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11.13. Storing Data Structures to Disk

You want to save your large, complex data
structure to disk so you don't have to reconstruct it from scratch
each time your program runs.

11.13.2. Solution

Use the Storable module's store and
retrieve
functions:

use Storable;
store(\%hash, "filename");
# later on...
$href = "filename"); # by ref
%hash = %{ "filename") }; # direct to hash

11.13.3. Discussion

The
store and retrieve functions
expect binary data using the machine's own byte-ordering. This means
files created with these functions cannot be shared across different
architectures. nstore does the same job
store does, but keeps data in canonical (network)
byte order, at a slight speed cost:

use Storable qw(nstore);
nstore(\%hash, "filename");
# later ...
$href = "filename");

No matter whether store or
nstore was used, you need to call the same
retrieve routine to restore the objects in memory.
The producer must commit to portability, but the consumer doesn't
have to. Code need only be changed in one place when the producer has
a change of heart; the code thus offers a consistent interface for
the consumer, who does not need to know or care.

The store and nstore functions
don't lock the files they work on. If you're worried about concurrent
access, open the file yourself, lock it using Recipe 7.18, and then use store_fd or
its slower but machine-independent version
nstore_fd.

Here's code to save a hash to a file, with locking. We don't open
with the O_TRUNC flag because we have to wait to
get the lock before we can clobber the file.

use Storable qw(nstore_fd);
use Fcntl qw(:DEFAULT :flock);
sysopen(DF, "/tmp/datafile", O_RDWR|O_CREAT, 0666)
or die "can't open /tmp/datafile: $!";
flock(DF, LOCK_EX) or die "can't lock /tmp/datafile: $!";
nstore_fd(\%hash, *DF)
or die "can't store hash\n";
truncate(DF, tell(DF));
close(DF);

Here's code to restore that hash from a file, with locking:

use Storable qw(retrieve_fd);
use Fcntl qw(:DEFAULT :flock);
open(DF, " < /tmp/datafile") or die "can't open /tmp/datafile: $!";
flock(DF, LOCK_SH) or die "can't lock /tmp/datafile: $!";
$href = >

With care, you can pass large data objects efficiently between
processes using this strategy, since a filehandle connected to a pipe
or socket is still a byte stream, just like a plain file.

Unlike the various DBM bindings, Storable does not restrict you to
using only hashes (or arrays, with DB_File). Arbitrary data
structures, including objects, can be stored to disk. The whole
structure must be read in or written out in its entirety.

11.13.4. See Also

The section on "Remote Procedure Calls (RPC)" in Chapter 13 of
Advanced Perl Programming, by Sriram
Srinivasan (O'Reilly); Recipe 11.14

11.12. Copying Data Structures		11.14. Transparently Persistent Data Structures

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