4. 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 messages matches the filter.

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

Expression Syntax

AMPS expressions are designed to work exactly as expected if you are familiar with XPath path specifiers and SQL-92 predicates. This section describes in detail how AMPS evaluates the syntax, operators, and functions available in the AMPS expression language.

AMPS expressions combine the following elements:

  • Identifiers specify a field in a message. When evaluating an expression, AMPS replaces identifiers with values from the message or set of messages being evaluated.
  • Literal values are explicit values in an AMPS expression, such as 'IBM' or 42
  • Operators and functions such as =, <, >, *, and UNIX_TIMESTAMP()

Every AMPS expression produces a value. The way that AMPS uses the value depends on the context in which AMPS evaluates the expression. For example, if the expression is used for a filter, the message is considered to match the filter when the expression returns true. When an expression is used to project a field, the result of the expression is used as the value of the projected field.

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. Because 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:

<Order update="full">
    <ClientID>12345</ClientID>
    <Symbol>IBM</Symbol>
    <OrderQty>1000</OrderQty>
</Order>

The following identifier specifies the Symbol element of an Order message:

/Order/Symbol

The following identifier specifies the update attribute of an Order message:

/Order/@update

For FIX and NVIX, 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:

/55

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:

[/Not Xpath 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. 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 with a FIX topic, even though FIX messages only use numeric tags, or an identifier like /DataPackage/RunDate with a BFlat topic, 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).

AMPS Data Types

Each value in AMPS is assigned a data type when the message type module parses the value. AMPS operators and functions attempt to convert values into compatible types, based on the type of operation. For example, the * operator (multiplication) will attempt to convert all values to numeric values, while the CONCAT function (string concatenation) will attempt to convert all values to strings. In effect, a value in AMPS can be treated as any type to which it can be meaningfully converted.

Internally, AMPS uses the following data types. As mentioned above, the message type module is responsible for assigning the type of a value from an incoming message as part of the parsing process. For some types, such as JSON, XML, FIX and NVFIX, the parser infers the type of the value from the field. For other types, such as BFLAT, Google Protocol Buffers, or BSON, the message itself contains information about the type of the field. Notice, however, that the AMPS expression language does not limit the value to the type assigned by the message type module. Instead, a value in AMPS can be used in any context.

For example, given the following JSON document:

{"a":1,"b":"47"}

The values of /a and /b can be used as either string values or numeric values. AMPS will automatically convert these values as necessary, and AMPS considers the string or numeric representation to be equally correct and valid.

Type Description Untyped Message Examples
NULL Unknown, untyped value (SQL-92 semantics)

[no field provided]

NFVIX: a=<SOH>

JSON: {"a":null}

XML: <a/>

Boolean True or false JSON: {"e":true}
Integer signed 64-bit integer

NVFIX: b=24

JSON: {"b":24}

XML: <b>24</b>

Floating point number 64-bit floating point number

NVFIX: c=24.0

JSON: {"c":24.0}

XML: <c>24.0</c>

String Arbitrary sequence of bytes of a specific length

NVFIX: d=Grilled cheese sandwich<SOH>

JSON: {"d":"Grilled cheese sandwich"}

XML: <d>Grilled cheese sandwich</d>

Table 4.1: AMPS data types

Numeric Types and Literals in AMPS Expressions

Numeric values in AMPS are always typed as either integers or floating point values. All numeric types in AMPS are signed. AMPS message types convert the original numeric types (or original representation for message types that do not have typed values) into the internal AMPS type system for the purposes of expression evaluation.

Within expressions, integer values are all numerals, with no decimal point, and can have a value in the same range as a 64-bit integer. For example:

42
149
-273

Within expressions, all numerals with a decimal point are floating-point numbers. AMPS interprets these numerals as double-precision floating point values. For example:

3.1415926535
98.6
-273.0

or, in scientific notation:

31.4e-1
6.022E23
2.998e8

AMPS automatically converts strings that contain numeric values to numbers when strings are used in a numeric comparison.

Type Promotion for Numeric Types

AMPS uses the following rules for type promotion when evaluating numeric expressions:

  1. If any of the values in the expression is NaN, the result is NaN.
  2. Otherwise, if any of the values in the expression is floating point, the result is floating point.
  3. Otherwise, all of the values in the expression are integers, and the result is an integer.

Notice that, for division in particular, the results returned are affected by the type of the values. For example, the expression 1 / 5 evaluates to 0 since the result is interpreted as an integer. In comparison, the expression 1.0 / 5 evaluates to 0.2 since the result is interpreted as a floating point value.

When a function or operator that expects a numeric type is provided with a string, AMPS will attempt to convert string values to numeric types as necessary. When converting string values, AMPS recognizes same numeric formats in message data as are supported in the AMPS expression language (see String Literals. If the string is in an unrecognized format, AMPS converts the string as NaN.

String Literals in AMPS Expressions

When creating expressions for AMPS, string literals are indicated with single or double quotes. For example:

/FIXML/Order/Instrmt/@Sym = 'IBM'

AMPS supports the following escape sequences within string literals:

Escape Sequence Definition
\a Alert
\b Backspace
\t Horizontal tab
\n Newline
\f Form feed
\r Carriage return
\xHH Hexadecimal digit where H is (0..9,a..f,A..F)
\OOO Octal Digit (0..7)

Table 4.2: Escape Sequences

Additionally, any character which follows a backslash will be treated as a literal character.

AMPS string operations have no restrictions on character set, and correctly handle embedded NULL characters (\x00) and characters outside of the 7-bit ASCII range. AMPS string operations are not unicode-aware.

NULL, NaN and IS NULL

XPath expressions are considered to be NULL when they evaluate to an empty or nonexistent field reference. In numeric expressions where the operands or results are not a valid number, the XPath expression evaluates to NaN (not a number). The rules for applying the AND and OR operators against NULL and NaN values are outlined in Table 4.3 and Table 4.4.

Operand1 Operand2 Result
TRUE NULL NULL
FALSE NULL FALSE
NULL NULL NULL

Table 4.3: Logical AND with NULL/NaN Values

Operand1 Operand2 Result
TRUE NULL TRUE
FALSE NULL NULL
NULL NULL NULL

Table 4.4: Logical OR with NULL/NaN Values

The NOT operator applied to a NULL value is also NULL, or “Unknown.” The only way to check for the existence of a NULL value reliably is to use the IS NULL predicate. There also exists an IS NAN predicate for checking that a value is NaN (not a number.)

caution To reliably check for existence of a NULL value, you must use the IS NULL predicate such as the filter: /Order/Comment IS NULL

AMPS also provides a COALESCE() function that accepts a set of values and returns the first value that is not NULL. For example, given the following filter expression:

COALESCE(/userCategory,
         /employeeCategory,
         /vendorCategory,
         'restricted') != 'restricted'

AMPS will return the first value that is not NULL, and compare that value to the constant string 'restricted'. Notice that, to make the intent of the filter clear, this example provides a constant value for AMPS to return from the COALESCE if all of the field values are NULL.

The COALESCE function, like other functions in AMPS, is not array-aware. This means that when one of the XPath expressions provided to COALESCE specifies an array in the original message, AMPS provides the first item in the array to the COALESCE function. See Working With Arrays for details.

Grouping and Order of Evaluation

AMPS expressions allow you to group parts of the expression using parentheses. Parts of an expression inside parentheses are evaluated together. 60East recommends using parentheses to group independent parts of an expression to ensure that expression is evaluated in the expected order. For example, in this expression:

( /counter % 3 ) == 0

The clause /counter % 3 is evaluated first, and the result of that evaluation is compared to 0.

Within a group, elements are evaluated left to right in precedence order. For example, given the filter below:

(expression1 OR expression2 AND expression3) OR (expression4 AND
            NOT expression5) ...

AMPS evalutes expression2, then expression3 (since AND has higher precedence than OR), and if they evaluate to false, then expression1 will be evaluated.

AMPS does not guarantee that all parts of an expression will be evaluated if the result of an expression can be determined after only evaluating part of the expression. For example, given the expression:

A_FUNCTION(/a) OR B_FUNCTION(/b)

AMPS only guarantees that B_FUNCTION(/b) will be evaluated if A_FUNCTION(/a) returns false.

Logical Operators

The logical operators are NOT, AND, and OR, in order of precedence. These operators have the usual Boolean logic semantics.

/FIXML/Order/Instrmt/@Sym = 'IBM' OR /FIXML/Order/Instrmt/@Sym = 'MSFT'

As with other operators, you can use parentheses to group operators and affect the order of evaluation

(/orderType = 'rush' AND /customerType IN ('silver', 'gold') )
OR  /customerType = 'platinum'

Arithmetic Operators

AMPS supports the arithmetic operators +, -, *, /, %, and MOD in expressions. The result of arithmetic operators where one of the operands is NULL is undefined and evaluates to NULL.

AMPS distinguishes between floating point and integral types. When an arithmetic operator uses two different types, AMPS will convert the integral type to a floating point value as described in Numeric Types and Literals.

Examples of filter expressions using arithmetic operators:

/6 * /14 < 1000

/Order/@Qty * /Order/@Prc >= 1000000

AMPS numeric types are signed, and the AMPS arithmetic operators correctly handle negative numbers. The MOD and % operators preserve the sign of the first argument to the operator. That is, -5 % 3 produces a result of -2, while 5 % -3 produces a result of 2.

caution When using mathematical operators in conjunction with filters, be careful about the placement of the operator. Some operators are used in the XPath expression as well as for mathematical operation (for example, the '/' operator in division). Therefore, it is important to separate mathematical operators with white space, to prevent interpretation as an XPath expression.

Comparison Operators

The comparison operators can be loosely grouped into equality comparisons and range comparisons. The basic equality comparison operators, in precedence order, are ==, =, >, >=, <, <=, !=, and <>. The == comparison and the = comparison are treated as the same operator and produce the same results.

If these binary operators are applied to two operands of different types, AMPS attempts to convert strings to numbers. If conversion succeeds, AMPS uses the numeric values. If conversion fails because the string cannot be meaningfully converted to a number, strings are always considered to be greater than numbers. The operators consider an empty string to be NULL.

The following table shows some examples of how AMPS compares different types.

Expression Result
1 < 2 TRUE
10 < '2' FALSE, ‘2’ can be converted to a number
'2.000' <> '2.0' TRUE, no conversion to numbers since both are strings
2 = 2.0 TRUE, numeric comparison
10 < 'Crank It Up' TRUE, strings are greater than numbers
10 < '' FALSE, an empty string is considered to be NULL
10 > '' FALSE, an empty string is considered to be NULL
'' = '' FALSE, an empty string is considered to be NULL
'' IS NULL TRUE, an empty string is considered to be NULL

Table 4.5: Comparison Operator Examples

There are also set and range comparison operators. The BETWEEN operator can be used to check the range values.

tip The range used in the BETWEEN operator is inclusive of both operands, meaning the expression /A BETWEEN 0 AND 100 is equivalent to /A >= 0 AND /A <= 100

For example:

/FIXML/Order/OrdQty/@Qty BETWEEN 0 AND 10000

/FIXML/Order/@Px NOT BETWEEN 90.0 AND 90.5

(/price * /qty) BETWEEN 0 AND 100000

The IN operator can be used to perform membership operations on sets of values. The IN operator returns true when the value on the left of the IN appears in the set of values in the IN clause. For example:

/Trade/OwnerID NOT IN ('JMB', 'BLH', 'CJB')

/21964 IN (/14*5, /6*/14, 1000, 2000)

/customer IN ('Bob', 'Phil', 'Brent')

The IN operator returns true for the set of records that would be returned by an equivalent set of = comparisons joined by OR. The following two statements return the same set of records:

/pet IN ('puppy', 'kitten', 'goldfish')
(/pet = 'puppy') OR (/pet = 'kitten') OR (/pet = 'goldfish)

This equivalence means that NULL values in either the field being evaluated, or the set of values provided to the IN clause, always return false.

This also means that, for string values, the IN operator performs exact, case-sensitive matching.

tip When evaluating against a set of values, the IN operator typically provides better performance than using a set of OR operators. That is, a filter written as /firstName IN ('Joe', 'Kathleen', 'Frank', 'Cindy', 'Mortimer') will typically perform better than an equivalent filter written as /firstName = 'Joe' OR /firstName = 'Kathleen' OR /firstName = 'Frank' OR /firstName = 'Cindy' OR /firstName = 'Mortimer'.

String Comparison Functions

AMPS includes several types of string comparison operators.

  • Case-sensitive exact matches. The IN, =, BEGINS WITH, ENDS WITH, and INSTR operators do literal matching on the contents of a string. These operators are case-sensitive.
  • Case-insensitive exact matches. AMPS also provides two case-insensitive operators: INSTR_I, a case-insensitive version of INSTR, and a case-insensitive equality operator, STREQUAL_I.
  • Regular expression matches. AMPS also provides full regular expression matching using the LIKE operator, described in regex matching and Chapter 5.

The = operator tests whether a field exactly matches the literal string provided.

/status = 'available'

/orderId = 'F327AC'

BEGINS WITH and ENDS WITH test whether a field begins or ends with the literal string provided. The operators return TRUE or FALSE:

/Department BEGINS WITH ('Engineering')

/path NOT BEGINS WITH ('/public/dropbox')

/filename ENDS WITH ('txt')

/price NOT ENDS WITH ('99')

AMPS allows you to use set comparisons with BEGINS WITH and ENDS WITH. In this case, the filter matches if the string in the field BEGINS WITH or ENDS WITH any of the strings in the set:

/Department BEGINS WITH ('Engineering', 'Research', 'Technical')

/filename ENDS WITH ('gif', 'png', 'jpg')

The INSTR operator allows you to check to see if one string occurs within another string. For this operator, you provide two string values. If the second string occurs within the first string, INSTR returns the position at which the second string starts, or 0 if the second string does not occur within the first string. Notice that the first character of the string is 1 (not 0). For example, the expression below tests whether the string critical occurs within the /eventLevels field.

INSTR(/eventLevels, "critical") != 0

AMPS also provides INSTR_I and STREQUAL_I functions for performing case-insensitive comparisons:

STREQUAL_I(/couponCode, 'QED')

INSTR_I(/symbolList, 'MSFT') != 0
Function or Operator Parameters Description
=

The string to be compared

The string to compare

Case-sensitive

Returns true if the string to be compared is identical to the string to compare.

/state = 'Ohio'
BEGINS WITH

The string to be compared

A list of strings to compare

Case-sensitive

Returns true if the string to be compared begins with any of the strings in the list.

/state
  BEGINS WITH ('North', 'South')
ENDS WITH

The string to be compared

A list of strings to compare

Case-sensitive

Returns true if the string to be compared ends with any of the strings in the list.

/state
  ENDS WITH ('Dakota', 'Carolina')
INSTR

The string to be compared

The string to compare

Case-sensitive

Returns the position at which the second string starts, or 0 if the second string does not occur within the first string.

INSTR(/state, 'i') != 0
INSTR_I

The string to be compared

The string to compare

Case-insensitive

Returns the position at which the second string starts, or 0 if the second string does not occur within the first string. This function is not unicode-aware.

INSTR_I(/state, 'i') != 0
STREQUAL_I

The string to be compared

The string to compare

Case-insensitive

Returns true if, when both strings are transformed to the same case, the string to be compared is identical to the string to compare. This function is not unicode-aware.

STREQUAL_I(/state, 'oHIO') != 0

Table 4.6: AMPS string comparison

Regular Expression Matching

AMPS also provides a regular expression comparison operator, LIKE, to provide regular expression matching on string values. A pattern is used for the right side of the LIKE operator. A pattern must be provided as a literal, quoted value. For more on regular expressions and the LIKE comparison operator, please see Chapter 4.

The string comparison operators described in the section called String Comparison Functions are usually more efficient than equivalent LIKE expressions, particularly when used to compare multiple literal patterns, or when the only purpose of the regular expression is to perform case-insensitive matching. Use LIKE operations when it is not practical to represent the filter condition with the string comparison operators.

Function or Operator Parameters Description
LIKE

The string to be compared

The pattern to evaluate the string against

Case-sensitive

Returns true if the string to be compared matches the pattern.

For example, the following filter uses a PCRE backreference to return true for any message where the /state field contains two identical characters in a row.

/state LIKE '(.)\1'

This operator is not unicode-aware.

Table 4.7: AMPS regular expression comparison

Conditional Operators

AMPS contains support for a ternary conditional IF operator which allows for a Boolean condition to be evaluated to true or false, and will return one of the two parameters. The general format of the IF statement is

IF (BOOLEAN_CONDITIONAL, VALUE_TRUE, VALUE_FALSE)

In this example, the BOOLEAN_CONDITIONAL will be evaluated, and if the result is true, the VALUE_TRUE value will be returned otherwise the VALUE_FALSE will be returned.

For example:

SUM( IF(( (/FIXML/Order/OrdQty/@Qty > 500) AND
          (/FIXML/Order/Instrmt/@Sym ='MSFT')), 1, 0 ))

The above example returns a count of the total number of orders that have been placed where the symbol is MSFT and the order contains a quantity more than 500.

The IF can also be used to evaluate results to determine if results are NULL or NaN. This is useful for calculating aggregates where some values may be NULL or NaN. The NULL and NaN values are discussed in more detail in the section called “NULL, NaN, and IS NULL”.

For example:

SUM(/FIXML/Order/Instrmt/@Qty * IF(
    /FIXML/Order/Instmt/@Price IS NOT NULL, 1, 0))

Working With Arrays

AMPS supports filters that operate on arrays in messages. There are two simple principles behind how AMPS treats arrays.

  • Binary operators that yield true or false (for example, =, <, LIKE) are array aware, as is the IN operator. These operators work on arrays as a whole, and evaluate every element in the array.
  • Arithmetic operators, functions, user-defined functions and other scalar operators, are not array aware, and use the first element in the array.

With these simple principles, you can predict how AMPS will evaluate an expression that uses an array. For any operator, an empty array evaluates to NULL.

Let’s look at some examples. For the purposes of this section, we will consider the following JSON document:

{
    "data"  : [1, 2, 3, "zebra", 5],
    "other" : [14, 34, 23, 5]
}

While these arrays are presented using JSON format for simplicity, the same principles apply to arrays in other message formats.

Here are some examples of ways to use an array in an AMPS filter:

  1. Determining if any element in an array meets a criteria. To determine this, you provide the identifier for the array, and use a comparison operator.

    Filter Evaluates as
    /data = 1 TRUE, /data contains 1
    /data = 'zebra' TRUE, /data contains 'zebra'
    /data != 'zebra' TRUE, /data contains an element that is not 'zebra'
    /data = 42 FALSE, /data does not contain 42
    /data LIKE 'z' TRUE, a member of /data matches 'z'
    /other > 30 TRUE, a member of /other is > 30
    /other > 50 FALSE, no member of /other is > 50

Table 4.8: Array contains element

  1. Determine whether a specific value is at a specific position. To determine this, use the subscript operator [] on the XPath identifier to specify the position, and use the equality operator to check the value at that position.

    Filter Evaluates as
    /data[0] = 1 TRUE, first element of /data is 1
    /data[3] = "zebra" TRUE, fourth element of /data is 'zebra'
    /data[1] != 1 TRUE, second element of /data is not 1
    /other[1] LIKE '4' TRUE, second element of /other matches '4'

Table 4.9: Element at specific position

  1. Determine whether any value in one array is present in another array.

    Filter Evaluates as
    /data = /other TRUE, a value in /data equals a value in /other
    /data != /other TRUE, a value in /data does not equal a value in /other

Table 4.10: Identical elements

  1. Determine whether an array contains one of a set of values.

    Filter Evaluates as
    3 IN (/data) TRUE, 3 is a member of /data
    /data IN (1, 2, 3) TRUE, a member of /data is in (1, 2, 3)
    /data IN ("zebra", "antelope", "li on") TRUE, a member of /data is in ("zebra", "antelope", "lion")

Table 4.11: Set of values in an array

Concatenating Strings

AMPS provides a function, CONCAT that can be used for constructing strings. The CONCAT function takes any number of parameters and returns a string constructed from those parameters. The function can accept both XPath identifiers and literal values.

The CONCAT function can be used in any AMPS expression that uses a string. For example, you could CONCAT in a filter as follows:

CONCAT(/firstName, " ", /lastName) = 'George Orwell'

CONCAT can be combined with other expressions, including conditional expressions. A mailingAddressName field in a view could be constructed as follows:

<Field>CONCAT(/firstName, " ", /lastName,
    IF(/suffix NOT NULL, CONCAT(", ", /suffix), "") )
    AS /mailingAddressName</Field>

Managing String Case

AMPS provides the UPPER and LOWER functions to produce a string in a specific case. This can be useful when constructing fields, or when an expression needs case-insensitive comparisons against a group of values using the IN clause.

As described above in String Comparison Functions, AMPS provides INSTR_I and STREQUAL_I functions for performing case-insensitive comparisons. In some cases, particularly when using strings with the IN clause, it is more efficient to simply convert the string to a known case.

The UPPER and LOWER functions are not unicode-aware; these functions will not produce the correct data when used with multibyte characters. For example, you might compare an incoming field of unknown case to a set of known values as follows:

UPPER(/ticker) IN ('MSFT', 'IBM', 'RHAT', 'DIS')
Function Parameters Description
UPPER The string to transform Returns the input string, transformed to upper case. This function is not unicode aware.
LOWER The string to transform Returns the input string, transformed to lower case. This function is not unicode aware.

Table 4.12: AMPS string manipulation functions

Replacing Text in Strings

AMPS provides a pair of functions, REPLACE and REGEXP_REPLACE, that replace text within strings.

Function Parameters Description
REPLACE string to transform, string to match, replacement text Returns the input string, with all occurrences of the string to match replaced with the replacement text
REGEXP_REPLACE string to transform, pattern to match, replacement text Returns the input string, with all occurrences of the pattern to match replaced with the replacement text

Table 4.13: AMPS string replacement functions

Working With Substrings

AMPS provides a function, SUBSTR, that can be used for returning a subset of a string. There are two forms of this function.

The first form takes the source string and the position at which to begin the substring. You can use a negative number to count backward from the end of the string. AMPS starts at the position specified, and returns a string that starts at the specified position and goes to the end of the string. If the provided position is before the beginning of the string, AMPS starts at the beginning of the string. If the provided position is past the end of the string, AMPS returns a zero-length string, which evaluates to NULL. If the provided position is before the beginning of the string, AMPS returns the full string.

For example, the following expressions are all TRUE:

SUBSTR("fandango", 4) == "dango"

SUBSTR("fandango", 1) == "fandango"

SUBSTR("fandango", -2) == "go"

SUBSTR("fandango", -99) == "fandango"

SUBSTR("fandango", 99) IS NULL

The second form of SUBSTR takes the source string, the position at which to begin the substring, and the length of the substring. Notice that SUBSTR considers the first character in the string to be position 1 (rather than position 0), as demonstrated below. AMPS will not return a string larger than the source string. As with the two-argument form, if the starting position is before the beginning of the string, AMPS starts at the beginning of the string. If the starting position is after the end of the source string, AMPS returns an empty string which evaluates to NULL.

For example, the following expressions are all TRUE:

SUBSTR("fandango", 1, 3) == "fan"

SUBSTR("fandango", -4, 2) == "an"

SUBSTR("fandango", -8, 8) == "fandango"

SUBSTR("fandango", -23, 3) == "fan"

SUBSTR("fandango", 99, 8) IS NULL

Timestamp Function

AMPS includes a function that returns the current Unix timestamp. Notice that AMPS also includes functions for working with date and time in the Legacy Messaging Compatibility layer.

Function Parameters Description
UNIX_TIMESTAMP none

Returns the current timestamp as a double.

Notice that a UNIX timestamp is seconds elapsed since 00:00 on January 1, 1970 in UTC and is independent of the timezone of the local system.

Table 4.14: AMPS Timestamp functions

Geospatial Functions

AMPS includes a function for calculating the distance from a signed latitude and longitude.

Function Parameters Description
GEO_DISTANCE first_latitude, first_longitude, Seccond_latitude, second_longitude

Returns a double that contains the distance between the point identified by first_latitude, first_longitude and second_latitude, second_longitude in meters.

For example, given a home point and a message containing /lat and /long fields, you could use the following expression to calculate the distance from home.

GEO_DISTANCE(
     /lat,
     /long,
     40.786337,
     -119.206508)

AMPS uses the haversine formula when computing distances.

Table 4.15: AMPS Geospatial Functions

Numeric Functions

AMPS includes the following functions for working with numbers.

Function Parameters Description
ABS number

Returns the absolute value of a number.

For example, the following filter will be TRUE when the difference between /a and /b is greater than 5, regardless of whether /a or /b is larger.

ABS(/a - /b) > 5
ROUND number, [number of decimal places]

Returns a number rounded to the specified number of decimal places.

The number of decimal places is optional. When not provided, the number defaults to 0.

The number of decimal places can be positive or negative. When the number is positive, the number specifies the number of digits to the right of the decimal place to round at. When the number is negative, the number specifies the number of digits to the left of the decimal place to round at.

For example, you could use the following expression in a view to limit the precision of the /price field of the source topic to 2 decimal places.

ROUND(/price, 2) AS
/price

Table 4.16: AMPS Numeric Functions

CRC32 Function

AMPS includes a function that computes a CRC32 checksum over a string. This function is useful for creating a numeric identifier from a string representation. This is commonly-used to create a shortened representation of the string, or to provide input for a MOD calculation.

Function Parameters Description
CRC32 _string_ Returns an integer calculated as a checksum of the provided string.

Table 4.17: AMPS CRC function

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 expressions and the destination identifier are separated by the AS keyword. The format for a field construction expression is as follows:

<source expression>
AS
<destination identifier>

For example, to create a field in a view that calculates the total value of an order by multiplying the /price field times the /quantity field, construct the field as shown below:

<Field>/price * /qty AS /total</Field>

This constructs a field using /price * /qty to 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.

Because 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:

<Field>IF(/source IN ('a','e','f'), /source, NULL)
       AS /source HINT OPTIONAL</Field>

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.

<Field>EXPENSIVE_UDF_CALL(/dataSet1, /dataSet2)
       AS /processedData HINT SET_CURRENT</Field>
<Field>IF(/processedData > 1000000,
           'A',
           'B') AS /resultClass</Field>

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:

<Field>EXPENSIVE_UDF_CALL(/dataSet1, /dataSet2)
       AS /processedData HINT SET_CURRENT,OPTIONAL
</Field>

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.

Because 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 indentifiers 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:

Modifier Description
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.

Table 4.18: XPath Identifier Modifiers for Enrichment

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:

<Field>IF(/source IN ('a','e','f'), /source, NULL)
       AS /source HINT OPTIONAL</Field>

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.

<Field>EXPENSIVE_UDF_CALL(/dataSet1, /dataSet2)
       AS /processedData HINT SET_CURRENT</Field>
<Field>IF(/processedData > 1000000,
           'A',
           'B') AS /resultClass</Field>

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:

<Field>EXPENSIVE_UDF_CALL(/dataSet1, /dataSet2)
       AS /processedData HINT SET_CURRENT,OPTIONAL
</Field>

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.

<Field>[nvfix].[orders]./quantity * [json].[items]./price AS /total</Field>

Aggregate Functions

AMPS provides a set of aggregation functions that can be used in a Field constructor. These functions return a single value for each distinct group of messages, as identified by distinct combinations of values in the Grouping clause.

Function Description
AVG Average over an expression. Returns the mean value of the values specified by the expression.
COUNT Count of values in an expression. Returns the number of values specified by the expression.
COUNT_DISTINCT Count of number of distinct values in an expression, ignoring NULL. Returns the number of distinct values in the expression. AMPS type conversion rules apply when determining distinct values.
MIN Minimum value. Returns the minimum out of the values specified by the expression.
MAX Maximum value. Returns the maximum out of the values specified by the expression.
STDDEV_POP Population standard deviation of an expression. Returns the calculated standard deviation.
STDDEV_SAMP Sample standard deviation of an expression. Returns the calculated standard deviation.
SUM Summation over an expression. Returns the total value of the values specified by the expression.

Table 4.19: AMPS Aggregation Functions

Null values are not included in aggregate expressions with AMPS, nor in ANSI SQL. COUNT will count only non-null values; SUM will add only non-null values; AVG will average only non-null values; and MIN and MAX ignore NULL values, and so on.

MIN and MAX can operate on either numbers or strings, or a combination of the two. AMPS compares values using the principles described for comparison operators. For MIN and MAX, AMPS determines order based on these rules:

  • Numbers sort in numeric order.
  • String values sort in ASCII order.
  • When comparing a number to a string, convert the string to a number, and use a numeric comparison. If that is not successful, the value of the string is higher than the value of the number.

For example, given a field that has the following values across a set of messages:

24, 020, 'cat', 75, 1.3, 200, '75', '42'

MIN will return 1.3, MAX will return 'cat'. Notice that different message types may have different support for converting strings to numeric values: AMPS relies on the parsing done by the message type to determine the numeric value of a string.