                       Tie::ShadowHash version 0.7
                (Merge multiple data sources into a hash)

  Copyright 1999, 2002 by Russ Allbery <rra@stanford.edu>.  This program
  is free software; you may redistribute it and/or modify it under the
  same terms as Perl itself.


INTRODUCTION

  If you have a bunch of separate sources of data in the form of Perl
  hashes, tied hashes (of whatever type, including on-disk databases tied
  with DB_File, GDBM_File, or similar modules), or text files that you
  want to turn into hashes, and you want to be able to query all of those
  sources of data at once for a particular key without having to check
  each one of them individually each time, this module is what you're
  looking for.  If you want to use a hash-like data source, even just one,
  but make modifications to its data over the course of your program that
  override its contents while your program runs but which don't have any
  permanent effect on it, then this module may be what you're looking for.

  Tie::ShadowHash lets you create a "shadow hash" that looks like a
  regular Perl hash to your program but behind the scenes queries a whole
  list of data sources.  All the data sources underneath have to also
  behave like Perl hashes, but that's the only constraint.  They can be
  regular Perl hashes or other tied hashes, including tied DB_File or
  GDBM_File hashes or the like to access on-disk databases.  All data
  sources are treated as read-only; modifications to any data is stored in
  the shadow hash itself, and subsequent accesses reflect any
  modifications, but none of the data sources are changed.

  See the POD documentation for complete details, features, and usage.


INSTALLATION

  Follow the standard installation procedure for Perl modules, which is to
  type the following commands:

          perl Makefile.PL
          make
          make test
          make install

  You'll probably need to do the last as root.  If instead you wish to
  install the module by hand, simply copy it into a directory named Tie in
  your Perl library directory.


THANKS

  To Chris Nandor <pudge@pobox.com> for testing this module on the Mac,
  pointing out that SDBM_File wasn't available there, mentioning that SDBM
  was byte-order-dependent anyway, and suggesting using AnyDBM_File
  instead.


CONTACTING ME

  Please send any comments, bug reports, feature requests, or patches to
  rra@stanford.edu.  You may want to look over TODO before doing so to get
  an idea of what I already have planned.

  Please allow at least a month for me to respond to mail regarding this
  module.  I get at least 1,000 mail messages a day, and I'm afraid this
  module is something that I only get a chance to work on once every few
  months.  I do intend to continue to maintain it, however, and I will get
  back to you eventually.
