istio.io/content/docs/reference/config/policy-and-telemetry/expression-language/index.md

title	description	weight	aliases
Expression Language	Mixer configuration expression language reference.	20	/docs/reference/config/mixer/expression-language.html

This page describes how to use the Mixer configuration expression language (CEXL).

Background

Mixer configuration uses an expression language (CEXL) to specify match expressions and mapping expressions. CEXL expressions map a set of typed attributes and constants to a typed value.

Syntax

CEXL accepts a subset of Go expressions, which defines the syntax. CEXL implements a subset of the Go operators that constrains the set of accepted Go expressions. CEXL also supports arbitrary parenthesization.

Functions

CEXL supports the following functions.

Operator/Function	Definition	Example	Description
==	Equals	request.size == 200
!=	Not Equals	request.auth.principal != "admin"
||	Logical OR	(request.size == 200) || (request.auth.principal == "admin")
&&	Logical AND	(request.size == 200) && (request.auth.principal == "admin")
[ ]	Map Access	request.headers["x-request-id"]
+	Add	request.host + request.path
>	Greater Than	response.code > 200
>=	Greater Than or Equal to	request.size >= 100
<	Less Than	response.code < 500
<=	Less Than or Equal to	request.size <= 100
|	First non empty	source.labels["app"] | source.labels["svc"] | "unknown"
match	Glob match	match(destination.service, "*.ns1.svc.cluster.local")	Matches prefix or suffix based on the location of *
email	Convert a textual e-mail into the EMAIL_ADDRESS type	email("awesome@istio.io")	Use the email function to create an EMAIL_ADDRESS literal.
dnsName	Convert a textual DNS name into the DNS_NAME type	dnsName("www.istio.io")	Use the dnsName function to create a DNS_NAME literal.
ip	Convert a textual IPv4 address into the IP_ADDRESS type	source.ip == ip("10.11.12.13")	Use the ip function to create an IP_ADDRESS literal.
timestamp	Convert a textual timestamp in RFC 3339 format into the TIMESTAMP type	timestamp("2015-01-02T15:04:35Z")	Use the timestamp function to create a TIMESTAMP literal.
uri	Convert a textual URI into the URI type	uri("http://istio.io")	Use the uri function to create a URI literal.
.matches	Regular expression match	"svc.*".matches(destination.service)	Matches destination.service against regular expression pattern "svc.*".
.startsWith	string prefix match	destination.service.startsWith("acme")	Checks whether destination.service starts with "acme".
.endsWith	string postfix match	destination.service.endsWith("acme")	Checks whether destination.service ends with "acme".
emptyStringMap	Create an empty string map	request.headers | emptyStringMap()	Use emptyStringMap to create an empty string map for default value of request.headers.
conditional	Simulate ternary operator	conditional((context.reporter.kind | "inbound") == "outbound", "client", "server")	Returns "client" if report kind is outbound otherwise returns "server".
toLower	Convert a string to lowercase letters	toLower("User-Agent")	Returns "user-agent".
size	Length of a string	size("admin")	Returns 5

Type checking

CEXL variables are attributes from the typed attribute vocabulary, constants are implicitly typed and, functions are explicitly typed.

Mixer validates a CEXL expression and resolves it to a type during configuration validation. Selectors must resolve to a boolean value and mapping expressions must resolve to the type they are mapping into. Configuration validation fails if a selector fails to resolve to a boolean or if a mapping expression resolves to an incorrect type.

For example, if an operator specifies a string label as request.size | 200, validation fails because the expression resolves to an integer.

Missing attributes

If an expression uses an attribute that is not available during request processing, the expression evaluation fails. Use the | operator to provide a default value if an attribute may be missing.

For example, the expression request.auth.principal == "user1" fails evaluation if the request.auth.principal attribute is missing. The | (OR) operator addresses the problem: (request.auth.principal | "nobody" ) == "user1".

Examples

Expression	Return Type	Description
request.size | 200	int	request.size if available, otherwise 200.
request.headers["x-forwarded-host"] == "myhost"	boolean
(request.headers["x-user-group"] == "admin") || (request.auth.principal == "admin")	boolean	True if the user is admin or in the admin group.
(request.auth.principal | "nobody" ) == "user1"	boolean	True if request.auth.principal is "user1", The expression will not error out if request.auth.principal is missing.
source.labels["app"]=="reviews" && source.labels["version"]=="v3"	boolean	True if app label is reviews and version label is v3, false otherwise.