Monitoring design document

commons-monitoring defines a limited set of classes (interface) for application instrumentation :

• Monitor is used to specify a monitored resource (with very large sense), defined by a Key. The key itslef is a composite of a name, a category (technical application layering) and subsystem (fonctionnal)
  
• Repository to get or create monitors
  
• StatValue to handle application state. It supports some basic statisticals. Derived interface Gauge and Counter provide more specific methods.
  

A monitor handle a set of values, identified by roles Strings. Some default roles are defined for PERFORMANCES and multi-thread CONCURRENCY.

TODO explain period-based monitoring like "per 5 minutes values" used to build monitoring graphs.