New god Conditions

If you haven't checked out god yet as an alternative to monit or other system/software-monitoring tools, do yourself a favour and head over to that link for a while and then come back... it's an awesome little monitoring tool written in ruby that has all kinds of cool features, including event-based conditions that will activate as soon as a process dies instead of needing a periodic check, etc...

I won't repeat what's on their site - suffice it to say that it's a pretty nifty little piece of software, and highly useful. As such, I'm already using it in numerous places, and have written a few custom conditions to extend what's packaged with it.

At the time of writing, these have been submitted to the maintainers for future inclusion (current version is 0.5.0, so look for them in 0.6.0 hopefully!), but I've posted the files for my wonderful readers so they can start using them right away ;)

All three of these have been tested with god v0.5.0, mysql_failed has been tested w/ mysql.rb v1.24, and disk_usage has been tested w/ an installed df v5.3.0 (linux 2.6.21.3).

mysql_failed (if at all) might end up in an auxillary god gem, since it's application specific and has external dependencies. Likewise, I'm going to try and re-write disk_usage (this was a quick one that I just wanted to get working) so that it's not dependant on external programs (df), but figures it out some other way, although I suspect this may still have to be dependant somehow on the environment (ie: /proc or something).

A few usage notes:

1) These files should be put in <god-gem-install-root>/lib/god/conditions.
2) You must edit <god-gem-install-root>/lib/god.rb to require them (at the top of the file) in order to be able to use them.

complex.rb:

This condition lets you combine other conditions into compound conditions... ie: this AND (that OR these). Usage is fairly straightforward, it works more or less like any other condition (see example below). Complex conditions can be nested ad infinitum (to emulate parentheses in 'real' compound logic statements) and the 'this()' method can be omitted if it makes your code DRYer...

on.condition(:complex) do |c|
 c.this(:memory_usage) do |c2|
   ...
 end

 c.or(:complex) do |c2|
   files = %w(file1.txt file2.txt file3.conf file4.bak)

   (0..3).to_a.each do |idx|
     c2.or(:some_kind_of_file_based_condition) do |c3|
       c3.filename = files[idx]
     end
   end
 end
end

mysql_failed.rb:

This condition is intended to test all aspects of a mysql dependancy that your app may have (ie: connection and privileges... others to be added in the future if necessary/requested, perhaps a possible mysql version check?). It's fairly simple to use, the only things requiring examples are the default config setup, which allows you to skip specifying any/all config info when setting up subsequent instances of this condition, and the way of specifying privileges for the connection...

on.condition(:mysql_failed) do |c|
  c.setUser('username')
  c.setPass('password')
  c.setHost('mysql.domain.com')
  c.setDB('database_name')
  c.setPrivs = {
    'select' => %w(table_one table_two),
    'delete' => ['table_three', 'table_four']
  }
end

on.condition(:mysql_failed) do |c|
  c.setHost('mysql2.domain.com')
end

Any info not explicitly set in secondary instances of the condition will inherit from the previously setup instance. So in the example above, the second mysql_failed condition will inherit the first mysql_failed's username, password, database name and privilege set to test for.

disk_usage.rb:

This one is simple enough to let the example do the talking...

on.condition(:disk_usage) do |c|
  c.limit = 90 # percentage of partition that needs to be full
  c.mount_point = '/usr'
end