should be easy to print, in most cases the framework should be able to print a sensible output with a single, provided, function. The DDL is used here to improve the standard one-size-fits-all methods.
  • Standardised packaging on the Puppet Forge using Puppet Modules.

We’ve provided full tutorials on Writing RPC Clients and Agents. Numerous full featured examples can be found in our GitHub Project dedicated to plugins.

A bit of code probably says more than lots of English, so here’s a simple hello world Agent, it just echo’s back everything you send it in the :msg argument:

module MCollective
  module Agent
    class Helloworld<RPC::Agent
      # Basic echo server
      action "echo" do
        validate :msg, String = request[:msg]

The nice thing about using a standard abstraction for clients is that you often won’t even need to write a client for it, we ship a standard client that you can use to call the agent above:

 % mco rpc helloworld echo msg="Welcome to MCollective RPC"
 Determining the amount of hosts matching filter for 2 seconds .... 1                          : OK
     "Welcome to MCollective RPC"

 ---- rpctest#echo call stats ----
            Nodes: 1
       Start Time: Wed Dec 23 20:49:14 +0000 2009
   Discovery Time: 0.00ms
       Agent Time: 54.35ms
       Total Time: 54.35ms

Note: Please see the agents and clients pages for a thorough walkthrough.

But you can still write your own clients, it’s incredibly simple, full details of a client is out of scope for the introduction - see the Writing Clients page instead for full details - but here is some sample code to do the same call as above including full discovery and help output:

#!/bin/env ruby

require "mcollective"

include MCollective::RPC

mc = rpcclient("helloworld")

printrpc mc.echo(:msg => "Welcome to MCollective RPC")


The above code effectively achieves the same as the command mco rpc helloworld echo msg="Welcome to MCollective RPC" and like that command you can pass --help to your script and do full filtering of which nodes to communicate with and more.