The annotation mechanism enables circuits to request and receive data from external user supplied processes.

A circuit component, the annotation node, can request data, causing an annotation request to be generated by the engine. The request is sent to the user supplied annotation server, via a socket stack.

The annotation server examines the request and sends response data back to the requesting annotation node.

More than one annotation server can connect to an engine, and an annotation server can connect to more than one engine.