Working with normalised events

The function of a decoding codec plug-in is to convert incoming messages into a standard normalised event format that can be processed by the Semantic Mapper. Events sent upstream to an encoding codec plug-in are provided to the plug-in in this same format.

Normalised events are essentially dictionaries of name-value pairs, where the names and values are both character strings. Each name-value pair nominally represents the name and content of a single field from an event, but users of the data structure are free to invent custom naming schemes to represent more complex event structures. Names must be unique within a given event. Values may be empty or NULL.

Note: When assigning names to fields in normalised events, keep in mind that the fields and transport attributes for event mapping conditions and event mapping rules both use a list of fields delimited by spaces or commas. This means, for example that <id fields="Exchange EX,foo" test="==" value="LSE"/> will successfully match a field called "Exchange", "EX" or "foo", but not a field called "Exchange EX,foo". While fields with spaces or commas in their names may be included in a payload dictionary in upstream or downstream directions, they cannot be referenced directly in mapping or id rules.

To construct strings for the normalised event fields representing container types (dictionaries, sequences, or nested events), use the EventWriter utility found in the AP_EventWriter.h header file, which is located in the include directory of the Apama installation. The following examples show how to add a sequence and a dictionary to a normalised event (note the escape character “ \ “ used in order to insert a quotation mark into a string).

#include <AP_EventWriter.h>
AP_EventWriter *map, *list;
AP_NormalisedEvent *event;
AP_EventWriterValue key, value;
list=AP_EventWriter_ctor(AP_SEQUENCE, NULL);
list->addString(list, "abc");
list->addString(list, "de\"f");
map=AP_EventWriter_ctor(AP_DICTIONARY, NULL);
key.stringValue="key1"; value.stringValue="value";
map->addDictValue(map, AP_STRING, key, AP_STRING, value);
key.stringValue="key\"{}2";
value.stringValue="value\"{}2";
map->addDictValue(map, AP_STRING, key, AP_STRING, value);
event=AP_NormalisedEvent_ctor();
event->functions->addQuick(event, "mySequenceField",
event->functions->list->toString(list));
event->functions->event->functions->addQuick(event,
"myDictionaryField", event->functions->map->toString(map));
AP_EventWriter_dtor(list);
AP_EventWriter_dtor(map);

Fields names and values of normalised events are in UTF-8 format. This means that the writer of the codec needs to ensure that downstream events are correctly formed and the codec should expect to handle UTF-8 coming upstream.

The NormalisedEvent.h header file defines objects and functions that make up a special programming interface for constructing and examining normalised events.

NormalisedEvent.h contains two main structures: AP_NormalisedEvent which represents a single normalised event, and AP_NormalisedEventIterator which can be used to step through the contents of a normalised event structure.