Puppet Module Writing 201

Modules 201
Writing Flexible and Scalable
Puppet

Eric Shamow | PuppetCamp NYC

Friday, April 27, 12

Who Am I?
• Senior Professional Services Engineer for
Puppet Labs
• Former Operations Manager
• Recovering Sysadmin
• Travel around helping people make Puppet
even more awesome
• Not Shamwow. If you came to the wrong
talk you can leave now

How Did We Get
Here?
The module seemed just ﬁne when I wrote
it...


How Did We Get
Here?
The module seemed just ﬁne when I wrote
it...
(I never thought RHEL 6 would come out)


I never thought...


I never thought...
• RHEL 6 would come out


I never thought...
• My company would switch to Debian


I never thought...
• Other people would want to reuse the
module


I never thought...
module
• I’d want to use only a part of the module


I never thought...
module
• I’d want to use only a part of the module
• ...maybe as a part of something else


Ur Doin It Wrong


Puppet is Declarative
Shoehorning conditional logic into
declarative language?


Please do not do this:


Please do not do this:
case $::operatingsystem {
‘redhat’: {
if $::fqdn == “bobmarley” {
file { ‘foo’:
...
} else {
...
...
}

When Logic Fails


Organizing Your Data



• Hiera



• Hiera
• External Node Classiﬁers



• Hiera
• External Node Classiﬁers
• Custom Functions


Code Models Reality


Code Models Reality

• Move complexity closer to where it is in
real life


Code Models Reality

real life
• If your CMDB contains lots of
exceptions, that’s where you should read
from


Code Models Reality

real life
• If your CMDB contains lots of
exceptions, that’s where you should read
from
• If there is contorted logic, keep it away
from nuts and bolts module mechanics


Please Don’t Make Me Edit
Your Module


Parameterized Classes

class motd (
$pci_enabled = true,
$owner = ‘bob’,
) {
...
}


params.pp Pattern

class motd::params {
$owner = ‘Bob’
}

class motd (
$owner = $motd::params::owner
) {


params.pp Pattern +
hiera

class motd::params {
$owner = hiera(‘owner’,‘Bob’)
}

class motd (
$owner = $motd::params::owner
) {


Outsource Logic to
Submodules
class mysql::params {
$server_package = $::operatingsystem ? {
‘redhat’ => ‘mysql-server’,
...
}
}

class mysql::server {
package { ‘mysql-server’:
name => $mysql::params::server_package,
...
}
}

Be as Modular as
Possible
class mysql::server {
package { ‘mysql-server’:
name => $mysql::params::server_package,
...
}
}
class mysql::client {
...
}
class mysql {
include mysql::client
include mysql::server
}

Limit Inheritance


Limit Inheritance

• Favor composition over inheritance


Limit Inheritance

• Inheritance + dynamic variable scoping =
PAIN


Limit Inheritance

PAIN
• Useful at the edges


Limit Inheritance

PAIN
• Useful at the edges
• Useful for overriding/extending in limited
circumstances


Stop Being Surprised by Change



• You don’t have to deﬁne every Operating
System or version



System or version
• Assume cutovers won’t be clean - you
will be 50% RHEL 5 and 50% RHEL 6 for
a while



System or version
• Assume cutovers won’t be clean - you
will be 50% RHEL 5 and 50% RHEL 6 for
a while
• And even when you aren’t, you’ll be 98%
RHEL 6 and 2% RHEL 5 until the end of
time.


Protect Yourself Against
Unintentional Defaults



• Always provide a default case



• In most cases that default case should be
failure



• In most cases that default case should be
failure

• Use the stdlib :fail method to fail
gracefully.



class mysql::params {
case $::operatingsystem {
‘redhat’: { $serverpkg = ‘mysql-server’ }
default: {
fail(‘MySQL Server package undefined.’)
}
}
}


Remember
You Don’t Have To Think of
Everything


Remember
Everything

Leave room for others to improve your modules
without refactoring them...


Remember
Everything

Leave room for others to improve your modules
without refactoring them...

...and then you get to beneﬁt and re-merge their
changes when they do.


And then we haz a community!


Thank You

Eric Shamow
eric@puppetlabs.com
http://opsrealist.info
@eshamow


Puppet Module Writing 201

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