MMeettaalliinnkk

Try not to download the same file twice. Improve cache efficiency and speed up downloads.

Take standard headers and knowledge about objects in the cache and potentially rewrite those headers so that a client will use a URL that's already cached instead of one that isn't. The headers are specified in [RFC 6249] (Metalink/HTTP: Mirrors and Hashes) and [RFC 3230] (Instance Digests in HTTP) and are sent by various download redirectors or content distribution networks.

11.. WWhhoo CCaarreess??

More important than saving a little bandwidth, this saves users from frustration.

A lot of download sites distribute the same files from many different mirrors and users don't know which mirrors are already cached. These sites often present users with a simple download button, but the button doesn't predictably access the same mirror, or a mirror that's already cached. To users it seems like the download works sometimes (takes seconds) and not others (takes hours), which is frustrating.

An extreme example of this happens when users share a limited, possibly unreliable internet connection, as is common in parts of Africa for example.

[How to cache openSUSE repositories with Squid] is another, different example of a use case where picking a URL that's already cached is valuable.

22.. WWhhaatt iitt DDooeess

When it sees a response with a "Location: ..." header and a "Digest: SHA-256=..." header, it checks if the URL in the Location header is already cached. If it isn't, then it tries to find a URL that is cached to use instead. It looks in the cache for some object that matches the digest in the Digest header and if it succeeds, then it rewites the Location header with that object's URL.

This way a client should get sent to a URL that's already cached and won't download the file again.

33.. HHooww ttoo UUssee iitt

Just build the plugin and add it to your plugin.config file.

The code is distributed along with recent versions of Traffic Server, in the plugins/experimental/metalink directory. To build it, pass the --enable-experimental-plugins option to the configure script when you build Traffic Server:

$ ./configure --enable-experimental-plugins

When you're done building Traffic Server, add "metalink.so" to your plugin.config file to start using the plugin.

44.. RReeaadd MMoorree

More details are on the [wiki page] in the Traffic Server wiki.

[RFC 6249] http://tools.ietf.org/html/rfc6249

[RFC 3230] http://tools.ietf.org/html/rfc3230

[How to cache openSUSE repositories with Squid] http://wiki.jessen.ch/index/How_to_cache_openSUSE_repositories_with_Squid

[wiki page] https://cwiki.apache.org/confluence/display/TS/Metalink