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Managing Infrastructure with Puppet by James Loope

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MCollective Commands

With both the servers and a client configured, we’re ready to start issuing MCollective commands. Let’s start off with the mc-find-hosts command. When run without any argument, mc-find-hosts will list all of the MCollective servers that are currently active and listening:

:> mc-find-hosts

We can also get some information about our individual MCollective nodes. mc-inventory will tell us what agents are available on a node, what Puppet classes that node is a member of, and assuming the Facter module is installed, a list out all of the available Facter facts about the node:

:> mc-inventory A.example.com

Inventory for A.example.com:

Server Statistics:
    Version: 1.0.1
    Start Time: Fri May 06 11:10:34 -0700 2011
    Config File: /etc/mcollective/server.cfg
    Process ID: 22338
    Total Messages: 143365
    Messages Passed Filters: 75428
    Messages Filtered: 67937
    Replies Sent: 75427
    Total Processor Time: 162.09 seconds
    System Time: 73.08 secondsAgents:
    discovery       filemgr        package 
    iptables        nrpe           rpcutil  
    process         puppetd        

Configuration Management Classes:
    ntp             php                apache2
    mysql-5            varnish

    architecture => x86_64
    domain => example.com
    facterversion => 1.5.7
    fqdn => A.example.com 
    hostname => A
    id => root
    is_virtual => true
    kernel => Linux
    kernelmajversion => 2.6
    kernelversion => 2.6.35

This is already a useful tool for diagnostics and inventory on all of your Puppet-managed servers, but MCollective also lets us execute agents on the target systems, filtered by any of these attributes, facts, agents, or classes. For example, if our servers run Apache and we need to restart all of the Apaches on all of our servers, we could use the mc-service agent to do this:

:> mc-service --with-class apache2 apache2 restart

This will place a message on the MCollective message bus that says: “All the servers with the apache2 Puppet class, use your service agent to restart apache2.” We can even add multiple filters like the following:

:> mc-service --with-class apache2 --with-fact architecture=x86_64 apache2 restart

This will let us restart Apache on only the 64bit “x86_64” architecture servers that have the Puppet apache2 class. These sorts of filters make remote execution of tasks on particular subsets of servers very easy.

Of particular interest to those of us running large infrastructures is MCollective’s built-in capacity to run the Puppet agent on the servers. Puppet’s client-server model, in its default configuration, will poll the Puppet Master once every half hour. This is not convenient, for instance, if you would like to use Puppet to coordinate an application release on a group of servers. If you would like some control over the sequence and timing of the Puppet runs, you can use the MCollective puppetd agent and forgo the polling behavior of the agent daemon. Since Puppet is built in to MCollective, it is not necessary to run the agent on boot either. So long as MCollective and Puppet are both installed, we can execute Puppet as we like.

The agent can be downloaded from GitHub at https://github.com/puppetlabs/mcollective-plugins/tree/master/agent/puppetd/ and, as with the Facter plug-in, should be copied to $libdir/mcollective on the servers, preferably using Puppet. Once it’s installed, you will be able to kick off a Puppet run on all or some of your servers with the following command:

:> mc-puppetd --with-class example runonce

If you don’t mind the default polling behavior of the Puppet agent, you can also use the puppetd MCollective agent to selectively enable or disable Puppet on sets of your instances as well as initiate one-off runs of the agent.


If you still want to have Puppet run on a regular basis to ensure configuration correctness, but need to avoid polling “stampedes,” take a look at the PuppetCommander project at http://projects.puppetlabs.com/projects/mcollective-plugins/wiki/ToolPuppetcommander. It uses MCollective’s puppetd module to centrally coordinate Puppet runs so as to avoid overwhelming a Puppet Master. It will also give you the power to specify which nodes or classes to run automatically.

Finally, there is an mc-rpc command that serves as a sort of metacommand, allowing access to all of the available agents. We can execute the puppetd agent, for example, with the following syntax:

:> mc-rpx --agent puppetd --with-class example runonce

Alternatively, we can use mc-rpc to read out the documentation for a particular agent:

:> mc-rpc --agent-help puppetd
SimpleRPC Puppet Agent

Agent to manage the puppet daemon

    Author: R.I.Pienaar
    Version: 1.3-sync
    License: Apache License 2.0
    Timeout: 120
Home Page: http://mcollective-plugins.googlecode.com/

disable, enable, runonce, status

disable action:
    Disables the Puppetd


            Description: String indicating status
            Display As: Statc
runonce action:
    Initiates a single Puppet run


            Description: Output from puppetd
            Display As: Output

status action:
    Status of the Puppet daemon


            Description: Is the agent enabled
            Display As: Enabled

            Description: When last did the agent run
            Display As: Last Run

            Description: String displaying agent status
            Display As: Status

            Description: Is the agent running
            Display As: Running

You’ve seen the basic features of MCollective in this chapter. It works as a great orchestration tool for Puppet, allowing you greater control over your Puppet agents and more insight into your configurations through Facter. Beyond this, the agents are fairly simple to write and can be used to accomplish any task that you might want to execute in a distributed fashion across all or part of your infrastructure. Puppet Labs provides documentation on extending MCollective with custom agents with SimpleRPC at http://docs.puppetlabs.com/mcollective/simplerpc/agents.html.

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