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 204         <div class="columnleft">
 205                 <h1>A Better Way to Monitor</h1>
 206                 <p>God is an easy to configure, easy to extend monitoring framework written in Ruby.</p>
 207                 <p>Keeping your server processes and tasks running should be a simple part of your deployment process. God aims to be the simplest, most powerful monitoring application available.</p>
 208         </div>
 209
 210         <div class="columnright">
 211         <h1>Features</h1>
 212         <ul class="features">
 213                 <li>Config file is written in Ruby</li>
 214                 <li>Write your own conditions in Ruby</li>
 215         </ul>
 216         </div>
 217
 218         <h1>Installation</h1>
 219         <p>God does not yet have a public release. You can read through these docs (which are likely to change), or if you're really antsy, peruse or clone the code from <a href="http://repo.or.cz/w/god.git">http://repo.or.cz/w/god.git</a></p>
 220
 221         <h1>Finally, a Config File that Makes Sense</h1>
 222         <p>The easiest way to understand how god will make your life better is by looking at a sample config file. The following configuration file is what I use at <a href="http://site.gravatar.com/">gravatar.com</a> to keep the mongrels running:</p>
 223
 224 <pre><code class="ruby"># file: gravatar.god
 225 #
 226 # This is the actual config file used to keep the mongrels of
 227 # gravatar.com running.
 228
 229 RAILS_ROOT = "/var/www/gravatar2/current"
 230
 231 God.meddle do |god|
 232   god.interval = 30 # seconds
 233
 234   %w{8200 8201 8202}.each do |port|
 235     god.watch do |w|
 236       w.name = "gravatar2-mongrel-#{port}"
 237       w.cwd = RAILS_ROOT
 238       w.start = "mongrel_rails cluster::start --only #{port}"
 239       w.stop = "mongrel_rails cluster::stop --only #{port}"
 240       w.grace = 10 # seconds
 241
 242       pid_file = File.join(RAILS_ROOT, "log/mongrel.#{port}.pid")
 243
 244       w.behavior(:clean_pid_file) do |b|
 245         b.pid_file = pid_file
 246       end
 247
 248       w.start_if do |start|
 249         start.condition(:process_not_running) do |c|
 250           c.pid_file = pid_file
 251         end
 252       end
 253
 254       w.restart_if do |restart|
 255         restart.condition(:memory_usage) do |c|
 256           c.pid_file = pid_file
 257           c.above = (150 * 1024) # 150mb
 258           c.times = [3, 5] # 3 out of 5 intervals
 259         end
 260
 261         restart.condition(:cpu_usage) do |c|
 262           c.pid_file = pid_file
 263           c.above = 50 # percent
 264           c.times = 5
 265         end
 266       end
 267     end
 268   end
 269 end</code></pre>
 270
 271 <p>That's a lot to take in at once, so I'll break it down by section and explain what's going on in each.</p>
 272
 273 <pre><code class="ruby">RAILS_ROOT = "/var/www/gravatar2/current"</code></pre>
 274
 275 <p>Here I've set a constant that is used throughout the file. Keeping the <code>RAILS_ROOT</code> value in a constant makes it easy to adapt this script to other applications. Because the config file is Ruby code, I can set whatever variables or constants I want that make the configuration more concise and easier to work with.</p>
 276
 277 <pre><code class="ruby">God.meddle do |god|
 278   god.interval = 30 # seconds
 279
 280   ...
 281 end</code></pre>
 282
 283 <p>The meat of the config file is defined inside a <code>God.meddle</code> block. At this level, you set system wide settings like <code>interval</code>. In this case, we've specified that god should do a check every 30 seconds.</p>
 284
 285 <pre><code class="ruby">  %w{8200 8201 8202}.each do |port|
 286     ...
 287   end</code></pre>
 288
 289 <p>Because the config file is written in actual Ruby code, we can construct loops and do other intelligent things that are impossible in your every day, run of the mill config file. I need to watch three mongrels, so I simply loop over their port numbers, eliminating duplication and making my life a whole lot easier.</p>
 290
 291 <pre><code class="ruby">    god.watch do |w|
 292       w.name = "gravatar2-mongrel-#{port}"
 293       w.cwd = RAILS_ROOT
 294       w.start = "mongrel_rails cluster::start --only #{port}"
 295       w.stop = "mongrel_rails cluster::stop --only #{port}"
 296       w.grace = 5 # seconds
 297
 298       ...
 299     end</code></pre>
 300
 301 <p>A <code>watch</code> represents a single process or task that has concrete start, stop, and/or restart operations. You can define as many watches as you like inside the <code>God.meddle</code> block. In the example above, I've got a Rails instance running in a Mongrel that I need to keep alive. Every watch must have a unique <code>name</code> so that it can be identified later on. The optional <code>cwd</code> attribute tells god to switch to that directory before executing any commands. The <code>start</code> and <code>stop</code> attributes specify the commands to start and stop the process. If no <code>restart</code> attribute is set, restart will be represented by a call to stop followed by a call to start. The optional <code>grace</code> attribute sets the amount of time following a start/stop/restart command to wait before resuming normal monitoring operations.</p>
 302
 303 <pre><code class="ruby">      pid_file = File.join(RAILS_ROOT, "log/mongrel.#{port}.pid")</code></pre>
 304
 305 <p>Another variable to hold the location of the PID file.</p>
 306
 307 <pre><code class="ruby">      w.behavior(:clean_pid_file) do |b|
 308         b.pid_file = pid_file
 309       end</code></pre>
 310
 311 <p>Behaviors allow you to execute additional commands around start/stop/restart commands. In our case, if the process dies it will leave a PID file behind. The next time a start command is issued, it will fail, complaining about the leftover PID file. We'd like that file cleaned up before a start command is issued. The built-in behavior <code>clean_pid_file</code> will do just that. All we have to do it specify the location of the PID file.</p>
 312
 313 <pre><code class="ruby">      w.start_if do |start|
 314         start.condition(:process_not_running) do |c|
 315           c.pid_file = pid_file
 316         end
 317       end</code></pre>
 318
 319 <p>Watches contain conditions grouped by the action to execute should they fail. I start with a <code>start_if</code> block that contains a single condition. Conditions are specified by calling <code>condition</code> with an identifier, in this case
 320 <code>:process_not_running</code>.
 321
 322 <pre><code class="ruby">      w.restart_if do |restart|
 323         restart.condition(:memory_usage) do |c|
 324           c.pid_file = pid_file
 325           c.above = (150 * 1024) # 150mb
 326           c.times = [3, 5] # 3 out of 5 intervals
 327         end
 328
 329         ...
 330       end</code></pre>
 331
 332 <p>Similar to <code>start_if</code> there is a <code>restart_if</code> command that groups conditions that should trigger a restart. The <code>memory_usage</code> condition will fail if the specified process is using too much memory. Once again, the <code>pid_file</code> must be set. The maximum allowable amount of memory is specified with the <code>above</code> attribute in units of kilobytes. The number of times the test needs to fail in order to trigger a restart is set with <code>times</code>. This can be either an integer or an array. An integer means it must fail that many times in a row while an array [x, y] means it must fail x times out of the last y tests.</p>
 333
 334 <pre><code class="ruby">      w.restart_if do |restart|
 335         ...
 336
 337         restart.condition(:cpu_usage) do |c|
 338           c.pid_file = pid_file
 339           c.above = 50 # percent
 340           c.times = 5
 341         end
 342       end</code></pre>
 343
 344 <p>To keep an eye on CPU usage, I've employed the <code>cpu_usage</code> condition. When CPU usage for a Mongrel process is over 50% for 5 consecutive intervals, it will be restarted by god.</p>
 345
 346 <h1>Extend God with your own Conditions</h1>
 347
 348 <p>God was designed from the start to allow you to easily write your own custom conditions, making it simple to add tests that are application specific.</p>
 349
 350 <pre><code class="ruby"></code></pre>
 351
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