AMPS Expressions

AMPS includes an expression language that combines elements of XPath and SQL-92's WHERE clause. This expression language is used whenever the AMPS server refers to the contents of a message, including:

  • Content filtering

  • Constructing fields for message enrichment

  • Creating projected fields for views

AMPS uses a common syntax for each of these purposes, and provides a common set of operators and functions. AMPS also provides special directives for message enrichment, and aggregation functions for projecting views.

For example, when an expression is used as a content filter, any message for which the expression returns true matches the content filter. When an expression is used to construct a field for message enrichment or view projection, the expression is evaluated and the result that the expression returns is used as the content of the field.

Expressions Overview

The quickest way to learn AMPS expressions is to think of each as a combination of identifiers that tell AMPS where to find data in a message, and operators that tell AMPS what to do with that data. Each AMPS expression produces a value. The way AMPS uses that value depends on where the expression is used. For example, in a content filter, AMPS uses the value of the expression to determine whether a message matches the filter. When constructing a field, AMPS uses the value of the expression as the contents of the field.

Consider a simple example of an expression used as a filter. Imagine AMPS receives the following JSON message:

{"name":"Gyro", "job":"kitten"}

Using an AMPS expression, you can easily construct a content filter that matches the message:

/name = 'Gyro'

There are three parts to this expression. The first part, /name, is an identifier that tells AMPS to look for the contents of the name field at the top level of the JSON document. The second part of the filter, =, is the equality operator, which tells AMPS to compare the values on either side of the operator and return true if the values match. The final part of the filter, 'Gyro', is a string literal for the equality operator to use in the comparison. When an expression is used in a content filter, a message matches the filter when the expression returns true. The expression returns true for the sample message, so the sample message matches the filter.

The identifier syntax is a subset of XPath, as described in the section on Identifiers. The comparison syntax is similar to SQL-92.

Notice that AMPS makes no rigid guarantees as to the number of times a given expression is evaluated or when that evaluation will take place. AMPS will evaluate the expression as needed.

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