MCollective Async Result Handling

08/19/2012

This ia a post in a series of posts I am doing about MCollective 2.0 and later.

Overview


The kind of application I tend to show with MCollective is very request-response orientated. You request some info from nodes and it shows you the data as they reply. This is not the typical thing people tend to do with middleware, instead what they do is create receivers for event streams processing those into databases or using it as a job queue.

The MCollective libraries can be used to build similar applications and today I’ll show a basic use case for this. It’s generally really easy creating a consumer for a job queue using Middleware as covered in my recent series of blog posts. It’s much harder doing it when you want to support multiple middleware brokers, support pluggable payload encryption, different serializers add some Authentication, Authorization and Auditing into the mix and soon it becomes a huge undertaking.

MCollective already has a rich sets of plugins for all of this so it would be great if you could reuse these to save yourself some time.

Request, but reply elsewhere


One of the features we added in 2.0.0 is more awareness of the classical reply-to behaviour common to middleware brokers to the core MCollective libraries. Now every request specifies a reply-to target and the nodes will send their replies there, this is how we get replies back from nodes and if the brokers support it this is typically done using temporary private queues.

But it’s not restricted to this, lets see how you can use this feature from the command line. First we’ll setup a listener on a specific queue using my stomp-irb application.

% stomp-irb -s stomp -p 6163
Interactive Ruby shell for STOMP
 
info> Attempting to connect to stomp://rip@stomp:6163
info> Connected to stomp://rip@stomp:6163
 
Type 'help' for usage instructions
 
>> subscribe :queue, "mcollective.nagios_passive_results"
Current Subscriptions:
        /queue/mcollective.nagios_passive_results
 
=> nil
>>

We’re now receiving all messages on /queue/mcollective.nagios_passive_results, lets see how we get all our machines to send some data there:

% mco rpc nrpe runcommand command=check_load --reply-to=/queue/mcollective.nagios_passive_results
Request sent with id: 61dcd7c8c4a354198289606fb55d5480 replies to /queue/mcollective.nagios_passive_results

Note this client recognised that you’re never going to get replies so it just publishes the request(s) and shows you the outcome. It’s real quick and doesn’t wait of care for the results.

And over in our stomp-irb we should see many messages like this one:

<<stomp>> BAh7CzoJYm9keSIB1QQIewg6CWRhdGF7CToNZXhpdGNvZGVpADoMY29tbWFu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What you’re looking at is a base64 encoded serialized MCollective reply message. This reply message is in this case signed using a SSL key for authenticity and has the whole MCollective reply in it.

MCollective to Nagios Passive Check bridge


So as you might have guessed from the use of the NRPE plugin and the queue name I chose the next step is to connect the MCollective NRPE results to Nagios using its passive check interface:

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require 'mcollective'
require 'pp'
 
# where the nagios command socket is
NAGIOSCMD = "/var/log/nagios/rw/nagios.cmd"
 
# to mcollective this is a client, load the client config and
# inform the security system we are a client
MCollective::Applications.load_config
MCollective::PluginManager["security_plugin"].initiated_by = :client
 
# connect to the middleware and subscribe
connector = MCollective::PluginManager["connector_plugin"]
connector.connect
connector.connection.subscribe("/queue/mcollective.nagios_passive_results")
 
# consume all the things...
loop do
  # get a mcollective Message object and configure it as a reply
  work = connector.receive
  work.type = :reply
 
  # decode it, this will go via the MCollective security system
  # and validate SSL etcetc
  work.decode!
 
  # Now we have the NRPE result, just save it to nagios
  result = work.payload
  data = result[:body][:data]
 
  unless data[:perfdata] == ""
    output = "%s|%s" % [data[:output], data[:perfdata]]
  else
    output = data[:output]
  end
 
  passive_check = "[%d] PROCESS_SERVICE_CHECK_RESULT;%s;%s;%d;%s" % [result[:msgtime], result[:senderid], data[:command].gsub("check_", ""), data[:exitcode], output]
 
  begin
    File.open(NAGIOSCMD, "w") {|nagios| nagios.puts passive_check }
  rescue => e
    puts "Could not write to #{NAGIOSCMD}: %s: %s" % [e.class, e.to_s]
  end
end

This code connects to the middleware using the MCollective Connector Plugin, subscribes to the specified queue and consumes the messages.

You’ll note there is very little being done here that’s actually middleware related we’re just using the MCollective libraries. The beauty of this code is that if we later wish to employ a different middleware or different security system or configure our middleware connections to use TLS to ActiveMQ nothing has to change here. All the hard stuff is done in MCollective config and libraries.

In this specific case I am using the SSL plugin for MCollective so the message is signed so no-one can edit the results in a MITM attack on the monitoring system. This came for free I didn’t have to write any code here to get this ability – just use MCollective.

Scheduling Nagios Checks and scaling them with MCollective


Now that we have a way to receive check results from the network lets look at how we can initiate checks. I’ll use the very awesome Rufus Scheduler Gem for this.

I want to create something simple that reads a simple config file of checks and repeatedly request my nodes – possibly matching mcollective filters – to do NRPE checks. Here’s a sample checks file:

nrpe "check_load", "1m", "monitored_by=monitor1"
nrpe "check_swap", "1m", "monitored_by=monitor1"
nrpe "check_disks", "1m", "monitored_by=monitor1"
nrpe "check_bacula_main", "6h", "bacula::node monitored_by=monitor1"

This will check load, swap and disks on all machines monitored by this monitoring box and do a bacula backup check on machines that has the bacula::node class included via puppet.

Here’s a simple bit of code that takes the above file and schedules the checks:

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require 'rubygems'
require 'mcollective'
require 'rufus/scheduler'
 
# (ab)use mcollective logger...
Log = MCollective::Log
 
class Scheduler
  include MCollective::RPC
 
  def initialize(destination, checks)
    @destination = destination
    @jobs = []
 
    @scheduler = Rufus::Scheduler.start_new
    @nrpe = rpcclient("nrpe")
 
    # this is where the magic happens, send all the results to the receiver...
    @nrpe.reply_to = destination
 
    instance_eval(File.read(checks))
  end
 
  # helper to schedule checks, this will create rufus jobs that does NRPE requests
  def nrpe(command, interval, filter=nil)
    options = {:first_in => "%ss" % rand(Rufus.parse_time_string(interval)),
               :blocking => true}
 
    Log.info("Adding a job for %s every %s matching '%s', first in %s" % [command, interval, filter, options[:first_in]])
 
    @jobs << @scheduler.every(interval.to_s, options) do
      Log.info("Publishing request for %s with filter '%s'" % [command, filter])
 
      @nrpe.reset_filter
      @nrpe.filter = parse_filter(filter)
      @nrpe.runcommand(:command => command.to_s)
    end
  end
 
  def parse_filter(filter)
    new_filter = MCollective::Util.empty_filter
 
    return new_filter unless filter
 
    filter.split(" ").each do |filter|
      begin
        fact_parsed = MCollective::Util.parse_fact_string(filter)
        new_filter["fact"] << fact_parsed
      rescue
        new_filter["cf_class"] << filter
      end
    end
 
    new_filter
  end
 
  def join
    @scheduler.join
  end
end
 
s = Scheduler.new("/queue/mcollective.nagios_passive_results", "checks.txt")
s.join

When I run it I get:

% ruby schedule.rb
info 2012/08/19 13:06:46: activemq.rb:96:in `on_connecting' TCP Connection attempt 0 to stomp://nagios@stomp:6163
info 2012/08/19 13:06:46: activemq.rb:101:in `on_connected' Conncted to stomp://nagios@stomp:6163
info 2012/08/19 13:06:46: schedule.rb:25:in `nrpe' Adding a job for check_load every 1m matching 'monitored_by=monitor1', first in 36s
info 2012/08/19 13:06:46: schedule.rb:25:in `nrpe' Adding a job for check_swap every 1m matching 'monitored_by=monitor1', first in 44s
info 2012/08/19 13:06:46: schedule.rb:25:in `nrpe' Adding a job for check_disks every 1m matching 'monitored_by=monitor1', first in 43s
info 2012/08/19 13:06:46: schedule.rb:25:in `nrpe' Adding a job for check_bacula_main every 6h matching 'bacula::node monitored_by=monitor1', first in 496s
info 2012/08/19 13:07:22: schedule.rb:28:in `nrpe' Publishing request for check_load with filter 'monitored_by=monitor1'
info 2012/08/19 13:07:29: schedule.rb:28:in `nrpe' Publishing request for check_disks with filter 'monitored_by=monitor1'
info 2012/08/19 13:07:30: schedule.rb:28:in `nrpe' Publishing request for check_swap with filter 'monitored_by=monitor1'
info 2012/08/19 13:08:22: schedule.rb:28:in `nrpe' Publishing request for check_load with filter 'monitored_by=monitor1'

All the checks are loaded, they are splayed a bit so they don’t cause a thundering herd and you can see the schedule is honoured. In my nagios logs I can see the passive results being submitted by the receiver.

MCollective NRPE Scaler


So taking these ideas I’ve knocked up a project that does this with some better code than above, it’s still in progress and I’ll blog later about it. For now you can check out the code on GitHub it includes all of the above but integrated better and should serve as a more complete example than I can realistically post on a blog post.

There are many advantages to this method that comes specifically from combining MCollective and Nagios. The Nagios scheduler visit hosts one by one meaning you get this moving view of status over a 5 minute resolution. Using MCollective to request the check on all your hosts means you get a 1 second resolution – all the load averages Nagios sees are from the same narrow time period. Receiving results on a queue has scaling benefits and the MCollective libraries are already multi broker aware and supports failover to standby brokers which means this isn’t a single point of failure.

Conclusion


So we’ve seen that we can reuse much of the MCollective internals and plugin system to setup a dedicated receiver of MCollective produced data and I’ve shown a simple use case where we’re requesting data from our managed nodes.

Today what I showed kept the request-response model but split the traditional MCollective client into two. One part scheduling requests and another part processing results. These parts could even be on different machines.

We can take this further and simply connect 2 bits of code together and flow arbitrary data between them but securing the communications using the MCollective protocol. A follow up blog post will look at that.