Expression Language

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


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.


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.


CEXL supports the following functions.

==Equalsrequest.size == 200 
matchGlob matchmatch(destination.service, "*.ns1.svc.cluster.local")Matches prefix or suffix based on the location of *
!=Not Equalsrequest.user != "admin" 
||Logical OR(request.size == 200) || (request.user == "admin") 
&&Logical AND(request.size == 200) && (request.user == "admin") 
[ ]Map Accessrequest.headers["x-id"] 
|First non emptysource.labels["app"] | source.labels["svc"] | "unknown" 
ipConvert a textual IPv4 address into the IP_ADDRESS typesource.ip == ip("")Use the ip function to create an IP_ADDRESS literal.

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 config validation. Selectors must resolve to a boolean value and mapping expressions must resolve to the type they are mapping into. Config 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.user == "user1" fails evaluation if the request.user attribute is missing. The | (OR) operator addresses the problem: (request.user | "nobody" ) == "user1".


ExpressionReturn TypeDescription
request.size| 200intrequest.size if available, otherwise 200.
request.header["X-FORWARDED-HOST"] == "myhost"boolean 
(request.header["x-user-group"] == "admin") || (request.user == "admin")booleanTrue if the user is admin or in the admin group.
(request.user | "nobody" ) == "user1"booleanTrue if request.user is “user1”, The expression will not error out if request.user is missing.
source.labels["app"]=="reviews" && source.labels["version"]=="v3"booleanTrue if app label is reviews and version label is v3, false otherwise.