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Kapacitor TICKscript lambda expressions

This page documents an earlier version of Kapacitor. Kapacitor v1.5 is the latest stable version. View this page in the v1.5 documentation.

Overview

TICKscript uses lambda expressions to define transformations on data points as well as define Boolean conditions that act as filters. Lambda expressions wrap mathematical operations, Boolean operations, internal function calls or a combination of all three. TICKscript tries to be similar to InfluxQL in that most expressions that you would use in an InfluxQL WHERE clause will work as expressions in TICKscript, but with its own syntax:

  • All field or tag identifiers must be double quoted.
  • The comparison operator for equality is == not =.

All lambda expressions in TICKscript begin with the lambda: keyword.

.where(lambda: "host" == 'server001.example.com')

In some nodes the results of a lambda expression can be captured into a new field as a named result using the property setter .as(). In this way they can be used in other nodes further down the pipeline.

The internal functions of lambda expressions can be either stateless or stateful. Stateful means that each time the function is evaluated the internal state can change and will persist until the next evaluation. For example the built-in function sigma calculates a running mean and standard deviation and returns the number of standard deviations the current data point is away from the mean.

Example 1 – the sigma function

sigma("value") > 3.0

Each time that the expression is evaluated it updates the running statistics and then returns the deviation. The simple expression in Example 1 evaluates to false while the stream of data points it has received remains within 3.0 standard deviations of the running mean. As soon as a value is processed that is more than 3.0 standard deviations from the mean it evaluates to true. Such an expression can be used inside of a TICKscript to define powerful alerts, as illustrated in Example 2 below.

Example 2 – TICKscript with lambda expression

stream
    |from()
    ...
    |alert()
        // use an expression to define when an alert should go critical.
        .crit(lambda: sigma("value") > 3.0)

Note on inadvertent type casting

Beware that numerical values declared in the TICKscript follow the parsing rules for literals introduced in the Syntax document. They may not be of a suitable type for the function or operation in which they will be used. Numerical values that include a decimal will be interpreted as floats. Numerical values without a decimal will be interpreted as integers. When integers and floats are used within the same expression the integer values need to use the float() type conversion function if a float result is desired. Failure to observe this rule can yield unexpected results. For example, when using a lambda expression to calculate a ratio between 0 and 1 of type float to use in generating a percentage; and when the fields are of type integer, it might be assumed that a subset field can be divided by the total field to get the ratio( e.g. subset/total * 100). Such an integer by integer division will result in an integer value of 0. Furthermore multiplication of the result of such an operation by the literal 100 (an integer) will also result in 0. Casting the integer values to float will result in a valid ratio in the range between 0 and 1, and then multiplication by the literal 100.0 (a float) will result in a valid percentage value. Correctly written, such an operation should look like this:

eval(lambda: float("total_error_responses")/float("total_responses") * 100.0).

If in the logs an error appears of the type E! mismatched type to binary operator..., check to ensure that the fields on both sides of the operator are of the same and the desired type.

In short, to ensure that the type of a field value is correct, use the built-in type conversion functions (see below).

Built-in functions

Stateful functions

Count

Count takes no arguments but returns the number of times the expression has been evaluated.

count() int64
Sigma

Computes the number of standard deviations a given value is away from the running mean. Each time the expression is evaluated the running mean and standard deviation are updated.

sigma(value float64) float64
Spread

Computes the running range of all values passed into it. The range is the difference between the maximum and minimum values received.

spread(value float64) float64

Stateless functions

Type conversion functions

Bool

Converts a string into a Boolean via Golang’s strconv.ParseBool function. Numeric types can also be converted to a bool where a 0 -> false and 1 -> true.

bool(value) bool
Int

Converts a string or float64 into an int64 via Golang’s strconv.ParseInt or simple int64() coercion. Strings are assumed to be decimal numbers. Durations are converted into an int64 with nanoseconds units. A Boolean is converted to an int64 where false -> 0 and true -> 1.

int(value) int64
Float

Converts a string or int64 into an float64 via Golang’s strconv.ParseFloat or simple float64() coercion. A Boolean is converted to a float64 where false -> 0.0 and true -> 1.0.

float(value) float64
String

Converts a bool, int64 or float64 into an string via Golang’s strconv.Format* functions. Durations are converted to a string representation of the duration.

string(value) string
Duration

Converts an int64 or a float64 into an duration assuming nanoseconds units. Strings are converted to duration of the form as duration literals in TICKscript.

duration(value) duration

Existence

IsPresent

Returns a Boolean value based on whether the specified field or tag key is present. Useful for filtering out data this is missing the specified field or tag.

|where(lambda: isPresent("myfield"))

This returns TRUE if myfield is a valid identifier and FALSE otherwise.

Time functions

Within each expression the time field contains the time of the current data point. The following functions can be used on the time field. Each function returns an int64.

FunctionDescription
minute(t time) int64the minute within the hour: range [0,59]
hour(t time) int64the hour within the day: range [0,23]
weekday(t time) int64the weekday within the week: range [0,6], 0 is Sunday
day(t time) int64the day within the month: range [1,31]
month(t time) int64the month within the year: range [1,12]
year(t time) int64the year

Example usage:

lambda: hour("time") >= 9 AND hour("time") < 19

The above expression evaluates to true if the hour of the day for the data point falls between 0900 hours and 1900 hours.

Math functions

The following mathematical functions are available. Each function is implemented via the equivalent Go function.

FunctionDescription
abs(x float64) float64Abs returns the absolute value of x.
acos(x float64) float64Acos returns the arccosine, in radians, of x.
acosh(x float64) float64Acosh returns the inverse hyperbolic cosine of x.
asin(x float64) float64Asin returns the arcsine, in radians, of x.
asinh(x float64) float64Asinh returns the inverse hyperbolic sine of x.
atan(x float64) float64Atan returns the arctangent, in radians, of x.
atan2(y, x float64) float64Atan2 returns the arc tangent of y/x, using the signs of the two to determine the quadrant of the return value.
atanh(x float64) float64Atanh returns the inverse hyperbolic tangent of x.
cbrt(x float64) float64Cbrt returns the cube root of x.
ceil(x float64) float64Ceil returns the least integer value greater than or equal to x.
cos(x float64) float64Cos returns the cosine of the radian argument x.
cosh(x float64) float64Cosh returns the hyperbolic cosine of x.
erf(x float64) float64Erf returns the error function of x.
erfc(x float64) float64Erfc returns the complementary error function of x.
exp(x float64) float64Exp returns e**x, the base-e exponential of x.
exp2(x float64) float64Exp2 returns 2**x, the base-2 exponential of x.
expm1(x float64) float64Expm1 returns e**x - 1, the base-e exponential of x minus 1. It is more accurate than Exp(x) - 1 when x is near zero.
floor(x float64) float64Floor returns the greatest integer value less than or equal to x.
gamma(x float64) float64Gamma returns the Gamma function of x.
hypot(p, q float64) float64Hypot returns Sqrt(p*p + q*q), taking care to avoid unnecessary overflow and underflow.
j0(x float64) float64J0 returns the order-zero Bessel function of the first kind.
j1(x float64) float64J1 returns the order-one Bessel function of the first kind.
jn(n int64, x float64) float64Jn returns the order-n Bessel function of the first kind.
log(x float64) float64Log returns the natural logarithm of x.
log10(x float64) float64Log10 returns the decimal logarithm of x.
log1p(x float64) float64Log1p returns the natural logarithm of 1 plus its argument x. It is more accurate than Log(1 + x) when x is near zero.
log2(x float64) float64Log2 returns the binary logarithm of x.
logb(x float64) float64Logb returns the binary exponent of x.
max(x, y float64) float64Max returns the larger of x or y.
min(x, y float64) float64Min returns the smaller of x or y.
mod(x, y float64) float64Mod returns the floating-point remainder of x/y. The magnitude of the result is less than y and its sign agrees with that of x.
pow(x, y float64) float64Pow returns x**y, the base-x exponential of y.
pow10(x int64) float64Pow10 returns 10**e, the base-10 exponential of e.
sin(x float64) float64Sin returns the sine of the radian argument x.
sinh(x float64) float64Sinh returns the hyperbolic sine of x.
sqrt(x float64) float64Sqrt returns the square root of x.
tan(x float64) float64Tan returns the tangent of the radian argument x.
tanh(x float64) float64Tanh returns the hyperbolic tangent of x.
trunc(x float64) float64Trunc returns the integer value of x.
y0(x float64) float64Y0 returns the order-zero Bessel function of the second kind.
y1(x float64) float64Y1 returns the order-one Bessel function of the second kind.
yn(n int64, x float64) float64Yn returns the order-n Bessel function of the second kind.

String functions

The following string manipulation functions are available. Each function is implemented via the equivalent Go function.

FunctionDescription
strContains(s, substr string) boolStrContains reports whether substr is within s.
strContainsAny(s, chars string) boolStrContainsAny reports whether any Unicode code points in chars are within s.
strCount(s, sep string) int64StrCount counts the number of non-overlapping instances of sep in s. If sep is an empty string, Count returns 1 + the number of Unicode code points in s.
strHasPrefix(s, prefix string) boolStrHasPrefix tests whether the string s begins with prefix.
strHasSuffix(s, suffix string) boolStrHasSuffix tests whether the string s ends with suffix.
strIndex(s, sep string) int64StrIndex returns the index of the first instance of sep in s, or -1 if sep is not present in s.
strIndexAny(s, chars string) int64StrIndexAny returns the index of the first instance of any Unicode code point from chars in s, or -1 if no Unicode code point from chars is present in s.
strLastIndex(s, sep string) int64StrLastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
strLastIndexAny(s, chars string) int64StrLastIndexAny returns the index of the last instance of any Unicode code point from chars in s, or -1 if no Unicode code point from chars is present in s.
strLength(s string) int64StrLength returns the length of the string.
strReplace(s, old, new string, n int64) stringStrReplace returns a copy of the string s with the first n non-overlapping instances of old replaced by new.
strSubstring(s string, start, stop int64) stringStrSubstring returns a substring based on the given indexes, strSubstring(str, start, stop) is equivalent to str[start:stop] in Go.
strToLower(s string) stringStrToLower returns a copy of the string s with all Unicode letters mapped to their lower case.
strToUpper(s string) stringStrToUpper returns a copy of the string s with all Unicode letters mapped to their upper case.
strTrim(s, cutset string) stringStrTrim returns a slice of the string s with all leading and trailing Unicode code points contained in cutset removed.
strTrimLeft(s, cutset string) stringStrTrimLeft returns a slice of the string s with all leading Unicode code points contained in cutset removed.
strTrimPrefix(s, prefix string) stringStrTrimPrefix returns s without the provided leading prefix string. If s doesn’t start with prefix, s is returned unchanged.
strTrimRight(s, cutset string) stringStrTrimRight returns a slice of the string s, with all trailing Unicode code points contained in cutset removed.
strTrimSpace(s string) stringStrTrimSpace returns a slice of the string s, with all leading and trailing white space removed, as defined by Unicode.
strTrimSuffix(s, suffix string) string)StrTrimSuffix returns s without the provided trailing suffix string. If s doesn’t end with suffix, s is returned unchanged.
regexReplace(r regex, s, pattern string) stringRegexReplace replaces matches of the regular expression in the input string with the output string. For example regexReplace(/a(b*)c/, ‘abbbc’, ‘group is $1’) -> ‘group is bbb’. The original string is returned if no matches are found.

Human string functions

HumanBytes

Converts an int64 or a float64 with units bytes into a human readable string representing the number of bytes.

humanBytes(value) string

Conditional functions

If

Returns the result of its operands depending on the value of the first argument. The second and third arguments must return the same type.

Example:

|eval(lambda: if("field" > threshold AND "field" != 0, 'true', 'false'))
    .as('value')

The value of the field value in the above example will be the string true or false, depending on the condition passed as the first argument.

The if function’s return type is the same type as its second and third arguments.

if(condition, true expression, false expression)

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