Expression Language

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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 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-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.