NAME
   Mirror::JSON - Mirror Configuration and Auto-Discovery

DESCRIPTION
   Mirror::JSON is a functionally-compatible JSON port of Mirror::YAML.

   A "mirror.json" file is used to allow a repository client to reliably
   and robustly locate, identify, validate and age a repository.

   It contains a timestamp for when the repository was last updated, the
   URI for the master repository, and a list of all the current mirrors at
   the time the repository was last updated.

   Mirror::JSON contains all the functionality requires to both create and
   read the mirror.json files, and the logic to select one or more mirrors
   entirely automatically.

   It currently scales cleanly for a dozen or so mirrors, but may be slow
   when used with very large repositories with a hundred or more mirrors.

 Methodology
   A variety of simple individual mechanisms are combined to provide a
   completely robust discovery and validation system.

   URI Validation

   The mirror.json file should exist in a standard location, typically at
   the root of the repository. The file is very small (no more than a few
   kilobytes at most) so the overhead of fetching one (or several) of them
   is negligable.

   The file is pulled via FTP or HTTP. Once pulled, the first three
   characters are examined to validate it is a JSON file and not a login
   page for a "captured hotspot" such as at hotels and airports.

   Responsiveness

   Because the mirror.json file is small (in simple cases only one or two
   packets) the download time can be used to measure the responsiveness of
   that mirror.

   By pulling the files from several mirrors, the comparative download
   times can be used as part of the process of selecting the fastest
   mirror.

   Timestamp

   The mirror.json file contains a timestamp that records the last update
   time for the repository. This timestamp should be updated every
   repository update cycle, even if there are no actual changes to the
   repository.

   Once a mirror.json file has been fetched correctly, the timestamp can
   then be used to verify the age of the mirror. Whereas a perfectly up to
   date mirror will show an age of less than an hour (assuming that the
   repository master updates every hour) a repository that has stopped
   updating will show an age that is greater than the longest mirror rate
   plus the update cycle time.

   Thus, any mirror that as "gone stale" can be filter out of the potential
   mirrors to use.

   For portability, the timestamp is recording in ISO format Zulu time.

   Master Repository URI

   The mirror.json file contains a link to the master repository.

   If the Mirror::JSON client has an out-of-date current state at some
   point, it will use the master repository URI in the current state to
   pull a fresh mirror.json from the master repository.

   This solves the most-simple case, but other cases require a little more
   complexity (which we'll address later).

   Mirror URI List

   The mirror.json file contains a simple list of all mirror URIs.

   Apart from filtering the list to try and find the best mirror to use,
   the mirror list allows the Mirror::JSON client to have backup options
   for locating the master repository if it moves, or the bootstrap
   mirror.json file has gotten old.

   If the client can't find the master repository (because it has moved)
   the client will scan the list of mirrors to try to find the location of
   the updated repository.

   The Bootstrap mirror.json

   To bootstrap the client, it should come with a default bootstrap
   mirror.json file built into it. When the client starts up for the first
   time, it will attempt to fetch an updated mirror.json from the master
   repository, and if that doesn't exist will pull from the default list of
   mirrors until it can find more than one up to date mirror that agrees on
   the real location of the master server.

   Anti-Hijacking Functionality

   On top of the straight forward mirror discovery functionality, the
   client algorithm contains additional logic to deal with either a mirror
   or the master server goes bad. While likely not 100% secure it heads off
   several attack scenarios to prevent anyone trying them, and provides as
   much as can be expected without resorting to cryto and certificates.

SUPPORT
   Bugs should be reported via the CPAN bug tracker at

   <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Mirror-JSON>

   For other issues, or commercial enhancement or support, contact the
   author.

AUTHOR
   Adam Kennedy <[email protected]>

SEE ALSO
   JSON

COPYRIGHT
   Copyright 2008 Adam Kennedy.

   This program is free software; you can redistribute it and/or modify it
   under the same terms as Perl itself.

   The full text of the license can be found in the LICENSE file included
   with this module.

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