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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
       2.0004

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:

       ·   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).

       ·   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.

       ·   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.

       ·   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:

       ·   dbh

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

       ·   dbi

	   This is a hashref containing:

	   ·   dsn

	   ·   username

	   ·   password

	   ·   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:

       ·   BDB (and other hash engines like memcached)

       ·   NoSQL engines (such as Tokyo Cabinet)

       ·   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.

       ·   type

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

	   ·   "DBM::Deep->TYPE_HASH"

	   ·   "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".

       ·   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).

       ·   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.

       ·   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.

	   See "TRANSACTIONS" below.

       ·   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.

       ·   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.

       ·   pack_size

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

	   ·   small

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

	   ·   medium (default)

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

	   ·   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.

       ·   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.

       ·   new() / clone()

	   These are the constructor and copy-functions.

       ·   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

       ·   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

       ·   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

       ·   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

       ·   clear()

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

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

       ·   lock() / unlock() / lock_exclusive() / lock_shared()

	   q.v. "LOCKING" for more info.

       ·   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.

       ·   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.

       ·   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.

       ·   begin_work() / commit() / rollback()

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

       ·   supports( $option )

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

	   ·   transactions

	   ·   unicode

       ·   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()".

       ·   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();

       ·   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()".

       ·   length()

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

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

       ·   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", {});

       ·   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();

       ·   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();

       ·   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", {});

       ·   splice()

	   Performs exactly like Perl's built-in function of the same name.
	   See "perldoc -f splice" 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:

       ·   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.

       ·   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.

       ·   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.

       ·   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 datbase, 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 hass 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:

       ·   begin_work()

	   This starts a transaction.

       ·   commit()

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

       ·   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.
       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.

       ·   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.

       ·   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.

       ·   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 "%{ ... }" 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
       <http://groups.google.com/group/DBM-Deep> or send email to
       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
       <http://github.com/robkinyon/dbm-deep>

MAINTAINERS
       Rob Kinyon, rkinyon@cpan.org

       Originally written by Joseph Huckaby, jhuckaby@cpan.org

SPONSORS
       Stonehenge Consulting (<http://www.stonehenge.com/>) sponsored the
       developement 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:

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

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

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

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

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

perl v5.14.1			  2011-07-15			  DBM::Deep(3)
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