As already described, Apama events are rigidly structured and need to comply with a precise event type definition. This describes the structure of a particular event: in particular its name, as well as the order, name, type and number of its constituent fields.

By contrast, external events, even when they are of the same ‘type’ or nature (e.g. all Trade events or News headlines) might vary in format and structure, even when originating from the same source or feed.

In order to accommodate this, Apama provides an optional payload field in Apama events. The payload field, typically the last field in an event type definition, can embed any number of additional optional fields in addition to the always-present primary fields.

For example, consider an external message that can appear in several guises, but where each always consists of a particular subset of critical fields together with a variable number of additional optional fields.

If it is desired that these varying guises are mapped to a single Apama event type, then this needs to be defined so that its fields correspond to the subset of critical (and always present) fields, followed by a payload field into which the additional (and optional) fields are embedded.