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DBM::Deep(3)	      User Contributed Perl Documentation	  DBM::Deep(3)

NAME
       DBM::Deep - A pure perl multi-level hash/array DBM that supports
       transactions

VERSION
       2.0016

SYNOPSIS
	 use DBM::Deep;
	 my $db	= DBM::Deep->new( "foo.db" );

	 $db->{key} = 'value';
	 print $db->{key};

	 $db->put('key'	=> 'value');
	 print $db->get('key');

	 # true	multi-level support
	 $db->{my_complex} = [
	     'hello', {	perl =>	'rules'	},
	     42, 99,
	 ];

	 $db->begin_work;

	 # Do stuff here

	 $db->rollback;
	 $db->commit;

	 tie my	%db, 'DBM::Deep', 'foo.db';
	 $db{key} = 'value';
	 print $db{key};

	 tied(%db)->put('key' => 'value');
	 print tied(%db)->get('key');

DESCRIPTION
       A unique	flat-file database module, written in pure perl. True multi-
       level hash/array	support	(unlike	MLDBM, which is	faked),	hybrid OO /
       tie() interface,	cross-platform FTPable files, ACID transactions, and
       is quite	fast.  Can handle millions of keys and unlimited levels
       without significant slow-down. Written from the ground-up in pure perl
       -- this is NOT a	wrapper	around a C-based DBM. Out-of-the-box
       compatibility with Unix,	Mac OS X and Windows.

VERSION	DIFFERENCES
       NOTE: 2.0000 introduces Unicode support in the File back	end. This
       necessitates a change in	the file format. The version 1.0003 format is
       still supported,	though,	so we have added a db_version()	method.	If you
       are using a database in the old format, you will	have to	upgrade	it to
       get Unicode support.

       NOTE: 1.0020 introduces different engines which are backed by different
       types of	storage. There is the original storage (called 'File') and a
       database	storage	(called	'DBI').	q.v. "PLUGINS" for more	information.

       NOTE: 1.0000 has	significant file format	differences from prior
       versions.  There	is a backwards-compatibility layer at
       "utils/upgrade_db.pl". Files created by 1.0000 or higher	are NOT
       compatible with scripts using prior versions.

PLUGINS
       DBM::Deep is a wrapper around different storage engines.	These are:

   File
       This is the traditional storage engine, storing the data	to a custom
       file format. The	parameters accepted are:

       o   file

	   Filename of the DB file to link the handle to. You can pass a full
	   absolute filesystem path, partial path, or a	plain filename if the
	   file	is in the current working directory. This is a required
	   parameter (though q.v. fh).

       o   fh

	   If you want,	you can	pass in	the fh instead of the file. This is
	   most	useful for doing something like:

	     my	$db = DBM::Deep->new( {	fh => \*DATA } );

	   You are responsible for making sure that the	fh has been opened
	   appropriately for your needs. If you	open it	read-only and attempt
	   to write, an	exception will be thrown. If you open it write-only or
	   append-only,	an exception will be thrown immediately	as DBM::Deep
	   needs to read from the fh.

       o   file_offset

	   This	is the offset within the file that the DBM::Deep db starts.
	   Most	of the time, you will not need to set this. However, it's
	   there if you	want it.

	   If you pass in fh and do not	set this, it will be set
	   appropriately.

       o   locking

	   Specifies whether locking is	to be enabled. DBM::Deep uses Perl's
	   flock() function to lock the	database in exclusive mode for writes,
	   and shared mode for reads. Pass any true value to enable. This
	   affects the base DB handle and any child hashes or arrays that use
	   the same DB file. This is an	optional parameter, and	defaults to 1
	   (enabled). See "LOCKING" below for more.

       When you	open an	existing database file,	the version of the database
       format will stay	the same. But if you are creating a new	file, it will
       be in the latest	format.

   DBI
       This is a storage engine	that stores the	data in	a relational database.
       Funnily enough, this engine doesn't work	with transactions (yet)	as
       InnoDB doesn't do what DBM::Deep	needs it to do.

       The parameters accepted are:

       o   dbh

	   This	is a DBH that's	already	been opened with "connect" in DBI.

       o   dbi

	   This	is a hashref containing:

	   o   dsn

	   o   username

	   o   password

	   o   connect_args

	   These correspond to the 4 parameters	"connect" in DBI takes.

       NOTE: This has only been	tested with MySQL and SQLite (with
       disappointing results). I plan on extending this	to work	with
       PostgreSQL in the near future. Oracle, Sybase, and other	engines	will
       come later.

   Planned engines
       There are plans to extend this functionality to (at least) the
       following:

       o   BDB (and other hash engines like memcached)

       o   NoSQL engines (such as Tokyo	Cabinet)

       o   DBIx::Class (and other ORMs)

SETUP
       Construction can	be done	OO-style (which	is the recommended way), or
       using Perl's tie() function. Both are examined here.

   OO Construction
       The recommended way to construct	a DBM::Deep object is to use the new()
       method, which gets you a	blessed	and tied hash (or array) reference.

	 my $db	= DBM::Deep->new( "foo.db" );

       This opens a new	database handle, mapped	to the file "foo.db". If this
       file does not exist, it will automatically be created. DB files are
       opened in "r+" (read/write) mode, and the type of object	returned is a
       hash, unless otherwise specified	(see "Options" below).

       You can pass a number of	options	to the constructor to specify things
       like locking, autoflush,	etc. This is done by passing an	inline hash
       (or hashref):

	 my $db	= DBM::Deep->new(
	     file      => "foo.db",
	     locking   => 1,
	     autoflush => 1
	 );

       Notice that the filename	is now specified inside	the hash with the
       "file" parameter, as opposed to being the sole argument to the
       constructor. This is required if	any options are	specified.  See
       "Options" below for the complete	list.

       You can also start with an array	instead	of a hash. For this, you must
       specify the "type" parameter:

	 my $db	= DBM::Deep->new(
	     file => "foo.db",
	     type => DBM::Deep->TYPE_ARRAY
	 );

       Note: Specifying	the "type" parameter only takes	effect when beginning
       a new DB	file. If you create a DBM::Deep	object with an existing	file,
       the "type" will be loaded from the file header, and an error will be
       thrown if the wrong type	is passed in.

   Tie Construction
       Alternately, you	can create a DBM::Deep handle by using Perl's built-in
       tie() function. The object returned from	tie() can be used to call
       methods,	such as	lock() and unlock(). (That object can be retrieved
       from the	tied variable at any time using	tied() - please	see perltie
       for more	info.)

	 my %hash;
	 my $db	= tie %hash, "DBM::Deep", "foo.db";

	 my @array;
	 my $db	= tie @array, "DBM::Deep", "bar.db";

       As with the OO constructor, you can replace the DB filename parameter
       with a hash containing one or more options (see "Options" just below
       for the complete	list).

	 tie %hash, "DBM::Deep", {
	     file => "foo.db",
	     locking =>	1,
	     autoflush => 1
	 };

   Options
       There are a number of options that can be passed	in when	constructing
       your DBM::Deep objects. These apply to both the OO- and tie- based
       approaches.

       o   type

	   This	parameter specifies what type of object	to create, a hash or
	   array. Use one of these two constants:

	   o   "DBM::Deep->TYPE_HASH"

	   o   "DBM::Deep->TYPE_ARRAY"

	   This	only takes effect when beginning a new file. This is an
	   optional parameter, and defaults to "DBM::Deep->TYPE_HASH".

       o   autoflush

	   Specifies whether autoflush is to be	enabled	on the underlying
	   filehandle.	This obviously slows down write	operations, but	is
	   required if you may have multiple processes accessing the same DB
	   file	(also consider enable locking).	 Pass any true value to
	   enable. This	is an optional parameter, and defaults to 1 (enabled).

       o   filter_*

	   See "FILTERS" below.

       The following parameters	may be specified in the	constructor the	first
       time the	datafile is created. However, they will	be stored in the
       header of the file and cannot be	overridden by subsequent openings of
       the file	- the values will be set from the values stored	in the
       datafile's header.

       o   num_txns

	   This	is the number of transactions that can be running at one time.
	   The default is one -	the HEAD. The minimum is one and the maximum
	   is 255. The more transactions, the larger and quicker the datafile
	   grows.

	   Simple access to a database,	regardless of how many processes are
	   doing it, already counts as one transaction (the HEAD). So, if you
	   want, say, 5	processes to be	able to	call begin_work	at the same
	   time, "num_txns" must be at least 6.

	   See "TRANSACTIONS" below.

       o   max_buckets

	   This	is the number of entries that can be added before a
	   reindexing. The larger this number is made, the larger a file gets,
	   but the better performance you will have. The default and minimum
	   number this can be is 16. The maximum is 256, but more than 64
	   isn't recommended.

       o   data_sector_size

	   This	is the size in bytes of	a given	data sector. Data sectors will
	   chain, so a value of	any size can be	stored.	However, chaining is
	   expensive in	terms of time. Setting this value to something close
	   to the expected common length of your scalars will improve your
	   performance.	If it is too small, your file will have	a lot of
	   chaining. If	it is too large, your file will	have a lot of dead
	   space in it.

	   The default for this	is 64 bytes. The minimum value is 32 and the
	   maximum is 256 bytes.

	   Note: There are between 6 and 10 bytes taken	up in each data	sector
	   for bookkeeping. (It's 4 + the number of bytes in your
	   "pack_size".) This is included within the data_sector_size, thus
	   the effective value is 6-10 bytes less than what you	specified.

	   Another note: If your strings contain any characters	beyond the
	   byte	range, they will be encoded as UTF-8 before being stored in
	   the file. This will make all	non-ASCII characters take up more than
	   one byte each.

       o   pack_size

	   This	is the size of the file	pointer	used throughout	the file. The
	   valid values	are:

	   o   small

	       This uses 2-byte	offsets, allowing for a	maximum	file size of
	       65 KB.

	   o   medium (default)

	       This uses 4-byte	offsets, allowing for a	maximum	file size of 4
	       GB.

	   o   large

	       This uses 8-byte	offsets, allowing for a	maximum	file size of
	       16 XB (exabytes). This can only be enabled if your Perl is
	       compiled	for 64-bit.

	   See "LARGEFILE SUPPORT" for more information.

       o   external_refs

	   This	is a boolean option. When enabled, it allows external
	   references to database entries to hold on to	those entries, even
	   when	they are deleted.

	   To illustrate, if you retrieve a hash (or array) reference from the
	   database,

	     $foo_hash = $db->{foo};

	   the hash reference is still tied to the database. So	if you

	     delete $db->{foo};

	   $foo_hash will point	to a location in the DB	that is	no longer
	   valid (we call this a stale reference). So if you try to retrieve
	   the data from $foo_hash,

	     for(keys %$foo_hash) {

	   you will get	an error.

	   The "external_refs" option causes $foo_hash to 'hang	on' to the DB
	   entry, so it	will not be deleted from the database if there is
	   still a reference to	it in a	running	program. It will be deleted,
	   instead, when the $foo_hash variable	no longer exists, or is
	   overwritten.

	   This	has the	potential to cause database bloat if your program
	   crashes, so it is not enabled by default. (See also the "export"
	   method for an alternative workaround.)

TIE INTERFACE
       With DBM::Deep you can access your databases using Perl's standard
       hash/array syntax. Because all DBM::Deep	objects	are tied to hashes or
       arrays, you can treat them as such (but see "external_refs", above, and
       "Stale References", below). DBM::Deep will intercept all	reads/writes
       and direct them to the right place -- the DB file. This has nothing to
       do with the "Tie	Construction" section above. This simply tells you how
       to use DBM::Deep	using regular hashes and arrays, rather	than calling
       functions like "get()" and "put()" (although those work too). It	is
       entirely	up to you how to want to access	your databases.

   Hashes
       You can treat any DBM::Deep object like a normal	Perl hash reference.
       Add keys, or even nested	hashes (or arrays) using standard Perl syntax:

	 my $db	= DBM::Deep->new( "foo.db" );

	 $db->{mykey} =	"myvalue";
	 $db->{myhash} = {};
	 $db->{myhash}->{subkey} = "subvalue";

	 print $db->{myhash}->{subkey} . "\n";

       You can even step through hash keys using the normal Perl "keys()"
       function:

	 foreach my $key (keys %$db) {
	     print "$key: " . $db->{$key} . "\n";
	 }

       Remember	that Perl's "keys()" function extracts every key from the hash
       and pushes them onto an array, all before the loop even begins. If you
       have an extremely large hash, this may exhaust Perl's memory. Instead,
       consider	using Perl's "each()" function,	which pulls keys/values	one at
       a time, using very little memory:

	 while (my ($key, $value) = each %$db) {
	     print "$key: $value\n";
	 }

       Please note that	when using "each()", you should	always pass a direct
       hash reference, not a lookup. Meaning, you should never do this:

	 # NEVER DO THIS
	 while (my ($key, $value) = each %{$db->{foo}})	{ # BAD

       This causes an infinite loop, because for each iteration, Perl is
       calling FETCH() on the $db handle, resulting in a "new" hash for	foo
       every time, so it effectively keeps returning the first key over	and
       over again. Instead, assign a temporary variable	to "$db->{foo}", then
       pass that to each().

   Arrays
       As with hashes, you can treat any DBM::Deep object like a normal	Perl
       array reference.	This includes inserting, removing and manipulating
       elements, and the "push()", "pop()", "shift()", "unshift()" and
       "splice()" functions.  The object must have first been created using
       type "DBM::Deep->TYPE_ARRAY", or	simply be a nested array reference
       inside a	hash. Example:

	 my $db	= DBM::Deep->new(
	     file => "foo-array.db",
	     type => DBM::Deep->TYPE_ARRAY
	 );

	 $db->[0] = "foo";
	 push @$db, "bar", "baz";
	 unshift @$db, "bah";

	 my $last_elem	 = pop @$db;   # baz
	 my $first_elem	 = shift @$db; # bah
	 my $second_elem = $db->[1];   # bar

	 my $num_elements = scalar @$db;

OO INTERFACE
       In addition to the tie()	interface, you can also	use a standard OO
       interface to manipulate all aspects of DBM::Deep	databases. Each	type
       of object (hash or array) has its own methods, but both types share the
       following common	methods: "put()", "get()", "exists()", "delete()" and
       "clear()". "fetch()" and	"store()" are aliases to "put()" and "get()",
       respectively.

       o   new() / clone()

	   These are the constructor and copy-functions.

       o   put() / store()

	   Stores a new	hash key/value pair, or	sets an	array element value.
	   Takes two arguments,	the hash key or	array index, and the new
	   value. The value can	be a scalar, hash ref or array ref. Returns
	   true	on success, false on failure.

	     $db->put("foo", "bar"); # for hashes
	     $db->put(1, "bar"); # for arrays

       o   get() / fetch()

	   Fetches the value of	a hash key or array element. Takes one
	   argument: the hash key or array index. Returns a scalar, hash ref
	   or array ref, depending on the data type stored.

	     my	$value = $db->get("foo"); # for	hashes
	     my	$value = $db->get(1); #	for arrays

       o   exists()

	   Checks if a hash key	or array index exists. Takes one argument: the
	   hash	key or array index. Returns true if it exists, false if	not.

	     if	($db->exists("foo")) { print "yay!\n"; } # for hashes
	     if	($db->exists(1)) { print "yay!\n"; } # for arrays

       o   delete()

	   Deletes one hash key/value pair or array element. Takes one
	   argument: the hash key or array index. Returns the data that	the
	   element used	to contain (just like Perl's "delete" function), which
	   is "undef" if it did	not exist. For arrays, the remaining elements
	   located after the deleted element are NOT moved over. The deleted
	   element is essentially just undefined, which	is exactly how Perl's
	   internal arrays work.

	     $db->delete("foo"); # for hashes
	     $db->delete(1); # for arrays

       o   clear()

	   Deletes all hash keys or array elements. Takes no arguments.	No
	   return value.

	     $db->clear(); # hashes or arrays

       o   lock() / unlock() / lock_exclusive()	/ lock_shared()

	   q.v.	"LOCKING" for more info.

       o   optimize()

	   This	will compress the datafile so that it takes up as little space
	   as possible.	 There is a freespace manager so that when space is
	   freed up, it	is used	before extending the size of the datafile.
	   But,	that freespace just sits in the	datafile unless	"optimize()"
	   is called.

	   "optimize" basically	copies everything into a new database, so, if
	   it is in version 1.0003 format, it will be upgraded.

       o   import()

	   Unlike simple assignment, "import()"	does not tie the right-hand
	   side. Instead, a copy of your data is put into the DB. "import()"
	   takes either	an arrayref (if	your DB	is an array) or	a hashref (if
	   your	DB is a	hash). "import()" will die if anything else is passed
	   in.

       o   export()

	   This	returns	a complete copy	of the data structure at the point you
	   do the export.  This	copy is	in RAM,	not on disk like the DB	is.

       o   begin_work()	/ commit() / rollback()

	   These are the transactional functions. "TRANSACTIONS" for more
	   information.

       o   supports( $option )

	   This	returns	a boolean indicating whether this instance of
	   DBM::Deep supports that feature. $option can	be one of:

	   o   transactions

	   o   unicode

       o   db_version()

	   This	returns	the version of the database format that	the current
	   database is in. This	is specified as	the earliest version of
	   DBM::Deep that supports it.

	   For the File	back end, this will be 1.0003 or 2.

	   For the DBI back end, it is currently always	1.0020.

   Hashes
       For hashes, DBM::Deep supports all the common methods described above,
       and the following additional methods: "first_key()" and "next_key()".

       o   first_key()

	   Returns the "first" key in the hash.	As with	built-in Perl hashes,
	   keys	are fetched in an undefined order (which appears random).
	   Takes no arguments, returns the key as a scalar value.

	     my	$key = $db->first_key();

       o   next_key()

	   Returns the "next" key in the hash, given the previous one as the
	   sole	argument.  Returns undef if there are no more keys to be
	   fetched.

	     $key = $db->next_key($key);

       Here are	some examples of using hashes:

	 my $db	= DBM::Deep->new( "foo.db" );

	 $db->put("foo", "bar");
	 print "foo: " . $db->get("foo") . "\n";

	 $db->put("baz", {}); #	new child hash ref
	 $db->get("baz")->put("buz", "biz");
	 print "buz: " . $db->get("baz")->get("buz") . "\n";

	 my $key = $db->first_key();
	 while ($key) {
	     print "$key: " . $db->get($key) . "\n";
	     $key = $db->next_key($key);
	 }

	 if ($db->exists("foo")) { $db->delete("foo"); }

   Arrays
       For arrays, DBM::Deep supports all the common methods described above,
       and the following additional methods: "length()", "push()", "pop()",
       "shift()", "unshift()" and "splice()".

       o   length()

	   Returns the number of elements in the array.	Takes no arguments.

	     my	$len = $db->length();

       o   push()

	   Adds	one or more elements onto the end of the array.	Accepts
	   scalars, hash refs or array refs. No	return value.

	     $db->push("foo", "bar", {});

       o   pop()

	   Fetches the last element in the array, and deletes it. Takes	no
	   arguments.  Returns undef if	array is empty.	Returns	the element
	   value.

	     my	$elem =	$db->pop();

       o   shift()

	   Fetches the first element in	the array, deletes it, then shifts all
	   the remaining elements over to take up the space. Returns the
	   element value. This method is not recommended with large arrays --
	   see "Large Arrays" below for	details.

	     my	$elem =	$db->shift();

       o   unshift()

	   Inserts one or more elements	onto the beginning of the array,
	   shifting all	existing elements over to make room. Accepts scalars,
	   hash	refs or	array refs.  No	return value. This method is not
	   recommended with large arrays -- see	<Large Arrays> below for
	   details.

	     $db->unshift("foo", "bar",	{});

       o   splice()

	   Performs exactly like Perl's	built-in function of the same name.
	   See "splice"	in perlfunc for	usage -- it is too complicated to
	   document here. This method is not recommended with large arrays --
	   see "Large Arrays" below for	details.

       Here are	some examples of using arrays:

	 my $db	= DBM::Deep->new(
	     file => "foo.db",
	     type => DBM::Deep->TYPE_ARRAY
	 );

	 $db->push("bar", "baz");
	 $db->unshift("foo");
	 $db->put(3, "buz");

	 my $len = $db->length();
	 print "length:	$len\n"; # 4

	 for (my $k=0; $k<$len;	$k++) {
	     print "$k:	" . $db->get($k) . "\n";
	 }

	 $db->splice(1,	2, "biz", "baf");

	 while (my $elem = shift @$db) {
	     print "shifted: $elem\n";
	 }

LOCKING
       Enable or disable automatic file	locking	by passing a boolean value to
       the "locking" parameter when constructing your DBM::Deep	object (see
       "SETUP" above).

	 my $db	= DBM::Deep->new(
	     file => "foo.db",
	     locking =>	1
	 );

       This causes DBM::Deep to	"flock()" the underlying filehandle with
       exclusive mode for writes, and shared mode for reads. This is required
       if you have multiple processes accessing	the same database file,	to
       avoid file corruption.  Please note that	"flock()" does NOT work	for
       files over NFS. See "DB over NFS" below for more.

   Explicit Locking
       You can explicitly lock a database, so it remains locked	for multiple
       actions.	This is	done by	calling	the "lock_exclusive()" method (for
       when you	want to	write) or the "lock_shared()" method (for when you
       want to read).  This is particularly useful for things like counters,
       where the current value needs to	be fetched, then incremented, then
       stored again.

	 $db->lock_exclusive();
	 my $counter = $db->get("counter");
	 $counter++;
	 $db->put("counter", $counter);
	 $db->unlock();

	 # or...

	 $db->lock_exclusive();
	 $db->{counter}++;
	 $db->unlock();

   Win32/Cygwin
       Due to Win32 actually enforcing the read-only status of a shared	lock,
       all locks on Win32 and cygwin are exclusive. This is because of how
       autovivification	currently works. Hopefully, this will go away in a
       future release.

IMPORTING/EXPORTING
       You can import existing complex structures by calling the "import()"
       method, and export an entire database into an in-memory structure using
       the "export()" method. Both are examined	here.

   Importing
       Say you have an existing	hash with nested hashes/arrays inside it.
       Instead of walking the structure	and adding keys/elements to the
       database	as you go, simply pass a reference to the "import()" method.
       This recursively	adds everything	to an existing DBM::Deep object	for
       you. Here is an example:

	 my $struct = {
	     key1 => "value1",
	     key2 => "value2",
	     array1 => [ "elem0", "elem1", "elem2" ],
	     hash1 => {
		 subkey1 => "subvalue1",
		 subkey2 => "subvalue2"
	     }
	 };

	 my $db	= DBM::Deep->new( "foo.db" );
	 $db->import( $struct );

	 print $db->{key1} . "\n"; # prints "value1"

       This recursively	imports	the entire $struct object into $db, including
       all nested hashes and arrays. If	the DBM::Deep object contains existing
       data, keys are merged with the existing ones, replacing if they already
       exist.  The "import()" method can be called on any database level (not
       just the	base level), and works with both hash and array	DB types.

       Note: Make sure your existing structure has no circular references in
       it.  These will cause an	infinite loop when importing. There are	plans
       to fix this in a	later release.

   Exporting
       Calling the "export()" method on	an existing DBM::Deep object will
       return a	reference to a new in-memory copy of the database. The export
       is done recursively, so all nested hashes/arrays	are all	exported to
       standard	Perl objects. Here is an example:

	 my $db	= DBM::Deep->new( "foo.db" );

	 $db->{key1} = "value1";
	 $db->{key2} = "value2";
	 $db->{hash1} =	{};
	 $db->{hash1}->{subkey1} = "subvalue1";
	 $db->{hash1}->{subkey2} = "subvalue2";

	 my $struct = $db->export();

	 print $struct->{key1} . "\n"; # prints	"value1"

       This makes a complete copy of the database in memory, and returns a
       reference to it.	The "export()" method can be called on any database
       level (not just the base	level),	and works with both hash and array DB
       types. Be careful of large databases -- you can store a lot more	data
       in a DBM::Deep object than an in-memory Perl structure.

       Note: Make sure your database has no circular references	in it.	These
       will cause an infinite loop when	exporting. There are plans to fix this
       in a later release.

FILTERS
       DBM::Deep has a number of hooks where you can specify your own Perl
       function	to perform filtering on	incoming or outgoing data. This	is a
       perfect way to extend the engine, and implement things like real-time
       compression or encryption. Filtering applies to the base	DB level, and
       all child hashes	/ arrays. Filter hooks can be specified	when your
       DBM::Deep object	is first constructed, or by calling the	"set_filter()"
       method at any time. There are four available filter hooks.

   set_filter()
       This method takes two parameters	- the filter type and the filter
       subreference.  The four types are:

       o   filter_store_key

	   This	filter is called whenever a hash key is	stored.	It is passed
	   the incoming	key, and expected to return a transformed key.

       o   filter_store_value

	   This	filter is called whenever a hash key or	array element is
	   stored. It is passed	the incoming value, and	expected to return a
	   transformed value.

       o   filter_fetch_key

	   This	filter is called whenever a hash key is	fetched	(i.e. via
	   "first_key()" or "next_key()"). It is passed	the transformed	key,
	   and expected	to return the plain key.

       o   filter_fetch_value

	   This	filter is called whenever a hash key or	array element is
	   fetched.  It	is passed the transformed value, and expected to
	   return the plain value.

       Here are	the two	ways to	setup a	filter hook:

	 my $db	= DBM::Deep->new(
	     file => "foo.db",
	     filter_store_value	=> \&my_filter_store,
	     filter_fetch_value	=> \&my_filter_fetch
	 );

	 # or...

	 $db->set_filter( "store_value", \&my_filter_store );
	 $db->set_filter( "fetch_value", \&my_filter_fetch );

       Your filter function will be called only	when dealing with SCALAR keys
       or values. When nested hashes and arrays	are being stored/fetched,
       filtering is bypassed. Filters are called as static functions, passed a
       single SCALAR argument, and expected to return a	single SCALAR value.
       If you want to remove a filter, set the function	reference to "undef":

	 $db->set_filter( "store_value", undef );

   Examples
       Please read DBM::Deep::Cookbook for examples of filters.

ERROR HANDLING
       Most DBM::Deep methods return a true value for success, and call	die()
       on failure. You can wrap	calls in an eval block to catch	the die.

	 my $db	= DBM::Deep->new( "foo.db" ); #	create hash
	 eval {	$db->push("foo"); }; # ILLEGAL -- push is array-only call

	 print $@;	     # prints error message

LARGEFILE SUPPORT
       If you have a 64-bit system, and	your Perl is compiled with both
       LARGEFILE and 64-bit support, you may be	able to	create databases
       larger than 4 GB.  DBM::Deep by default uses 32-bit file	offset tags,
       but these can be	changed	by specifying the 'pack_size' parameter	when
       constructing the	file.

	 DBM::Deep->new(
	     file      => $filename,
	     pack_size => 'large',
	 );

       This tells DBM::Deep to pack all	file offsets with 8-byte (64-bit) quad
       words instead of	32-bit longs. After setting these values your DB files
       have a theoretical maximum size of 16 XB	(exabytes).

       You can also use	"pack_size => 'small'" in order	to use 16-bit file
       offsets.

       Note: Changing these values will	NOT work for existing database files.
       Only change this	for new	files. Once the	value has been set, it is
       stored in the file's header and cannot be changed for the life of the
       file. These parameters are per-file, meaning you	can access 32-bit and
       64-bit files, as	you choose.

       Note: We	have not personally tested files larger	than 4 GB -- all our
       systems have only a 32-bit Perl.	However, we have received user reports
       that this does indeed work.

LOW-LEVEL ACCESS
       If you require low-level	access to the underlying filehandle that
       DBM::Deep uses, you can call the	"_fh()"	method,	which returns the
       handle:

	 my $fh	= $db->_fh();

       This method can be called on the	root level of the database, or any
       child hashes or arrays. All levels share	a root structure, which
       contains	things like the	filehandle, a reference	counter, and all the
       options specified when you created the object. You can get access to
       this file object	by calling the "_storage()" method.

	 my $file_obj =	$db->_storage();

       This is useful for changing options after the object has	already	been
       created,	such as	enabling/disabling locking. You	can also store your
       own temporary user data in this structure (be wary of name collision),
       which is	then accessible	from any child hash or array.

CIRCULAR REFERENCES
       DBM::Deep has full support for circular references. Meaning you can
       have a nested hash key or array element that points to a	parent object.
       This relationship is stored in the DB file, and is preserved between
       sessions.  Here is an example:

	 my $db	= DBM::Deep->new( "foo.db" );

	 $db->{foo} = "bar";
	 $db->{circle} = $db; #	ref to self

	 print $db->{foo} . "\n"; # prints "bar"
	 print $db->{circle}->{foo} . "\n"; # prints "bar" again

       This also works as expected with	array and hash references. So, the
       following works as expected:

	 $db->{foo} = [	1 .. 3 ];
	 $db->{bar} = $db->{foo};

	 push @{$db->{foo}}, 42;
	 is( $db->{bar}[-1], 42	); # Passes

       This, however, does not extend to assignments from one DB file to
       another.	 So, the following will	throw an error:

	 my $db1 = DBM::Deep->new( "foo.db" );
	 my $db2 = DBM::Deep->new( "bar.db" );

	 $db1->{foo} = [];
	 $db2->{foo} = $db1->{foo}; # dies

       Note: Passing the object	to a function that recursively walks the
       object tree (such as Data::Dumper or even the built-in "optimize()" or
       "export()" methods) will	result in an infinite loop. This will be fixed
       in a future release by adding singleton support.

TRANSACTIONS
       As of 1.0000, DBM::Deep has ACID	transactions. Every DBM::Deep object
       is completely transaction-ready - it is not an option you have to turn
       on. You do have to specify how many transactions	may run	simultaneously
       (q.v. "num_txns").

       Three new methods have been added to support them. They are:

       o   begin_work()

	   This	starts a transaction.

       o   commit()

	   This	applies	the changes done within	the transaction	to the
	   mainline and	ends the transaction.

       o   rollback()

	   This	discards the changes done within the transaction to the
	   mainline and	ends the transaction.

       Transactions in DBM::Deep are done using	a variant of the MVCC method,
       the same	method used by the InnoDB MySQL	engine.

MIGRATION
       As of 1.0000, the file format has changed. To aid in upgrades, a
       migration script	is provided within the CPAN distribution, called
       utils/upgrade_db.pl.

       NOTE: This script is not	installed onto your system because it carries
       a copy of every version prior to	the current version.

       As of version 2.0000, databases created by old versions back to 1.0003
       can be read, but	new features may not be	available unless the database
       is upgraded first.

TODO
       The following are items that are	planned	to be added in future
       releases. These are separate from the "CAVEATS, ISSUES &	BUGS" below.

   Sub-Transactions
       Right now, you cannot run a transaction within a	transaction. Removing
       this restriction	is technically straightforward,	but the	combinatorial
       explosion of possible usecases hurts my head. If	this is	something you
       want to see immediately,	please submit many testcases.

   Caching
       If a client is willing to assert	upon opening the file that this
       process will be the only	consumer of that datafile, then	there are a
       number of caching possibilities that can	be taken advantage of. This
       does, however, mean that	DBM::Deep is more vulnerable to	losing data
       due to unflushed	changes. It also means a much larger in-memory
       footprint. As such, it's	not clear exactly how this should be done.
       Suggestions are welcome.

   Ram-only
       The techniques used in DBM::Deep	simply require a seekable contiguous
       datastore. This could just as easily be a large string as a file. By
       using substr, the STM capabilities of DBM::Deep could be	used within a
       single-process. I have no idea how I'd specify this, though.
       Suggestions are welcome.

   Different contention	resolution mechanisms
       Currently, the only contention resolution mechanism is last-write-wins.
       This is the mechanism used by most RDBMSes and should be	good enough
       for most	uses.  For advanced uses of STM, other contention mechanisms
       will be needed. If you have an idea of how you'd	like to	see contention
       resolution in DBM::Deep,	please let me know.

CAVEATS, ISSUES	& BUGS
       This section describes all the known issues with	DBM::Deep. These are
       issues that are either intractable or depend on some feature within
       Perl working exactly right. It you have found something that is not
       listed below, please send an e-mail to bug-DBM-Deep@rt.cpan.org
       <mailto:bug-DBM-Deep@rt.cpan.org>.  Likewise, if	you think you know of
       a way around one	of these issues, please	let me know.

   References
       (The following assumes a	high level of Perl understanding, specifically
       of references. Most users can safely skip this section.)

       Currently, the only references supported	are HASH and ARRAY. The	other
       reference types (SCALAR,	CODE, GLOB, and	REF) cannot be supported for
       various reasons.

       o   GLOB

	   These are things like filehandles and other sockets.	They can't be
	   supported because it's completely unclear how DBM::Deep should
	   serialize them.

       o   SCALAR / REF

	   The discussion here refers to the following type of example:

	     my	$x = 25;
	     $db->{key1} = \$x;

	     $x	= 50;

	     # In some other process ...

	     my	$val = ${ $db->{key1} };

	     is( $val, 50, "What actually gets stored in the DB	file?" );

	   The problem is one of synchronization. When the variable being
	   referred to changes value, the reference isn't notified, which is
	   kind	of the point of	references. This means that the	new value
	   won't be stored in the datafile for other processes to read.	There
	   is no TIEREF.

	   It is theoretically possible	to store references to values already
	   within a DBM::Deep object because everything	already	is
	   synchronized, but the change	to the internals would be quite	large.
	   Specifically, DBM::Deep would have to tie every single value	that
	   is stored. This would bloat the RAM footprint of DBM::Deep at least
	   twofold (if not more) and be	a significant performance drain, all
	   to support a	feature	that has never been requested.

       o   CODE

	   Data::Dump::Streamer	provides a mechanism for serializing coderefs,
	   including saving off	all closure state. This	would allow for
	   DBM::Deep to	store the code for a subroutine. Then, whenever	the
	   subroutine is read, the code	could be "eval()"'ed into being.
	   However, just as for	SCALAR and REF,	that closure state may change
	   without notifying the DBM::Deep object storing the reference.
	   Again, this would generally be considered a feature.

   External references and transactions
       If you do "my $x	= $db->{foo};",	then start a transaction, $x will be
       referencing the database	from outside the transaction. A	fix for	this
       (and other issues with how external references into the database) is
       being looked into. This is the skipped set of tests in
       t/39_singletons.t and a related issue is	the focus of
       t/37_delete_edge_cases.t

   File	corruption
       The current level of error handling in DBM::Deep	is minimal. Files are
       checked for a 32-bit signature when opened, but any other form of
       corruption in the datafile can cause segmentation faults. DBM::Deep may
       try to "seek()" past the	end of a file, or get stuck in an infinite
       loop depending on the level and type of corruption. File	write
       operations are not checked for failure (for speed), so if you happen to
       run out of disk space, DBM::Deep	will probably fail in a	bad way. These
       things will be addressed	in a later version of DBM::Deep.

   DB over NFS
       Beware of using DBM::Deep files over NFS. DBM::Deep uses	flock(), which
       works well on local filesystems,	but will NOT protect you from file
       corruption over NFS. I've heard about setting up	your NFS server	with a
       locking daemon, then using "lockf()" to lock your files,	but your
       mileage may vary	there as well.	From what I understand,	there is no
       real way	to do it. However, if you need access to the underlying
       filehandle in DBM::Deep for using some other kind of locking scheme
       like "lockf()", see the "LOW-LEVEL ACCESS" section above.

   Copying Objects
       Beware of copying tied objects in Perl. Very strange things can happen.
       Instead,	use DBM::Deep's	"clone()" method which safely copies the
       object and returns a new, blessed and tied hash or array	to the same
       level in	the DB.

	 my $copy = $db->clone();

       Note: Since clone() here	is cloning the object, not the database
       location, any modifications to either $db or $copy will be visible to
       both.

   Stale References
       If you take a reference to an array or hash from	the database, it is
       tied to the database itself. This means that if the datum in question
       is subsequently deleted from the	database, the reference	to it will
       point to	an invalid location and	unpredictable things will happen if
       you try to use it.

       So a seemingly innocuous	piece of code like this:

	 my %hash = %{ $db->{some_hash}	};

       can fail	if another process deletes or clobbers "$db->{some_hash}"
       while the data are being	extracted, since "%{A ...A }" is not atomic.
       (This actually happened.) The solution is to lock the database before
       reading the data:

	 $db->lock_exclusive;
	 my %hash = %{ $db->{some_hash}	};
	 $db->unlock;

       As of version 1.0024, if	you assign a stale reference to	a location in
       the database, DBM::Deep will warn, if you have uninitialized warnings
       enabled,	and treat the stale reference as "undef". An attempt to	use a
       stale reference as an array or hash reference will cause	an error.

   Large Arrays
       Beware of using "shift()", "unshift()" or "splice()" with large arrays.
       These functions cause every element in the array	to move, which can be
       murder on DBM::Deep, as every element has to be fetched from disk, then
       stored again in a different location. This will be addressed in a
       future version.

       This has	been somewhat addressed	so that	the cost is constant,
       regardless of what is stored at those locations.	So, small arrays with
       huge data structures in them are	faster.	But, large arrays are still
       large.

   Writeonly Files
       If you pass in a	filehandle to new(), you may have opened it in either
       a readonly or writeonly mode. STORE will	verify that the	filehandle is
       writable.  However, there doesn't seem to be a good way to determine if
       a filehandle is readable. And, if the filehandle	isn't readable,	it's
       not clear what will happen. So, don't do	that.

   Assignments Within Transactions
       The following will not work as one might	expect:

	 my $x = { a =>	1 };

	 $db->begin_work;
	 $db->{foo} = $x;
	 $db->rollback;

	 is( $x->{a}, 1	); # This will fail!

       The problem is that the moment a	reference used as the rvalue to	a
       DBM::Deep object's lvalue, it becomes tied itself. This is so that
       future changes to $x can	be tracked within the DBM::Deep	file and is
       considered to be	a feature. By the time the rollback occurs, there is
       no knowledge that there had been	an $x or what memory location to
       assign an "export()" to.

       NOTE: This does not affect importing because imports do a walk over the
       reference to be imported	in order to explicitly leave it	untied.

CODE COVERAGE
       Devel::Cover is used to test the	code coverage of the tests. Below is
       the Devel::Cover	report on this distribution's test suite.

	 ---------------------------- ------ ------ ------ ------ ------ ------	------
	 File				stmt   bran   cond    sub    pod   time	 total
	 ---------------------------- ------ ------ ------ ------ ------ ------	------
	 blib/lib/DBM/Deep.pm	       100.0   89.1   82.9  100.0  100.0   32.5	  98.1
	 blib/lib/DBM/Deep/Array.pm    100.0   94.4  100.0  100.0  100.0    5.2	  98.8
	 blib/lib/DBM/Deep/Engine.pm   100.0   92.9  100.0  100.0  100.0    7.4	 100.0
	 ...ib/DBM/Deep/Engine/DBI.pm	95.0   73.1  100.0  100.0  100.0    1.5	  90.4
	 ...b/DBM/Deep/Engine/File.pm	92.3   78.5   88.9  100.0  100.0    4.9	  90.3
	 blib/lib/DBM/Deep/Hash.pm     100.0  100.0  100.0  100.0  100.0    3.8	 100.0
	 .../lib/DBM/Deep/Iterator.pm  100.0	n/a    n/a  100.0  100.0    0.0	 100.0
	 .../DBM/Deep/Iterator/DBI.pm  100.0  100.0    n/a  100.0  100.0    1.2	 100.0
	 ...DBM/Deep/Iterator/File.pm	92.5   84.6    n/a  100.0   66.7    0.6	  90.0
	 ...erator/File/BucketList.pm  100.0   75.0    n/a  100.0   66.7    0.4	  93.8
	 ...ep/Iterator/File/Index.pm  100.0  100.0    n/a  100.0  100.0    0.2	 100.0
	 blib/lib/DBM/Deep/Null.pm	87.5	n/a    n/a   75.0    n/a    0.0	  83.3
	 blib/lib/DBM/Deep/Sector.pm	91.7	n/a    n/a   83.3    0.0    6.7	  74.4
	 ...ib/DBM/Deep/Sector/DBI.pm	96.8   83.3    n/a  100.0    0.0    1.0	  89.8
	 ...p/Sector/DBI/Reference.pm  100.0   95.5  100.0  100.0    0.0    2.2	  91.2
	 ...Deep/Sector/DBI/Scalar.pm  100.0  100.0    n/a  100.0    0.0    1.1	  92.9
	 ...b/DBM/Deep/Sector/File.pm	96.0   87.5  100.0   92.3   25.0    2.2	  91.0
	 ...Sector/File/BucketList.pm	98.2   85.7   83.3  100.0    0.0    3.3	  89.4
	 .../Deep/Sector/File/Data.pm  100.0	n/a    n/a  100.0    0.0    0.1	  90.9
	 ...Deep/Sector/File/Index.pm  100.0   80.0   33.3  100.0    0.0    0.8	  83.1
	 .../Deep/Sector/File/Null.pm  100.0  100.0    n/a  100.0    0.0    0.0	  91.7
	 .../Sector/File/Reference.pm  100.0   90.0   80.0  100.0    0.0    1.4	  91.5
	 ...eep/Sector/File/Scalar.pm	98.4   87.5    n/a  100.0    0.0    0.8	  91.9
	 blib/lib/DBM/Deep/Storage.pm  100.0	n/a    n/a  100.0  100.0    0.0	 100.0
	 ...b/DBM/Deep/Storage/DBI.pm	97.3   70.8    n/a  100.0   38.5    6.7	  87.0
	 .../DBM/Deep/Storage/File.pm	96.6   77.1   80.0   95.7  100.0   16.0	  91.8
	 Total				99.3   85.2   84.9   99.8   63.3  100.0	  97.6
	 ---------------------------- ------ ------ ------ ------ ------ ------	------

MORE INFORMATION
       Check out the DBM::Deep Google Group at
       <http://groups.google.com/group/DBM-Deep> or send email to
       DBM-Deep@googlegroups.com <mailto:DBM-Deep@googlegroups.com>.  You can
       also visit #dbm-deep on irc.perl.org

       The source code repository is at	<http://github.com/robkinyon/dbm-deep>

MAINTAINERS
       Rob Kinyon, rkinyon@cpan.org <mailto:rkinyon@cpan.org>

       Originally written by Joseph Huckaby, jhuckaby@cpan.org
       <mailto:jhuckaby@cpan.org>

SPONSORS
       Stonehenge Consulting (<http://www.stonehenge.com/>) sponsored the
       development of transactions and freespace management, leading to	the
       1.0000 release. A great debt of gratitude goes out to them for their
       continuing leadership in	and support of the Perl	community.

CONTRIBUTORS
       The following have contributed greatly to make DBM::Deep	what it	is
       today:

       o   Adam	Sah and	Rich Gaushell for innumerable contributions early on.

       o   Dan Golden and others at YAPC::NA 2006 for helping me design
	   through transactions.

       o   James Stanley for bug fix

       o   David Steinbrunner for fixing typos and adding repository cpan
	   metadata

       o   H. Merijn Brandt for	fixing the POD escapes.

       o   Breno G. de Oliveira	for minor packaging tweaks

SEE ALSO
       DBM::Deep::Cookbook(3)

       perltie(1), Tie::Hash(3), Fcntl(3), flock(2), lockf(3), nfs(5)

LICENSE
       Copyright (c) 2007-14 Rob Kinyon. All Rights Reserved.  This is free
       software, you may use it	and distribute it under	the same terms as Perl
       itself.

perl v5.32.0			  2018-05-20			  DBM::Deep(3)

NAME | VERSION | SYNOPSIS | DESCRIPTION | VERSION DIFFERENCES | PLUGINS | SETUP | TIE INTERFACE | OO INTERFACE | LOCKING | IMPORTING/EXPORTING | FILTERS | ERROR HANDLING | LARGEFILE SUPPORT | LOW-LEVEL ACCESS | CIRCULAR REFERENCES | TRANSACTIONS | MIGRATION | TODO | CAVEATS, ISSUES & BUGS | CODE COVERAGE | MORE INFORMATION | MAINTAINERS | SPONSORS | CONTRIBUTORS | SEE ALSO | LICENSE

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