Use-As-Dictionary header

Syntax
Directives
Examples
Path prefix
Versioned directories
Destinations
Id
Type
Specifications
Browser compatibility
See also

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

The browser uses the dictionary only while the response that carries this header is fresh, or while stale-while-revalidate still allows that response to be served stale. A response sent with no-cache or no-store is never used as a dictionary, and one sent with must-revalidate is used only until its max-age runs out. See Dictionary freshness for more details.

See the Compression Dictionary Transport guide for more information.

Syntax

Use-As-Dictionary: match="<url-pattern>"
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="<url-pattern>", 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. Regular expression capturing groups are not allowed, so URLPattern.hasRegExpGroups must be false.
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

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 uses match-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.

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/*", 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:

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

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

Type

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

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