7.5.2.1 Load polling

Each host object is able to determine the system load of its corresponding host on demand. Via a so called spy process which is simply a remote shell process (sh) that listens on stdin, it is issuing uptime commands that return the system load. Polling is initiated by a call of the virtual host's :trigger-spy method. As soon as the load has been determined successfully the object signals this circumstance by means of the new-load event. Typically the balancing authority, and the GUI are listening for the new-load event.

It should be stressed that this is done asynchronously, which means that availability of the result of the uptime command is notified by the spy process. This has a strong influence on the robustness of this load balancing mechanism since it might easily be possible that a host is not able to respond for some reason. In such a situation SIESTA would be stalled if the communication were synchronous.