Adding flexible subscription options to EPICS

The need for a mechanism to control and filter subscriptions to control system variables by the client was described in a paper at the ICALEPCS-2009 conference. The implementation follows a plug-in design that allows the insertion of plug-in instances into the event stream on the server side. The client can instantiate and configure these plug-ins when opening a subscription, by adding modifiers to the channel name using JSON notation. This paper describes the design and implementation of a modular server-side plug-in framework for Channel Access, and shows examples for plug-ins as well as their use within an EPICS (Experimental Physics and Industrial Control System) control system. It appears that the addition of the plug-in framework to the EPICS tool-kit adds valuable functionality to the IOC. It opens yet another way to customize the system, allowing to add exactly the operations needed for a specific installation, and configure them at run time individually for the appropriate connections