Expression Language
- Background
- Syntax
- Functions
- Type checking
- Missing attributes
- Examples

Expression Language

This page describes how to use the Mixer configuration 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 typedvalue.

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-request-id"]`
`+`	Add	`request.host + request.path`
`>`	Greater Than	`response.code > 200`
`>=`	Greater Than or Equal to	`request.size >= 100`
`<`	Less Than	`response.code < 500`
`<=`	Less Than or Equal to	`request.size <= 100`
`\|`	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("awesome@istio.io")`	Use the `email` function to create an `EMAIL_ADDRESS` literal.
`dnsName`	Convert a textual DNS name into the `DNS_NAME` type	`dnsName("www.istio.io")`	Use the `dnsName` function to create a `DNS_NAME` literal.
`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.
`uri`	Convert a textual URI into the `URI` type	`uri("http://istio.io")`	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"`.
`emptyStringMap`	Create an empty string map	`request.headers` `\|` `emptyStringMap()`	Use `emptyStringMap` to create an empty string map for default value of `request.headers`.
`conditional`	Simulate ternary operator	`conditional((context.reporter.kind` `\|` `"inbound") == "outbound", "client", "server")`	Returns `"client"` if report kind is `outbound` otherwise returns `"server"`.
`toLower`	Convert a string to lowercase letters	`toLower("User-Agent")`	Returns `"user-agent"`.
`size`	Length of a string	`size("admin")`	Returns 5

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 configuration validation.Selectors must resolve to a boolean value and mapping expressions must resolve to the type they are mapping into. Configuration 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.