r11686@rob-kinyons-powerbook58: rob | 2006-04-29 23:23:17 -0400

[dbsrgits/DBM-Deep.git] / README

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

SYNOPSIS
      use DBM::Deep;
      my $db = new DBM::Deep "foo.db";
  
      $db->{key} = 'value'; # tie() style
      print $db->{key};
  
      $db->put('key', 'value'); # OO style
      print $db->get('key');
  
      # true multi-level support
      $db->{my_complex} = [
            'hello', { perl => 'rules' }, 
            42, 99 ];

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, and quite fast. Can
    handle millions of keys and unlimited hash 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.

INSTALLATION
    Hopefully you are using CPAN's excellent Perl module, which will
    download and install the module for you. If not, get the tarball, and
    run these commands:

            tar zxf DBM-Deep-*
            cd DBM-Deep-*
            perl Makefile.PL
            make
            make test
            make install

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, tied hash or array reference.

            my $db = new DBM::Deep "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:

            my $db = new DBM::Deep(
                    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 = new DBM::Deep(
                    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 ignored if it is passed
    to the constructor.

  TIE CONSTRUCTION
    Alternatively, you can create a DBM::Deep handle by using Perl's
    built-in tie() function. This is not ideal, because you get only a
    basic, tied hash (or array) which is not blessed, so you can't call any
    functions on it.

            my %hash;
            tie %hash, "DBM::Deep", "foo.db";
        
            my @array;
            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.

    * mode
        File open mode (read-only, read-write, etc.) string passed to Perl's
        FileHandle module. This is an optional parameter, and defaults to
        "r+" (read/write). Note: If the default (r+) mode is selected, the
        file will also be auto- created if it doesn't exist.

    * type
        This parameter specifies what type of object to create, a hash or
        array. Use one of these two constants: "DBM::Deep::TYPE_HASH" or
        "DBM::Deep::TYPE_ARRAY". This only takes effect when beginning a new
        file. This is an optional parameter, and defaults to hash.

    * locking
        Specifies whether locking is to be enabled. DBM::Deep uses Perl's
        Fnctl 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 0 (disabled). 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* or at least *volatile*). Pass
        any true value to enable. This is an optional parameter, and
        defaults to 0 (disabled).

    * volatile
        If *volatile* mode is enabled, DBM::Deep will stat() the DB file
        before each STORE() operation. This is required if an outside force
        may change the size of the file between transactions. Locking also
        implicitly enables volatile. This is useful if you want to use a
        different locking system or write your own. Pass any true value to
        enable. This is an optional parameter, and defaults to 0 (disabled).

    * autobless
        If *autobless* mode is enabled, DBM::Deep will preserve blessed
        hashes, and restore them when fetched. This is an experimental
        feature, and does have side-effects. Basically, when hashes are
        re-blessed into their original classes, they are no longer blessed
        into the DBM::Deep class! So you won't be able to call any DBM::Deep
        methods on them. You have been warned. This is an optional
        parameter, and defaults to 0 (disabled).

    * filter_*
        See FILTERS below.

    * debug
        Setting *debug* mode will make all errors non-fatal, dump them out
        to STDERR, and continue on. This is for debugging purposes only, and
        probably not what you want. This is an optional parameter, and
        defaults to 0 (disabled).

TIE INTERFACE
    With DBM::Deep you can access your databases using Perl's standard
    hash/array syntax. Because all Deep objects are *tied* to hashes or
    arrays, you can treat them as such. 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 = new DBM::Deep "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 extra 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 = new DBM::Deep(
                    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()".

    * put()
        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()
        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. Please note that the space occupied by the deleted
        key/value or element is not reused again -- see "UNUSED SPACE
        RECOVERY" below for details and workarounds.

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

    * clear()
        Deletes all hash keys or array elements. Takes no arguments. No
        return value. Please note that the space occupied by the deleted
        keys/values or elements is not reused again -- see "UNUSED SPACE
        RECOVERY" below for details and workarounds.

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

  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 = new DBM::Deep "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 = new DBM::Deep(
                    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 automatic file locking by passing a true value to the "locking"
    parameter when constructing your DBM::Deep object (see SETUP above).

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

    This causes Deep to "flock()" the underlying FileHandle object 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
    transactions. 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( DBM::Deep::LOCK_SH );
            # something here
            $db->unlock();

    If you want to implement your own file locking scheme, be sure to create
    your DBM::Deep objects setting the "volatile" option to true. This hints
    to Deep that the DB file may change between transactions. See "LOW-LEVEL
    ACCESS" below for more.

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 = new DBM::Deep "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.

  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 = new DBM::Deep "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.

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 = new DBM::Deep(
                    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 $cipher = new Crypt::CBC({
                    'key'             => 'my secret key',
                    'cipher'          => 'Blowfish',
                    'iv'              => '$KJh#(}q',
                    'regenerate_key'  => 0,
                    'padding'         => 'space',
                    'prepend_iv'      => 0
            });
        
            my $db = new DBM::Deep(
                    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 = new DBM::Deep(
                    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. Also,
    the actual error message is stored in an internal scalar, which can be
    fetched by calling the "error()" method.

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

    You can then call "clear_error()" to clear the current error state.

            $db->clear_error();

    If you set the "debug" option to true when creating your DBM::Deep
    object, all errors are considered NON-FATAL, and dumped to STDERR. This
    is only for debugging purposes.

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 2 GB. DBM::Deep by default uses 32-bit file offset tags, but
    these can be changed by calling the static "set_pack()" method before
    you do anything else.

            DBM::Deep::set_pack(8, 'Q');

    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).

    Note: Changing these values will NOT work for existing database files.
    Only change this for new files, and make sure it stays set consistently
    throughout the file's life. If you do set these values, you can no
    longer access 32-bit DB files. You can, however, call "set_pack(4, 'N')"
    to change back to 32-bit mode.

    Note: I have not personally tested files > 2 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 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 your options
    you specified when you created the object. You can get access to this
    root structure by calling the "root()" method.

            my $root = $db->root();

    This is useful for changing options after the object has already been
    created, such as enabling/disabling locking, volatile or debug modes.
    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 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 calling the static
    "set_digest()" function, passing a reference to a subroutine, and the
    length of the algorithm's hashes (in bytes). This is a global static
    function, which affects ALL Deep objects. 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.

            use DBM::Deep;
            use Digest::SHA256;
        
            my $context = Digest::SHA256::new(256);
        
            DBM::Deep::set_digest( \&my_digest, 32 );
        
            my $db = new DBM::Deep "foo-sha.db";
        
            $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 "set_digest()" function (in this case 32).

CIRCULAR REFERENCES
    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 = new DBM::Deep "foo.db";
        
            $db->{foo} = "bar";
            $db->{circle} = $db; # ref to self
        
            print $db->{foo} . "\n"; # prints "foo"
            print $db->{circle}->{foo} . "\n"; # prints "foo" again

    One catch is, 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. The
    other catch is, if you fetch the *key* of a circular reference (i.e.
    using the "first_key()" or "next_key()" methods), you will get the
    *target object's key*, not the ref's key. This gets even more
    interesting with the above example, where the *circle* key points to the
    base DB object, which technically doesn't have a key. So I made
    DBM::Deep return "[base]" as the key name in that special case.

CAVEATS / ISSUES / BUGS
    This section describes all the known issues with DBM::Deep. It you have
    found something that is not listed here, please send e-mail to
    jhuckaby@cpan.org.

  UNUSED SPACE RECOVERY
    One major caveat with Deep is that space occupied by existing keys and
    values is not recovered when they are deleted. Meaning if you keep
    deleting and adding new keys, your file will continuously grow. I am
    working on this, but in the meantime you can call the built-in
    "optimize()" method from time to time (perhaps in a crontab or
    something) to recover all your unused space.

            $db->optimize(); # returns true on success

    This rebuilds the ENTIRE database into a new file, then moves it on top
    of the original. The new file will have no unused space, thus it will
    take up as little disk space as possible. Please note that this
    operation can take a long time for large files, and you need enough disk
    space to temporarily hold 2 copies of your DB file. The temporary file
    is created in the same directory as the original, named with a ".tmp"
    extension, and is deleted when the operation completes. Oh, and if
    locking is enabled, the DB is automatically locked for the entire
    duration of the copy.

    WARNING: Only call optimize() on the top-level node of the database, and
    make sure there are no child references lying around. Deep keeps a
    reference counter, and if it is greater than 1, optimize() will abort
    and return undef.

  AUTOVIVIFICATION
    Unfortunately, autovivification doesn't work with tied hashes. This
    appears to be a bug in Perl's tie() system, as *Jakob Schmidt*
    encountered the very same issue with his *DWH_FIle* module (see
    <http://search.cpan.org/search?module=DWH_File>), and it is also
    mentioned in the BUGS section for the *MLDBM* module <see
    <http://search.cpan.org/search?module=MLDBM>). Basically, on a new db
    file, this does not work:

            $db->{foo}->{bar} = "hello";

    Since "foo" doesn't exist, you cannot add "bar" to it. You end up with
    "foo" being an empty hash. Try this instead, which works fine:

            $db->{foo} = { bar => "hello" };

    As of Perl 5.8.7, this bug still exists. I have walked very carefully
    through the execution path, and Perl indeed passes an empty hash to the
    STORE() method. Probably a bug in Perl.

  FILE CORRUPTION
    The current level of error handling in Deep is minimal. Files *are*
    checked for a 32-bit signature on open(), but other corruption in files
    can cause segmentation faults. Deep may try to seek() past the end of a
    file, or get stuck in an infinite loop depending on the level of
    corruption. File write operations are not checked for failure (for
    speed), so if you happen to run out of disk space, 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 DB files over NFS. 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 milage 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 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 Deep's "clone()" method which safely copies the object and
    returns a new, blessed, tied hash or array to the same level in the DB.

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

  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 may be addressed in a later
    version.

PERFORMANCE
    This section discusses DBM::Deep's speed and memory usage.

  SPEED
    Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs,
    such as the almighty *BerkeleyDB*. But it makes up for it in features
    like true multi-level hash/array support, and cross-platform FTPable
    files. Even so, DBM::Deep is still pretty fast, and the speed stays
    fairly consistent, even with huge databases. Here is some test data:

            Adding 1,000,000 keys to new DB file...
        
            At 100 keys, avg. speed is 2,703 keys/sec
            At 200 keys, avg. speed is 2,642 keys/sec
            At 300 keys, avg. speed is 2,598 keys/sec
            At 400 keys, avg. speed is 2,578 keys/sec
            At 500 keys, avg. speed is 2,722 keys/sec
            At 600 keys, avg. speed is 2,628 keys/sec
            At 700 keys, avg. speed is 2,700 keys/sec
            At 800 keys, avg. speed is 2,607 keys/sec
            At 900 keys, avg. speed is 2,190 keys/sec
            At 1,000 keys, avg. speed is 2,570 keys/sec
            At 2,000 keys, avg. speed is 2,417 keys/sec
            At 3,000 keys, avg. speed is 1,982 keys/sec
            At 4,000 keys, avg. speed is 1,568 keys/sec
            At 5,000 keys, avg. speed is 1,533 keys/sec
            At 6,000 keys, avg. speed is 1,787 keys/sec
            At 7,000 keys, avg. speed is 1,977 keys/sec
            At 8,000 keys, avg. speed is 2,028 keys/sec
            At 9,000 keys, avg. speed is 2,077 keys/sec
            At 10,000 keys, avg. speed is 2,031 keys/sec
            At 20,000 keys, avg. speed is 1,970 keys/sec
            At 30,000 keys, avg. speed is 2,050 keys/sec
            At 40,000 keys, avg. speed is 2,073 keys/sec
            At 50,000 keys, avg. speed is 1,973 keys/sec
            At 60,000 keys, avg. speed is 1,914 keys/sec
            At 70,000 keys, avg. speed is 2,091 keys/sec
            At 80,000 keys, avg. speed is 2,103 keys/sec
            At 90,000 keys, avg. speed is 1,886 keys/sec
            At 100,000 keys, avg. speed is 1,970 keys/sec
            At 200,000 keys, avg. speed is 2,053 keys/sec
            At 300,000 keys, avg. speed is 1,697 keys/sec
            At 400,000 keys, avg. speed is 1,838 keys/sec
            At 500,000 keys, avg. speed is 1,941 keys/sec
            At 600,000 keys, avg. speed is 1,930 keys/sec
            At 700,000 keys, avg. speed is 1,735 keys/sec
            At 800,000 keys, avg. speed is 1,795 keys/sec
            At 900,000 keys, avg. speed is 1,221 keys/sec
            At 1,000,000 keys, avg. speed is 1,077 keys/sec

    This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 &
    Perl 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash
    keys and values were between 6 - 12 chars in length. The DB file ended
    up at 210MB. Run time was 12 min 3 sec.

  MEMORY USAGE
    One of the great things about DBM::Deep is that it uses very little
    memory. Even with huge databases (1,000,000+ keys) you will not see much
    increased memory on your process. Deep relies solely on the filesystem
    for storing and fetching data. Here is output from */usr/bin/top* before
    even opening a database handle:

              PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME COMMAND
            22831 root      11   0  2716 2716  1296 R     0.0  0.2   0:07 perl

    Basically the process is taking 2,716K of memory. And here is the same
    process after storing and fetching 1,000,000 keys:

              PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME COMMAND
            22831 root      14   0  2772 2772  1328 R     0.0  0.2  13:32 perl

    Notice the memory usage increased by only 56K. Test was performed on a
    700mHz x86 box running Linux RedHat 7.2 & Perl 5.6.1.

DB FILE FORMAT
    In case you were interested in the underlying DB file format, it is
    documented here in this section. You don't need to know this to use the
    module, it's just included for reference.

  SIGNATURE
    DBM::Deep files always start with a 32-bit signature to identify the
    file type. This is at offset 0. The signature is "DPDB" in network byte
    order. This is checked upon each file open().

  TAG
    The DBM::Deep file is in a *tagged format*, meaning each section of the
    file has a standard header containing the type of data, the length of
    data, and then the data itself. The type is a single character (1 byte),
    the length is a 32-bit unsigned long in network byte order, and the data
    is, well, the data. Here is how it unfolds:

  MASTER INDEX
    Immediately after the 32-bit file signature is the *Master Index*
    record. This is a standard tag header followed by 1024 bytes (in 32-bit
    mode) or 2048 bytes (in 64-bit mode) of data. The type is *H* for hash
    or *A* for array, depending on how the DBM::Deep object was constructed.

    The index works by looking at a *MD5 Hash* of the hash key (or array
    index number). The first 8-bit char of the MD5 signature is the offset
    into the index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The
    value of the index element is a file offset of the next tag for the
    key/element in question, which is usually a *Bucket List* tag (see
    below).

    The next tag *could* be another index, depending on how many
    keys/elements exist. See RE-INDEXING below for details.

  BUCKET LIST
    A *Bucket List* is a collection of 16 MD5 hashes for keys/elements, plus
    file offsets to where the actual data is stored. It starts with a
    standard tag header, with type *B*, and a data size of 320 bytes in
    32-bit mode, or 384 bytes in 64-bit mode. Each MD5 hash is stored in
    full (16 bytes), plus the 32-bit or 64-bit file offset for the *Bucket*
    containing the actual data. When the list fills up, a *Re-Index*
    operation is performed (See RE-INDEXING below).

  BUCKET
    A *Bucket* is a tag containing a key/value pair (in hash mode), or a
    index/value pair (in array mode). It starts with a standard tag header
    with type *D* for scalar data (string, binary, etc.), or it could be a
    nested hash (type *H*) or array (type *A*). The value comes just after
    the tag header. The size reported in the tag header is only for the
    value, but then, just after the value is another size (32-bit unsigned
    long) and then the plain key itself. Since the value is likely to be
    fetched more often than the plain key, I figured it would be *slightly*
    faster to store the value first.

    If the type is *H* (hash) or *A* (array), the value is another *Master
    Index* record for the nested structure, where the process begins all
    over again.

  RE-INDEXING
    After a *Bucket List* grows to 16 records, its allocated space in the
    file is exhausted. Then, when another key/element comes in, the list is
    converted to a new index record. However, this index will look at the
    next char in the MD5 hash, and arrange new Bucket List pointers
    accordingly. This process is called *Re-Indexing*. Basically, a new
    index tag is created at the file EOF, and all 17 (16 + new one)
    keys/elements are removed from the old Bucket List and inserted into the
    new index. Several new Bucket Lists are created in the process, as a new
    MD5 char from the key is being examined (it is unlikely that the keys
    will all share the same next char of their MD5s).

    Because of the way the *MD5* algorithm works, it is impossible to tell
    exactly when the Bucket Lists will turn into indexes, but the first
    round tends to happen right around 4,000 keys. You will see a *slight*
    decrease in performance here, but it picks back up pretty quick (see
    SPEED above). Then it takes a lot more keys to exhaust the next level of
    Bucket Lists. It's right around 900,000 keys. This process can continue
    nearly indefinitely -- right up until the point the *MD5* signatures
    start colliding with each other, and this is EXTREMELY rare -- like
    winning the lottery 5 times in a row AND getting struck by lightning
    while you are walking to cash in your tickets. Theoretically, since
    *MD5* hashes are 128-bit values, you *could* have up to
    340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I
    believe this is 340 unodecillion, but don't quote me).

  STORING
    When a new key/element is stored, the key (or index number) is first ran
    through *Digest::MD5* to get a 128-bit signature (example, in hex:
    b05783b0773d894396d475ced9d2f4f6). Then, the *Master Index* record is
    checked for the first char of the signature (in this case *b*). If it
    does not exist, a new *Bucket List* is created for our key (and the next
    15 future keys that happen to also have *b* as their first MD5 char).
    The entire MD5 is written to the *Bucket List* along with the offset of
    the new *Bucket* record (EOF at this point, unless we are replacing an
    existing *Bucket*), where the actual data will be stored.

  FETCHING
    Fetching an existing key/element involves getting a *Digest::MD5* of the
    key (or index number), then walking along the indexes. If there are
    enough keys/elements in this DB level, there might be nested indexes,
    each linked to a particular char of the MD5. Finally, a *Bucket List* is
    pointed to, which contains up to 16 full MD5 hashes. Each is checked for
    equality to the key in question. If we found a match, the *Bucket* tag
    is loaded, where the value and plain key are stored.

    Fetching the plain key occurs when calling the *first_key()* and
    *next_key()* methods. In this process the indexes are walked
    systematically, and each key fetched in increasing MD5 order (which is
    why it appears random). Once the *Bucket* is found, the value is skipped
    the plain key returned instead. Note: Do not count on keys being fetched
    as if the MD5 hashes were alphabetically sorted. This only happens on an
    index-level -- as soon as the *Bucket Lists* are hit, the keys will come
    out in the order they went in -- so it's pretty much undefined how the
    keys will come out -- just like Perl's built-in hashes.

AUTHOR
    Joseph Huckaby, jhuckaby@cpan.org

    Special thanks to Adam Sah and Rich Gaushell! You know why :-)

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) 2002-2005 Joseph Huckaby. All Rights Reserved. This is
    free software, you may use it and distribute it under the same terms as
    Perl itself.