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Phil Taprogge
Asked: 2016-09-06 04:27:58 +0800 CST

puppet: symlinks for directories in yet-to-be-installed packages

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We currently use puppet 3.8.7 to provision a large number of AWS EC2 instances.

Since these machines have small and slow root volumes, we symlink certain directories from e.g. /var to /mnt.

This leads to manifests riddles with segments like this:

group { 'postgres':
  ensure  => present,
  system  => true,
}

user { 'postgres':
  ensure  => present,
  comment => 'PostgreSQL administrator',
  gid     => 'postgres',
  home    => '/var/lib/postgresql',
  require => Group['postgres'],
}

file { '/mnt/postgresql':
  ensure  => directory,
  owner   => 'postgres',
  group   => 'postgres',
  mode    => '0755',
  require => User['postgres'],
}

file { '/var/lib/postgresql':
  ensure  => link,
  target  => '/mnt/postgresql',
  require => File['/mnt/postgresql'],
  before  => Class['dbserver'],
}

This is

  1. ugly

  2. overly verbose

    and

  3. error prone if an implicit dependency (e.g. on postgresql-client) creates the directory before the file statement is evaluated.

So I'm wondering, how do other people tackle this?

Is there a better way to ensure that

  • such symlinks always get created before the package containing the directory gets installed

but

  • they only get created if a package containing the directory is going to be installed?
amazon-ec2 puppet

1 Answers

  1. The puppet files are reflecting your system configuration, and are thus a bit ugly. I suggest one of the following solutions:

    • Restructuring your system configuration might help a bit. Do not mount your device to /mnt/postgres, but to /var/lib/postgresql, or maybe use a /var partition.

    • If your only problem is postgres being installed before your symlink creation, this is easy: Enforce creation of the symlink before the postgres installation.

      package { 'postgresql':
  ensure  => installed,
  require => File['/var/lib/postgresql'],
}

      If you want to fix systems where your problem already persists, I recommend implementing a fixing exec statement, similar to this:

      exec { '/usr/local/bin/fix_postgres_installation.sh':
  user   => 'root,
  onlyif => 'test -d /var/lib/postgresql',
  before => File['/var/lib/postgresql'],
}

      The script could possibly shutdown the postgres, sync the data to /mnt/postgres, delete /var/lib/postgres.

    • If you simply want to force director deletion for the file resource, you can use the force option, as described in the Puppet documentation

    • Change the data directories of your applications. Most applications allow this. For Postgres, the settign data_directory() should help you, also see the PostgreSQL Documentation.

    • 1