Puppet Module Writing 201

Modules 201

Eric Shamow | PuppetCamp NYC

Writing Flexible and Scalable Puppet

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...




• I’d want to use only a part of the module


I never thought...




• I’d want to use only a part of the module

• ...maybe as a part of something else


Ur Doin It Wrong


Puppet is DeclarativeShoehorning 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


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


Code Models Reality


• 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


Limit Inheritance

• Favor composition over inheritance


Limit Inheritance


• Inheritance + dynamic variable scoping = PAIN


Limit Inheritance



• Useful at the edges


Limit Inheritance



• 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




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




• 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.’) } }}


RememberYou Don’t Have To Think of

Everything



Everything

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



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 [email protected]://opsrealist.info

@eshamow


mailto:[email protected]

mailto:[email protected]

http://opsrealist.info

http://opsrealist.info

Entertainment & Humor

Puppet Module Writing 201