istio.io/_docs/reference/config/mixer/expression-language.md

title	overview	order	layout	type
Expression Language	Mixer config expression language reference.	43	docs	markdown

{% capture mixerConfig %}{{home}}/docs/concepts/policy-and-control/mixer-config.html{% endcapture %}

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

Background

Mixer configuration uses an expression language (CEXL) to specify selectors 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 |Description |Example |------------------------------------ |== |Equals |request.size == 200 |== |Equals Prefix|service.name == "svc1.*" |== |Equals Suffix|service.name == "*.ns1.svc.cluster.local" |!= |Not Equals |request.user != "admin" ||| |Logical OR | (request.size == 200) || (request.user == "admin") |&& |Logical AND | (request.size == 200) && (request.user == "admin") |[ ] |Map Access | request.headers["x-id"] || |First non empty | source.labels["app"] | source.labels["svc"] | "unknown"

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".

Examples

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