Blog of Sylvain Le Gall

I have a recurring problem when trying to use augeas on a complex node: trying to edit a specific entry in a list which is uniquely defined by many attributes.

You probably don't know that you have this problem, but it is easy to spot it into your augeas/puppet resource.

Here are some symptoms of this problem:

• you need to use onlyif with multiple constraint on the selection
• you use last() and last() +1

 
 augeas {
   "setup-shorewall":
     changes =>
       [
         "set entry[last() + 1]/source 'all'",
         "set entry[last()]/dest 'all'",
         "set entry[last()]/policy 'REJECT'",
         "set entry[last()]/log_level 'info'",
       ],
     onlyif => "match entry[source = 'all'][dest = 'all'][policy = 'REJECT'] size == 0";

For a long time, I thought it was the only solution. But last week, I read again the documentation and found another solution.

My main concerns are the onlyif and last() parts, it doesn't look clean to me. The problem is that I cannot define the entry all at once and if I use a value that will be set late, the node cannot be targeted in between.

The clean way to do this was to define first the target attribute. Typically, in augeas changes:

 set spec[user = '$name']/user '$name'

This way if the node doesn't exist it is created and you can then use it directly:

 set spec[user = '$name']/host_group/host 'ALL'
 set spec[user = '$name']/host_group/command1 'ALL'
 set spec[user = '$name']/host_group/command1/tag 'PASSWD'

But sometimes it is not possible to set the attribute directly -- typically when you need to use multiple attribute. The solution in this case is to use defnode:

 defnode target entry[#comment = 'puppet: <%= name %>']/ "<%= name %>"
 set $target/action '<%= action %>'
 set $target/source '<%= source %>'
 set $target/#comment 'puppet: <%= name %>'
 clear $target

The big trick here is that defnode needs a value, but most of the time you cannot set a value for the node -- because it has none. To solve this, you set a value with defnode, process with your change and you clear the node at the end.

This recent discovery has simplify a lot some augeas changes I use.

Feel free to leave comment on your personal technique to deal with augeas and puppet.