MEL Quick Reference

24 Jul 2024
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MEL Quick Reference

Updated on 24 Jul 2024
8 Minutes to read

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Introduction

You can use the CDNI Metadata Expression Language (MEL) when configuring the following site configuration features:

Source Attribution

This article includes information copied from the document CDNI Metadata Expression Language, for your convenience when configuring a site for content delivery. It is meant to provide a quick reference to the MEL expression syntax. The full original document is publicly available at: https://datatracker.ietf.org/doc/draft-power-metadata-expression-language/.

Quick Reference

The following is a list of the SVA Open Caching Configuration - CDNI MEL components supported by Qwilt, with descriptions provided in the subsequent sections of this document.

Variables	Operators	Keywords	Functions
req.h.<name> req.uri req.uri.path req.uri.pathquery req.uri.query req.uri.query.<key> req.uri.querykv.<key> req.method req.scheme resp.h.<name> resp.status req.clientip req.clientport var.<user-variable>	== Equality test != Inequality test ! Logical NOT operator > Greater than test < Less than test >= Greater than or equal test <= Less than or equal = Glob style match ~= Regular expression match + Addition - Subtraction Multiplication / Division % Modulus . Concatenation ( ) Grouping ? : Conditional operator: <e> ? <v1> : <v2>	and or not nil true false	string(e) upper(e) lower(e) match(string Input, string Match) match_replace(string Input, string Match, string Replace) add_query(string Input, string q, string v) add_query_multi(string Input, string qvs) remove_query(string Input, string q) remove_query_multi(string Input, string qs) keep_query_multi(string Input, string qs) path_element(string Input, integer n) path_element(string Input, integer n, integer m)

Variables

Operators

Keywords

Functions

req.h.<name>

req.uri

req.uri.path

req.uri.pathquery

req.uri.query

req.uri.query.<key>

req.uri.querykv.<key>

req.method

req.scheme

resp.h.<name>

resp.status

req.clientip

req.clientport

var.<user-variable>

== Equality test

!= Inequality test

! Logical NOT operator

> Greater than test

< Less than test

>= Greater than or equal test

<= Less than or equal

*= Glob style match

~= Regular expression match

+ Addition

- Subtraction

* Multiplication

/ Division

% Modulus

. Concatenation

( ) Grouping

? : Conditional operator: <e> ? <v1> : <v2>

and

not

nil

true

false

string(e)

upper(e)

lower(e)

match(string Input, string Match)

match_replace(string Input, string Match, string Replace)

add_query(string Input, string q, string v)

add_query_multi(string Input, string qvs)

remove_query(string Input, string q)

remove_query_multi(string Input, string qs) keep_query_multi(string Input, string qs)

path_element(string Input, integer n)

path_element(string Input, integer n, integer m)

Expression Usage in CDNI Metadata Model

Source Attribution
The information in this section is copied from section 3 of the document CDNI Metadata Expression Language. Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.

The CDNI Metadata Expression Language (MEL) provides a syntax with a rich set of variables, operators, and built-in functions to facilitate two key use cases within the CDNI metadata model:

Match Expressions: Expressions that evaluate to a Boolean value are used to dynamically determine if metadata should be applied based on the evaluation of aspects of an inbound HTTP request (matching to a header value, for example).
Value Expressions: Enable the dynamic construction of a value to be used in scenarios such as constructing a cache key, setting an HTTP response header or status code, rewriting a request Uniform Resource Identifier (URI), or dynamically generating a response body.

Refer to the Processing Stages Metadata Specification [SVTA 2032] for example usage of both match expressions and value expressions within the Processing Stages Model.

Refer to the Cache Control Metadata Specification [SVTA2033] for example usage of value expressions to dynamically generate cache keys.

Core Variables

Source Attribution
The information in this section is copied from section 5 of the document CDNI Metadata Expression Language. Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.

The expression language supports two namespaces of core variables, each with a distinct prefix:

req. - A prefix for accessing attributes of the HTTP request object.
resp. - A prefix for accessing attributes of the HTTP response object.

Variable	Meaning	Type
req.h.<name>	Request header <name>	String
req.uri	Request URI (includes query string and fragment identifier, if any.	String
req.uri.path	Request URI path	String
req.uri.pathquery	Request path and query string.	String
req.uri.query	Request query string	String
req.uri.query.<key>	Request query string value associated with <key>. If the key is not present in the URI, nil is returned. If the key is present with no value, as in "a=", then an empty string is returned.	String
req.uri.querykv.<key>	Request query string key and value associated with <key>, returned as a single string exactly as-is from the request URL. For example, when used with a URI containing a query string "key=xxx", the expression would return the string "key=xxx". When used with a URI containing a query string "key=", the expression would return the string"key=".	String
req.method	Request HTTP method (GET, POST, etc.)	String
req.scheme	Request scheme (http or https)	String
req.clientip	IP address of the client that made the request. Note: IPv6 addresses MUST NOT be enclosed in square brackets [ ].	String
req.clientport	Request port number	Unsigned
resp.h.<name>	Response header <name>	String
resp.status	Response status code	Unsigned

Operators and Keywords

Source Attribution
The information in this section is copied from section 6 of the document CDNI Metadata Expression Language. Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.

*Note that the ipmatch operator is not supported by Qwilt.

Operators

Operator	Type	Result Type	Meaning
==	infix	Boolean	Equality test
!=	infix	Boolean	Inequality test
!	infix	Boolean	Logical NOT operator
>	infix	Boolean	Greater than test
<	infix	Boolean	Less than test
>=	infix	Boolean	Greater than or equal test
<=	infix	Boolean	Less than or equal test
*=	infix	Boolean	Glob style match
~=	infix	Boolean	Regular expression match (See [PCRE] for details on PCRE RegEx matching/)
ipmatch - Not supported by Qwilt.	infix	Boolean	Match against IP address or CIDR (IPv4 and IPv6).
+	infix	Numeric	Addition
-	infix	Numeric	Subtraction
*	infix	Numeric	Multiplication
/	infix	Numeric	Division
%	infix	Unsigned or Integer	Modulus
.	infix	String	Concatenation Note: There MUST be at least 1 space before/after the dot operator.
? :	ternary	*	Conditional operator: <e> ? <v1> : <v2>Evaluates <v1> if <e> is true, <v2> otherwise.
( )	grouping		Used to override precedence and used for function calls.

Keywords

Keyword	Meaning
and	Logical AND
or	Logical OR
not	Logical NOT (see also the "!" operator)
nil	No value (distinct from empty value)
true	Boolean constant: true
false	Boolean constant: false

Built-In Functions

Source Attribution
The information in this section is copied from section 7 of the document CDNI Metadata Expression Language. Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.

*Note that the integer, real, and Boolean functions are not supported by Qwilt.

To enable the types of expressions typically used in content delivery scenarios to evaluate and generate or modify HTTP headers, the following set of built-in functions are defined to facilitate format conversions, matching, and query string management. Any dCDN that advertises support for a metadata property that leverages MEL MUST provide implementations of these built-in functions.

Type Conversions

Function	Action	Argument(s)	Returns
integer(e) - Not supported by Qwilt.	Converts expression to integer.	1	integer
real (e) - Not supported by Qwilt.	Converts expression to real.	1	real
string (e)	Converts expression to string.	1	string
boolean (e) - Not supported by Qwilt.	Converts expression to Boolean.	1	Boolean

String Conversions

Function	Action	Argument(s)	Returns
upper (e)	Converts a string to uppercase. Useful for case-insensitive comparisons.	1	string
lower (e)	Converts a string to lowercase. Useful for case-insensitive comparisons.	1	string

Convenience Functions

Function	Action	Args	Returns
match (string Input, string Match)	Regular expression 'Match' is applied to Input and the matching element (if any) is returned. An empty string is returned if there is no match. See [PCRE] for details on PCRE RegEx matching.	2	string
match_replace (string Input, string Match, string Replace)	Regular expression 'Match' is applied to the Input argument and replaced with the Replace argument upon successful match. It returns the updated (replaced) version of Input.	3	string
add_query (string Input, string q, string v)	Add query string element q with value v to the Input string. If v is nil, just add the query string element q. The query element q and value v MUST conform to the format defined in [RFC3986].	2	string
add_query_multi (string input, string qvs)	Add all the query value elements from qvs to the input string. For example, if qvs = "k1=v1, k2=v2, k3=v3"), parameters k1, k2, k3 and associated values would be added to input. If a qvs element only has a key but no value, the existing value of that key will be kept.	2	string
remove_query (string Input,string q)	Remove all occurrences of query string element q from the Input string.	2	string
remove_query_multi (string input, string qs)	Remove all occurrences of the query string elements referenced in parameter qs from the input string. For example, if qs= "k1, k2, k3", all occurrences of k1, k2, k3 would be removed from input.	2	string
keep_query_multi (string input, string qs)	Remove all query string elements from input except for the elements referenced in parameter qs. For example, if qs = "k1, k2, k3", all query string elements would be removed from input except for k1, k2, k3.	2	string
path_element (string Input, integer n)	Return the path element n from Input. -1 returns the last element.	2	string
path_elements (string Input, integer n, integer m)	Return the path elements from position n to m.	3	string

User-Defined Variables

Source Attribution
The information in this section is copied from section 8 of document CDNI Metadata Expression Language. Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.

In addition to the core variable namespaces that address HTTP request and response objects (req. and resp.), MEL supports user-defined variables that can be referenced via the following prefix:

var. Prefix for accessing user-defined variables set via MI.SetVariable

A user defined variable is available for access in MEL expressions from the moment it is assigned, throughout the duration of the transaction. If a user variable is accessed before it has been set, it returns the value of nil.

MI.SetVariable

MI.SetVariable is a GenericMetadata object that allows one to set user-defined variables that can be accessed from MEL. Variables set via this mechanism can be accessed in matching and value construction expressions using a var. prefix before the variable name. Note: Variable names MUST be string literals.

Property: variable-name

Description: The name of the variable.
Type: String. Alphanumeric with both uppercase and lowercase characters; the property MUST start with a letter.
Mandatory-to-Specify: Yes

Property: variable-value

Description: The string representation of the value to set for the user-defined variable.
Type: String, either the static value or a MEL expression, determined by the value-is-expression property.
Mandatory-to-Specify: Yes

Property: value-is-expression

Description: A flag to signal whether the value is a static string literal or a MEL expression that needs to be dynamically evaluated.
Type: Boolean
Mandatory-to-Specify: No. The default is "False", indicating that the value is a string literal and does not need to be evaluated.

The following example illustrates the setting of a user-defined variable whose value is a MEL expression suffixed by a static literal string:

{
  "generic-metadata-type": "MI.SetVariable",
  "generic-metadata-value": {
    "variable-name": "myvar",
    "variable-value": "req.h.host . 'is my host from player'"
    "value-is-expression": true
  }
}

The following example illustrates the setting of a user-defined variable whose value is another user-defined variable suffixed by a static literal string:

{
  "generic-metadata-type": "MI.SetVariable",
  "generic-metadata-value": {
    "variable-name": "myvar1",
    "variable-value": "req.host . 'is my host from player'"
    "value-is-expression": true
  }
}
{
  "generic-metadata-type": "MI.SetVariable",
  "generic-metadata-value": {
    "variable-name": "myvar2",
    "variable-value": "var.myvar1 . 'and forwarded to Origin'"
    "value-is-expression": true
  }
}

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Rule Order

Table of contents

Introduction
Source Attribution
Quick Reference
Expression Usage in CDNI Metadata Model
Core Variables
Operators and Keywords
Built-In Functions
User-Defined Variables