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 event parser/builder found in the util5.1.jar file, which is located in the Apama installation’s lib directory 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).

List<String> list = new ArrayList<String>();
list.add("abc");
list.add("de\"f");
Map<String,String> map = new HashMap<String,String>();
map.put("key1", "value1");
map.put("key\"{}2", "value\"{}2");
final SequenceFieldType STRING_SEQUENCE_FIELD_TYPE =
    new SequenceFieldType(StringFieldType.TYPE);
final DictionaryFieldType STRING_DICT_FIELD_TYPE =
    new DictionaryFieldType(StringFieldType.TYPE, StringFieldType.TYPE);
NormalisedEvent event = new NormalisedEvent();
event.add("mySequenceField",
    STRING_SEQUENCE_FIELD_TYPE.format(list));
event.add("myDictionaryField", STRING_DICT_FIELD_TYPE.format(map));

The programming interface for constructing and using normalised events is made up of three Java classes: NormalisedEvent, NormalisedEventIterator and NormalisedEventException. NormalisedEvent is the most important part of the interface, and encapsulates the data and operations that can be performed on a single normalised event. Some of these operations return NormalisedEventIterator objects, which support the process of stepping through the name-value pairs in the normalised event. Any errors encountered result in instances of NormalisedEventException being thrown.

This section provides an overview of the capabilities of the two main classes. Please see the Javadoc documentation for full information on the normalised event interface.