Knife Search Negative Query

Are you trying to look for Chef Nodes that don't include a particular result? Do you need to avoid pulling back nodes with specific values but need something more complex than NOT? Fear not, the solution exists! It is not very well documented that I can find so here goes.

I did find a couple of references to what I was looking for in negative queries, which I've linked below. It appears this request was fixed, but I couldn't find the merge with the details of what/how. You have to look very careful on the Chef.io page for knife search. If you look carefully under the "Fuzzy Matching" portion in the "About Operators" section you can find a quick reference to the use of AND and NOT in the same search. Their example is "knife search sample "id:foo AND -id:bar""

Anyway, on to the examples.

Search for any instance that is linux and not in the role [webserver]
knife search node -q "os:linux AND -roles:webserver" -i

If using psearch instead of search
knife psearch node "(NOT (os:linux OR roles:webserver)

Note: These are not the same logical results, it is only to show how to use the NOT syntax with the AND operator.


https://tickets.opscode.com/browse/CHEF-1821
https://docs.chef.io/knife_search.html#about-patterns

Chef Runtime Error Nil:NilClass

Today, an error came up with our Chef environment. The chef-client on new servers was having NoMethodError responses and failing during the chef-client run. 

The problem

The error message was pointing at a particular piece of code that has not given us trouble in the past. That code would look like the code below.

variables({
    :attribute_one => node[:node][:attribute][:one],
    :attribute_two => node[:node][:attribute][:two]
})

Fun fact, in Chef Client 12 the notation above no longer works. This is what is cause the nil:NilClass error. The node[:node][:attribute] values are being resolved to nil. That nil value is then stored in the :attribute_one and :attribute_two which means the value ends as a null method.

The fix

variables({
    :attribute_one => node['node']['attribute']['one'],
    :attribute_two => node['node']['attribute']['two']
})

Nginx 502 Bad Gateway

This morning I awoke to a 502 Bad Gateway error from Nginx. Luckily it was only one user that made me aware of this! The current setup I'm running is Nginx as a proxy for Apache.

To resolve this, I first checked to see if httpd was running. Surprisingly, httpd was not running. I went ahead and did "service httpd restart" and all was good. 

After fixing the issue, I set out to see why it may not have been running. The issue was caused by the httpd being set to stop at boot time. I fixed this by using "chkconfig httpd on" which set it to start at boot time. Also accomplished with "sudo /sbin/chkconfig --add httpd"

Curly Brackets in Chef/Ruby

Surprise, it helps to know the syntax of the language you are writing in.

The issue I ran into this time has to deal with the curly brackets in Ruby. 

Code Example 1

execute 'policy_selinux' do

  command "semodule -i /tmp/policy.pp"

  action :run

  not_if { 'semodule -l | grep policy' }

end

Code Example 2

execute 'policy_selinux' do

  command "semodule -i /tmp/policy.pp"

  action :run

  not_if 'semodule -l | grep policy' 

end

The key to note between example 1 and 2 is that in the second example there is no use of the curly "{}" brackets. This is for a very good reason. The curly brackets, in our case, are being used like a code block. This means that in example 1, the not_if is evaluating the {'semodule -l | grep policy'} as a code block. The code block evaluates as true all the time because it is evaluating a string that has no arguments to evaluate it(tautology). 

The code example 2 has no "{}" which means that the string is going to be used by the not_if statement. This will not be evaluated as a code block, but a string that is truly a command with a return status. This will allow us to get our return status like we desire.

Below is a good resource that helped lead me to my conclusion.
http://stackoverflow.com/questions/12565/what-do-the-different-brackets-in-ruby-mean