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Storable - persistence for Perl data structures
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 = fd_retrieve(\*SOCKET); $hashref = fd_retrieve(\*SOCKET);
# Serializing to memory
$serialized = freeze \%table;
%table_clone = %{ thaw($serialized) };
# Deep (recursive) cloning $cloneref = dclone($ref);
# Advisory locking
use Storable qw(lock_store lock_nstore lock_retrieve)
lock_store \%table, 'file';
lock_nstore \%table, 'file';
$hashref = lock_retrieve('file');
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. The objects stored into that file are recreated into memory for you, and a reference to the root object is 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 "fd_retrieve". Those names aren't imported by default, so you will have to do that explicitly 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 = fd_retrieve(*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. Double values are stored stringified to ensure portability as well, at the slight risk of loosing some precision in the last decimals.
When using "fd_retrieve", 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).
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:
dclone(.) = thaw(freeze(.))
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 immediately thaws it out.
As with any advisory locking scheme, the protection only works if you systematically use "lock_store" and "lock_retrieve". If one side of your application uses "store" whilst the other uses "lock_retrieve", you will get no protection at all.
The internal advisory locking is implemented using Perl's flock() routine. If your system does not support any form of flock(), or if you share your files across NFS, you might wish to use other forms of locking by using modules such as LockFile::Simple which lock a file using a filesystem entry, instead of locking the file descriptor.
Canonical order does not imply network order; those are two orthogonal settings.
If $Storable::Deparse and/or $Storable::Eval are set to false values, then the value of $Storable::forgive_me (see below) is respected while serializing and deserializing.
This version of Storable will defer croaking until it encounters a data type in the file that it does not recognize. This means that it will continue to read files generated by newer Storable modules which are careful in what they write out, making it easier to upgrade Storable modules in a mixed environment.
The old behaviour of immediate croaking can be re-instated by setting $Storable::accept_future_minor to some "FALSE" value.
All these variables have no effect on a newer Perl which supports the relevant feature.
When Storable croaks, it tries to report the error via the "logcroak()" routine from the "Log::Agent" package, if it is available.
Normal errors are reported by having store() or retrieve() return "undef". Such errors are usually I/O errors (or truncated stream errors at retrieval).
Since we said earlier:
dclone(.) = thaw(freeze(.))
everything we say about hooks should also hold for deep cloning. However, hooks get to know whether the operation is a mere serialization, or a cloning.
Therefore, when serializing hooks are involved,
dclone(.) <> thaw(freeze(.))
Well, you could keep them in sync, but there's no guarantee it will always hold on classes somebody else wrote. Besides, there is little to gain in doing so: a serializing hook could keep only one attribute of an object, which is probably not what should happen during a deep cloning of that same object.
Here is the hooking interface:
Arguments: obj
Returned value: A LIST "($serialized, $ref1, $ref2, ...)" where $serialized is the serialized form to be used, and the optional $ref1, $ref2, etc... are extra references that you wish to let the Storable engine serialize.
At deserialization time, you will be given back the same LIST, but all the extra references will be pointing into the deserialized structure.
The first time the hook is hit in a serialization flow, you may have it return an empty list. That will signal the Storable engine to further discard that hook for this class and to therefore revert to the default serialization of the underlying Perl data. The hook will again be normally processed in the next serialization.
Unless you know better, serializing hook should always say:
sub STORABLE_freeze {
my ($self, $cloning) = @_;
return if $cloning; # Regular default serialization
....
}
in order to keep reasonable dclone() semantics.
Wrong: the Storable engine creates an empty one for you. If you know Eiffel, you can view
This means the hook can be inherited like any other method, and that obj is your blessed reference for this particular instance.
The other arguments should look familiar if you know "STORABLE_freeze": cloning is true when we're part of a deep clone operation, serialized is the serialized string you returned to the engine in "STORABLE_freeze", and there may be an optional list of references, in the same order you gave them at serialization time, pointing to the deserialized objects (which have been processed courtesy of the Storable engine).
When the Storable engine does not find any "STORABLE_thaw" hook routine, it tries to load the class by requiring the package dynamically (using the blessed package name), and then re-attempts the lookup. If at that time the hook cannot be located, the engine croaks. Note that this mechanism will fail if you define several classes in the same file, but perlmod warned you.
It is up to you to use this information to populate obj the way you want.
Returned value: none.
The alternative "STORABLE_attach" method provides a solution for these shared objects. Instead of "STORABLE_freeze" --E<GT> "STORABLE_thaw", you implement "STORABLE_freeze" --E<GT> "STORABLE_attach" instead.
Arguments: class is the class we are attaching to, cloning is a flag indicating whether we're in a dclone() or a regular de-serialization via thaw(), and serialized is the stored string for the resource object.
Because these resource objects are considered to be owned by the entire process/system, and not the ``property'' of whatever is being serialized, no references underneath the object should be included in the serialized string. Thus, in any class that implements "STORABLE_attach", the "STORABLE_freeze" method cannot return any references, and "Storable" will throw an error if "STORABLE_freeze" tries to return references.
All information required to ``attach'' back to the shared resource object must be contained only in the "STORABLE_freeze" return string. Otherwise, "STORABLE_freeze" behaves as normal for "STORABLE_attach" classes.
Because "STORABLE_attach" is passed the class (rather than an object), it also returns the object directly, rather than modifying the passed object.
Returned value: object of type "class"
There are a few things you need to know, however:
That's why "STORABLE_freeze" lets you provide a list of references to serialize. The engine guarantees that those will be serialized in the same context as the other objects, and therefore that shared objects will stay shared.
In the above [A, C] example, the "STORABLE_freeze" hook could return:
("something", $self->{B})
and the B part would be serialized by the engine. In "STORABLE_thaw", you would get back the reference to the B' object, deserialized for you.
Therefore, recursion should normally be avoided, but is nonetheless supported.
use Storable qw(store retrieve freeze thaw dclone);
%color = ('Blue' => 0.1, 'Red' => 0.8, 'Black' => 0, 'White' => 1);
store(\%color, 'mycolors') or die "Can't store %a in mycolors!\n";
$colref = retrieve('mycolors');
die "Unable to retrieve from mycolors!\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.
Serialization of CODE references and deserialization in a safe compartment:
use Storable qw(freeze thaw);
use Safe;
use strict;
my $safe = new Safe;
# because of opcodes used in "use strict":
$safe->permit(qw(:default require));
local $Storable::Deparse = 1;
local $Storable::Eval = sub { $safe->reval($_[0]) };
my $serialized = freeze(sub { 42 });
my $code = thaw($serialized);
$code->() == 42;
It won't work across a sequence of "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 temporary 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.
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.
When storing doubles in network order, their value is stored as text. However, you should also not expect non-numeric floating-point values such as infinity and ``not a number'' to pass successfully through a nstore()/retrieve() pair.
As Storable neither knows nor cares about character sets (although it does know that characters may be more than eight bits wide), any difference in the interpretation of character codes between a host and a target system is your problem. In particular, if host and target use different code points to represent the characters used in the text representation of floating-point numbers, you will not be able be able to exchange floating-point data, even with nstore().
"Storable::drop_utf8" is a blunt tool. There is no facility either to return all strings as utf8 sequences, or to attempt to convert utf8 data back to 8 bit and "croak()" if the conversion fails.
Prior to Storable 2.01, no distinction was made between signed and unsigned integers on storing. By default Storable prefers to store a scalars string representation (if it has one) so this would only cause problems when storing large unsigned integers that had never been coverted to string or floating point. In other words values that had been generated by integer operations such as logic ops and then not used in any string or arithmetic context before storing.
Storable writes a file header which contains the sizes of various C language types for the C compiler that built Storable (when not writing in network order), and will refuse to load files written by a Storable not on the same (or compatible) architecture. This check and a check on machine byteorder is needed because the size of various fields in the file are given by the sizes of the C language types, and so files written on different architectures are incompatible. This is done for increased speed. (When writing in network order, all fields are written out as standard lengths, which allows full interworking, but takes longer to read and write)
Perl 5.6.x introduced the ability to optional configure the perl interpreter to use C's "long long" type to allow scalars to store 64 bit integers on 32 bit systems. However, due to the way the Perl configuration system generated the C configuration files on non-Windows platforms, and the way Storable generates its header, nothing in the Storable file header reflected whether the perl writing was using 32 or 64 bit integers, despite the fact that Storable was storing some data differently in the file. Hence Storable running on perl with 64 bit integers will read the header from a file written by a 32 bit perl, not realise that the data is actually in a subtly incompatible format, and then go horribly wrong (possibly crashing) if it encountered a stored integer. This is a design failure.
Storable has now been changed to write out and read in a file header with information about the size of integers. It's impossible to detect whether an old file being read in was written with 32 or 64 bit integers (they have the same header) so it's impossible to automatically switch to a correct backwards compatibility mode. Hence this Storable defaults to the new, correct behaviour.
What this means is that if you have data written by Storable 1.x running on perl 5.6.0 or 5.6.1 configured with 64 bit integers on Unix or Linux then by default this Storable will refuse to read it, giving the error Byte order is not compatible. If you have such data then you you should set $Storable::interwork_56_64bit to a true value to make this Storable read and write files with the old header. You should also migrate your data, or any older perl you are communicating with, to this current version of Storable.
If you don't have data written with specific configuration of perl described above, then you do not and should not do anything. Don't set the flag - not only will Storable on an identically configured perl refuse to load them, but Storable a differently configured perl will load them believing them to be correct for it, and then may well fail or crash part way through reading them.
Jarkko Hietaniemi <jhi@iki.fi>
Ulrich Pfeifer <pfeifer@charly.informatik.uni-dortmund.de>
Benjamin A. Holzman <bah@ecnvantage.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>
Marc Lehmann <pcg@opengroup.org>
Justin Banks <justinb@wamnet.com>
Jarkko Hietaniemi <jhi@iki.fi> (AGAIN, as perl 5.7.0 Pumpkin!)
Salvador Ortiz Garcia <sog@msg.com.mx>
Dominic Dunlop <domo@computer.org>
Erik Haugan <erik@solbors.no>
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. Marc Lehmann added overloading and references to tied items support.
Please e-mail us with problems, bug fixes, comments and complaints, although if you have complements you should send them to Raphael. Please don't e-mail Raphael with problems, as he no longer works on Storable, and your message will be delayed while he forwards it to us.
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