What is a ’Poll plugin’ ?

Poll plugins have the task of periodically poll the servers in the server farm, also, this plugins may (or may not) collect stats and rebuild the servers database (e.g. mark a server as unavailable if the server can’t be reached, or bring back up a server if it can be reached by the poll tests). This plugins can be stacked, this means that you can catenate the tests together, using the previous result in the following plugin (note that all the results are stored in the Load Balancer server database).

We have the following poll plugins available in Load Balancer:

plugin name	Description
poll_generic.so	This is the generic poll plugin. This plugin trasverses all the servers in all the clusters, and issues a open/close connection, so it tests that the machine is alive, and we can connect and close to this port. Also knows when a server can be marked enable or disabled. At last, this plugin collects some information about the time spent between the connection and the close (this allows to stimate the servers’ response time). configuration options server_timeout: the number of seconds required to mark this server as unrecheable
poll_http.so	This is a ’smart’ plugin that understands the HTTP protocol. poll_http transverses all the servers in all the clusters, and issues a HTTP request to the server. Then it processes the result, and scan the return value to check if it is a hit (code 2xx or 3xx) or if its an error (4xx or above). Also knows when a server can be marked enable or disabled. At last, this plugin collects some information about the time spent between the connection and the close (this allows to stimate the servers’ response time). This poll plugin is very useful to check a HTTP server, not only if it is recheable or not, it also test if the documents are accesible. configuration options server_timeout: the number of seconds required to mark this server as unrecheable. url: the url that the plugin will poll to check if the HTTP server is returning the data ok.

poll_generic.so

  <poll_generic 
       server_timeout="2"
  />

poll_http.so

  <poll_http 
       server_timeout="2" 
       url="/" 
  />