The codec layer is a layer of abstraction between the transport layer and the IAF's Semantic Mapper. It consists of one or more plug-in libraries that perform message encodinganddecoding. Decoding involves translating downstream messages retrieved by the transport layer into the standard ‘normalised event' format on which the Semantic Mapper's rules run; encoding works in the opposite direction, converting upstream normalized events into an appropriate format for transport layer plug-ins to send on. Note that unlike the situation with C/C++, in Java codec plug-ins are always both encoders and decoders. See The Integration Adapter Framework for a full introduction to codec plug-ins and the IAF's architecture.

This chapter includes the Codec Plug-in Development Specification for Java and additional information for developers of Java event codecs. C/C++ Codec Plug-in Development provides analogous information about developing codec plug-ins using C/C++.

Before developing a new codec plug-in, it is worth considering whether one of the standard Apama plug-ins could be used instead. Using Adapter Plug-ins provides more information on the standard IAF codec plug-ins: JStringCodec and JNullCodec. The JStringCodec plug-in codes normalized events as formatted text strings. The JNullCodec plug-in is useful in situations where it does not make sense to decouple the codec and transport layers, and allows transport plug-ins to communicate with the Semantic Mapper directly using normalized events.