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.

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-id"]
| 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("") Use the email function to create an EMAIL_ADDRESS literal.
dnsName Convert a textual DNS name into the DNS_NAME type dnsName("") Use the dnsName function to create a DNS_NAME literal.
ip Convert a textual IPv4 address into the IP_ADDRESS type source.ip == ip("") 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("") 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".

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


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.