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 Storable - persistency for perl data structures


NAME

Storable - persistency for perl data structures


SUPPORTED PLATFORMS

  • Linux
  • Solaris
  • Windows
This module is not included with the standard ActivePerl distribution. It is available as a separate download using PPM.

SYNOPSIS

 use Storable;
 store \%table, 'file';
 $hashref = retrieve('file');
 use Storable qw(nstore store_fd nstore_fd freeze thaw dclone);
 # Network order
 nstore \%table, 'file';
 $hashref = retrieve('file');   # There is NO nretrieve()
 # Storing to and retrieving from an already opened file
 store_fd \@array, \*STDOUT;
 nstore_fd \%table, \*STDOUT;
 $aryref = retrieve_fd(\*SOCKET);
 $hashref = retrieve_fd(\*SOCKET);
 # Serializing to memory
 $serialized = freeze \%table;
 %table_clone = %{ thaw($serialized) };
 # Deep (recursive) cloning
 $cloneref = dclone($ref);


DESCRIPTION

The Storable package brings persistency to your perl data structures containing SCALAR, ARRAY, HASH or REF objects, i.e. anything that can be convenientely stored to disk and retrieved at a later time.

It can be used in the regular procedural way by calling store with a reference to the object to be stored, along with the file name where the image should be written. The routine returns undef for I/O problems or other internal error, a true value otherwise. Serious errors are propagated as a die exception.

To retrieve data stored to disk, use retrieve with a file name, and the objects stored into that file are recreated into memory for you, a reference to the root object being returned. In case an I/O error occurs while reading, undef is returned instead. Other serious errors are propagated via die.

Since storage is performed recursively, you might want to stuff references to objects that share a lot of common data into a single array or hash table, and then store that object. That way, when you retrieve back the whole thing, the objects will continue to share what they originally shared.

At the cost of a slight header overhead, you may store to an already opened file descriptor using the store_fd routine, and retrieve from a file via retrieve_fd. Those names aren't imported by default, so you will have to do that explicitely if you need those routines. The file descriptor you supply must be already opened, for read if you're going to retrieve and for write if you wish to store.

        store_fd(\%table, *STDOUT) || die "can't store to stdout\n";
        $hashref = retrieve_fd(*STDIN);

You can also store data in network order to allow easy sharing across multiple platforms, or when storing on a socket known to be remotely connected. The routines to call have an initial n prefix for network, as in nstore and nstore_fd. At retrieval time, your data will be correctly restored so you don't have to know whether you're restoring from native or network ordered data.

When using retrieve_fd, objects are retrieved in sequence, one object (i.e. one recursive tree) per associated store_fd.

If you're more from the object-oriented camp, you can inherit from Storable and directly store your objects by invoking store as a method. The fact that the root of the to-be-stored tree is a blessed reference (i.e. an object) is special-cased so that the retrieve does not provide a reference to that object but rather the blessed object reference itself. (Otherwise, you'd get a reference to that blessed object).


MEMORY STORE

The Storable engine can also store data into a Perl scalar instead, to later retrieve them. This is mainly used to freeze a complex structure in some safe compact memory place (where it can possibly be sent to another process via some IPC, since freezing the structure also serializes it in effect). Later on, and maybe somewhere else, you can thaw the Perl scalar out and recreate the original complex structure in memory.

Surprisingly, the routines to be called are named freeze and thaw. If you wish to send out the frozen scalar to another machine, use nfreeze instead to get a portable image.

Note that freezing an object structure and immediately thawing it actually achieves a deep cloning of that structure. Storable provides you with a dclone interface which does not create that intermediary scalar but instead freezes the structure in some internal memory space and then immediatly thaws it out.


SPEED

The heart of Storable is written in C for decent speed. Extra low-level optimization have been made when manipulating perl internals, to sacrifice encapsulation for the benefit of a greater speed.

Storage is now slightly slower than retrieval since the former has to also store data in a hash table to keep track of which objects have been stored already, whilst the latter uses an array instead of a hash table.

On my HP 9000/712 machine running HPUX 9.03 and with perl 5.004, I can store 0.85 Mbyte/s and I can retrieve at 0.90 Mbytes/s, approximatively (CPU + system time). This was measured with Benchmark and the Magic: The Gathering database from Tom Christiansen (1.6 Mbytes on disk).


CANONICAL REPRESENTATION

Normally Storable stores elements of hashes in the order they are stored internally by Perl, i.e. pseudo-randomly. If you set $Storable::canonical to some TRUE value, Storable will store hashes with the elements sorted by their key. This allows you to compare data structures by comparing their frozen representations (or even the compressed frozen representations), which can be useful for creating lookup tables for complicated queries.

Canonical order does not imply network order, those are two orthogonal settings.


WIZARDS ONLY

The Storable::last_op_in_netorder() predicate will tell you whether network order was used in the last store or retrieve operation. If you don't know how to use this, just forget about it.


EXAMPLES

Here are some code samples showing a possible usage of Storable:

        use Storable qw(store retrieve freeze thaw dclone);
        %color = ('Blue' => 0.1, 'Red' => 0.8, 'Black' => 0, 'White' => 1);
        store(\%color, '/tmp/colors') or die "Can't store %a in /tmp/colors!\n";
        $colref = retrieve('/tmp/colors');
        die "Unable to retrieve from /tmp/colors!\n" unless defined $colref;
        printf "Blue is still %lf\n", $colref->{'Blue'};
        $colref2 = dclone(\%color);
        $str = freeze(\%color);
        printf "Serialization of %%color is %d bytes long.\n", length($str);
        $colref3 = thaw($str);

which prints (on my machine):

        Blue is still 0.100000
        Serialization of %color is 102 bytes long.


WARNING

If you're using references as keys within your hash tables, you're bound to disapointment when retrieving your data. Indeed, Perl stringifies references used as hash table keys. If you later wish to access the items via another reference stringification (i.e. using the same reference that was used for the key originally to record the value into the hash table), it will work because both references stringify to the same string.

It won't work across a store and retrieve operations however, because the addresses in the retrieved objects, which are part of the stringified references, will probably differ from the original addresses. The topology of your structure is preserved, but not hidden semantics like those.

On platforms where it matters, be sure to call binmode() on the descriptors that you pass to Storable functions.

Storing data canonically that contains large hashes can be significantly slower than storing the same data normally, as temprorary arrays to hold the keys for each hash have to be allocated, populated, sorted and freed. Some tests have shown a halving of the speed of storing -- the exact penalty will depend on the complexity of your data. There is no slowdown on retrieval.


BUGS

You can't store GLOB, CODE, FORMLINE, etc... If you can define semantics for those operations, feel free to enhance Storable so that it can deal with them.

The store functions will croak if they run into such references unless you set $Storable::forgive_me to some TRUE value. In that case, the fatal message is turned in a warning and some meaningless string is stored instead.

Setting $Storable::canonical may not yield frozen strings that compare equal due to possible stringification of numbers. When the string version of a scalar exists, it is the form stored, therefore if you happen to use your numbers as strings between two freezing operations on the same data structures, you will get different results.

Due to the aforementionned optimizations, Storable is at the mercy of perl's internal redesign or structure changes. If that bothers you, you can try convincing Larry that what is used in Storable should be documented and consistently kept in future revisions.


CREDITS

Thank you to (in chronological order):

        Jarkko Hietaniemi <jhi@iki.fi>
        Ulrich Pfeifer <pfeifer@charly.informatik.uni-dortmund.de>
        Benjamin A. Holzman <benjamin.a.holzman@bender.com>
        Andrew Ford <A.Ford@ford-mason.co.uk>
        Gisle Aas <gisle@aas.no>
        Jeff Gresham <gresham_jeffrey@jpmorgan.com>
        Murray Nesbitt <murray@activestate.com>

for their bug reports, suggestions and contributions.

Benjamin Holzman contributed the tied variable support, Andrew Ford contributed the canonical order for hashes, and Gisle Aas fixed a few misunderstandings of mine regarding the Perl internals, and optimized the emission of ``tags'' in the output streams by simply counting the objects instead of tagging them (leading to a binary incompatibility for the Storable image starting at version 0.6--older images are of course still properly understood). Murray Nesbitt made Storable thread-safe.


TRANSLATIONS

There is a Japanese translation of this man page available at http://member.nifty.ne.jp/hippo2000/perltips/storable.htm , courtesy of Kawai, Takanori <kawai@nippon-rad.co.jp>.


AUTHOR

Raphael Manfredi <Raphael_Manfredi@pobox.com>

 Storable - persistency for perl data structures