cache-digest

Nearly literal implementation of the algorithms described in the Cache Digest proposed specification.

API

computeDigestValue(validators, urls, p)

Specification: https://tools.ietf.org/html/draft-kazuho-h2-cache-digest-01#section-2.1.1

• validators is a boolean. Defaults to false. Any truthy value is currently unsupported. See below.

• urls is an array of [url, etag] tuples, where url and etag are strings. Etags are not currently supported. See below.

• p is an integer. Represents the probability of collisions.

computeHashValue([url, etag], validators, n, p)

Specification: https://tools.ietf.org/html/draft-kazuho-h2-cache-digest-01#section-2.1.2

Returns the hash as an integer. May throw errors on invalid or unsupported input.

• url is a string representing the URL. Defaults to empty string. Example: https://example.com/foo/bar.js

• etag is a string. Defaults to empty string. Currently unsupported. See below.

• validators is a boolean. Defaults to false. Any truthy value is currently unsupported. See below.

• n is an integer. Defaults to 1. Represents the number of elements in the digest. Affects the size of the hash.

• p is an integer. Defaults to 1. Represents the probability of collisions. Affects the size of the hash.

queryDigestValue(digestValue, url, etag, validators)

Specification: https://tools.ietf.org/html/draft-kazuho-h2-cache-digest-01#section-2.2.1

Returns true if the URL is present in the cache digest. Throws an error otherwise.

• digestValue is a Uint8Array containing the digest.

• url is a string representing the URL. Defaults to empty string. Example: https://example.com/foo/bar.js

• etag is a string. Defaults to empty string. Currently unsupported. See below.

• validators is a boolean. Defaults to false. Any truthy value is currently unsupported. See below.

Known Limitations

31 bit maximum hash size

Though the specification allows up to 62 bits, this implementation is limitd to 31 bits.

Due to JavaScript's boolean logic operators returning 32-bit signed integers, where one bit is lost to the sign, the maximum size of a hash value is 31 bits.

Keep an eye on the N (number of items in the cache digest) and P (probability of collisions) values. If their sum exceeds 31, an error will be throw.

Note: This is only applicable to the hash size. Any size of digest value can be generated and queried.

No Etags support

This may be implemented someday. I am looking for clarification from the specification authors, ideally including test cases (given/expected) to validate the implementation.

See Also

• http2server — Webserver using this package to support Cache Digests via the cache-digest header or cookie.
• cache-digest-immutable — Service worker to calculate cache digests client side. Does HTTP caching with special support for cache-control: immutable.
• H2O — Webserver with C-language Cache Digest implementation.
• cache-digest.js — Original JavaScript implementation. Borrowing its BitCoder.