NAME
DBM::Deep - A pure perl multi-level hash/array DBM that supports
transactions
SYNOPSIS
use DBM::Deep;
my $db = DBM::Deep->new( "foo.db" );
$db->{key} = 'value';
print $db->{key};
$db->put('key' => 'value');
print $db->get('key');
# true multi-level support
$db->{my_complex} = [
'hello', { perl => 'rules' },
42, 99,
];
$db->begin_work;
# Do stuff here
$db->rollback;
$db->commit;
tie my %db, 'DBM::Deep', 'foo.db';
$db{key} = 'value';
print $db{key};
tied(%db)->put('key' => 'value');
print tied(%db)->get('key');
DESCRIPTION
A unique flat-file database module, written in pure perl. True
multi-level hash/array support (unlike MLDBM, which is faked), hybrid OO
/ tie() interface, cross-platform FTPable files, ACID transactions, and
is quite fast. Can handle millions of keys and unlimited levels without
significant slow-down. Written from the ground-up in pure perl -- this
is NOT a wrapper around a C-based DBM. Out-of-the-box compatibility with
Unix, Mac OS X and Windows.
VERSION DIFFERENCES
NOTE: 0.99_03 has significant file format differences from prior
versions. THere will be a backwards-compatibility layer in 1.00, but
that is slated for a later 0.99_x release. This version is NOT backwards
compatible with any other release of DBM::Deep.
NOTE: 0.99_01 and above have significant file format differences from
0.983 and before. There will be a backwards-compatibility layer in 1.00,
but that is slated for a later 0.99_x release. This version is NOT
backwards compatible with 0.983 and before.
SETUP
Construction can be done OO-style (which is the recommended way), or
using Perl's tie() function. Both are examined here.
OO Construction
The recommended way to construct a DBM::Deep object is to use the new()
method, which gets you a blessed *and* tied hash (or array) reference.
my $db = DBM::Deep->new( "foo.db" );
This opens a new database handle, mapped to the file "foo.db". If this
file does not exist, it will automatically be created. DB files are
opened in "r+" (read/write) mode, and the type of object returned is a
hash, unless otherwise specified (see OPTIONS below).
You can pass a number of options to the constructor to specify things
like locking, autoflush, etc. This is done by passing an inline hash (or
hashref):
my $db = DBM::Deep->new(
file => "foo.db",
locking => 1,
autoflush => 1
);
Notice that the filename is now specified *inside* the hash with the
"file" parameter, as opposed to being the sole argument to the
constructor. This is required if any options are specified. See OPTIONS
below for the complete list.
You can also start with an array instead of a hash. For this, you must
specify the "type" parameter:
my $db = DBM::Deep->new(
file => "foo.db",
type => DBM::Deep->TYPE_ARRAY
);
Note: Specifing the "type" parameter only takes effect when beginning a
new DB file. If you create a DBM::Deep object with an existing file, the
"type" will be loaded from the file header, and an error will be thrown
if the wrong type is passed in.
Tie Construction
Alternately, you can create a DBM::Deep handle by using Perl's built-in
tie() function. The object returned from tie() can be used to call
methods, such as lock() and unlock(). (That object can be retrieved from
the tied variable at any time using tied() - please see perltie for more
info.
my %hash;
my $db = tie %hash, "DBM::Deep", "foo.db";
my @array;
my $db = tie @array, "DBM::Deep", "bar.db";
As with the OO constructor, you can replace the DB filename parameter
with a hash containing one or more options (see OPTIONS just below for
the complete list).
tie %hash, "DBM::Deep", {
file => "foo.db",
locking => 1,
autoflush => 1
};
Options
There are a number of options that can be passed in when constructing
your DBM::Deep objects. These apply to both the OO- and tie- based
approaches.
* file
Filename of the DB file to link the handle to. You can pass a full
absolute filesystem path, partial path, or a plain filename if the
file is in the current working directory. This is a required
parameter (though q.v. fh).
* fh
If you want, you can pass in the fh instead of the file. This is
most useful for doing something like:
my $db = DBM::Deep->new( { fh => \*DATA } );
You are responsible for making sure that the fh has been opened
appropriately for your needs. If you open it read-only and attempt
to write, an exception will be thrown. If you open it write-only or
append-only, an exception will be thrown immediately as DBM::Deep
needs to read from the fh.
* file_offset
This is the offset within the file that the DBM::Deep db starts.
Most of the time, you will not need to set this. However, it's there
if you want it.
If you pass in fh and do not set this, it will be set appropriately.
* type
This parameter specifies what type of object to create, a hash or
array. Use one of these two constants:
* "DBM::Deep->TYPE_HASH"
* "DBM::Deep->TYPE_ARRAY".
This only takes effect when beginning a new file. This is an
optional parameter, and defaults to "DBM::Deep->TYPE_HASH".
* locking
Specifies whether locking is to be enabled. DBM::Deep uses Perl's
flock() function to lock the database in exclusive mode for writes,
and shared mode for reads. Pass any true value to enable. This
affects the base DB handle *and any child hashes or arrays* that use
the same DB file. This is an optional parameter, and defaults to 1
(enabled). See LOCKING below for more.
* autoflush
Specifies whether autoflush is to be enabled on the underlying
filehandle. This obviously slows down write operations, but is
required if you may have multiple processes accessing the same DB
file (also consider enable *locking*). Pass any true value to
enable. This is an optional parameter, and defaults to 1 (enabled).
* filter_*
See "FILTERS" below.
The following parameters may be specified in the constructor the first
time the datafile is created. However, they will be stored in the header
of the file and cannot be overridden by subsequent openings of the file
- the values will be set from the values stored in the datafile's
header.
* num_txns
This is the number of transactions that can be running at one time.
The default is one - the HEAD. The minimum is one and the maximum is
255. The more transactions, the larger and quicker the datafile
grows.
See "TRANSACTIONS" below.
* max_buckets
This is the number of entries that can be added before a reindexing.
The larger this number is made, the larger a file gets, but the
better performance you will have. The default and minimum number
this can be is 16. The maximum is 256, but more than 64 isn't
recommended.
* data_sector_size
This is the size in bytes of a given data sector. Data sectors will
chain, so a value of any size can be stored. However, chaining is
expensive in terms of time. Setting this value to something close to
the expected common length of your scalars will improve your
performance. If it is too small, your file will have a lot of
chaining. If it is too large, your file will have a lot of dead
space in it.
The default for this is 64 bytes. The minimum value is 32 and the
maximum is 256 bytes.
Note: There are between 6 and 10 bytes taken up in each data sector
for bookkeeping. (It's 4 + the number of bytes in your "pack_size".)
This is included within the data_sector_size, thus the effective
value is 6-10 bytes less than what you specified.
* pack_size
This is the size of the file pointer used throughout the file. The
valid values are:
* small
This uses 2-byte offsets, allowing for a maximum file size of 65
KB.
* medium (default)
This uses 4-byte offsets, allowing for a maximum file size of 4
GB.
* large
This uses 8-byte offsets, allowing for a maximum file size of 16
XB (exabytes). This can only be enabled if your Perl is compiled
for 64-bit.
See "LARGEFILE SUPPORT" for more information.
TIE INTERFACE
With DBM::Deep you can access your databases using Perl's standard
hash/array syntax. Because all DBM::Deep objects are *tied* to hashes or
arrays, you can treat them as such. DBM::Deep will intercept all
reads/writes and direct them to the right place -- the DB file. This has
nothing to do with the "TIE CONSTRUCTION" section above. This simply
tells you how to use DBM::Deep using regular hashes and arrays, rather
than calling functions like "get()" and "put()" (although those work
too). It is entirely up to you how to want to access your databases.
Hashes
You can treat any DBM::Deep object like a normal Perl hash reference.
Add keys, or even nested hashes (or arrays) using standard Perl syntax:
my $db = DBM::Deep->new( "foo.db" );
$db->{mykey} = "myvalue";
$db->{myhash} = {};
$db->{myhash}->{subkey} = "subvalue";
print $db->{myhash}->{subkey} . "\n";
You can even step through hash keys using the normal Perl "keys()"
function:
foreach my $key (keys %$db) {
print "$key: " . $db->{$key} . "\n";
}
Remember that Perl's "keys()" function extracts *every* key from the
hash and pushes them onto an array, all before the loop even begins. If
you have an extremely large hash, this may exhaust Perl's memory.
Instead, consider using Perl's "each()" function, which pulls
keys/values one at a time, using very little memory:
while (my ($key, $value) = each %$db) {
print "$key: $value\n";
}
Please note that when using "each()", you should always pass a direct
hash reference, not a lookup. Meaning, you should never do this:
# NEVER DO THIS
while (my ($key, $value) = each %{$db->{foo}}) { # BAD
This causes an infinite loop, because for each iteration, Perl is
calling FETCH() on the $db handle, resulting in a "new" hash for foo
every time, so it effectively keeps returning the first key over and
over again. Instead, assign a temporary variable to "$db-"{foo}>, then
pass that to each().
Arrays
As with hashes, you can treat any DBM::Deep object like a normal Perl
array reference. This includes inserting, removing and manipulating
elements, and the "push()", "pop()", "shift()", "unshift()" and
"splice()" functions. The object must have first been created using type
"DBM::Deep->TYPE_ARRAY", or simply be a nested array reference inside a
hash. Example:
my $db = DBM::Deep->new(
file => "foo-array.db",
type => DBM::Deep->TYPE_ARRAY
);
$db->[0] = "foo";
push @$db, "bar", "baz";
unshift @$db, "bah";
my $last_elem = pop @$db; # baz
my $first_elem = shift @$db; # bah
my $second_elem = $db->[1]; # bar
my $num_elements = scalar @$db;
OO INTERFACE
In addition to the *tie()* interface, you can also use a standard OO
interface to manipulate all aspects of DBM::Deep databases. Each type of
object (hash or array) has its own methods, but both types share the
following common methods: "put()", "get()", "exists()", "delete()" and
"clear()". "fetch()" and "store(" are aliases to "put()" and "get()",
respectively.
* new() / clone()
These are the constructor and copy-functions.
* put() / store()
Stores a new hash key/value pair, or sets an array element value.
Takes two arguments, the hash key or array index, and the new value.
The value can be a scalar, hash ref or array ref. Returns true on
success, false on failure.
$db->put("foo", "bar"); # for hashes
$db->put(1, "bar"); # for arrays
* get() / fetch()
Fetches the value of a hash key or array element. Takes one
argument: the hash key or array index. Returns a scalar, hash ref or
array ref, depending on the data type stored.
my $value = $db->get("foo"); # for hashes
my $value = $db->get(1); # for arrays
* exists()
Checks if a hash key or array index exists. Takes one argument: the
hash key or array index. Returns true if it exists, false if not.
if ($db->exists("foo")) { print "yay!\n"; } # for hashes
if ($db->exists(1)) { print "yay!\n"; } # for arrays
* delete()
Deletes one hash key/value pair or array element. Takes one
argument: the hash key or array index. Returns true on success,
false if not found. For arrays, the remaining elements located after
the deleted element are NOT moved over. The deleted element is
essentially just undefined, which is exactly how Perl's internal
arrays work.
$db->delete("foo"); # for hashes
$db->delete(1); # for arrays
* clear()
Deletes all hash keys or array elements. Takes no arguments. No
return value.
$db->clear(); # hashes or arrays
* lock() / unlock()
q.v. Locking.
* optimize()
Recover lost disk space. This is important to do, especially if you
use transactions.
* import() / export()
Data going in and out.
* begin_work() / commit() / rollback()
These are the transactional functions. "TRANSACTIONS" for more
information.
Hashes
For hashes, DBM::Deep supports all the common methods described above,
and the following additional methods: "first_key()" and "next_key()".
* first_key()
Returns the "first" key in the hash. As with built-in Perl hashes,
keys are fetched in an undefined order (which appears random). Takes
no arguments, returns the key as a scalar value.
my $key = $db->first_key();
* next_key()
Returns the "next" key in the hash, given the previous one as the
sole argument. Returns undef if there are no more keys to be
fetched.
$key = $db->next_key($key);
Here are some examples of using hashes:
my $db = DBM::Deep->new( "foo.db" );
$db->put("foo", "bar");
print "foo: " . $db->get("foo") . "\n";
$db->put("baz", {}); # new child hash ref
$db->get("baz")->put("buz", "biz");
print "buz: " . $db->get("baz")->get("buz") . "\n";
my $key = $db->first_key();
while ($key) {
print "$key: " . $db->get($key) . "\n";
$key = $db->next_key($key);
}
if ($db->exists("foo")) { $db->delete("foo"); }
Arrays
For arrays, DBM::Deep supports all the common methods described above,
and the following additional methods: "length()", "push()", "pop()",
"shift()", "unshift()" and "splice()".
* length()
Returns the number of elements in the array. Takes no arguments.
my $len = $db->length();
* push()
Adds one or more elements onto the end of the array. Accepts
scalars, hash refs or array refs. No return value.
$db->push("foo", "bar", {});
* pop()
Fetches the last element in the array, and deletes it. Takes no
arguments. Returns undef if array is empty. Returns the element
value.
my $elem = $db->pop();
* shift()
Fetches the first element in the array, deletes it, then shifts all
the remaining elements over to take up the space. Returns the
element value. This method is not recommended with large arrays --
see "LARGE ARRAYS" below for details.
my $elem = $db->shift();
* unshift()
Inserts one or more elements onto the beginning of the array,
shifting all existing elements over to make room. Accepts scalars,
hash refs or array refs. No return value. This method is not
recommended with large arrays -- see <LARGE ARRAYS> below for
details.
$db->unshift("foo", "bar", {});
* splice()
Performs exactly like Perl's built-in function of the same name. See
"perldoc -f splice" for usage -- it is too complicated to document
here. This method is not recommended with large arrays -- see "LARGE
ARRAYS" below for details.
Here are some examples of using arrays:
my $db = DBM::Deep->new(
file => "foo.db",
type => DBM::Deep->TYPE_ARRAY
);
$db->push("bar", "baz");
$db->unshift("foo");
$db->put(3, "buz");
my $len = $db->length();
print "length: $len\n"; # 4
for (my $k=0; $k<$len; $k++) {
print "$k: " . $db->get($k) . "\n";
}
$db->splice(1, 2, "biz", "baf");
while (my $elem = shift @$db) {
print "shifted: $elem\n";
}
LOCKING
Enable or disable automatic file locking by passing a boolean value to
the "locking" parameter when constructing your DBM::Deep object (see
SETUP above).
my $db = DBM::Deep->new(
file => "foo.db",
locking => 1
);
This causes DBM::Deep to "flock()" the underlying filehandle with
exclusive mode for writes, and shared mode for reads. This is required
if you have multiple processes accessing the same database file, to
avoid file corruption. Please note that "flock()" does NOT work for
files over NFS. See "DB OVER NFS" below for more.
Explicit Locking
You can explicitly lock a database, so it remains locked for multiple
actions. This is done by calling the "lock()" method, and passing an
optional lock mode argument (defaults to exclusive mode). This is
particularly useful for things like counters, where the current value
needs to be fetched, then incremented, then stored again.
$db->lock();
my $counter = $db->get("counter");
$counter++;
$db->put("counter", $counter);
$db->unlock();
# or...
$db->lock();
$db->{counter}++;
$db->unlock();
You can pass "lock()" an optional argument, which specifies which mode
to use (exclusive or shared). Use one of these two constants:
"DBM::Deep->LOCK_EX" or "DBM::Deep->LOCK_SH". These are passed directly
to "flock()", and are the same as the constants defined in Perl's Fcntl
module.
$db->lock( $db->LOCK_SH );
# something here
$db->unlock();
IMPORTING/EXPORTING
You can import existing complex structures by calling the "import()"
method, and export an entire database into an in-memory structure using
the "export()" method. Both are examined here.
Importing
Say you have an existing hash with nested hashes/arrays inside it.
Instead of walking the structure and adding keys/elements to the
database as you go, simply pass a reference to the "import()" method.
This recursively adds everything to an existing DBM::Deep object for
you. Here is an example:
my $struct = {
key1 => "value1",
key2 => "value2",
array1 => [ "elem0", "elem1", "elem2" ],
hash1 => {
subkey1 => "subvalue1",
subkey2 => "subvalue2"
}
};
my $db = DBM::Deep->new( "foo.db" );
$db->import( $struct );
print $db->{key1} . "\n"; # prints "value1"
This recursively imports the entire $struct object into $db, including
all nested hashes and arrays. If the DBM::Deep object contains exsiting
data, keys are merged with the existing ones, replacing if they already
exist. The "import()" method can be called on any database level (not
just the base level), and works with both hash and array DB types.
Note: Make sure your existing structure has no circular references in
it. These will cause an infinite loop when importing. There are plans to
fix this in a later release.
Exporting
Calling the "export()" method on an existing DBM::Deep object will
return a reference to a new in-memory copy of the database. The export
is done recursively, so all nested hashes/arrays are all exported to
standard Perl objects. Here is an example:
my $db = DBM::Deep->new( "foo.db" );
$db->{key1} = "value1";
$db->{key2} = "value2";
$db->{hash1} = {};
$db->{hash1}->{subkey1} = "subvalue1";
$db->{hash1}->{subkey2} = "subvalue2";
my $struct = $db->export();
print $struct->{key1} . "\n"; # prints "value1"
This makes a complete copy of the database in memory, and returns a
reference to it. The "export()" method can be called on any database
level (not just the base level), and works with both hash and array DB
types. Be careful of large databases -- you can store a lot more data in
a DBM::Deep object than an in-memory Perl structure.
Note: Make sure your database has no circular references in it. These
will cause an infinite loop when exporting. There are plans to fix this
in a later release.
FILTERS
DBM::Deep has a number of hooks where you can specify your own Perl
function to perform filtering on incoming or outgoing data. This is a
perfect way to extend the engine, and implement things like real-time
compression or encryption. Filtering applies to the base DB level, and
all child hashes / arrays. Filter hooks can be specified when your
DBM::Deep object is first constructed, or by calling the "set_filter()"
method at any time. There are four available filter hooks, described
below:
* filter_store_key
This filter is called whenever a hash key is stored. It is passed
the incoming key, and expected to return a transformed key.
* filter_store_value
This filter is called whenever a hash key or array element is
stored. It is passed the incoming value, and expected to return a
transformed value.
* filter_fetch_key
This filter is called whenever a hash key is fetched (i.e. via
"first_key()" or "next_key()"). It is passed the transformed key,
and expected to return the plain key.
* filter_fetch_value
This filter is called whenever a hash key or array element is
fetched. It is passed the transformed value, and expected to return
the plain value.
Here are the two ways to setup a filter hook:
my $db = DBM::Deep->new(
file => "foo.db",
filter_store_value => \&my_filter_store,
filter_fetch_value => \&my_filter_fetch
);
# or...
$db->set_filter( "filter_store_value", \&my_filter_store );
$db->set_filter( "filter_fetch_value", \&my_filter_fetch );
Your filter function will be called only when dealing with SCALAR keys
or values. When nested hashes and arrays are being stored/fetched,
filtering is bypassed. Filters are called as static functions, passed a
single SCALAR argument, and expected to return a single SCALAR value. If
you want to remove a filter, set the function reference to "undef":
$db->set_filter( "filter_store_value", undef );
Real-time Encryption Example
Here is a working example that uses the *Crypt::Blowfish* module to do
real-time encryption / decryption of keys & values with DBM::Deep
Filters. Please visit
<
http://search.cpan.org/search?module=Crypt::Blowfish> for more on
*Crypt::Blowfish*. You'll also need the *Crypt::CBC* module.
use DBM::Deep;
use Crypt::Blowfish;
use Crypt::CBC;
my $cipher = Crypt::CBC->new({
'key' => 'my secret key',
'cipher' => 'Blowfish',
'iv' => '$KJh#(}q',
'regenerate_key' => 0,
'padding' => 'space',
'prepend_iv' => 0
});
my $db = DBM::Deep->new(
file => "foo-encrypt.db",
filter_store_key => \&my_encrypt,
filter_store_value => \&my_encrypt,
filter_fetch_key => \&my_decrypt,
filter_fetch_value => \&my_decrypt,
);
$db->{key1} = "value1";
$db->{key2} = "value2";
print "key1: " . $db->{key1} . "\n";
print "key2: " . $db->{key2} . "\n";
undef $db;
exit;
sub my_encrypt {
return $cipher->encrypt( $_[0] );
}
sub my_decrypt {
return $cipher->decrypt( $_[0] );
}
Real-time Compression Example
Here is a working example that uses the *Compress::Zlib* module to do
real-time compression / decompression of keys & values with DBM::Deep
Filters. Please visit
<
http://search.cpan.org/search?module=Compress::Zlib> for more on
*Compress::Zlib*.
use DBM::Deep;
use Compress::Zlib;
my $db = DBM::Deep->new(
file => "foo-compress.db",
filter_store_key => \&my_compress,
filter_store_value => \&my_compress,
filter_fetch_key => \&my_decompress,
filter_fetch_value => \&my_decompress,
);
$db->{key1} = "value1";
$db->{key2} = "value2";
print "key1: " . $db->{key1} . "\n";
print "key2: " . $db->{key2} . "\n";
undef $db;
exit;
sub my_compress {
return Compress::Zlib::memGzip( $_[0] ) ;
}
sub my_decompress {
return Compress::Zlib::memGunzip( $_[0] ) ;
}
Note: Filtering of keys only applies to hashes. Array "keys" are
actually numerical index numbers, and are not filtered.
ERROR HANDLING
Most DBM::Deep methods return a true value for success, and call die()
on failure. You can wrap calls in an eval block to catch the die.
my $db = DBM::Deep->new( "foo.db" ); # create hash
eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
print $@; # prints error message
LARGEFILE SUPPORT
If you have a 64-bit system, and your Perl is compiled with both
LARGEFILE and 64-bit support, you *may* be able to create databases
larger than 4 GB. DBM::Deep by default uses 32-bit file offset tags, but
these can be changed by specifying the 'pack_size' parameter when
constructing the file.
DBM::Deep->new(
filename => $filename,
pack_size => 'large',
);
This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad
words instead of 32-bit longs. After setting these values your DB files
have a theoretical maximum size of 16 XB (exabytes).
You can also use "pack_size => 'small'" in order to use 16-bit file
offsets.
Note: Changing these values will NOT work for existing database files.
Only change this for new files. Once the value has been set, it is
stored in the file's header and cannot be changed for the life of the
file. These parameters are per-file, meaning you can access 32-bit and
64-bit files, as you choose.
Note: We have not personally tested files larger than 4 GB -- all my
systems have only a 32-bit Perl. However, I have received user reports
that this does indeed work.
LOW-LEVEL ACCESS
If you require low-level access to the underlying filehandle that
DBM::Deep uses, you can call the "_fh()" method, which returns the
handle:
my $fh = $db->_fh();
This method can be called on the root level of the datbase, or any child
hashes or arrays. All levels share a *root* structure, which contains
things like the filehandle, a reference counter, and all the options
specified when you created the object. You can get access to this file
object by calling the "_storage()" method.
my $file_obj = $db->_storage();
This is useful for changing options after the object has already been
created, such as enabling/disabling locking. You can also store your own
temporary user data in this structure (be wary of name collision), which
is then accessible from any child hash or array.
CUSTOM DIGEST ALGORITHM
DBM::Deep by default uses the *Message Digest 5* (MD5) algorithm for
hashing keys. However you can override this, and use another algorithm
(such as SHA-256) or even write your own. But please note that DBM::Deep
currently expects zero collisions, so your algorithm has to be
*perfect*, so to speak. Collision detection may be introduced in a later
version.
You can specify a custom digest algorithm by passing it into the
parameter list for new(), passing a reference to a subroutine as the
'digest' parameter, and the length of the algorithm's hashes (in bytes)
as the 'hash_size' parameter. Here is a working example that uses a
256-bit hash from the *Digest::SHA256* module. Please see
<
http://search.cpan.org/search?module=Digest::SHA256> for more
information.
use DBM::Deep;
use Digest::SHA256;
my $context = Digest::SHA256::new(256);
my $db = DBM::Deep->new(
filename => "foo-sha.db",
digest => \&my_digest,
hash_size => 32,
);
$db->{key1} = "value1";
$db->{key2} = "value2";
print "key1: " . $db->{key1} . "\n";
print "key2: " . $db->{key2} . "\n";
undef $db;
exit;
sub my_digest {
return substr( $context->hash($_[0]), 0, 32 );
}
Note: Your returned digest strings must be EXACTLY the number of bytes
you specify in the hash_size parameter (in this case 32).
Note: If you do choose to use a custom digest algorithm, you must set it
every time you access this file. Otherwise, the default (MD5) will be
used.
CIRCULAR REFERENCES
NOTE: DBM::Deep 0.99_03 has turned off circular references pending
evaluation of some edge cases. I hope to be able to re-enable circular
references in a future version after 1.00. This means that circular
references are NO LONGER available.
DBM::Deep has experimental support for circular references. Meaning you
can have a nested hash key or array element that points to a parent
object. This relationship is stored in the DB file, and is preserved
between sessions. Here is an example:
my $db = DBM::Deep->new( "foo.db" );
$db->{foo} = "bar";
$db->{circle} = $db; # ref to self
print $db->{foo} . "\n"; # prints "bar"
print $db->{circle}->{foo} . "\n"; # prints "bar" again
Note: Passing the object to a function that recursively walks the object
tree (such as *Data::Dumper* or even the built-in "optimize()" or
"export()" methods) will result in an infinite loop. This will be fixed
in a future release.
TRANSACTIONS
New in 0.99_01 is ACID transactions. Every DBM::Deep object is
completely transaction-ready - it is not an option you have to turn on.
You do have to specify how many transactions may run simultaneously
(q.v. "num_txns").
Three new methods have been added to support them. They are:
* begin_work()
This starts a transaction.
* commit()
This applies the changes done within the transaction to the mainline
and ends the transaction.
* rollback()
This discards the changes done within the transaction to the
mainline and ends the transaction.
Transactions in DBM::Deep are done using a variant of the MVCC method,
the same method used by the InnoDB MySQL engine.
Software-Transactional Memory
The addition of transactions to this module provides the basis for STM
within Perl 5. Contention is resolved using a default last-write-wins.
Currently, this default cannot be changed, but it will be addressed in a
future version.
PERFORMANCE
Because DBM::Deep is a conncurrent datastore, every change is flushed to
disk immediately and every read goes to disk. This means that DBM::Deep
functions at the speed of disk (generally 10-20ms) vs. the speed of RAM
(generally 50-70ns), or at least 150-200x slower than the comparable
in-memory datastructure in Perl.
There are several techniques you can use to speed up how DBM::Deep
functions.
* Put it on a ramdisk
The easiest and quickest mechanism to making DBM::Deep run faster is
to create a ramdisk and locate the DBM::Deep file there. Doing this
as an option may become a feature of DBM::Deep, assuming there is a
good ramdisk wrapper on CPAN.
* Work at the tightest level possible
It is much faster to assign the level of your db that you are
working with to an intermediate variable than to re-look it up every
time. Thus
# BAD
while ( my ($k, $v) = each %{$db->{foo}{bar}{baz}} ) {
...
}
# GOOD
my $x = $db->{foo}{bar}{baz};
while ( my ($k, $v) = each %$x ) {
...
}
* Make your file as tight as possible
If you know that you are not going to use more than 65K in your
database, consider using the "pack_size => 'small'" option. This
will instruct DBM::Deep to use 16bit addresses, meaning that the
seek times will be less.
TODO
The following are items that are planned to be added in future releases.
These are separate from the "CAVEATS, ISSUES & BUGS" below.
Sub-Transactions
Right now, you cannot run a transaction within a transaction. Removing
this restriction is technically straightforward, but the combinatorial
explosion of possible usecases hurts my head. If this is something you
want to see immediately, please submit many testcases.
Caching
If a user is willing to assert upon opening the file that this process
will be the only consumer of that datafile, then there are a number of
caching possibilities that can be taken advantage of. This does,
however, mean that DBM::Deep is more vulnerable to losing data due to
unflushed changes. It also means a much larger in-memory footprint. As
such, it's not clear exactly how this should be done. Suggestions are
welcome.
Ram-only
The techniques used in DBM::Deep simply require a seekable contiguous
datastore. This could just as easily be a large string as a file. By
using substr, the STM capabilities of DBM::Deep could be used within a
single-process. I have no idea how I'd specify this, though. Suggestions
are welcome.
Importing using Data::Walker
Right now, importing is done using "Clone::clone()" to make a complete
copy in memory, then tying that copy. It would be much better to use
Data::Walker to walk the data structure instead, particularly in the
case of large datastructures.
Different contention resolution mechanisms
Currently, the only contention resolution mechanism is last-write-wins.
This is the mechanism used by most RDBMSes and should be good enough for
most uses. For advanced uses of STM, other contention mechanisms will be
needed. If you have an idea of how you'd like to see contention
resolution in DBM::Deep, please let me know.
CAVEATS, ISSUES & BUGS
This section describes all the known issues with DBM::Deep. These are
issues that are either intractable or depend on some feature within Perl
working exactly right. It you have found something that is not listed
below, please send an e-mail to
[email protected]. Likewise, if you think
you know of a way around one of these issues, please let me know.
References
(The following assumes a high level of Perl understanding, specifically
of references. Most users can safely skip this section.)
Currently, the only references supported are HASH and ARRAY. The other
reference types (SCALAR, CODE, GLOB, and REF) cannot be supported for
various reasons.
* GLOB
These are things like filehandles and other sockets. They can't be
supported because it's completely unclear how DBM::Deep should
serialize them.
* SCALAR / REF
The discussion here refers to the following type of example:
my $x = 25;
$db->{key1} = \$x;
$x = 50;
# In some other process ...
my $val = ${ $db->{key1} };
is( $val, 50, "What actually gets stored in the DB file?" );
The problem is one of synchronization. When the variable being
referred to changes value, the reference isn't notified, which is
kind of the point of references. This means that the new value won't
be stored in the datafile for other processes to read. There is no
TIEREF.
It is theoretically possible to store references to values already
within a DBM::Deep object because everything already is
synchronized, but the change to the internals would be quite large.
Specifically, DBM::Deep would have to tie every single value that is
stored. This would bloat the RAM footprint of DBM::Deep at least
twofold (if not more) and be a significant performance drain, all to
support a feature that has never been requested.
* CODE
Data::Dump::Streamer provides a mechanism for serializing coderefs,
including saving off all closure state. This would allow for
DBM::Deep to store the code for a subroutine. Then, whenever the
subroutine is read, the code could be "eval()"'ed into being.
However, just as for SCALAR and REF, that closure state may change
without notifying the DBM::Deep object storing the reference. Again,
this would generally be considered a feature.
File corruption
The current level of error handling in DBM::Deep is minimal. Files *are*
checked for a 32-bit signature when opened, but any other form of
corruption in the datafile can cause segmentation faults. DBM::Deep may
try to "seek()" past the end of a file, or get stuck in an infinite loop
depending on the level and type of corruption. File write operations are
not checked for failure (for speed), so if you happen to run out of disk
space, DBM::Deep will probably fail in a bad way. These things will be
addressed in a later version of DBM::Deep.
DB over NFS
Beware of using DBM::Deep files over NFS. DBM::Deep uses flock(), which
works well on local filesystems, but will NOT protect you from file
corruption over NFS. I've heard about setting up your NFS server with a
locking daemon, then using "lockf()" to lock your files, but your
mileage may vary there as well. From what I understand, there is no real
way to do it. However, if you need access to the underlying filehandle
in DBM::Deep for using some other kind of locking scheme like "lockf()",
see the "LOW-LEVEL ACCESS" section above.
Copying Objects
Beware of copying tied objects in Perl. Very strange things can happen.
Instead, use DBM::Deep's "clone()" method which safely copies the object
and returns a new, blessed and tied hash or array to the same level in
the DB.
my $copy = $db->clone();
Note: Since clone() here is cloning the object, not the database
location, any modifications to either $db or $copy will be visible to
both.
Large Arrays
Beware of using "shift()", "unshift()" or "splice()" with large arrays.
These functions cause every element in the array to move, which can be
murder on DBM::Deep, as every element has to be fetched from disk, then
stored again in a different location. This will be addressed in a future
version.
Writeonly Files
If you pass in a filehandle to new(), you may have opened it in either a
readonly or writeonly mode. STORE will verify that the filehandle is
writable. However, there doesn't seem to be a good way to determine if a
filehandle is readable. And, if the filehandle isn't readable, it's not
clear what will happen. So, don't do that.
Assignments Within Transactions
The following will *not* work as one might expect:
my $x = { a => 1 };
$db->begin_work;
$db->{foo} = $x;
$db->rollback;
is( $x->{a}, 1 ); # This will fail!
The problem is that the moment a reference used as the rvalue to a
DBM::Deep object's lvalue, it becomes tied itself. This is so that
future changes to $x can be tracked within the DBM::Deep file and is
considered to be a feature. By the time the rollback occurs, there is no
knowledge that there had been an $x or what memory location to assign an
"export()" to.
NOTE: This does not affect importing because imports do a walk over the
reference to be imported in order to explicitly leave it untied.
CODE COVERAGE
Devel::Cover is used to test the code coverage of the tests. Below is
the Devel::Cover report on this distribution's test suite.
---------------------------- ------ ------ ------ ------ ------ ------ ------
File stmt bran cond sub pod time total
---------------------------- ------ ------ ------ ------ ------ ------ ------
blib/lib/DBM/Deep.pm 96.8 87.9 90.5 100.0 89.5 4.5 95.2
blib/lib/DBM/Deep/Array.pm 100.0 94.3 100.0 100.0 100.0 4.8 98.7
blib/lib/DBM/Deep/Engine.pm 97.2 86.4 86.0 100.0 0.0 56.8 91.0
blib/lib/DBM/Deep/File.pm 98.1 83.3 66.7 100.0 0.0 31.4 88.0
blib/lib/DBM/Deep/Hash.pm 100.0 100.0 100.0 100.0 100.0 2.5 100.0
Total 97.7 88.1 86.6 100.0 31.6 100.0 93.0
---------------------------- ------ ------ ------ ------ ------ ------ ------
MORE INFORMATION
Check out the DBM::Deep Google Group at
<
http://groups.google.com/group/DBM-Deep> or send email to
[email protected]. You can also visit #dbm-deep on irc.perl.org
The source code repository is at <
http://svn.perl.org/modules/DBM-Deep>
MAINTAINER(S)
Rob Kinyon,
[email protected]
Originally written by Joseph Huckaby,
[email protected]
CONTRIBUTORS
The following have contributed greatly to make DBM::Deep what it is
today:
* Adam Sah and Rich Gaushell
* Stonehenge for sponsoring the 1.00 release
* Dan Golden and others at YAPC::NA 2006 for helping me design through
transactions.
SEE ALSO
perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3),
nfs(5), Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
LICENSE
Copyright (c) 2007 Rob Kinyon. All Rights Reserved. This is free
software, you may use it and distribute it under the same terms as Perl
itself.
