fragments

Chef Provisioning is great for provisioning virtual machines when a clear definition of a cluster or platform is available. If such a definition will never be available at the time a Provisioning cookbook is written then it can be a bit constrictive. In organisations where there are hundreds of services and just as many combinations they may be combined a different approach may be necessary.

The fragments cookbook is an attempt at moving from thinking about virtual machines as being the unit that should be defined to thinking more about sections of a large platform and how they are defined and relate depend on each other. From now on these sections will be referred to as fragments.

A fragment is defined using an LWRP called fragment. For instance, a simple web application may have the following fragments.

cluster 'greatwebapp'

fragment 'database' do
  memory_weight 100
  run_list %w(mysqld::default)
end

fragment 'webapp' do
  memory_weight 250
  run_list %w(webapp::default)
  required_fragment %w(database)
end

fragment 'nginx' do
  memory_weight 50
  run_list %w(nginx::default)
  required_fragment %w(webapp)
end

packer 'default' do
  action [:pack, # Calculate number of virtual machines and pack fragments
          :berkshelf_vendor, # Retrieve and upload cookbooks to Chef Server
          :provision, # Create the virtual machines
          :converge # Converge the virtual machines
         ]
end

So rather than define a virtual machine with a run list that contains the above recipes, fragments are defined. The cookbook then includes facilities to pack these fragments into as many virtual machines as are required to host them.

Usage

There is more explanation below about the many variables that may be used to control the packing process. However, a simle example will help in getting used to the ideas.

Authors

fragments was written by Robin Ridler, while working at DataSift