Hash tests pass again with header being read and cached

[dbsrgits/DBM-Deep.git] / README

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

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: 0.99_03 has significant file format differences from prior
    versions. THere will be a backwards-compatibility layer in 1.00, but
    that is slated for a later 0.99_x release. This version is NOT backwards
    compatible with any other release of DBM::Deep.

    NOTE: 0.99_01 and above have significant file format differences from
    0.983 and before. There will be a backwards-compatibility layer in 1.00,
    but that is slated for a later 0.99_x release. This version is NOT
    backwards compatible with 0.983 and before.

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: Specifing 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.

    * 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.

    * 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".

    * 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.

    * 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.

    * 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.

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. 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 true on success,
        false if not found. 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()
        q.v. Locking.

    * optimize()
        Recover lost disk space. This is important to do, especially if you
        use transactions.

    * import() / export()
        Data going in and out.

    * begin_work() / commit() / rollback()
        These are the transactional functions. "TRANSACTIONS" for more
        information.

  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()" method, and passing an
    optional lock mode argument (defaults to exclusive mode). This is
    particularly useful for things like counters, where the current value
    needs to be fetched, then incremented, then stored again.

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

      # or...

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

    You can pass "lock()" an optional argument, which specifies which mode
    to use (exclusive or shared). Use one of these two constants:
    "DBM::Deep->LOCK_EX" or "DBM::Deep->LOCK_SH". These are passed directly
    to "flock()", and are the same as the constants defined in Perl's Fcntl
    module.

      $db->lock( $db->LOCK_SH );
      # something here
      $db->unlock();

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 exsiting
    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, described
    below:

    * 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( "filter_store_value", \&my_filter_store );
      $db->set_filter( "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( "filter_store_value", undef );

  Real-time Encryption Example
    Here is a working example that uses the *Crypt::Blowfish* module to do
    real-time encryption / decryption of keys & values with DBM::Deep
    Filters. Please visit
    <http://search.cpan.org/search?module=Crypt::Blowfish> for more on
    *Crypt::Blowfish*. You'll also need the *Crypt::CBC* module.

      use DBM::Deep;
      use Crypt::Blowfish;
      use Crypt::CBC;

      my $cipher = Crypt::CBC->new({
          'key'             => 'my secret key',
          'cipher'          => 'Blowfish',
          'iv'              => '$KJh#(}q',
          'regenerate_key'  => 0,
          'padding'         => 'space',
          'prepend_iv'      => 0
      });

      my $db = DBM::Deep->new(
          file => "foo-encrypt.db",
          filter_store_key => \&my_encrypt,
          filter_store_value => \&my_encrypt,
          filter_fetch_key => \&my_decrypt,
          filter_fetch_value => \&my_decrypt,
      );

      $db->{key1} = "value1";
      $db->{key2} = "value2";
      print "key1: " . $db->{key1} . "\n";
      print "key2: " . $db->{key2} . "\n";

      undef $db;
      exit;

      sub my_encrypt {
          return $cipher->encrypt( $_[0] );
      }
      sub my_decrypt {
          return $cipher->decrypt( $_[0] );
      }

  Real-time Compression Example
    Here is a working example that uses the *Compress::Zlib* module to do
    real-time compression / decompression of keys & values with DBM::Deep
    Filters. Please visit
    <http://search.cpan.org/search?module=Compress::Zlib> for more on
    *Compress::Zlib*.

      use DBM::Deep;
      use Compress::Zlib;

      my $db = DBM::Deep->new(
          file => "foo-compress.db",
          filter_store_key => \&my_compress,
          filter_store_value => \&my_compress,
          filter_fetch_key => \&my_decompress,
          filter_fetch_value => \&my_decompress,
      );

      $db->{key1} = "value1";
      $db->{key2} = "value2";
      print "key1: " . $db->{key1} . "\n";
      print "key2: " . $db->{key2} . "\n";

      undef $db;
      exit;

      sub my_compress {
          return Compress::Zlib::memGzip( $_[0] ) ;
      }
      sub my_decompress {
          return Compress::Zlib::memGunzip( $_[0] ) ;
      }

    Note: Filtering of keys only applies to hashes. Array "keys" are
    actually numerical index numbers, and are not filtered.

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(
          filename  => $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 my
    systems have only a 32-bit Perl. However, I 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.

CUSTOM DIGEST ALGORITHM
    DBM::Deep by default uses the *Message Digest 5* (MD5) algorithm for
    hashing keys. However you can override this, and use another algorithm
    (such as SHA-256) or even write your own. But please note that DBM::Deep
    currently expects zero collisions, so your algorithm has to be
    *perfect*, so to speak. Collision detection may be introduced in a later
    version.

    You can specify a custom digest algorithm by passing it into the
    parameter list for new(), passing a reference to a subroutine as the
    'digest' parameter, and the length of the algorithm's hashes (in bytes)
    as the 'hash_size' parameter. Here is a working example that uses a
    256-bit hash from the *Digest::SHA256* module. Please see
    <http://search.cpan.org/search?module=Digest::SHA256> for more
    information.

      use DBM::Deep;
      use Digest::SHA256;

      my $context = Digest::SHA256::new(256);

      my $db = DBM::Deep->new(
          filename => "foo-sha.db",
          digest => \&my_digest,
          hash_size => 32,
      );

      $db->{key1} = "value1";
      $db->{key2} = "value2";
      print "key1: " . $db->{key1} . "\n";
      print "key2: " . $db->{key2} . "\n";

      undef $db;
      exit;

      sub my_digest {
          return substr( $context->hash($_[0]), 0, 32 );
      }

    Note: Your returned digest strings must be EXACTLY the number of bytes
    you specify in the hash_size parameter (in this case 32).

    Note: If you do choose to use a custom digest algorithm, you must set it
    every time you access this file. Otherwise, the default (MD5) will be
    used.

CIRCULAR REFERENCES
    NOTE: DBM::Deep 0.99_03 has turned off circular references pending
    evaluation of some edge cases. I hope to be able to re-enable circular
    references in a future version after 1.00. This means that circular
    references are NO LONGER available.

    DBM::Deep has experimental 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

    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.

TRANSACTIONS
    New in 0.99_01 is 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.

  Software-Transactional Memory
    The addition of transactions to this module provides the basis for STM
    within Perl 5. Contention is resolved using a default last-write-wins.
    Currently, this default cannot be changed, but it will be addressed in a
    future version.

PERFORMANCE
    Because DBM::Deep is a conncurrent datastore, every change is flushed to
    disk immediately and every read goes to disk. This means that DBM::Deep
    functions at the speed of disk (generally 10-20ms) vs. the speed of RAM
    (generally 50-70ns), or at least 150-200x slower than the comparable
    in-memory datastructure in Perl.

    There are several techniques you can use to speed up how DBM::Deep
    functions.

    * Put it on a ramdisk
        The easiest and quickest mechanism to making DBM::Deep run faster is
        to create a ramdisk and locate the DBM::Deep file there. Doing this
        as an option may become a feature of DBM::Deep, assuming there is a
        good ramdisk wrapper on CPAN.

    * Work at the tightest level possible
        It is much faster to assign the level of your db that you are
        working with to an intermediate variable than to re-look it up every
        time. Thus

          # BAD
          while ( my ($k, $v) = each %{$db->{foo}{bar}{baz}} ) {
            ...
          }

          # GOOD
          my $x = $db->{foo}{bar}{baz};
          while ( my ($k, $v) = each %$x ) {
            ...
          }

    * Make your file as tight as possible
        If you know that you are not going to use more than 65K in your
        database, consider using the "pack_size => 'small'" option. This
        will instruct DBM::Deep to use 16bit addresses, meaning that the
        seek times will be less.

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 user 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.

  Importing using Data::Walker
    Right now, importing is done using "Clone::clone()" to make a complete
    copy in memory, then tying that copy. It would be much better to use
    Data::Walker to walk the data structure instead, particularly in the
    case of large datastructures.

  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 rkinyon@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.

  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.

  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.

  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           96.8   87.9   90.5  100.0   89.5    4.5   95.2
      blib/lib/DBM/Deep/Array.pm    100.0   94.3  100.0  100.0  100.0    4.8   98.7
      blib/lib/DBM/Deep/Engine.pm    97.2   86.4   86.0  100.0    0.0   56.8   91.0
      blib/lib/DBM/Deep/File.pm      98.1   83.3   66.7  100.0    0.0   31.4   88.0
      blib/lib/DBM/Deep/Hash.pm     100.0  100.0  100.0  100.0  100.0    2.5  100.0
      Total                          97.7   88.1   86.6  100.0   31.6  100.0   93.0
      ---------------------------- ------ ------ ------ ------ ------ ------ ------

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. You can also visit #dbm-deep on irc.perl.org

    The source code repository is at <http://svn.perl.org/modules/DBM-Deep>

MAINTAINER(S)
    Rob Kinyon, rkinyon@cpan.org

    Originally written by Joseph Huckaby, jhuckaby@cpan.org

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

    * Adam Sah and Rich Gaushell
    * Stonehenge for sponsoring the 1.00 release
    * Dan Golden and others at YAPC::NA 2006 for helping me design through
    transactions.

SEE ALSO
    perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3),
    nfs(5), Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)

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.