File¶

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

• file

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

• fh

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

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

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

• file_offset

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

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

• locking

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

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

DBI¶

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

The parameters accepted are:

• dbh

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

• dbi

  This is a hashref containing:

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

Planned engines¶

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

• BDB (and other hash engines like memcached)
• NoSQL engines (such as Tokyo Cabinet)
• DBIx::Class (and other ORMs)

OO Construction¶

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

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

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

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

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

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

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

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

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

Tie Construction¶

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

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

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

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

Options¶

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

•

type

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

• 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.

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

  See "TRANSACTIONS" below.

• max_buckets

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

• data_sector_size

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

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

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

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

• pack_size

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

•

external_refs

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

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

  $foo_hash = $db->{foo};
    

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

  delete $db->{foo};
    

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

  for(keys %$foo_hash) {
    

you will get an error.

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

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

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;

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 "splice" in perlfunc for usage -- it is too complicated to document here. This method is not recommended with large arrays -- see "Large Arrays" below for details.

Here are some examples of using arrays:

  my $db = DBM::Deep->new(
      file => "foo.db",
      type => DBM::Deep->TYPE_ARRAY
  );
  $db->push("bar", "baz");
  $db->unshift("foo");
  $db->put(3, "buz");
  my $len = $db->length();
  print "length: $len\n"; # 4
  for (my $k=0; $k<$len; $k++) {
      print "$k: " . $db->get($k) . "\n";
  }
  $db->splice(1, 2, "biz", "baf");
  while (my $elem = shift @$db) {
      print "shifted: $elem\n";
  }

Explicit Locking¶

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

  $db->lock_exclusive();
  my $counter = $db->get("counter");
  $counter++;
  $db->put("counter", $counter);
  $db->unlock();
  # or...
  $db->lock_exclusive();
  $db->{counter}++;
  $db->unlock();

Win32/Cygwin¶

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

Importing¶

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

  my $struct = {
      key1 => "value1",
      key2 => "value2",
      array1 => [ "elem0", "elem1", "elem2" ],
      hash1 => {
          subkey1 => "subvalue1",
          subkey2 => "subvalue2"
      }
  };
  my $db = DBM::Deep->new( "foo.db" );
  $db->import( $struct );
  print $db->{key1} . "\n"; # prints "value1"

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

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

Exporting¶

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

  my $db = DBM::Deep->new( "foo.db" );
  $db->{key1} = "value1";
  $db->{key2} = "value2";
  $db->{hash1} = {};
  $db->{hash1}->{subkey1} = "subvalue1";
  $db->{hash1}->{subkey2} = "subvalue2";
  my $struct = $db->export();
  print $struct->{key1} . "\n"; # prints "value1"

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

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

set_filter()¶

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

• filter_store_key

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

• filter_store_value

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

• filter_fetch_key

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

• filter_fetch_value

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

Here are the two ways to setup a filter hook:

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

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

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

Examples¶

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

Sub-Transactions¶

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

Caching¶

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

Ram-only¶

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

Different contention resolution mechanisms¶

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

References¶

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

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

• GLOB

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

• SCALAR / REF

  The discussion here refers to the following type of example:

    my $x = 25;
    $db->{key1} = \$x;
    $x = 50;
    # In some other process ...
    my $val = ${ $db->{key1} };
    is( $val, 50, "What actually gets stored in the DB file?" );
      

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

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

• CODE

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

External references and transactions¶

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

File corruption¶

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

DB over NFS¶

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

Copying Objects¶

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

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

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

Stale References¶

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

So a seemingly innocuous piece of code like this:

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

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

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

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

Large Arrays¶

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

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

Writeonly Files¶

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

Assignments Within Transactions¶

The following will not work as one might expect:

  my $x = { a => 1 };
  $db->begin_work;
  $db->{foo} = $x;
  $db->rollback;
  is( $x->{a}, 1 ); # This will fail!

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

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