chef_dns_zone_with_serial_number.rb

# Using chef to create files with a time stamp or similar - in this case,
# the serial number in a DNS zone file - presents a problem, because the
# time stamp interferes with with idempotency. Here is a trick for solving
# this (thanks to Noah Kantrowitz for his input and suggestion!)

# The trick is to use the same template twice, once with a constant serial
# number to check for idempotency, and a second time with the "real" serial
# number. The actual serial number can be stored in an attribute.

# Initial serial number for the zone
node.default['myzone']['serial'] = 1

# Use node.set so the default is only used the first time round.
# Don't use node.override because that would not be saved to the
# node!
ruby_block "dns trick for myzone" do
  block do
    node.set['myzone']['serial'] = node['myzone']['serial'] + 1
  end
  action :nothing
end

# the template for checking for idempotency.
# If you want to manually force incrementing the serial number,
# you simply delete /tmp/myzone.withoutserial, and the next
# chef run will regenerate it and trigger incrementing the
# serial number, as well
template "/tmp/myzone.withoutserial" do
  source "zonetemplate.erb"
  variables(
    :serialnumber => 0,
    :data => zonedata
  )
  notifies :run, "ruby_block[dns trick for myzone]", :immediate
end

# the real template is almost identical to the one without serial number
# lazy evaluation so we pick up the new serial number.
template "/tmp/myzone" do
  source "zonetemplate.erb"
  variables lazy {
    {
      :serialnumber => node['myzone']['serial'],
      :data => zonedata
    }
  }
  # notify BIND or whatever else you may need to do here
end