This is a way of extending functionality in Centreon Engine without having to alter main code. Modules are based on shared code libraries. Modules are hooked into the Centreon Engine process. Modules uses callback routines which have to be served by the modules. These routines are executed when special events occur in the Centreon Engine server process.
The external commands system was moved on a module. To use it, you need to add line on Centreon Engine configuration: