Abstract: Provided are a system and method for supporting communication between application programs and the processing of messages by those programs. A table driven approach is used to select appropriate components of modular application programs to process received messages. The selection is carried out in dependence on associations between message types (e.g. request, inform, reply) and other characteristics of the message (e.g. whether the message is an expected reply (as identified by an identifier value). Also used in said selection are dynamic characteristics of the message or the system (e.g. possibly expiry of a timeout, or the state of an application). Rules combine these criteria and determine the conditions for invoking an application program component.Also provided is a mechanism for differentiating between reply messages which are received out of serial sequence but are still current and messages which are invalid.