Support for repeating fields

[kleihan]

Orchestra supports repeating groups with one or more elements and a group cardinality range with the attributes minOccurs and maxOccurs. FIX Protocol has repeating groups with a field as single element. ISO 20022 supports the definition of an element cardinality range, e.g. 0-1, 1-1, 1-∞. The minimum value of the range in ISO 20022 is semantically equivalent to the presence attribute values optional (0) and required (1) in Orchestra.

Orchestra syntax does not support the attributes minOccurs and maxOccurs for field references. This could be useful to express a logical array of values, avoiding the workaround of a repeating group with one field. The field values can be an Orchestra datatype or code set. Mapping to an encoding depends on the capabilities of the encoding:

• TagValue: not supported, needs to be a repeating group with one field (requires explicit NumInGroup field)
• FIXML: supported using xs:group and xs:sequence without attributes
• SBE: partially supported using <set> if it is a code set that can be mapped to an SBE primitive datatype
• JSON: supporting using type "array"
• ISO 20022 XML: supported using element attributes minOccurs and maxOccurs

Note that Orchestra currently requires a numInGroup element for a group element (does it? It has minOccurs="0" in the schema) although it is specific to TagValue encoding. Maybe it should be shifted to the encoding section of a group.