man DBM::Deep () - A pure perl multi-level hash/array DBM


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


  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 ];


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.


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 test
        make install


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


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 CWtype parameter:

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

Note: Specifing the CWtype parameter only takes effect when beginning a new DB file. If you create a DBM::Deep object with an existing file, the CWtype will be loaded from the file header, and ignored if it is passed to the constructor.


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


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: CWDBM::Deep::TYPE_HASH or CWDBM::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).
* filter_*
See FILTERS below.
* debug
Currently, debug mode does nothing more than print all errors to STDERR. However, it may be expanded in the future to log more debugging information. Pass any true value to enable. This is an optional paramter, and defaults to 0 (disabled).


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 CWget() and CWput() (although those work too). It is entirely up to you how to want to access your databases.


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 CWkeys() function:

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

Remember that Perl's CWkeys() 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 CWeach() function, which pulls keys/values one at a time, using very little memory:

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


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 CWpush(), CWpop(), CWshift(), CWunshift() and CWsplice() functions. The object must have first been created using type CWDBM::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;


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: CWput(), CWget(), CWexists(), CWdelete() and CWclear().

* 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


For hashes, DBM::Deep supports all the common methods described above, and the following additional methods: CWfirst_key() and CWnext_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"); }


For arrays, DBM::Deep supports all the common methods described above, and the following additional methods: CWlength(), CWpush(), CWpop(), CWshift(), CWunshift() and CWsplice().

* 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->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";


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

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

This causes Deep to CWflock() 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 CWflock() does NOT work for files over NFS. See DB OVER NFS below for more.


You can explicitly lock a database, so it remains locked for multiple transactions. This is done by calling the CWlock() 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.

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

        # or...


You can pass CWlock() an optional argument, which specifies which mode to use (exclusive or shared). Use one of these two constants: CWDBM::Deep::LOCK_EX or CWDBM::Deep::LOCK_SH. These are passed directly to CWflock(), and are the same as the constants defined in Perl's CWFcntl module.

        $db->lock( DBM::Deep::LOCK_SH );
        # something here

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


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


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 CWimport() 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 CW$struct object into CW$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 CWimport() 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.


Calling the CWexport() method on an existing DBM::Deep object will return a reference to a new in-memory copy of the datbase. 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 CWexport() 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.


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 CWset_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 CWfirst_key() or CWnext_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 CWundef:

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


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 <> 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;

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


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 <> 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;

        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.


Most DBM::Deep methods return a true value for success, and a false value for failure. Upon failure, the actual error message is stored in an internal scalar, which can be fetched by calling the CWerror() method.

        my $db = new DBM::Deep "foo.db"; # hash
        $db->push("foo"); # ILLEGAL -- array only func

        print $db->error(); # prints error message

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


It is always a good idea to check the error state upon object creation. Deep immediately tries to CWopen() the FileHandle, so if you don't have sufficient permissions or some other filesystem error occurs, you should act accordingly before trying to access the database.

        my $db = new DBM::Deep("foo.db");
        if ($db->error()) {
                die "ERROR: " . $db->error();

If you set the CWdebug option to true when creating your DBM::Deep object, all errors are printed to STDERR.


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 CWset_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 CWset_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. If anyone tries this, please tell me what happens!


If you require low-level access to the underlying FileHandle that Deep uses, you can call the CWfh() 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 CWroot() 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.


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 CWset_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 <> for more.

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

        my $context = Digest::SHA256::new(2);

        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;

        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 CWset_digest() function (in this case 32).


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 CWoptimize() or CWexport() methods) will result in an infinite loop. The other catch is, if you fetch the key of a circular reference (i.e. using the CWfirst_key() or CWnext_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.


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


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 CWoptimize() 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.


Unfortunately, autovivification doesn't always work. 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 <>), and it is also mentioned in the BUGS section for the MLDBM module <see <>). Basically, your milage may vary when issuing statements like this:

        $db->{a} = { b => [ 1, 2, { c => [ 'd', { e => 'f' } ] } ] };

This causes 3 hashes and 2 arrays to be created in the database all in one fell swoop, and all nested within each other. Perl may choke on this, and fail to create one or more of the nested structures. This doesn't appear to be a bug in DBM::Deep, but I am still investigating it. The problem is intermittent. For safety, I recommend creating nested structures using a series of commands instead of just one, which will always work:

        $db->{a} = {};
        $db->{a}->{b} = [];

        my $b = $db->{a}->{b};
        $b->[0] = 1;
        $b->[1] = 2;
        $b->[2] = {};
        $b->[2]->{c} = [];

        my $c = $b->[2]->{c};
        $c->[0] = 'd';
        $c->[1] = {};
        $c->[1]->{e} = 'f';

        undef $c;
        undef $b;

Also, you can just create the whole structure in memory using a temporary variable, then use DBM::Deep's CWimport() method to import the entire thing into the database:

        my $temp = {
                a => { b => [ 1, 2, { c => [ 'd', { e => 'f' } ] } ] }
        $db->import( $temp );

Note: I have yet to recreate this bug with Perl 5.8.1. Perhaps the issue has been resolved? Will update as events warrant.


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.


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.


Beware of copying tied objects in Perl. Very strange things can happen. Instead, use Deep's CWclone() 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();


Beware of using CWshift(), CWunshift() or CWsplice() 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.


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


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.


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:

        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:

        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.


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.


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().


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:


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.


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).


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.


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).


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 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.


Joseph Huckaby,

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



Copyright (c) 2002-2004 Joseph Huckaby. All Rights Reserved. This is free software, you may use it and distribute it under the same terms as Perl itself.