Expression Language
This page describes how to use the Mixer config 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-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 * |
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. |
.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".
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. |