Use-As-Dictionary

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

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

See the Compression Dictionary Transport guide for more information.

Syntax

http

Use-As-Dictionary: match="<urlpattern>"
Use-As-Dictionary: match-dest=("<destination1>" "<destination2>", …)
Use-As-Dictionary: id="<string-identifier>"
Use-As-Dictionary: type="raw"

// Multiple, in any order
Content-Encoding: match="<urlpattern>", match-dest=("<destination1>")

Directives

match: A string value containing a URL Pattern: only resources whose URLs match this pattern may use this resource as a dictionary.
match-dest: A space-separated list of strings, with each string in quotes and the whole value enclosed in parentheses, that provides a list of Fetch 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 the Dictionary-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 only raw is supported (which is the default) so this is more for future compatibility.

Examples

Path prefix

http

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

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

Versioned directories

http

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

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

Destinations

http

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

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

http

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

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

Id

http

Use-As-Dictionary: match="/product/*", id="dictionary-12345"

When Use-As-Dictionary includes an id directive, as in this example, the id value will be included in the Dictionary-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 the Available-Dictionary header:

http

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

Type

http

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

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

Specifications

Specification
Compression Dictionary Transport # name-use-as-dictionary