OpenNMS’ Metadata DSL (domain specific language) allows you to use dynamic configuration in parameter values to interpolate metadata into the parameter. The syntax allows for the use of patterns in an expression, whereby the metadata is replaced with a corresponding value during the collection process.
You can use this feature with provisioning (service detectors), service assurance (pollerd), and performance management (collectd) in OpenNMS. Using metadata in these situations can streamline the configuration of things that require customization and items, such as protocol port numbers, that are unique to a node.
Ronny Trommer and Marcel Fuhrmann have written a use case that shows how to use metadata to monitor websites. The use case presents a scenario of running a blackbox test on a web server that hosts three websites, and a detailed explanation of how to configure OpenNMS to do this.
Link: Monitoring Websites Using Metadata