This document contains a presentation on writing flexible and scalable Puppet modules. The presentation discusses common issues that arise when modules are not designed to be flexible, such as unexpected changes in operating systems or use cases. It provides guidance on how to design modules to be more modular, parameterized, and able to handle unexpected changes over time through techniques like using Hiera for configuration, separating logic into submodules, and favoring composition over inheritance. The goal is to create modules that can be easily adapted and improved by both their original authors and other users.
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Puppet Module Writing 201
1. Modules 201
Writing Flexible and Scalable
Puppet
Eric Shamow | PuppetCamp NYC
Friday, April 27, 12
2. 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
Friday, April 27, 12
3. How Did We Get
Here?
The module seemed just fine when I wrote
it...
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4. How Did We Get
Here?
The module seemed just fine when I wrote
it...
(I never thought RHEL 6 would come out)
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7. I never thought...
• RHEL 6 would come out
• My company would switch to Debian
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8. I never thought...
• RHEL 6 would come out
• My company would switch to Debian
• Other people would want to reuse the
module
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9. I never thought...
• RHEL 6 would come out
• My company would switch to Debian
• Other people would want to reuse the
module
• I’d want to use only a part of the module
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10. I never thought...
• RHEL 6 would come out
• My company would switch to Debian
• Other people would want to reuse the
module
• I’d want to use only a part of the module
• ...maybe as a part of something else
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12. Puppet is Declarative
Shoehorning conditional logic into
declarative language?
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13. Puppet is Declarative
Shoehorning conditional logic into
declarative language?
Please do not do this:
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14. Puppet is Declarative
Shoehorning conditional logic into
declarative language?
Please do not do this:
case $::operatingsystem {
‘redhat’: {
if $::fqdn == “bobmarley” {
file { ‘foo’:
...
} else {
...
...
}
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21. Code Models Reality
• Move complexity closer to where it is in
real life
Friday, April 27, 12
22. Code Models Reality
• Move complexity closer to where it is in
real life
• If your CMDB contains lots of
exceptions, that’s where you should read
from
Friday, April 27, 12
23. Code Models Reality
• Move complexity closer to where it is in
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
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25. Parameterized Classes
class motd (
$pci_enabled = true,
$owner = ‘bob’,
) {
...
}
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26. params.pp Pattern
class motd::params {
$owner = ‘Bob’
}
class motd (
$owner = $motd::params::owner
) {
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27. params.pp Pattern +
hiera
class motd::params {
$owner = hiera(‘owner’,‘Bob’)
}
class motd (
$owner = $motd::params::owner
) {
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28. Outsource Logic to
Submodules
class mysql::params {
$server_package = $::operatingsystem ? {
‘redhat’ => ‘mysql-server’,
...
}
}
class mysql::server {
package { ‘mysql-server’:
name => $mysql::params::server_package,
...
}
}
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29. 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
}
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38. Stop Being Surprised by Change
• You don’t have to define every Operating
System or version
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39. Stop Being Surprised by Change
• You don’t have to define every Operating
System or version
• Assume cutovers won’t be clean - you
will be 50% RHEL 5 and 50% RHEL 6 for
a while
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40. Stop Being Surprised by Change
• You don’t have to define every Operating
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.
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42. Protect Yourself Against
Unintentional Defaults
• Always provide a default case
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43. Protect Yourself Against
Unintentional Defaults
• Always provide a default case
• In most cases that default case should be
failure
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44. Protect Yourself Against
Unintentional Defaults
• Always provide a default case
• In most cases that default case should be
failure
• Use the stdlib :fail method to fail
gracefully.
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45. Protect Yourself Against
Unintentional Defaults
class mysql::params {
case $::operatingsystem {
‘redhat’: { $serverpkg = ‘mysql-server’ }
default: {
fail(‘MySQL Server package undefined.’)
}
}
}
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46. Remember
You Don’t Have To Think of
Everything
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47. Remember
You Don’t Have To Think of
Everything
Leave room for others to improve your modules
without refactoring them...
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48. Remember
You Don’t Have To Think of
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.
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49. And then we haz a community!
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50. Thank You
Eric Shamow
eric@puppetlabs.com
http://opsrealist.info
@eshamow
Friday, April 27, 12
