Constructing Fields
For views, aggregated subscriptions, and SOW topic enrichment, AMPS allows you to construct new fields based on existing data.
When you construct a field, there are two components required:
A source expression that produces a value. This expression can include XPath identifiers that extract values from a message, literal values, operators, and functions.
A destination identifier that specifies the identifier where the message type will serialize the value produced by the source expression.
The source expression and the destination identifier are separated by the AS keyword. The format for a field construction expression is as follows:
For example, to create a field in a view that calculates the total value of an order by multiplying the /price field times the /qty field, construct the field as shown below:
This constructs a field using /price * /qty as the source expression. Both /price and /qty are taken from the incoming message. When the result of this expression is computed, the value will be produced with the XPath identifier /total as the destination. That value will then be serialized to a message (with the exact format and syntax determined by the message type).
Notice that the grammar for constructing fields does not specify precisely how the field is represented in the message. AMPS constructs the value and provides the XPath identifier to the message type. The message type itself is responsible for serializing the value into the correct representation and structure for that message type.
All of the AMPS operators and functions that are available for filters are available to use in source expressions, including any user-defined functions loaded into the instance.
Depending on the context for field construction, there are additional capabilities available when constructing fields, as described in the following sections.
Constructing Preprocessing Fields
Preprocessing field constructors operate on a single message and construct fields based on that message. The results of the preprocessing field constructor are merged into the incoming message. Any field in the source message that is not changed or removed during preprocessing is left unchanged, so it is not necessary to include all fields in the message in the Preprocessing block.
Since preprocessing fields apply to a specific message, preprocessing fields cannot specify the topic or message type in an XPath identifier. All identifiers in the source expression are evaluated as identifiers in the message being preprocessed. Preprocessing fields are evaluated during the preprocessing phase, so they cannot refer to the previous state of a message.
Using HINT to Control Field Construction
Preprocessing can be used to remove fields from a message. By default, AMPS serializes any field that has an empty string or NULL value after preprocessing. Preprocessing fields can include a directive that specifies that a field that contains a NULL value should be removed from the set of fields rather than serialized with a NULL value. The directive HINT OPTIONAL applied to the XPath identifier specifies that if the result of the source expression is NULL, AMPS does not provide the value for the message type to serialize. For example, the following field constructor removes the /source field from the message if the value provided is not in a specific list of values:
By default, AMPS considers the results of field construction (the processed message) to be distinct from the current message. AMPS rewrites the current message after preprocessing is completed. This means that, by default, the results of fields constructed during preprocessing are not available to other fields within preprocessing. The HINT SET_CURRENT option immediately inserts or updates values in the current message, which makes the new value available to all subsequent Field declarations.
In the sample below, AMPS enriches the message by performing an expensive operation (implemented as a user-defined function) on two input fields, and immediately updates the current message with the output of that operation. AMPS then sets other fields in the processed message using the updated value in the current message.
Notice that using HINT SET_CURRENT requires AMPS to process Field declarations in order, which may prevent future optimizations.
Hints can be combined as follows:
In this case, if the projected field would be NULL, the field is removed from the current message.
Constructing Enrichment Fields
Enrichment field constructors operate on a single message and construct fields based on that message. Enrichment expressions operate on the current message and change the current message. The results of the enrichment directives are merged into the incoming message. Any field in the source message that is not changed or removed during preprocessing is left unchanged, so it is not necessary to include all fields in the message in the Enrichment directive.
Since enrichment fields apply to a specific message, enrichment fields cannot specify the topic or message type in an XPath identifier. All identifiers in the source expression are evaluated as identifiers in the message being enriched.
Enrichment fields are constructed during the enrichment phase, so enrichment fields can refer to the previous state of a message. Within an enrichment expression, AMPS provides two special modifiers for XPath identifiers that specify whether an XPath identifier refers to the current incoming message or the previous state of the message. These modifiers apply only to the source expression, and cannot be used in the destination identifier. The modifiers are as follows:
OF CURRENT
Specify that the XPath identifier refers to the incoming message.
OF PREVIOUS
Specify that the XPath identifier refers to the previous state of the message in the SOW.
If there is no record in the SOW for this message, all identifiers that specify OF PREVIOUS return NULL.
Using HINT to Control Field Construction
Enrichment can be used to remove fields from a message. By default, AMPS serializes any field that has an empty string or NULL value after enrichment. Enrichment Field elements can include a directive that specifies that a field that contains a NULL value should be removed from the message rather than serialized with a NULL value. The directive HINT OPTIONAL applied to the XPath identifier specifies that if the result of the source expression is NULL, AMPS does not provide the value for the message type to serialize. For example, the following field constructor removes the /source field from the message if the value provided is not in a specific list of values:
By default, AMPS considers the results of field construction (the enriched message) to be distinct from the current message. AMPS rewrites the current message after enrichment is completed. This means that, by default, the results of fields constructed during enrichment are not available to other fields within enrichment. The HINT SET_CURRENT option immediately inserts or updates values in the current message, which makes the new value available to all subsequent Field declarations.
In the sample below, AMPS enriches the message by performing an expensive operation (implemented as a user-defined function) on two input fields, and immediately updates the current message with the output of that operation. AMPS then sets other fields in the processed message using the updated value in the current message.
Notice that using HINT SET_CURRENT requires AMPS to process Field declarations in order, which may prevent future optimizations.
Hints can be combined as follows:
In this case, if the projected field would be NULL, the field is removed from the current message.
Constructing View Fields
View field constructors operate over groups of messages, and construct a single output message for each distinct group, as specified by the Grouping element in the View configuration.
When constructing a field in a view, all identifiers used in the source expression must be in one of the underlying topics for the view. When the view uses a Join, the identifiers must include the topic identifier. If the topics in the Join are of different message types, the identifiers must include both the message type and the topic identifier.
For example, the following Field definition multiplies the /quantity from the NVFIX topic orders by the /price from the JSON topic items, and projects the result into the /total field of the view.
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