Adding the self-healing page

.gitignore

@@ -7,6 +7,7 @@
 target
 doc/website/faq.html
 doc/website/configuring-and-tuning.html
+doc/website/self-healing.html
 examples/ruby/tmp
 test-output
 .dotest

doc/website/self-healing.markdown

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+Title: Selenium Grid Self-Healing
+CSS: stylesheets/site.css stylesheets/document.css stylesheets/faq.css
+Use numbered headers: false
+
+<div class="header">
+  <a href="index.html"><img alt="Selenium_grid_logo_large" src="images/selenium-grid-logo-large.png"/></a>
+  <p>Self-Healing</p>
+</div>
+
+Table Of Content:
+{: class=toc}
+
+* This will become a table of contents (this text will be scraped).
+{:toc}
+
+Heartbeat Mechanism
+===================
+
+ To ease the maintenance of a Selenium Grid deployment:
+
+ * Selenium Grid Hub sends periodic heartbeat messages to all registered
+   Remote Controls to make sure that they are still up-and-running.
+
+ * Selenium Grid Remote Controls send periodic heartbeat messages to the Hub
+   to check that it still considers them as registered.
+
+  Keep in mind that **there is obviously a performance tradeoff in how often
+  these heartbeat messages are sent**:
+
+ * If you send them too often they will use most of your network bandwidth for
+   heartbeat management messages as opposed to actual testing commands.
+
+ * If you wait too long to send heartbeat messages, it might take longer for
+   the Hub and the Remote Controls to automatically unregister or reconnect.
+
+
+Hub: Automatic Unregistering of Unresponsive Remote Controls
+===========================================================
+
+  Remote Controls sometimes die, are killed or just hang. To better
+  cope with these situations, Selenium Grid Hub checks the status of registered
+  Remote Controls. It will automatically unregister:
+
+ * A Remote Control detected as **unavailable when reserving it to start a new
+   test session**
+
+ * A Remote Control that **does not respond to a heartbeat message request**
+   (periodic poll every `remoteControlPollingIntervalInSeconds` seconds,
+   default is 2 minutes)
+
+ * A _reserved_ Remote Control whose **testing session has been idle for more
+   than `sessionMaxIdleTimeInSeconds` seconds** (3 minutes by default).
+   Sessions are checked every `remoteControlPollingIntervalInSeconds` seconds
+   (at the same time heartbeat messages are sent).
+
+ `remoteControlPollingIntervalInSeconds` and `sessionMaxIdleTimeInSeconds` are 
+ configuration parameters that can be set in `grid_configuration.yml`. For instance:
+
+    hub:
+       port: 4444
+       remoteControlPollingIntervalInSeconds: 180
+       sessionMaxIdleTimeInSeconds: 300
+       environments:
+           - name:    "Firefox on Windows"
+             browser: "*firefox"
+      
+
+Remote Control: Automatic Re-registering to the Hub
+===================================================
+
+  Sometimes you might want to restart Selenium Grid Hub without restarting all
+  the Remote Controls that were previously registered. Or the Hub might
+  temporarily get disconnected from the remote controls. To better cope with
+  these situations, the Remote Controls periodically poll the Hub (every
+  `hubPollerIntervalInSeconds` seconds, default is 3 minutes).
+
+  If the Hub can be contacted but does not have any registered Remote Control 
+  matching the information sent by the Remote Control submitting the
+  heartbeat request, **the Remote Control will re-register itself to the Hub
+  automatically**.
+
+  You can specify `hubPollerIntervalInSeconds` when starting the 
+  Remote Controls. For instance:
+
+    ant -DhubPollerIntervalInSeconds=120 launch-hub  
+
+