NAME

File::DigestStore - Digested hierarchical storage of files

SYNOPSIS

my $store = new File::DigestStore root => '/var/lib/digeststore';

# stores the file and returns a short-ish ID
my $id = $store->store_file('/etc/motd');
# Will output a hex string like '110fe...'
print "$id\n";
# returns a filename that has the same contents as the stored file
my $path = $store->fetch_file($id);
# Will return something like '/var/lib/digeststore/1/2/110fe...'
print "$path\n";

DESCRIPTION

This module is used to take large files (or strings) and stash them away in a backend data store, returning a short key that may then be stored in a database. This avoids having to store large BLOBs in your database.

The backend data store should be considered opaque as far as your Perl program is concerned, but it actually consists of the files hashed and then stored in a multi-level directory structure for fast access. Files are never moved around the tree so if the stash is particularly large, you can place subtrees on their own filesystem if required. Directories are created on-demand and so do not need to be pre-created.

FUNCTIONS

new

my $store = new File::DigestStore root => '/var/lib/digeststore';

This creates a handle to a new digested storage area. Arguments are given to it to define the layout of the storage area:

root (required)

The base directory that is used to store the files. This will be created if it does not exist, and the stashed files stored underneath it.

levels (optional, default "8,256")

The number of directory entries in each level of the tree. For example, "8,256" means that the top-level will have eight directories (called "0" through "7") and each of those directories will have 256 sub-directories. The stashed data files appear under those.

algorithm (optional, default "SHA-512")

The digest algorithm used to hash the files. This is passed to Digest->new(). The file's content is hashed and then stored using that name, so you should select an algorithm that does not generate collisions.

dir_mask (optional, default 0777)

The directory creation mask for the stash directories. This is merged with yoru umask so the default is usually fine.

file_mask (optional, default 0666)

The file creation mask for the stashed files. This is merged with your umask setting so the default is usually fine.

store_file

my $id = $store->store_file('/etc/motd');

my ($id, $size) = $store->store_file('/etc/passwd');

This copies the file's contents into the stash. In scalar context it returns the file's ID. In list context it returns an (ID, file size) tuple. (The latter saves you having to stat() your file.)

store_string

my $id = $store->store_string('Hello, world');

This copies the string's contents into the stash. In scalar context it returns the file's ID. In list context it returns an (ID, string length) tuple.

fetch_file

my $path = $store->fetch_file($id);

Given an ID, will return the path to the stashed copy of the file, or undef if no file with that ID has ever been stored.

Note that the path refers to the master copy of the file within the stash so you will need to copy the file if you are going to potentially modify it.

fetch_string

my $string = $store->fetch_string($id);

Given an ID, will return the string which was previously stashed to that ID or undef if no string with that ID has ever been stored.

exists

if($store->exists($id)) {
   # ...
}

Returns true if anything was stashed with this the given ID, otherwise false.

BUGS

This does not currently check for hash collisions.

You cannot provide a hashing algorithm that is not a Digest::* derivative.

SEE ALSO

File::HStore implements a similar idea.

AUTHOR

All code and documentation by Peter Corlett <abuse@cabal.org.uk>.

COPYRIGHT

Copyright (C) 2008 Peter Corlett <abuse@cabal.org.uk>. All rights reserved.

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

SUPPORT / WARRANTY

This is free software. IT COMES WITHOUT WARRANTY OF ANY KIND.