The HTTPUse-As-Dictionary response header lists the matching criteria that theCompression Dictionary Transport dictionary can be used for, for future requests.

See theCompression Dictionary Transport guide for more information.

Syntax

Use-As-Dictionary: match="<url-pattern>"Use-As-Dictionary: match-dest=("<destination1>" "<destination2>", …)Use-As-Dictionary:Use-As-Dictionary: type="raw"// Multiple, in any orderContent-Encoding: match="<url-pattern>", match-dest=("<destination1>")

Directives

match

A string value containing aURL Pattern: only resources whose URLs match this pattern may use this resource as a dictionary. Regular expression capturing groups are not allowed, soURLPattern.hasRegExpGroups must befalse.

match-dest

A space-separated list of strings, with each string in quotes and the whole value enclosed in parentheses, that provides a list ofFetch request destinations that requests must match if they are to use this dictionary.

id

A string value that specifies a server identifier for the dictionary. This ID value will then be added in theDictionary-ID request header when the browser requests a resource which can use this dictionary.

type

A string value that describes the file format of the supplied dictionary. Currently onlyraw is supported (which is the default) so this is more for future compatibility.

Examples

Path prefix

Use-As-Dictionary: match="/product/*"

This says the dictionary is only to be used for URLs starting with/product/.

Versioned directories

Use-As-Dictionary: match="/app/*/main.js"

This uses a wildcard to match multiple versions of a file.

Destinations

Use-As-Dictionary: match="/product/*", match-dest=("document")

This usesmatch-dest to ensure the dictionary is only used fordocument requests so<script src="/product/js/app.js"> resource requests for example would not match.

Use-As-Dictionary: match="/product/*", match-dest=("document" "frame")

This would allow the dictionary to match both top-level documents and iframes.

Id

Use-As-Dictionary: match="/product/*",

WhenUse-As-Dictionary includes anid directive, as in this example, theid value will be included in theDictionary-ID request header for resources that can use this dictionary. The resource request will also include the SHA-256 hash of the dictionary surrounded by colons in theAvailable-Dictionary header:

Accept-Encoding: gzip, br, zstd, dcb, dczAvailable-Dictionary: :pZGm1Av0IEBKARczz7exkNYsZb8LzaMrV7J32a2fFG4=:Dictionary-ID: "dictionary-12345"

The server must still check the hash from theAvailable-Dictionary header — theDictionary-ID is additional information for the server to identify the dictionary but does not replace the need for theAvailable-Dictionary header.

Type

Use-As-Dictionary: match="/product/*", type="raw"

Currently, onlyraw is supported (which is the default) so this is more for future compatibility.

Specifications

Browser compatibility

See also

• Compression Dictionary Transport guide
• Available-Dictionary
• Dictionary-ID