While the transport layer communicates with the custom message sources and sinks and extracts messages, such as stock trade data, from them, the responsibility of the codec layer is to correctly interpret and decode each message into a ‘normalized' format on which the semantic mapping rules can be run; similarly in the upstream direction a codec may be responsible for encoding a normalized message in an appropriate format for transmission by particular transport(s).

Message sources like middleware message buses typically use proprietary representation of messages. Messages might appear as strings (possibly human readable or otherwise encoded) or sequences of binary characters. Messages might also be self-describing (possibly in XML or through some other proprietary descriptive format) or else be structured according to a schema available elsewhere.

Producing a universal generic normalized format from these messages requires the codec layer to understand the particular format of the messages. In the upstream direction the codec layer needs to encode the messages correctly according to the destination message sink.

As with the transport layer, in order to enable this custom functionality, the IAF is designed to dynamically load codec plug-ins that are capable of decoding and encoding the messages being received, when supplied with any required configuration properties. Apama provides some generic codec plug-ins, such as the StringCodec codec. This can decode most string based name-value representations of messages once it is configured with the syntactic elements used to delimit the elements in a message. In addition the user may develop proprietary codec plug-ins. See C/C++ Codec Plug-in Development and Java Codec Plug-in Development for the Codec plug-in Development Specifications for C/C++ and for Java, which describe how custom codec plug-ins may be developed using the C, C++ and Java programming languages.

An adapter can load multiple codec plug-ins (to deal with different message types). These are loaded at startup and the set of loaded codecs can be changed while the adapter is running. Individual codec plug-ins may also be re-configured at any time. If a codec plug-in requires startup or re-configuration parameters, these need to be supplied in the IAF configuration file as documented in The IAF configuration file.

This manual uses the terms codec layer plug-in and event codec interchangeably.