Managing the order that your resources execute in is one of the most important things you will do when writing a class. Unfortunately, the more complex and longer a class becomes, the harder it is to “see” those relationships. Chaining is a neat feature that can help clarify the dependencies. Both before/requires and notify type relationships can be specified by using directional arrows between the related classes.
In the first example, I’ve used a straight arrow to specify that the
postgres package should be managed before its
associated config file. In the next, the tilde arrow specifies that the
postgres config file should notify the service. A more
powerful feature is demonstrated on the last line, where a collection is
used to specify that the rubygems package should be
managed before any package that uses the gem provider. In this way, we can
constuct concise one-to-many dependency between our managed gems and the
gem program required:
Package['postgresql-8.4'] -> File ['/etc/postgresql/8.4/main/postgresql.conf']
File['/etc/postgresql/8.4/main/postgresql.conf'] ~> Service['postgres']
Package['rubygems'] -> Package <| provider == gem |>Resources can be chained on declaration, as well as by reference like in these examples. However, this will often make the declarations more confusing than using the alternative require or notify parameters.