NAME
- DBM::Deep - A pure perl multi-level hash/array DBM
+ DBM::Deep - A pure perl multi-level hash/array DBM that supports
+ transactions
SYNOPSIS
use DBM::Deep;
- my $db = new DBM::Deep "foo.db";
-
- $db->{key} = 'value'; # tie() style
+ my $db = DBM::Deep->new( "foo.db" );
+
+ $db->{key} = 'value';
print $db->{key};
-
- $db->put('key', 'value'); # OO style
+
+ $db->put('key' => 'value');
print $db->get('key');
-
+
# true multi-level support
$db->{my_complex} = [
- 'hello', { perl => 'rules' },
- 42, 99 ];
+ '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, and quite fast. Can
- handle millions of keys and unlimited hash levels without significant
- slow-down. Written from the ground-up in pure perl -- this is NOT a
- wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
- Mac OS X and Windows.
-
-INSTALLATION
- Hopefully you are using CPAN's excellent Perl module, which will
- download and install the module for you. If not, get the tarball, and
- run these commands:
-
- tar zxf DBM-Deep-*
- cd DBM-Deep-*
- perl Makefile.PL
- make
- make test
- make install
+ / 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
+ OO Construction
The recommended way to construct a DBM::Deep object is to use the new()
- method, which gets you a blessed, tied hash or array reference.
+ method, which gets you a blessed *and* tied hash (or array) reference.
- my $db = new DBM::Deep "foo.db";
+ 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
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:
+ like locking, autoflush, etc. This is done by passing an inline hash (or
+ hashref):
- my $db = new DBM::Deep(
- file => "foo.db",
- locking => 1,
- autoflush => 1
- );
+ 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
You can also start with an array instead of a hash. For this, you must
specify the "type" parameter:
- my $db = new DBM::Deep(
- file => "foo.db",
- type => DBM::Deep::TYPE_ARRAY
- );
+ 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 ignored if it is passed
- to the constructor.
+ "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.
- TIE CONSTRUCTION
- Alternatively, you can create a DBM::Deep handle by using Perl's
- built-in tie() function. This is not ideal, because you get only a
- basic, tied hash (or array) which is not blessed, so you can't call any
- functions on it.
+ my %hash;
+ my $db = tie %hash, "DBM::Deep", "foo.db";
- my %hash;
- tie %hash, "DBM::Deep", "foo.db";
-
- my @array;
- tie @array, "DBM::Deep", "bar.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
- };
+ tie %hash, "DBM::Deep", {
+ file => "foo.db",
+ locking => 1,
+ autoflush => 1
+ };
- OPTIONS
+ 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.
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.
+ 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 } );
- * 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.
+ 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" or
- "DBM::Deep::TYPE_ARRAY". This only takes effect when beginning a new
- file. This is an optional parameter, and defaults to hash.
+ 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
- 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.
+ 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
+ filehandle. This obviously slows down write operations, but is
required if you may have multiple processes accessing the same DB
- file (also consider enable *locking* or at least *volatile*). Pass
- any true value to enable. This is an optional parameter, and
- defaults to 0 (disabled).
-
- * volatile
- If *volatile* mode is enabled, DBM::Deep will stat() the DB file
- before each STORE() operation. This is required if an outside force
- may change the size of the file between transactions. Locking also
- implicitly enables volatile. This is useful if you want to use a
- different locking system or write your own. Pass any true value to
- enable. This is an optional parameter, and defaults to 0 (disabled).
-
- * autobless
- If *autobless* mode is enabled, DBM::Deep will preserve blessed
- hashes, and restore them when fetched. This is an experimental
- feature, and does have side-effects. Basically, when hashes are
- re-blessed into their original classes, they are no longer blessed
- into the DBM::Deep class! So you won't be able to call any DBM::Deep
- methods on them. You have been warned. This is an optional
- parameter, and defaults to 0 (disabled).
+ 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.
-
- * debug
- Setting *debug* mode will make all errors non-fatal, dump them out
- to STDERR, and continue on. This is for debugging purposes only, and
- probably not what you want. This is an optional parameter, and
- defaults to 0 (disabled).
+ 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 Deep objects are *tied* to hashes or
- arrays, you can treat them as such. Deep will intercept all reads/writes
- and direct them to the right place -- the DB file. This has nothing to
- do with the "TIE CONSTRUCTION" section above. This simply tells you how
- to use DBM::Deep using regular hashes and arrays, rather than calling
- functions like "get()" and "put()" (although those work too). It is
- entirely up to you how to want to access your databases.
-
- HASHES
+ 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 = new DBM::Deep "foo.db";
-
- $db->{mykey} = "myvalue";
- $db->{myhash} = {};
- $db->{myhash}->{subkey} = "subvalue";
+ my $db = DBM::Deep->new( "foo.db" );
+
+ $db->{mykey} = "myvalue";
+ $db->{myhash} = {};
+ $db->{myhash}->{subkey} = "subvalue";
- print $db->{myhash}->{subkey} . "\n";
+ 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";
- }
+ foreach my $key (keys %$db) {
+ print "$key: " . $db->{$key} . "\n";
+ }
Remember that Perl's "keys()" function extracts *every* key from the
hash and pushes them onto an array, all before the loop even begins. If
- you have an extra large hash, this may exhaust Perl's memory. Instead,
- consider using Perl's "each()" function, which pulls keys/values one at
- a time, using very little memory:
+ 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: $value\n";
- }
+ while (my ($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
+ # 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 $db handle, resulting in a "new" hash for foo
over again. Instead, assign a temporary variable to "$db-"{foo}>, then
pass that to each().
- ARRAYS
+ 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
+ "DBM::Deep->TYPE_ARRAY", or simply be a nested array reference inside a
hash. Example:
- my $db = new DBM::Deep(
- file => "foo-array.db",
- type => DBM::Deep::TYPE_ARRAY
- );
-
- $db->[0] = "foo";
- push @$db, "bar", "baz";
- unshift @$db, "bah";
-
- my $last_elem = pop @$db; # baz
- my $first_elem = shift @$db; # bah
- my $second_elem = $db->[1]; # bar
-
- my $num_elements = scalar @$db;
+ 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()".
+ "clear()". "fetch()" and "store(" are aliases to "put()" and "get()",
+ respectively.
- * put()
+ * 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
+ $db->put("foo", "bar"); # for hashes
+ $db->put(1, "bar"); # for arrays
- * get()
+ * 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
+ 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 ($db->exists(1)) { print "yay!\n"; } # for arrays
+ if ($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
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.
+ arrays work.
- $db->delete("foo"); # for hashes
- $db->delete(1); # for arrays
+ $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.
+ return value.
+
+ $db->clear(); # hashes or arrays
- $db->clear(); # hashes or arrays
+ * lock() / unlock()
+ q.v. Locking.
- HASHES
+ * 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()".
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();
+ 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);
+ $key = $db->next_key($key);
Here are some examples of using hashes:
- my $db = new DBM::Deep "foo.db";
-
- $db->put("foo", "bar");
- print "foo: " . $db->get("foo") . "\n";
-
- $db->put("baz", {}); # new child hash ref
- $db->get("baz")->put("buz", "biz");
- print "buz: " . $db->get("baz")->get("buz") . "\n";
-
- my $key = $db->first_key();
- while ($key) {
- print "$key: " . $db->get($key) . "\n";
- $key = $db->next_key($key);
- }
-
- if ($db->exists("foo")) { $db->delete("foo"); }
-
- ARRAYS
+ 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();
+ 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", {});
+ $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();
+ my $elem = $db->pop();
* shift()
Fetches the first element in the array, deletes it, then shifts all
element value. This method is not recommended with large arrays --
see "LARGE ARRAYS" below for details.
- my $elem = $db->shift();
+ my $elem = $db->shift();
* unshift()
Inserts one or more elements onto the beginning of the array,
recommended with large arrays -- see <LARGE ARRAYS> below for
details.
- $db->unshift("foo", "bar", {});
+ $db->unshift("foo", "bar", {});
* splice()
Performs exactly like Perl's built-in function of the same name. See
Here are some examples of using arrays:
- my $db = new DBM::Deep(
- file => "foo.db",
- type => DBM::Deep::TYPE_ARRAY
- );
-
- $db->push("bar", "baz");
- $db->unshift("foo");
- $db->put(3, "buz");
-
- my $len = $db->length();
- print "length: $len\n"; # 4
-
- for (my $k=0; $k<$len; $k++) {
- print "$k: " . $db->get($k) . "\n";
- }
-
- $db->splice(1, 2, "biz", "baf");
-
- while (my $elem = shift @$db) {
- print "shifted: $elem\n";
- }
+ 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 automatic file locking by passing a true value to the "locking"
- parameter when constructing your DBM::Deep object (see SETUP above).
+ 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 = new DBM::Deep(
- file => "foo.db",
- locking => 1
- );
+ my $db = DBM::Deep->new(
+ file => "foo.db",
+ locking => 1
+ );
- This causes Deep to "flock()" the underlying FileHandle object with
+ 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
+ Explicit Locking
You can explicitly lock a database, so it remains locked for multiple
- transactions. This is done by calling the "lock()" method, and passing
- an optional lock mode argument (defaults to exclusive mode). This is
+ 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();
+ $db->lock();
+ my $counter = $db->get("counter");
+ $counter++;
+ $db->put("counter", $counter);
+ $db->unlock();
+
+ # or...
- # or...
-
- $db->lock();
- $db->{counter}++;
- $db->unlock();
+ $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.
+ "DBM::Deep->LOCK_EX" or "DBM::Deep->LOCK_SH". These are passed directly
+ to "flock()", and are the same as the constants defined in Perl's Fcntl
+ module.
- $db->lock( DBM::Deep::LOCK_SH );
- # something here
- $db->unlock();
-
- If you want to implement your own file locking scheme, be sure to create
- your DBM::Deep objects setting the "volatile" option to true. This hints
- to Deep that the DB file may change between transactions. See "LOW-LEVEL
- ACCESS" below for more.
+ $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
+ Importing
Say you have an existing hash with nested hashes/arrays inside it.
Instead of walking the structure and adding keys/elements to the
database as you go, simply pass a reference to the "import()" method.
This recursively adds everything to an existing DBM::Deep object for
you. Here is an example:
- my $struct = {
- key1 => "value1",
- key2 => "value2",
- array1 => [ "elem0", "elem1", "elem2" ],
- hash1 => {
- subkey1 => "subvalue1",
- subkey2 => "subvalue2"
- }
- };
-
- my $db = new DBM::Deep "foo.db";
- $db->import( $struct );
-
- print $db->{key1} . "\n"; # prints "value1"
+ 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
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.
+ it. These will cause an infinite loop when importing. There are plans to
+ fix this in a later release.
- EXPORTING
+ Exporting
Calling the "export()" method on an existing DBM::Deep object will
return a reference to a new in-memory copy of the database. The export
is done recursively, so all nested hashes/arrays are all exported to
standard Perl objects. Here is an example:
- my $db = new DBM::Deep "foo.db";
-
- $db->{key1} = "value1";
- $db->{key2} = "value2";
- $db->{hash1} = {};
- $db->{hash1}->{subkey1} = "subvalue1";
- $db->{hash1}->{subkey2} = "subvalue2";
-
- my $struct = $db->export();
-
- print $struct->{key1} . "\n"; # prints "value1"
+ 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
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.
+ 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
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 );
+ 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,
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 );
+ $db->set_filter( "filter_store_value", undef );
- REAL-TIME ENCRYPTION EXAMPLE
+ Real-time Encryption Example
Here is a working example that uses the *Crypt::Blowfish* module to do
real-time encryption / decryption of keys & values with DBM::Deep
Filters. Please visit
<http://search.cpan.org/search?module=Crypt::Blowfish> for more on
*Crypt::Blowfish*. You'll also need the *Crypt::CBC* module.
- use DBM::Deep;
- use Crypt::Blowfish;
- use Crypt::CBC;
-
- my $cipher = new Crypt::CBC({
- 'key' => 'my secret key',
- 'cipher' => 'Blowfish',
- 'iv' => '$KJh#(}q',
- 'regenerate_key' => 0,
- 'padding' => 'space',
- 'prepend_iv' => 0
- });
-
- my $db = new DBM::Deep(
- file => "foo-encrypt.db",
- filter_store_key => \&my_encrypt,
- filter_store_value => \&my_encrypt,
- filter_fetch_key => \&my_decrypt,
- filter_fetch_value => \&my_decrypt,
- );
-
- $db->{key1} = "value1";
- $db->{key2} = "value2";
- print "key1: " . $db->{key1} . "\n";
- print "key2: " . $db->{key2} . "\n";
-
- undef $db;
- exit;
-
- sub my_encrypt {
- return $cipher->encrypt( $_[0] );
- }
- sub my_decrypt {
- return $cipher->decrypt( $_[0] );
- }
-
- REAL-TIME COMPRESSION EXAMPLE
+ 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 = new DBM::Deep(
- file => "foo-compress.db",
- filter_store_key => \&my_compress,
- filter_store_value => \&my_compress,
- filter_fetch_key => \&my_decompress,
- filter_fetch_value => \&my_decompress,
- );
-
- $db->{key1} = "value1";
- $db->{key2} = "value2";
- print "key1: " . $db->{key1} . "\n";
- print "key2: " . $db->{key2} . "\n";
-
- undef $db;
- exit;
-
- sub my_compress {
- return Compress::Zlib::memGzip( $_[0] ) ;
- }
- sub my_decompress {
- return Compress::Zlib::memGunzip( $_[0] ) ;
- }
+ 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. Also,
- the actual error message is stored in an internal scalar, which can be
- fetched by calling the "error()" method.
+ on failure. You can wrap calls in an eval block to catch the die.
- my $db = new DBM::Deep "foo.db"; # create hash
- eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
-
- print $db->error(); # prints error message
+ my $db = DBM::Deep->new( "foo.db" ); # create hash
+ eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
- You can then call "clear_error()" to clear the current error state.
-
- $db->clear_error();
-
- If you set the "debug" option to true when creating your DBM::Deep
- object, all errors are considered NON-FATAL, and dumped to STDERR. This
- is only for debugging purposes.
+ 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 2 GB. DBM::Deep by default uses 32-bit file offset tags, but
- these can be changed by calling the static "set_pack()" method before
- you do anything else.
+ 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::set_pack(8, 'Q');
+ 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, and make sure it stays set consistently
- throughout the file's life. If you do set these values, you can no
- longer access 32-bit DB files. You can, however, call "set_pack(4, 'N')"
- to change back to 32-bit mode.
+ 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: I have not personally tested files > 2 GB -- all my systems have
- only a 32-bit Perl. However, I have received user reports that this does
- indeed work!
+ 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 Deep
- uses, you can call the "fh()" method, which returns the handle:
+ 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();
+ my $fh = $db->_fh();
This method can be called on the root level of the datbase, or any child
hashes or arrays. All levels share a *root* structure, which contains
- things like the FileHandle, a reference counter, and all your options
- you specified when you created the object. You can get access to this
- root structure by calling the "root()" method.
+ 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 $root = $db->root();
+ my $file_obj = $db->_storage();
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.
+ 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 Deep
+ (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 calling the static
- "set_digest()" function, passing a reference to a subroutine, and the
- length of the algorithm's hashes (in bytes). This is a global static
- function, which affects ALL Deep objects. Here is a working example that
- uses a 256-bit hash from the *Digest::SHA256* module. Please see
- <http://search.cpan.org/search?module=Digest::SHA256> for more.
-
- use DBM::Deep;
- use Digest::SHA256;
-
- my $context = Digest::SHA256::new(256);
-
- DBM::Deep::set_digest( \&my_digest, 32 );
-
- my $db = new DBM::Deep "foo-sha.db";
-
- $db->{key1} = "value1";
- $db->{key2} = "value2";
- print "key1: " . $db->{key1} . "\n";
- print "key2: " . $db->{key2} . "\n";
-
- undef $db;
- exit;
-
- sub my_digest {
- return substr( $context->hash($_[0]), 0, 32 );
- }
+ 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 "set_digest()" function (in this case 32).
+ 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 = new DBM::Deep "foo.db";
-
- $db->{foo} = "bar";
- $db->{circle} = $db; # ref to self
-
- print $db->{foo} . "\n"; # prints "foo"
- print $db->{circle}->{foo} . "\n"; # prints "foo" again
-
- One catch is, passing the object to a function that recursively walks
- the object tree (such as *Data::Dumper* or even the built-in
- "optimize()" or "export()" methods) will result in an infinite loop. The
- other catch is, if you fetch the *key* of a circular reference (i.e.
- using the "first_key()" or "next_key()" methods), you will get the
- *target object's key*, not the ref's key. This gets even more
- interesting with the above example, where the *circle* key points to the
- base DB object, which technically doesn't have a key. So I made
- DBM::Deep return "[base]" as the key name in that special case.
-
-CAVEATS / ISSUES / BUGS
- This section describes all the known issues with DBM::Deep. It you have
- found something that is not listed here, please send e-mail to
- jhuckaby@cpan.org.
-
- UNUSED SPACE RECOVERY
- One major caveat with Deep is that space occupied by existing keys and
- values is not recovered when they are deleted. Meaning if you keep
- deleting and adding new keys, your file will continuously grow. I am
- working on this, but in the meantime you can call the built-in
- "optimize()" method from time to time (perhaps in a crontab or
- something) to recover all your unused space.
-
- $db->optimize(); # returns true on success
-
- This rebuilds the ENTIRE database into a new file, then moves it on top
- of the original. The new file will have no unused space, thus it will
- take up as little disk space as possible. Please note that this
- operation can take a long time for large files, and you need enough disk
- space to temporarily hold 2 copies of your DB file. The temporary file
- is created in the same directory as the original, named with a ".tmp"
- extension, and is deleted when the operation completes. Oh, and if
- locking is enabled, the DB is automatically locked for the entire
- duration of the copy.
-
- WARNING: Only call optimize() on the top-level node of the database, and
- make sure there are no child references lying around. Deep keeps a
- reference counter, and if it is greater than 1, optimize() will abort
- and return undef.
-
- AUTOVIVIFICATION
- Unfortunately, autovivification doesn't work with tied hashes. This
- appears to be a bug in Perl's tie() system, as *Jakob Schmidt*
- encountered the very same issue with his *DWH_FIle* module (see
- <http://search.cpan.org/search?module=DWH_File>), and it is also
- mentioned in the BUGS section for the *MLDBM* module <see
- <http://search.cpan.org/search?module=MLDBM>). Basically, on a new db
- file, this does not work:
-
- $db->{foo}->{bar} = "hello";
-
- Since "foo" doesn't exist, you cannot add "bar" to it. You end up with
- "foo" being an empty hash. Try this instead, which works fine:
-
- $db->{foo} = { bar => "hello" };
-
- As of Perl 5.8.7, this bug still exists. I have walked very carefully
- through the execution path, and Perl indeed passes an empty hash to the
- STORE() method. Probably a bug in Perl.
-
- FILE CORRUPTION
- The current level of error handling in Deep is minimal. Files *are*
- checked for a 32-bit signature on open(), but other corruption in files
- can cause segmentation faults. Deep may try to seek() past the end of a
- file, or get stuck in an infinite loop depending on the level of
- corruption. File write operations are not checked for failure (for
- speed), so if you happen to run out of disk space, Deep will probably
- fail in a bad way. These things will be addressed in a later version of
- DBM::Deep.
-
- DB OVER NFS
- Beware of using DB files over NFS. Deep uses flock(), which works well
- on local filesystems, but will NOT protect you from file corruption over
- NFS. I've heard about setting up your NFS server with a locking daemon,
- then using lockf() to lock your files, but your milage may vary there as
- well. From what I understand, there is no real way to do it. However, if
- you need access to the underlying FileHandle in Deep for using some
- other kind of locking scheme like lockf(), see the "LOW-LEVEL ACCESS"
- section above.
-
- COPYING OBJECTS
+ my $db = DBM::Deep->new( "foo.db" );
+
+ $db->{foo} = "bar";
+ $db->{circle} = $db; # ref to self
+
+ print $db->{foo} . "\n"; # prints "bar"
+ print $db->{circle}->{foo} . "\n"; # prints "bar" again
+
+ Note: Passing the object to a function that recursively walks the object
+ tree (such as *Data::Dumper* or even the built-in "optimize()" or
+ "export()" methods) will result in an infinite loop. This will be fixed
+ in a future release.
+
+TRANSACTIONS
+ New in 0.99_01 is ACID transactions. Every DBM::Deep object is
+ completely transaction-ready - it is not an option you have to turn on.
+ You do have to specify how many transactions may run simultaneously
+ (q.v. "num_txns").
+
+ Three new methods have been added to support them. They are:
+
+ * begin_work()
+ This starts a transaction.
+
+ * commit()
+ This applies the changes done within the transaction to the mainline
+ and ends the transaction.
+
+ * rollback()
+ This discards the changes done within the transaction to the
+ mainline and ends the transaction.
+
+ Transactions in DBM::Deep are done using a variant of the MVCC method,
+ the same method used by the InnoDB MySQL engine.
+
+ Software-Transactional Memory
+ The addition of transactions to this module provides the basis for STM
+ within Perl 5. Contention is resolved using a default last-write-wins.
+ Currently, this default cannot be changed, but it will be addressed in a
+ future version.
+
+PERFORMANCE
+ Because DBM::Deep is a conncurrent datastore, every change is flushed to
+ disk immediately and every read goes to disk. This means that DBM::Deep
+ functions at the speed of disk (generally 10-20ms) vs. the speed of RAM
+ (generally 50-70ns), or at least 150-200x slower than the comparable
+ in-memory datastructure in Perl.
+
+ There are several techniques you can use to speed up how DBM::Deep
+ functions.
+
+ * Put it on a ramdisk
+ The easiest and quickest mechanism to making DBM::Deep run faster is
+ to create a ramdisk and locate the DBM::Deep file there. Doing this
+ as an option may become a feature of DBM::Deep, assuming there is a
+ good ramdisk wrapper on CPAN.
+
+ * Work at the tightest level possible
+ It is much faster to assign the level of your db that you are
+ working with to an intermediate variable than to re-look it up every
+ time. Thus
+
+ # BAD
+ while ( my ($k, $v) = each %{$db->{foo}{bar}{baz}} ) {
+ ...
+ }
+
+ # GOOD
+ my $x = $db->{foo}{bar}{baz};
+ while ( my ($k, $v) = each %$x ) {
+ ...
+ }
+
+ * Make your file as tight as possible
+ If you know that you are not going to use more than 65K in your
+ database, consider using the "pack_size => 'small'" option. This
+ will instruct DBM::Deep to use 16bit addresses, meaning that the
+ seek times will be less.
+
+TODO
+ The following are items that are planned to be added in future releases.
+ These are separate from the "CAVEATS, ISSUES & BUGS" below.
+
+ Sub-Transactions
+ Right now, you cannot run a transaction within a transaction. Removing
+ this restriction is technically straightforward, but the combinatorial
+ explosion of possible usecases hurts my head. If this is something you
+ want to see immediately, please submit many testcases.
+
+ Caching
+ If a user is willing to assert upon opening the file that this process
+ will be the only consumer of that datafile, then there are a number of
+ caching possibilities that can be taken advantage of. This does,
+ however, mean that DBM::Deep is more vulnerable to losing data due to
+ unflushed changes. It also means a much larger in-memory footprint. As
+ such, it's not clear exactly how this should be done. Suggestions are
+ welcome.
+
+ Ram-only
+ The techniques used in DBM::Deep simply require a seekable contiguous
+ datastore. This could just as easily be a large string as a file. By
+ using substr, the STM capabilities of DBM::Deep could be used within a
+ single-process. I have no idea how I'd specify this, though. Suggestions
+ are welcome.
+
+ Importing using Data::Walker
+ Right now, importing is done using "Clone::clone()" to make a complete
+ copy in memory, then tying that copy. It would be much better to use
+ Data::Walker to walk the data structure instead, particularly in the
+ case of large datastructures.
+
+ Different contention resolution mechanisms
+ Currently, the only contention resolution mechanism is last-write-wins.
+ This is the mechanism used by most RDBMSes and should be good enough for
+ most uses. For advanced uses of STM, other contention mechanisms will be
+ needed. If you have an idea of how you'd like to see contention
+ resolution in DBM::Deep, please let me know.
+
+CAVEATS, ISSUES & BUGS
+ This section describes all the known issues with DBM::Deep. These are
+ issues that are either intractable or depend on some feature within Perl
+ working exactly right. It you have found something that is not listed
+ below, please send an e-mail to rkinyon@cpan.org. Likewise, if you think
+ you know of a way around one of these issues, please let me know.
+
+ References
+ (The following assumes a high level of Perl understanding, specifically
+ of references. Most users can safely skip this section.)
+
+ Currently, the only references supported are HASH and ARRAY. The other
+ reference types (SCALAR, CODE, GLOB, and REF) cannot be supported for
+ various reasons.
+
+ * GLOB
+ These are things like filehandles and other sockets. They can't be
+ supported because it's completely unclear how DBM::Deep should
+ serialize them.
+
+ * SCALAR / REF
+ The discussion here refers to the following type of example:
+
+ my $x = 25;
+ $db->{key1} = \$x;
+
+ $x = 50;
+
+ # In some other process ...
+
+ my $val = ${ $db->{key1} };
+
+ is( $val, 50, "What actually gets stored in the DB file?" );
+
+ The problem is one of synchronization. When the variable being
+ referred to changes value, the reference isn't notified, which is
+ kind of the point of references. This means that the new value won't
+ be stored in the datafile for other processes to read. There is no
+ TIEREF.
+
+ It is theoretically possible to store references to values already
+ within a DBM::Deep object because everything already is
+ synchronized, but the change to the internals would be quite large.
+ Specifically, DBM::Deep would have to tie every single value that is
+ stored. This would bloat the RAM footprint of DBM::Deep at least
+ twofold (if not more) and be a significant performance drain, all to
+ support a feature that has never been requested.
+
+ * CODE
+ Data::Dump::Streamer provides a mechanism for serializing coderefs,
+ including saving off all closure state. This would allow for
+ DBM::Deep to store the code for a subroutine. Then, whenever the
+ subroutine is read, the code could be "eval()"'ed into being.
+ However, just as for SCALAR and REF, that closure state may change
+ without notifying the DBM::Deep object storing the reference. Again,
+ this would generally be considered a feature.
+
+ File corruption
+ The current level of error handling in DBM::Deep is minimal. Files *are*
+ checked for a 32-bit signature when opened, but any other form of
+ corruption in the datafile can cause segmentation faults. DBM::Deep may
+ try to "seek()" past the end of a file, or get stuck in an infinite loop
+ depending on the level and type of corruption. File write operations are
+ not checked for failure (for speed), so if you happen to run out of disk
+ space, DBM::Deep will probably fail in a bad way. These things will be
+ addressed in a later version of DBM::Deep.
+
+ DB over NFS
+ Beware of using DBM::Deep files over NFS. DBM::Deep uses flock(), which
+ works well on local filesystems, but will NOT protect you from file
+ corruption over NFS. I've heard about setting up your NFS server with a
+ locking daemon, then using "lockf()" to lock your files, but your
+ mileage may vary there as well. From what I understand, there is no real
+ way to do it. However, if you need access to the underlying filehandle
+ in DBM::Deep for using some other kind of locking scheme like "lockf()",
+ see the "LOW-LEVEL ACCESS" section above.
+
+ Copying Objects
Beware of copying tied objects in Perl. Very strange things can happen.
- Instead, use Deep's "clone()" method which safely copies the object and
- returns a new, blessed, tied hash or array to the same level in the DB.
+ 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();
- 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
+ Large Arrays
Beware of using "shift()", "unshift()" or "splice()" with large arrays.
These functions cause every element in the array to move, which can be
murder on DBM::Deep, as every element has to be fetched from disk, then
- stored again in a different location. This may be addressed in a later
+ stored again in a different location. This will be addressed in a future
version.
-PERFORMANCE
- This section discusses DBM::Deep's speed and memory usage.
-
- SPEED
- Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs,
- such as the almighty *BerkeleyDB*. But it makes up for it in features
- like true multi-level hash/array support, and cross-platform FTPable
- files. Even so, DBM::Deep is still pretty fast, and the speed stays
- fairly consistent, even with huge databases. Here is some test data:
-
- Adding 1,000,000 keys to new DB file...
-
- At 100 keys, avg. speed is 2,703 keys/sec
- At 200 keys, avg. speed is 2,642 keys/sec
- At 300 keys, avg. speed is 2,598 keys/sec
- At 400 keys, avg. speed is 2,578 keys/sec
- At 500 keys, avg. speed is 2,722 keys/sec
- At 600 keys, avg. speed is 2,628 keys/sec
- At 700 keys, avg. speed is 2,700 keys/sec
- At 800 keys, avg. speed is 2,607 keys/sec
- At 900 keys, avg. speed is 2,190 keys/sec
- At 1,000 keys, avg. speed is 2,570 keys/sec
- At 2,000 keys, avg. speed is 2,417 keys/sec
- At 3,000 keys, avg. speed is 1,982 keys/sec
- At 4,000 keys, avg. speed is 1,568 keys/sec
- At 5,000 keys, avg. speed is 1,533 keys/sec
- At 6,000 keys, avg. speed is 1,787 keys/sec
- At 7,000 keys, avg. speed is 1,977 keys/sec
- At 8,000 keys, avg. speed is 2,028 keys/sec
- At 9,000 keys, avg. speed is 2,077 keys/sec
- At 10,000 keys, avg. speed is 2,031 keys/sec
- At 20,000 keys, avg. speed is 1,970 keys/sec
- At 30,000 keys, avg. speed is 2,050 keys/sec
- At 40,000 keys, avg. speed is 2,073 keys/sec
- At 50,000 keys, avg. speed is 1,973 keys/sec
- At 60,000 keys, avg. speed is 1,914 keys/sec
- At 70,000 keys, avg. speed is 2,091 keys/sec
- At 80,000 keys, avg. speed is 2,103 keys/sec
- At 90,000 keys, avg. speed is 1,886 keys/sec
- At 100,000 keys, avg. speed is 1,970 keys/sec
- At 200,000 keys, avg. speed is 2,053 keys/sec
- At 300,000 keys, avg. speed is 1,697 keys/sec
- At 400,000 keys, avg. speed is 1,838 keys/sec
- At 500,000 keys, avg. speed is 1,941 keys/sec
- At 600,000 keys, avg. speed is 1,930 keys/sec
- At 700,000 keys, avg. speed is 1,735 keys/sec
- At 800,000 keys, avg. speed is 1,795 keys/sec
- At 900,000 keys, avg. speed is 1,221 keys/sec
- At 1,000,000 keys, avg. speed is 1,077 keys/sec
-
- This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 &
- Perl 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash
- keys and values were between 6 - 12 chars in length. The DB file ended
- up at 210MB. Run time was 12 min 3 sec.
-
- MEMORY USAGE
- One of the great things about DBM::Deep is that it uses very little
- memory. Even with huge databases (1,000,000+ keys) you will not see much
- increased memory on your process. Deep relies solely on the filesystem
- for storing and fetching data. Here is output from */usr/bin/top* before
- even opening a database handle:
-
- PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
- 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
-
- Basically the process is taking 2,716K of memory. And here is the same
- process after storing and fetching 1,000,000 keys:
-
- PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
- 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
-
- Notice the memory usage increased by only 56K. Test was performed on a
- 700mHz x86 box running Linux RedHat 7.2 & Perl 5.6.1.
-
-DB FILE FORMAT
- In case you were interested in the underlying DB file format, it is
- documented here in this section. You don't need to know this to use the
- module, it's just included for reference.
-
- SIGNATURE
- DBM::Deep files always start with a 32-bit signature to identify the
- file type. This is at offset 0. The signature is "DPDB" in network byte
- order. This is checked upon each file open().
-
- TAG
- The DBM::Deep file is in a *tagged format*, meaning each section of the
- file has a standard header containing the type of data, the length of
- data, and then the data itself. The type is a single character (1 byte),
- the length is a 32-bit unsigned long in network byte order, and the data
- is, well, the data. Here is how it unfolds:
-
- MASTER INDEX
- Immediately after the 32-bit file signature is the *Master Index*
- record. This is a standard tag header followed by 1024 bytes (in 32-bit
- mode) or 2048 bytes (in 64-bit mode) of data. The type is *H* for hash
- or *A* for array, depending on how the DBM::Deep object was constructed.
-
- The index works by looking at a *MD5 Hash* of the hash key (or array
- index number). The first 8-bit char of the MD5 signature is the offset
- into the index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The
- value of the index element is a file offset of the next tag for the
- key/element in question, which is usually a *Bucket List* tag (see
- below).
-
- The next tag *could* be another index, depending on how many
- keys/elements exist. See RE-INDEXING below for details.
-
- BUCKET LIST
- A *Bucket List* is a collection of 16 MD5 hashes for keys/elements, plus
- file offsets to where the actual data is stored. It starts with a
- standard tag header, with type *B*, and a data size of 320 bytes in
- 32-bit mode, or 384 bytes in 64-bit mode. Each MD5 hash is stored in
- full (16 bytes), plus the 32-bit or 64-bit file offset for the *Bucket*
- containing the actual data. When the list fills up, a *Re-Index*
- operation is performed (See RE-INDEXING below).
-
- BUCKET
- A *Bucket* is a tag containing a key/value pair (in hash mode), or a
- index/value pair (in array mode). It starts with a standard tag header
- with type *D* for scalar data (string, binary, etc.), or it could be a
- nested hash (type *H*) or array (type *A*). The value comes just after
- the tag header. The size reported in the tag header is only for the
- value, but then, just after the value is another size (32-bit unsigned
- long) and then the plain key itself. Since the value is likely to be
- fetched more often than the plain key, I figured it would be *slightly*
- faster to store the value first.
-
- If the type is *H* (hash) or *A* (array), the value is another *Master
- Index* record for the nested structure, where the process begins all
- over again.
-
- RE-INDEXING
- After a *Bucket List* grows to 16 records, its allocated space in the
- file is exhausted. Then, when another key/element comes in, the list is
- converted to a new index record. However, this index will look at the
- next char in the MD5 hash, and arrange new Bucket List pointers
- accordingly. This process is called *Re-Indexing*. Basically, a new
- index tag is created at the file EOF, and all 17 (16 + new one)
- keys/elements are removed from the old Bucket List and inserted into the
- new index. Several new Bucket Lists are created in the process, as a new
- MD5 char from the key is being examined (it is unlikely that the keys
- will all share the same next char of their MD5s).
-
- Because of the way the *MD5* algorithm works, it is impossible to tell
- exactly when the Bucket Lists will turn into indexes, but the first
- round tends to happen right around 4,000 keys. You will see a *slight*
- decrease in performance here, but it picks back up pretty quick (see
- SPEED above). Then it takes a lot more keys to exhaust the next level of
- Bucket Lists. It's right around 900,000 keys. This process can continue
- nearly indefinitely -- right up until the point the *MD5* signatures
- start colliding with each other, and this is EXTREMELY rare -- like
- winning the lottery 5 times in a row AND getting struck by lightning
- while you are walking to cash in your tickets. Theoretically, since
- *MD5* hashes are 128-bit values, you *could* have up to
- 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I
- believe this is 340 unodecillion, but don't quote me).
-
- STORING
- When a new key/element is stored, the key (or index number) is first ran
- through *Digest::MD5* to get a 128-bit signature (example, in hex:
- b05783b0773d894396d475ced9d2f4f6). Then, the *Master Index* record is
- checked for the first char of the signature (in this case *b*). If it
- does not exist, a new *Bucket List* is created for our key (and the next
- 15 future keys that happen to also have *b* as their first MD5 char).
- The entire MD5 is written to the *Bucket List* along with the offset of
- the new *Bucket* record (EOF at this point, unless we are replacing an
- existing *Bucket*), where the actual data will be stored.
-
- FETCHING
- Fetching an existing key/element involves getting a *Digest::MD5* of the
- key (or index number), then walking along the indexes. If there are
- enough keys/elements in this DB level, there might be nested indexes,
- each linked to a particular char of the MD5. Finally, a *Bucket List* is
- pointed to, which contains up to 16 full MD5 hashes. Each is checked for
- equality to the key in question. If we found a match, the *Bucket* tag
- is loaded, where the value and plain key are stored.
-
- Fetching the plain key occurs when calling the *first_key()* and
- *next_key()* methods. In this process the indexes are walked
- systematically, and each key fetched in increasing MD5 order (which is
- why it appears random). Once the *Bucket* is found, the value is skipped
- the plain key returned instead. Note: Do not count on keys being fetched
- as if the MD5 hashes were alphabetically sorted. This only happens on an
- index-level -- as soon as the *Bucket Lists* are hit, the keys will come
- out in the order they went in -- so it's pretty much undefined how the
- keys will come out -- just like Perl's built-in hashes.
-
-AUTHOR
- Joseph Huckaby, jhuckaby@cpan.org
-
- Special thanks to Adam Sah and Rich Gaushell! You know why :-)
+ Writeonly Files
+ If you pass in a filehandle to new(), you may have opened it in either a
+ readonly or writeonly mode. STORE will verify that the filehandle is
+ writable. However, there doesn't seem to be a good way to determine if a
+ filehandle is readable. And, if the filehandle isn't readable, it's not
+ clear what will happen. So, don't do that.
+
+ Assignments Within Transactions
+ The following will *not* work as one might expect:
+
+ my $x = { a => 1 };
+
+ $db->begin_work;
+ $db->{foo} = $x;
+ $db->rollback;
+
+ is( $x->{a}, 1 ); # This will fail!
+
+ The problem is that the moment a reference used as the rvalue to a
+ DBM::Deep object's lvalue, it becomes tied itself. This is so that
+ future changes to $x can be tracked within the DBM::Deep file and is
+ considered to be a feature. By the time the rollback occurs, there is no
+ knowledge that there had been an $x or what memory location to assign an
+ "export()" to.
+
+ NOTE: This does not affect importing because imports do a walk over the
+ reference to be imported in order to explicitly leave it untied.
+
+CODE COVERAGE
+ Devel::Cover is used to test the code coverage of the tests. Below is
+ the Devel::Cover report on this distribution's test suite.
+
+ ---------------------------- ------ ------ ------ ------ ------ ------ ------
+ File stmt bran cond sub pod time total
+ ---------------------------- ------ ------ ------ ------ ------ ------ ------
+ blib/lib/DBM/Deep.pm 96.8 87.9 90.5 100.0 89.5 4.5 95.2
+ blib/lib/DBM/Deep/Array.pm 100.0 94.3 100.0 100.0 100.0 4.8 98.7
+ blib/lib/DBM/Deep/Engine.pm 97.2 86.4 86.0 100.0 0.0 56.8 91.0
+ blib/lib/DBM/Deep/File.pm 98.1 83.3 66.7 100.0 0.0 31.4 88.0
+ blib/lib/DBM/Deep/Hash.pm 100.0 100.0 100.0 100.0 100.0 2.5 100.0
+ Total 97.7 88.1 86.6 100.0 31.6 100.0 93.0
+ ---------------------------- ------ ------ ------ ------ ------ ------ ------
+
+MORE INFORMATION
+ Check out the DBM::Deep Google Group at
+ <http://groups.google.com/group/DBM-Deep> or send email to
+ DBM-Deep@googlegroups.com. You can also visit #dbm-deep on irc.perl.org
+
+ The source code repository is at <http://svn.perl.org/modules/DBM-Deep>
+
+MAINTAINER(S)
+ Rob Kinyon, rkinyon@cpan.org
+
+ Originally written by Joseph Huckaby, jhuckaby@cpan.org
+
+CONTRIBUTORS
+ The following have contributed greatly to make DBM::Deep what it is
+ today:
+
+ * Adam Sah and Rich Gaushell
+ * Stonehenge for sponsoring the 1.00 release
+ * Dan Golden and others at YAPC::NA 2006 for helping me design through
+ transactions.
SEE ALSO
perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3),
nfs(5), Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
LICENSE
- Copyright (c) 2002-2005 Joseph Huckaby. All Rights Reserved. This is
- free software, you may use it and distribute it under the same terms as
- Perl itself.
+ 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.
projects / dbsrgits/DBM-Deep.git / commitdiff