A problem I ran into early when I was bringing services into Puppet was slightly different configurations on servers with different specs. For example, I run Tomcat on three servers but one is also running some other services. On this one server the JVM maximum heap size needs to be lower than on the others, but the rest of the Tomcat configuration is the same. To manage this without making a second class definition I used the template system in Puppet.

There are four steps to making this work. First you need to define a default for the variable. Next you need to write the template. Third, connect the template to a file on the client. Finally override the variable where you need it.

Define a default

The default variable definition goes in the module’s manifest file before the class definition. Putting it before the class definition makes sure that it is in scope for overriding in the node definition. The top of the manifest file will look like this:

$heapsize = 1000

class jvm {
...

Note that variables have leading dollar signs.

Write the template

Puppet uses Ruby’s ERB templating system. The template language is powerful, but I’m only going to cover variable substitution here.

Start with putting the configuration file in the templates subdirectory of the module, i.e. <module name>/templates/<config> so that Puppet knows where to find it. To substitute the variables, use < %= name %>. Here the variable name does not start with a dollar sign. Continuing the earlier example, this is part of our template:

export JAVA_OPTS="-Xmx<%= heapsize %>M"

Connecting the template to a file

Using the template in a file declaration is not as simple as just changing the path from the static source. Instead, you have to use the content parameter and the template function. The parameter to the template function is a weird path fragment: the first segment is the module name, and the rest is the path to your file underneath the templates directory. Continuing our example, assuming that the module is named jvm and the template is at modules/jvm/templates/options.erb, this declaration is in the jvm class:

file {"/usr/local/bin/jvm_options.sh":
  mode    => "664",
  content => template("jvm/options.erb"),
}

Notice that the name of the template file doesn’t have to match the target name on the client.

Override the variable

Finally, you have to add a line to the node for the server which needs a different configuration. In the node definition before including the class, redefine the variable to what you need. Example:

node "lowmem" {
  $heapsize = 750
  include jvm
}

Nodes that don’t override the variable will get the default value defined in the module’s manifest file.

Puppet’s templates can do a lot more than simple variable substitution, but this should get you started. Beyond this it’s mostly ERB.

Puppet uses a declarative syntax built on top of ruby for defining rules. The documentation is mediocre at best, so you may struggle at first learning how to write the rules. Just read through the example recipes and test it out with a non-production server. The easiest way I found to test a rule is to run

sudo puppetd --test

on the server that should be getting the configuration. It will tell you there if there are any errors.

The version of puppet in Ubuntu 9.10 doesn’t support defining node names with regular expressions, so you might want to grab the version in Debian sid.

I’m still trying to figure out the best way to split up modules and organize my node definitions, so if you’ve used Puppet leave a comment and let me know what you think.

U.S. Marshals.

Every week Bear gets dropped in a wilderness location (like Les Stroud really did) and has to evade U.S. Marshals for seven days. The locals will be told that he’s a real fugitive. Set the dogs on him. Heck, they should shoot him on sight, just to add a bit more excitement. The real fun begins when Bear has to betray his camera crew to buy some more time.

I plan to start posting a wider variety of content rather than trying to stick to Linux, development, physics, and LaTeX. I’ve become more active on twitter and Facebook, so I’m dumping short stuff there. I also got a tumblr account as a temporary landing site while this one was down, and I think I’ll be keeping it for meaningless drivel that’s between tweet and blog length. I’ll lovingly handcraft longer, meatier, vastly more important drivel for this site. You’ll get more of my personality, my raw thoughts, and let’s be honest, I haven’t quite been prolific with the geeky material to date.

One step towards the personalization of my personal blog has been to import my old LiveJournal posts. These date back to the heady days of 2001 when “blog” was a new term that was still considered a contraction of “web log.” I was a freshly laid-off refugee of the dot-com bust back then, and I was still too dumb to know I’m not funny. There might be some gems in the archives, but as you dig through try not to get lost in the self-important ramblings of an arrogant young man. If the going is too tough, I promise to keep the site updated with a steady stream of self-important ramblings from an arrogant, slightly less young man.

So thanks for looking at the main site after you stumbled across one of my old LaTeX posts. I hope this isn’t the last update for another two years.

The problem is that the lookAt method defaults to using the world y-axis as “up” for the billboards, which isn’t usually correct. Of course we don’t really care about the world y-axis with billboards. We just want them to be vertical in the camera. Here’s some code that does it.

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1. Catholic Easter mass takes forever

   
   My sister received full communion into the Catholic church the night before Easter, so I got a cool, three for one bonus: Easter mass, confirmations, and baptisms. Sweet! I can’t tell you how excited I was, but the three (four?) hour ceremony exceeded all my expectations.



2. There are kids who don’t enjoy hunting for eggs on Easter

   
   This is the first Easter that Connor is aware of, and my niece was with us too, so we had a lot of fun getting ready.
   

Then, typing

gives you: The [111] crystal planes are 3.2×10^-10 m apart. whether or not you’re already in a math environment. If the exponent is just one number, you can omit the braces, like this: 3\e8 m/s. Cool, huh?

(Of course, for 10^-10 m you can just use Angstroms, \AA. If you’re in a math environment, use \text{\AA}, or else the circle won’t line up with the A. That is, if you’re okay with non-SI units.)

This also makes unit vectors (typeset with \hat) bold.

Click on “more” to play the game.

This week’s Pizza Hut code

Here’s what they’re giving away:

• $30k for a car
• 5 entertainment centers worth $4k each
• 20 $400 Pizza Hut gift cards
• 500 $5 music download gift cards