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