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Puppet dry run

Is the web site down?” asked my boss. Probably the five words I least want to hear, along with “I’ve run over your cat” and “I’ll pay you next month”.

Yes, the site was down - I’d made a minor Puppet change, and assumed this would roll out without any problems. Of course, the problem with assumptions is that they make an ASS out of U in front of your BOSS. Whenever I use Puppet to make changes to production servers, I like to do it by running Puppet manually, rather than waiting for the half-hourly auto-run, and I like to do a dry run first to see what’s going to happen.

Puppet’s dry-run feature is a powerful tool that’s often overlooked by busy sysadmins. Even if you test your Puppet manifests on a virtualised replica of your production site, which many people don’t have the time or the budget to do, pushing changes out live can have unforeseen side effects which are best avoided.

To dry-run Puppet, use the --noop flag:

# puppetd --test --noop
info: Caching catalog at /var/puppet/state/localconfig.yaml
notice: Starting catalog run
762a763
> This change will screw up your Apache server.
notice: //Node[soupnazi]/nagios::server/apache/File[/etc/httpd/conf/httpd.conf]/source: 
  is {md5}327dc6b2d2337e4467840c007a0edd61, should be puppet:///apache/httpd.conf (noop)
info: //Node[soupnazi]/nagios::server/apache/File[/etc/httpd/conf/httpd.conf]: Scheduling refresh 
  of Service[httpd]
notice: //Node[soupnazi]/nagios::server/apache/Service[httpd]: Would have triggered refresh from 1 
  dependencies
notice: Finished catalog run in 8.93 seconds

Puppet’s ‘noop’ (no-operation) mode shows you what would happen, but doesn’t actually do it. As you can see, Puppet reports that it would have updated the httpd.conf file and restarted Apache, and it helpfully shows us the line it would have inserted into the file:

> This change will screw up your Apache server.

Aren’t you glad you added the --noop flag? Of course, whenever you’re contemplating rolling out changes to a live server like this, you’ll want to stop the Puppet daemon running on the box temporarily (or disable the cron job if you run Puppet out of cron). The last thing you want is Puppet running automatically in “really do it” mode while you’re scrambling to fix the broken manifest.

Puppet’s dry run mode is not perfect - resource Y may fail because it requires resource X, for example, and resource X has not really been applied. Puppet isn’t so clever as to model the state of the box if each resource had been really applied - which is not surprising, because that’s a hard problem. Luckily, it doesn’t need to be perfect: dry-running Puppet in this way will catch a lot of small but potentially disastrous errors before they happen.

Opinions differ as to how meaningful or correct a dry-run mode can be, since changing the state of the box makes things unpredictable (Chef doesn’t have a dry run option at all), but I’ll take avoiding a disaster over academic arguments any day.

Thanks for getting the site back up,” said my boss. “Now, something else I meant to tell you. I’ll pay you next month.” Read more »

Puppet resources

Puppet is an indispensable configuration management and Linux automation tool for the system administrator who prefers to spend her time improving the infrastructure, rather than doing tedious manual labour. Chef, Puppet, Fabric and other tools can be a huge boost to productivity. From small beginnings, you can grow your Puppet installation to manage a whole network, or many networks. However, with great power comes a steep learning curve. There are lots of great resources and tutorials available if you want to learn Puppet - here’s a brief guide for aspiring devops.

Puppet help

Need an introduction to Puppet? Start here.

What is Puppet?

Are You Looking For Trouble?

Do you know where your servers are right now? Good sysadmins and devops use automation and monitoring, but good sysadmins don’t rely on them. The problem with automated monitoring, as we know from the movie Jurassic Park, is that you only find what you’re looking for.

In this article I’ll explain why paranoia, suspicion and snooping are essential items in your sysadmin toolbox, and give you a few hints on how to find trouble you didn’t know you had.

Jurassic Park: when monitoring isn’t enough.

Puppet Drupal recipes

Drupal, Puppet. Puppet, meet Drupal

Puppet and Drupal make a great combination. Drupal is an amazing tool for quickly constructing attractive, functional web sites. It lets you manage large numbers of web sites from a single installation, and (via add-on modules) provides almost any CMS or blog feature you could want.

However, like any powerful tool, Drupal takes some learning. It also needs a certain amount of discipline to manage Drupal servers without getting into a chaotic mess. The Drupal sysadmin can end up trying to navigate a spaghetti of ad-hoc symlinks and face problems upgrading, maintaining, monitoring and backing up a large Drupal installation. Aegir can help with this (I’ll look at Aegir vs. Puppet in a future article) but first we need to get Drupal itself under control.

Fortunately, Puppet can help you tame Drupal and use the power of configuration management to bring your Drupal sites under control. In this article I’ll explain some techniques and Puppet recipes I use to manage Drupal sites and servers, including my own sites, including this one! Read more »

Scaling Puppet with Git

Scaling Puppet with Git

More and more people are turning to systems automation tools like Puppet and Chef to get the most out of their environments, and to create time to focus on delivering business benefits. Scaling Puppet is most commonly done using client/server mode, in which every client is issued with an SSL certificate, and conversations take place between clients and a server, over HTTP, and manifests and assets are served over the network, and applied by a locally running Puppet daemon. However, is there a better way? We present an alternative to the traditional Puppetmaster solution which we like to call ‘Git Puppet’.

Guest article by Stephen Nelson-Smith

Update: There is a fuller and more up-to-date description of my Git-based Puppet infrastructure in The Puppet Cookbook - so after you read this introduction, you may find it helpful to read the book too. Read more »

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