Identifiers
AMPS identifiers use a subset of XPath to specify values in a message. AMPS identifiers specify the value of an attribute or element in an XML message, and the value of a field in a JSON, FIX or NVFIX message. Given that the identifier syntax is only used to specify values, the subset of XPath used by AMPS does not include wildcards, relative paths, array manipulation, predicates or functions.
For example, when messages are in this XML format:
The following identifier specifies the Symbol element of an Order message:
The following identifier specifies the update attribute of an Order message:
For FIX and NVFIX, you specify fields using / and the tag name. AMPS interprets FIX and NVFIX messages as though they were an XML fragment with no root element. For example, to specify the value of FIX tag 55 (symbol), use the following identifier:
Likewise, for JSON or other types that represent an object, you navigate through the object structure using the / to indicate each level of nesting.
AMPS only guarantees support for field identifiers that are valid step names in XPath. For example, AMPS does not guarantee that it can process or filter on a field named Fits&Starts.
AMPS also supports an optional bracketed field identifier syntax that extends the characters available for field names. For example, the following step name:
refers to a field name of Not Xpath Name at the root level of the message. This syntax allows spaces to be used in field names in AMPS expressions, even though this is not a valid step name in XPath. Notice that not all message types support field names with embedded spaces or other special characters. For example, the Not Xpath Name identifier is not a valid element name in XML, nor would it be a valid field name in Google Protocol Buffers.
AMPS checks the syntax of identifiers when parsing an expression. AMPS does not try to predict whether an identifier will match messages within a particular topic. It is not an error to submit an identifier that can never match due to the limitations of the message type. For example, AMPS allows you to use an identifier like /OrderQty in a filter submitted for a FIX connection, even though FIX messages only use numeric tags, or an identifier like /DataPackage/RunDate in a filter submitted for a BFlat connection, even though BFlat does not support nested elements.
The message type is responsible for constructing a set of identifiers from a message. In most cases, the mapping is simple. However, see the documentation for the message type for details, or if the mapping is unclear. For example, a composite-local message type adds the number of the part to the beginning of each XPath within the part (so, a top-level field of /name in the first part of the message has an identifier of /0/name).
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