The message handler is a simple interface for processing messages:

public interface IMessageHandler
    boolean handleMessage(@NotNull CEnvelope aEnvelope,
                          @NotNull CRecord aRecord) throws Exception;

It receives the components of the message: the envelope with the information needed for the message transport, and the Record as payload.

The return value indicates whether the message was finally processed (true), or not (false).

The message handler is used in target, where it is either registered for a message ID, or where it serves as a null handler as a handler for all unspecified messages. - Innovative Distributed System