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 208         <div class="columnleft">
 209                 <h1>A Better Way to Monitor</h1>
 210                 <p>God is an easy to configure, easy to extend monitoring framework written in Ruby.</p>
 211                 <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>
 212                 <p style="text-align: right">Tom Preston-Werner<br />tom at rubyisawesome dot com</p>
 213         </div>
 214
 215         <div class="columnright">
 216         <h1>Features</h1>
 217         <ul class="features">
 218                 <li>Config file is written in Ruby</li>
 219                 <li>Easily write your own custom conditions in Ruby</li>
 220                 <li>Supports both poll and event based conditions</li>
 221                 <li>Different poll conditions can have different intervals</li>
 222                 <li>Easily control non-daemonized processes</li>
 223         </ul>
 224         </div>
 225
 226         <h1>Installation (v 0.4.0)</h1>
 227         <p>The best way to get god is via rubygems:</p>
 228         <pre>$ sudo gem install god</pre>
 229         <p>You can also peruse or clone the code from <a href="http://repo.or.cz/w/god.git">http://repo.or.cz/w/god.git</a></p>
 230
 231         <h1>Requirements</h1>
 232
 233         <p>God currently only works on <b>Linux (kernel 2.6.15+), BSD,</b> and <b>Darwin</b> systems. No support for Windows is planned. Event based conditions on Linux systems require the <code>cn</code> (connector) kernel module loaded or compiled in to the kernel and god must be run as root.</p>
 234
 235         <p>The following systems have been tested. Help us test it on others!</p>
 236
 237         <ul>
 238           <li>Darwin 10.4.10</li>
 239           <li>RedHat Fedora Core 6</li>
 240           <li>Ubuntu Dapper (no events)</li>
 241           <li>Ubuntu Feisty</li>
 242           <li>CentOS 4.5 (no events)</li>
 243         </ul>
 244
 245         <h1>Finally, a Config File that Makes Sense</h1>
 246         <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>
 247
 248 <pre><code class="ruby"># file:      gravatar.god
 249 # run with:  god -c /path/to/gravatar.god
 250 #
 251 # This is the actual config file used to keep the mongrels of
 252 # gravatar.com running.
 253
 254 RAILS_ROOT = "/var/www/gravatar2/current"
 255
 256 %w{8200 8201 8202}.each do |port|
 257   God.watch do |w|
 258     w.name = "gravatar2-mongrel-#{port}"
 259     w.interval = 30.seconds # default
 260     w.start = "mongrel_rails cluster::start --only #{port} \
 261       -C #{RAILS_ROOT}/config/mongrel_cluster.yml"
 262     w.stop = "mongrel_rails cluster::stop --only #{port} \
 263       -C #{RAILS_ROOT}/config/mongrel_cluster.yml"
 264     w.grace = 10.seconds
 265     w.pid_file = File.join(RAILS_ROOT, "log/mongrel.#{port}.pid")
 266
 267     w.behavior(:clean_pid_file)
 268
 269     w.start_if do |start|
 270       start.condition(:process_running) do |c|
 271         c.interval = 5.seconds
 272         c.running = false
 273       end
 274     end
 275
 276     w.restart_if do |restart|
 277       restart.condition(:memory_usage) do |c|
 278         c.above = 150.megabytes
 279         c.times = [3, 5] # 3 out of 5 intervals
 280       end
 281
 282       restart.condition(:cpu_usage) do |c|
 283         c.above = 50.percent
 284         c.times = 5
 285       end
 286     end
 287   end
 288 end</code></pre>
 289
 290 <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>
 291
 292 <pre><code class="ruby">RAILS_ROOT = "/var/www/gravatar2/current"</code></pre>
 293
 294 <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>
 295
 296 <pre><code class="ruby">%w{8200 8201 8202}.each do |port|
 297   ...
 298 end</code></pre>
 299
 300 <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>
 301
 302 <pre><code class="ruby">  God.watch do |w|
 303     w.name = "gravatar2-mongrel-#{port}"
 304     w.interval = 30.seconds # default
 305     w.start = "mongrel_rails cluster::start --only #{port} \
 306       -C #{RAILS_ROOT}/config/mongrel_cluster.yml"
 307     w.stop = "mongrel_rails cluster::stop --only #{port} \
 308       -C #{RAILS_ROOT}/config/mongrel_cluster.yml"
 309     w.grace = 10.seconds
 310     pid_file = File.join(RAILS_ROOT, "log/mongrel.#{port}.pid")
 311
 312     ...
 313   end</code></pre>
 314
 315 <p>A <code>watch</code> represents a single process that has concrete start, stop, and/or restart operations. You can define as many watches as you like. 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 <code>interval</code> option sets the default poll interval (this can be overridden in each condition). 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. If the process you're watches runs as a daemon, you'll need to set the <code>pid_file</code> attribute in order for the behaviors and conditions to function.</p>
 316
 317 <pre><code class="ruby">    w.behavior(:clean_pid_file)</code></pre>
 318
 319 <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 the PID file cleaned up before a start command is issued. The built-in behavior <code>clean_pid_file</code> will do just that.</p>
 320
 321 <pre><code class="ruby">    w.start_if do |start|
 322       start.condition(:process_running) do |c|
 323         c.interval = 5.seconds
 324         c.running = false
 325       end
 326     end</code></pre>
 327
 328 <p>Watches contain conditions grouped by the action to execute should they return <code>true</code>. 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
 329 <code>:process_running</code>. Each condition can specify a poll interval that will override the default watch interval. In this case, I want to check that the process is still running every 5 seconds instead of the 30 second interval that other conditions will inherit. The ability to set condition specific poll intervals makes it possible to run costly tests less often then cheap tests.</p>
 330
 331 <pre><code class="ruby">    w.restart_if do |restart|
 332       restart.condition(:memory_usage) do |c|
 333         c.above = 150.megabytes
 334         c.times = [3, 5] # 3 out of 5 intervals
 335       end
 336
 337       ...
 338     end</code></pre>
 339
 340 <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. The maximum allowable amount of memory is specified with the <code>above</code> attribute (you can use the kilobytes, megabytes, or gigabytes helpers). 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>
 341
 342 <pre><code class="ruby">    w.restart_if do |restart|
 343       ...
 344
 345       restart.condition(:cpu_usage) do |c|
 346         c.above = 50.percent
 347         c.times = 5
 348       end
 349     end</code></pre>
 350
 351 <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.</p>
 352
 353 <p>That's it! Simple, huh?</p>
 354
 355 <!-- ------------------------------------------------------------------------- -->
 356
 357 <h1>Changing UID/GID for processes</h1>
 358
 359 <p>It is possible to have god run your start/stop/restart commands as a specific user/group. This can be done by setting the <code>uid</code> and/or <code>gid</code> attributes of a watch.</p>
 360
 361 <pre><code class="ruby">  God.watch do |w|
 362     ...
 363
 364     w.uid = 'tom'
 365     w.gid = 'devs'
 366
 367     ...
 368   end</code></pre>
 369
 370 </p>This only works for commands specified as a string. Lambda commands are unaffected.</p>
 371
 372
 373 <!-- ------------------------------------------------------------------------- -->
 374
 375 <h1>Lambda commands</h1>
 376
 377 <p>In addition to specifying start/stop/restart commands as strings (to be executed via the shell), you can specify a lambda that will be called.</p>
 378
 379 <pre><code class="ruby">  God.watch do |w|
 380     ...
 381
 382     w.start = lambda { ENV['APACHE'] ? `apachectl -k graceful` : `lighttpd restart` }
 383
 384     ...
 385   end</code></pre>
 386
 387 <!-- ------------------------------------------------------------------------- -->
 388
 389 <h1>Starting and Controlling God</h1>
 390
 391 <p>To start the god monitoring process as a daemon simply run the <code>god</code> executable passing in the path to the config file (you need to sudo if you're using events on Linux or want to use the setuid/setgid functionality):</p>
 392
 393 <pre>$ sudo god -c /path/to/config.god</pre>
 394
 395 <p>While you're writing your config file, it can be helpful to run god in the foreground so you can see the log messages. You can do that with:</p>
 396
 397 <pre>$ sudo god -c /path/to/config.god -D</pre>
 398
 399 <p>You can start/restart/stop/monitor/unmonitor your Watches with the same utility like so:</p>
 400
 401 <pre>$ sudo god stop 'gravatar2-mongrel-8200'</pre>
 402
 403 <!-- ------------------------------------------------------------------------- -->
 404
 405 <h1>Grouping Watches</h1>
 406
 407 <p>Watches can be assigned to groups. These groups can then be controlled together from the command line.</p>
 408
 409 <pre><code class="ruby">  God.watch do |w|
 410     ...
 411
 412     w.group = 'mongrels'
 413
 414     ...
 415   end</code></pre>
 416
 417 <p>The above configuration now allows you to control the watch (and any others that are in the group) with a single command:</p>
 418
 419 <pre>$ sudo god stop 'mongrels'</pre>
 420
 421 <!-- ------------------------------------------------------------------------- -->
 422
 423 <h1>Advanced Configuration with Transitions and Events</h1>
 424
 425 <p>So far you've been introduced to a simple poll-based config file and seen how to run it. Poll-based monitoring works great for simple things, but falls short for highly critical tasks. God has native support for kqueue/netlink events on BSD/Darwin/Linux systems. For instance, instead of using the <code>process_running</code> condition to poll for the status of your process, you can use the <code>process_exits</code> condition that will be notified <b>immediately</b> upon the exit of your process. This means less load on your system and shorter downtime after a crash.</p>
 426
 427 <p>While the configuration syntax you saw in the previous example is very simple, it lacks the power that we need to deal with event based monitoring. In fact, the <code>start_if</code> and <code>restart_if</code> methods are really just calling out to a lower-level API. If we use the low-level API directly, we can harness the full power of god's event based lifecycle system. Let's look at another example config file.</p>
 428
 429 <pre><code class="ruby">RAILS_ROOT = "/Users/tom/dev/git/helloworld"
 430
 431 God.watch do |w|
 432   w.name = "local-3000"
 433   w.interval = 5.seconds # default
 434   w.start = "mongrel_rails start -P ./log/mongrel.pid -c #{RAILS_ROOT}"
 435   w.stop = "mongrel_rails stop -P ./log/mongrel.pid -c #{RAILS_ROOT}"
 436   w.pid_file = File.join(RAILS_ROOT, "log/mongrel.pid")
 437
 438   # clean pid files before start if necessary
 439   w.behavior(:clean_pid_file)
 440
 441   # determine the state on startup
 442   w.transition(:init, { true => :up, false => :start }) do |on|
 443     on.condition(:process_running) do |c|
 444       c.running = true
 445     end
 446   end
 447
 448   # determine when process has finished starting
 449   w.transition([:start, :restart], :up) do |on|
 450     on.condition(:process_running) do |c|
 451       c.running = true
 452     end
 453
 454     # failsafe
 455     on.condition(:tries) do |c|
 456       c.times = 3
 457       c.transition = :start
 458     end
 459   end
 460
 461   # start if process is not running
 462   w.transition(:up, :start) do |on|
 463     on.condition(:process_exits)
 464   end
 465
 466   # restart if memory or cpu is too high
 467   w.transition(:up, :restart) do |on|
 468     on.condition(:memory_usage) do |c|
 469       c.interval = 20
 470       c.above = 50.megabytes
 471       c.times = [3, 5]
 472     end
 473
 474     on.condition(:cpu_usage) do |c|
 475       c.interval = 10
 476       c.above = 10.percent
 477       c.times = [3, 5]
 478     end
 479   end
 480 end
 481 </code></pre>
 482
 483 <p>A bit longer, I know, but very straighforward once you understand how the <code>transition</code> calls work. The <code>name</code>, <code>interval</code>, <code>start</code>, <code>stop</code>, and <code>pid_file</code> attributes should be familiar. We also specify the <code>clean_pid_file</code> behavior.</p>
 484
 485 <p>Before jumping into the code, it's important to understand the different states that a Watch can have, and how that state changes over time. At any given time, a Watch will be in one of the <code>init</code>, <code>up</code>, <code>start</code>, or <code>restart</code> states. As different conditions are satisfied, the Watch will progress from state to state, enabling and disabling conditions along the way.</p>
 486
 487 <p>When god first starts, each Watch is placed in the <code>init</code> state.</p>
 488
 489 <p>You'll use the <code>transition</code> method to tell god how to transition between states. It takes two arguments. The first argument may be either a symbol or an array of symbols representing the state or states during which the specified conditions should be enabled. The second argument may be either a symbol or a hash. If it is a symbol, then that is the state that will be transitioned to if any of the conditions return <code>true</code>. If it is a hash, then that hash must have both <code>true</code> and <code>false</code> keys, each of which point to a symbol that represents the state to transition to given the corresponding return from the single condition that must be specified.</p>
 490
 491 <pre><code class="ruby">  # determine the state on startup
 492   w.transition(:init, { true => :up, false => :start }) do |on|
 493     on.condition(:process_running) do |c|
 494       c.running = true
 495     end
 496   end</code></pre>
 497
 498 <p>The first transition block tells god what to do when the Watch is in the <code>init</code> state (first argument). This is where I tell god how to determine if my task is already running. Since I'm monitoring a process, I can use the <code>process_running</code> condition to determine whether the process is running. If the process is running, it will return true, otherwise it will return false. Since I sent a hash as the second argument to <code>transition</code>, the return from <code>process_running</code> will determine which of the two states will be transitioned to. If the process is running, the return is true and god will put the Watch into the <code>up</code> state. If the process is not running, the return is false and god will put the Watch into the <code>start</code> state.</p>
 499
 500 <pre><code class="ruby">  # determine when process has finished starting
 501   w.transition([:start, :restart], :up) do |on|
 502     on.condition(:process_running) do |c|
 503       c.running = true
 504     end
 505
 506     ...
 507   end</code></pre>
 508
 509 <p>If god has determined that my process isn't running, the Watch will be put into the <code>start</code> state. Upon entering this state, the <code>start</code> command that I specified on the Watch will be called. In addition, the above transition specifies a condition that should be enabled when in either the <code>start</code> or <code>restart</code> states. The condition is another <code>process_running</code>, however this time I'm only interested in moving to another state once it returns <code>true</code>. A <code>true</code> return from this condition means that the process is running and it's ok to transition to the <code>up</code> state (second argument to <code>transition</code>).</p>
 510
 511 <pre><code class="ruby">  # determine when process has finished starting
 512   w.transition([:start, :restart], :up) do |on|
 513     ...
 514
 515     # failsafe
 516     on.condition(:tries) do |c|
 517       c.times = 3
 518       c.transition = :start
 519     end
 520   end</code></pre>
 521
 522 <p>The other half of this transition uses the <code>tries</code> condition to ensure that god doesn't get stuck in this state. It's possible that the process could go down while the transition is being made, in which case god would end up polling forever to see if the process is up. Here I've specified that if this condition is called three times, god should override the normal transition destination and move to the <code>start</code> state instead. If you specify a <code>transition</code> attribute on any condition, that state will be transferred to instead of the normal transfer destination.</p>
 523
 524 <pre><code class="ruby">  # start if process is not running
 525   w.transition(:up, :start) do |on|
 526     on.condition(:process_exits)
 527   end</code></pre>
 528
 529 <p>This is where the event based system comes into play. Once in the <code>up</code> state, I want to be notified when my process exits. The <code>process_exits</code> condition registers a callback that will trigger a transition change when it is fired off. Event conditions (like this one) cannot be used in transitions that have a hash for the second argument (as they do not return true or false).</p>
 530
 531 <pre><code class="ruby">  # restart if memory or cpu is too high
 532   w.transition(:up, :restart) do |on|
 533     on.condition(:memory_usage) do |c|
 534       c.interval = 20
 535       c.above = 50.megabytes
 536       c.times = [3, 5]
 537     end
 538
 539     on.condition(:cpu_usage) do |c|
 540       c.interval = 10
 541       c.above = 10.percent
 542       c.times = [3, 5]
 543     end
 544   end</code></pre>
 545
 546 <p>Notice that I can have multiple transitions with the same start state. In this case, I want to have the <code>memory_usage</code> and <code>cpu_usage</code> poll conditions going at the same time that I listen for the process exit event. In the case of runaway CPU or memory usage, however, I want to transition to the <code>restart</code> state. When a Watch enters the <code>restart</code> state it will either call the <code>restart</code> command that you specified, or if none has been set, call the <code>stop</code> and then <code>start</code> commands.</p>
 547
 548 <!-- ------------------------------------------------------------------------- -->
 549
 550 <h1>Watching Non-Daemon Processes</h1>
 551
 552 <p>Need to watch a script that doesn't have built in daemonization? No problem! God will daemonize and keep track of your process for you. If you don't specify a <code>pid_file</code> attribute for a watch, it will be auto-daemonized and a PID file will be stored for it in <code>/var/run/god</code>. If you'd rather have the PID file stored in a different location, you can set it in a <code>God.init</code> block at the top of your config:</p>
 553
 554 <pre><code class="ruby">God.init do |god|
 555   god.pid_file_directory = '/home/tom/pids'
 556 end
 557
 558 God.watch do |w|
 559   # watch with no pid_file attribute set
 560 end</code></pre>
 561
 562 <p>The directory you specify must be writable by god.</p>
 563
 564 <!-- ------------------------------------------------------------------------- -->
 565
 566 <h1>Loading Other Config Files</h1>
 567
 568 <p>You should feel free to separate your god configs into separate files for easier organization. You can load in other configs using Ruby's normal <code>load</code> method, or use the convenience method <code>God.load</code> which allows for glob-style paths:</p>
 569
 570 <pre><code class="ruby"># load in all god configs
 571 God.load "/usr/local/conf/*.god"</code></pre>
 572
 573 <p>God won't start it's monitoring operations until all configurations have been loaded.</p>
 574
 575 <!-- ------------------------------------------------------------------------- -->
 576
 577 <h1>Dynamically Loading Config Files Into an Already Running God</h1>
 578
 579 <p>God allows you to load or reload configurations into an already running instance. There are a few things to consider when doing this:</p>
 580
 581 <ul>
 582   <li>Existng Watches with the same <code>name</code> as the incoming Watches will be overidden by the new config.</li>
 583   <li>All paths must be either absolute or relative to the path from which god was started.</li>
 584 </ul>
 585
 586 <p>To load a config into a running god, issue the following command:</p>
 587
 588 <pre>$ sudo god load path/to/config.god</pre>
 589
 590 <p>Config files that are loaded dynamically can contain anything that a normal config file contains, however, <code>God.init</code> blocks will be ignored.</p>
 591
 592 <!-- ------------------------------------------------------------------------- -->
 593
 594 <h1>Getting Logs for a Single Watch</h1>
 595
 596 <p>Sifting through the god logs for statements specific to a specific Watch can be frustrating when you have many of them. You can get the realtime logs for a single Watch via the command line:</p>
 597
 598 <pre>$ sudo god log 'local-3000'</pre>
 599
 600 <p>This will display the last 100 lines of log for the 'local-3000' Watch and update every second with new log messages.</p>
 601
 602 <!-- ------------------------------------------------------------------------- -->
 603
 604 <h1>Extend God with your own Conditions</h1>
 605
 606 <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>
 607
 608 <pre><code class="ruby">module God
 609   module Conditions
 610
 611     class ProcessRunning < PollCondition
 612       attr_accessor :running
 613
 614       def valid?
 615         valid = true
 616         valid &= complain("You must specify the 'pid_file' attribute \
 617           on the Watch for :process_running") if self.watch.pid_file.nil?
 618         valid &= complain("You must specify the 'running' attribute \
 619           for :process_running") if self.running.nil?
 620         valid
 621       end
 622
 623       def test
 624         return !self.running unless File.exist?(self.watch.pid_file)
 625
 626         pid = File.read(self.watch.pid_file).strip
 627         active = System::Process.new(pid).exists?
 628
 629         (self.running && active) || (!self.running && !active)
 630       end
 631     end
 632
 633   end
 634 end</code></pre>
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