BerkeleyDB(3) User Contributed Perl Documentation BerkeleyDB(3)
NAME
BerkeleyDB - Perl extension for Berkeley DB version 2, 3 or 4
SYNOPSIS
use BerkeleyDB;
$env = new BerkeleyDB::Env [OPTIONS] ;
$db = tie %hash, 'BerkeleyDB::Hash', [OPTIONS] ;
$db = new BerkeleyDB::Hash [OPTIONS] ;
$db = tie %hash, 'BerkeleyDB::Btree', [OPTIONS] ;
$db = new BerkeleyDB::Btree [OPTIONS] ;
$db = tie @array, 'BerkeleyDB::Recno', [OPTIONS] ;
$db = new BerkeleyDB::Recno [OPTIONS] ;
$db = tie @array, 'BerkeleyDB::Queue', [OPTIONS] ;
$db = new BerkeleyDB::Queue [OPTIONS] ;
$db = new BerkeleyDB::Unknown [OPTIONS] ;
$status = BerkeleyDB::db_remove [OPTIONS]
$status = BerkeleyDB::db_rename [OPTIONS]
$status = BerkeleyDB::db_verify [OPTIONS]
$hash{$key} = $value ;
$value = $hash{$key} ;
each %hash ;
keys %hash ;
values %hash ;
$status = $db->db_get()
$status = $db->db_put() ;
$status = $db->db_del() ;
$status = $db->db_sync() ;
$status = $db->db_close() ;
$status = $db->db_pget()
$hash_ref = $db->db_stat() ;
$status = $db->db_key_range();
$type = $db->type() ;
$status = $db->status() ;
$boolean = $db->byteswapped() ;
$status = $db->truncate($count) ;
$status = $db->compact($start, $stop, $c_data, $flags, $end);
$bool = $env->cds_enabled();
$bool = $db->cds_enabled();
$lock = $db->cds_lock();
$lock->cds_unlock();
($flag, $old_offset, $old_length) = $db->partial_set($offset, $length) ;
($flag, $old_offset, $old_length) = $db->partial_clear() ;
$cursor = $db->db_cursor([$flags]) ;
$newcursor = $cursor->c_dup([$flags]);
$status = $cursor->c_get() ;
$status = $cursor->c_put() ;
$status = $cursor->c_del() ;
$status = $cursor->c_count() ;
$status = $cursor->c_pget() ;
$status = $cursor->status() ;
$status = $cursor->c_close() ;
$cursor = $db->db_join() ;
$status = $cursor->c_get() ;
$status = $cursor->c_close() ;
$status = $env->txn_checkpoint()
$hash_ref = $env->txn_stat()
$status = $env->setmutexlocks()
$status = $env->set_flags()
$txn = $env->txn_begin() ;
$db->Txn($txn);
$txn->Txn($db1, $db2,...);
$status = $txn->txn_prepare()
$status = $txn->txn_commit()
$status = $txn->txn_abort()
$status = $txn->txn_id()
$status = $txn->txn_discard()
$status = $env->set_lg_dir();
$status = $env->set_lg_bsize();
$status = $env->set_lg_max();
$status = $env->set_data_dir() ;
$status = $env->set_tmp_dir() ;
$status = $env->set_verbose() ;
$db_env_ptr = $env->DB_ENV() ;
$BerkeleyDB::Error
$BerkeleyDB::db_version
# DBM Filters
$old_filter = $db->filter_store_key ( sub { ... } ) ;
$old_filter = $db->filter_store_value( sub { ... } ) ;
$old_filter = $db->filter_fetch_key ( sub { ... } ) ;
$old_filter = $db->filter_fetch_value( sub { ... } ) ;
# deprecated, but supported
$txn_mgr = $env->TxnMgr();
$status = $txn_mgr->txn_checkpoint()
$hash_ref = $txn_mgr->txn_stat()
$txn = $txn_mgr->txn_begin() ;
DESCRIPTION
NOTE: This document is still under construction. Expect it to be incomplete in places.
This Perl module provides an interface to most of the functionality available in Berkeley DB versions
2, 3 and 4. In general it is safe to assume that the interface provided here to be identical to the
Berkeley DB interface. The main changes have been to make the Berkeley DB API work in a Perl way.
Note that if you are using Berkeley DB 2.x, the new features available in Berkeley DB 3.x or DB 4.x
are not available via this module.
The reader is expected to be familiar with the Berkeley DB documentation. Where the interface
provided here is identical to the Berkeley DB library and the... TODO
The db_appinit, db_cursor, db_open and db_txn man pages are particularly relevant.
The interface to Berkeley DB is implemented with a number of Perl classes.
The BerkeleyDB::Env Class
The BerkeleyDB::Env class provides an interface to the Berkeley DB function db_appinit in Berkeley DB
2.x or db_env_create and DBENV->open in Berkeley DB 3.x/4.x. Its purpose is to initialise a number of
sub-systems that can then be used in a consistent way in all the databases you make use of in the
environment.
If you don't intend using transactions, locking or logging, then you shouldn't need to make use of
BerkeleyDB::Env.
Note that an environment consists of a number of files that Berkeley DB manages behind the scenes for
you. When you first use an environment, it needs to be explicitly created. This is done by including
"DB_CREATE" with the "Flags" parameter, described below.
Synopsis
$env = new BerkeleyDB::Env
[ -Home => $path, ]
[ -Server => $name, ]
[ -CacheSize => $number, ]
[ -Config => { name => value, name => value }, ]
[ -ErrFile => filename, ]
[ -ErrPrefix => "string", ]
[ -Flags => number, ]
[ -SetFlags => bitmask, ]
[ -LockDetect => number, ]
[ -SharedMemKey => number, ]
[ -Verbose => boolean, ]
[ -Encrypt => { Password => "string",
Flags => number }, ]
All the parameters to the BerkeleyDB::Env constructor are optional.
-Home
If present, this parameter should point to an existing directory. Any files that aren't
specified with an absolute path in the sub-systems that are initialised by the BerkeleyDB::Env
class will be assumed to live in the Home directory.
For example, in the code fragment below the database "fred.db" will be opened in the directory
"/home/databases" because it was specified as a relative path, but "joe.db" will be opened in
"/other" because it was part of an absolute path.
$env = new BerkeleyDB::Env
-Home => "/home/databases"
...
$db1 = new BerkeleyDB::Hash
-Filename = "fred.db",
-Env => $env
...
$db2 = new BerkeleyDB::Hash
-Filename = "/other/joe.db",
-Env => $env
...
-Server
If present, this parameter should be the hostname of a server that is running the Berkeley DB
RPC server. All databases will be accessed via the RPC server.
-Encrypt
If present, this parameter will enable encryption of all data before it is written to the
database. This parameters must be given a hash reference. The format is shown below.
-Encrypt => { -Password => "abc", Flags => DB_ENCRYPT_AES }
Valid values for the Flags are 0 or "DB_ENCRYPT_AES".
This option requires Berkeley DB 4.1 or better.
-Cachesize
If present, this parameter sets the size of the environments shared memory buffer pool.
-SharedMemKey
If present, this parameter sets the base segment ID for the shared memory region used by
Berkeley DB.
This option requires Berkeley DB 3.1 or better.
Use "$env->get_shm_key($id)" to find out the base segment ID used once the environment is open.
-Config
This is a variation on the "-Home" parameter, but it allows finer control of where specific
types of files will be stored.
The parameter expects a reference to a hash. Valid keys are: DB_DATA_DIR, DB_LOG_DIR and
DB_TMP_DIR
The code below shows an example of how it can be used.
$env = new BerkeleyDB::Env
-Config => { DB_DATA_DIR => "/home/databases",
DB_LOG_DIR => "/home/logs",
DB_TMP_DIR => "/home/tmp"
}
...
-ErrFile
Expects a filename or filenhandle. Any errors generated internally by Berkeley DB will be logged
to this file. A useful debug setting is to open environments with either
-ErrFile => *STDOUT
or
-ErrFile => *STDERR
-ErrPrefix
Allows a prefix to be added to the error messages before they are sent to -ErrFile.
-Flags
The Flags parameter specifies both which sub-systems to initialise, as well as a number of
environment-wide options. See the Berkeley DB documentation for more details of these options.
Any of the following can be specified by OR'ing them:
DB_CREATE
If any of the files specified do not already exist, create them.
DB_INIT_CDB
Initialise the Concurrent Access Methods
DB_INIT_LOCK
Initialise the Locking sub-system.
DB_INIT_LOG
Initialise the Logging sub-system.
DB_INIT_MPOOL
Initialise the ...
DB_INIT_TXN
Initialise the ...
DB_MPOOL_PRIVATE
Initialise the ...
DB_INIT_MPOOL is also specified.
Initialise the ...
DB_NOMMAP
Initialise the ...
DB_RECOVER
DB_RECOVER_FATAL
DB_THREAD
DB_TXN_NOSYNC
DB_USE_ENVIRON
DB_USE_ENVIRON_ROOT
-SetFlags
Calls ENV->set_flags with the supplied bitmask. Use this when you need to make use of
DB_ENV->set_flags before DB_ENV->open is called.
Only valid when Berkeley DB 3.x or better is used.
-LockDetect
Specifies what to do when a lock conflict occurs. The value should be one of
DB_LOCK_DEFAULT
DB_LOCK_OLDEST
DB_LOCK_RANDOM
DB_LOCK_YOUNGEST
-Verbose
Add extra debugging information to the messages sent to -ErrFile.
Methods
The environment class has the following methods:
$env->errPrefix("string") ;
This method is identical to the -ErrPrefix flag. It allows the error prefix string to be changed
dynamically.
$env->set_flags(bitmask, 1|0);
$txn = $env->TxnMgr()
Constructor for creating a TxnMgr object. See "TRANSACTIONS" for more details of using
transactions.
This method is deprecated. Access the transaction methods using the txn_ methods below from the
environment object directly.
$env->txn_begin()
TODO
$env->txn_stat()
TODO
$env->txn_checkpoint()
TODO
$env->status()
Returns the status of the last BerkeleyDB::Env method.
$env->DB_ENV()
Returns a pointer to the underlying DB_ENV data structure that Berkeley DB uses.
$env->get_shm_key($id)
Writes the base segment ID for the shared memory region used by the Berkeley DB environment into
$id. Returns 0 on success.
This option requires Berkeley DB 4.2 or better.
Use the "-SharedMemKey" option when opening the environemt to set the base segment ID.
$env->status()
Returns the status of the last BerkeleyDB::Env method.
Examples
TODO.
Global Classes
$status = BerkeleyDB::db_remove [OPTIONS]
$status = BerkeleyDB::db_rename [OPTIONS]
$status = BerkeleyDB::db_verify [OPTIONS]
THE DATABASE CLASSES
BerkeleyDB supports the following database formats:
BerkeleyDB::Hash
This database type allows arbitrary key/value pairs to be stored in data files. This is
equivalent to the functionality provided by other hashing packages like DBM, NDBM, ODBM, GDBM,
and SDBM. Remember though, the files created using BerkeleyDB::Hash are not compatible with any
of the other packages mentioned.
A default hashing algorithm, which will be adequate for most applications, is built into
BerkeleyDB. If you do need to use your own hashing algorithm it is possible to write your own in
Perl and have BerkeleyDB use it instead.
BerkeleyDB::Btree
The Btree format allows arbitrary key/value pairs to be stored in a B+tree.
As with the BerkeleyDB::Hash format, it is possible to provide a user defined Perl routine to
perform the comparison of keys. By default, though, the keys are stored in lexical order.
BerkeleyDB::Recno
TODO.
BerkeleyDB::Queue
TODO.
BerkeleyDB::Unknown
This isn't a database format at all. It is used when you want to open an existing Berkeley DB
database without having to know what type is it.
Each of the database formats described above is accessed via a corresponding BerkeleyDB class. These
will be described in turn in the next sections.
BerkeleyDB::Hash
Equivalent to calling db_open with type DB_HASH in Berkeley DB 2.x and calling db_create followed by
DB->open with type DB_HASH in Berkeley DB 3.x or greater.
Two forms of constructor are supported:
$db = new BerkeleyDB::Hash
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
# BerkeleyDB::Hash specific
[ -Ffactor => number,]
[ -Nelem => number,]
[ -Hash => code reference,]
[ -DupCompare => code reference,]
and this
[$db =] tie %hash, 'BerkeleyDB::Hash',
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
# BerkeleyDB::Hash specific
[ -Ffactor => number,]
[ -Nelem => number,]
[ -Hash => code reference,]
[ -DupCompare => code reference,]
When the "tie" interface is used, reading from and writing to the database is achieved via the tied
hash. In this case the database operates like a Perl associative array that happens to be stored on
disk.
In addition to the high-level tied hash interface, it is possible to make use of the underlying
methods provided by Berkeley DB
Options
In addition to the standard set of options (see "COMMON OPTIONS") BerkeleyDB::Hash supports these
options:
-Property
Used to specify extra flags when opening a database. The following flags may be specified by
bitwise OR'ing together one or more of the following values:
DB_DUP
When creating a new database, this flag enables the storing of duplicate keys in the database.
If DB_DUPSORT is not specified as well, the duplicates are stored in the order they are created
in the database.
DB_DUPSORT
Enables the sorting of duplicate keys in the database. Ignored if DB_DUP isn't also specified.
-Ffactor
-Nelem
See the Berkeley DB documentation for details of these options.
-Hash
Allows you to provide a user defined hash function. If not specified, a default hash function is
used. Here is a template for a user-defined hash function
sub hash
{
my ($data) = shift ;
...
# return the hash value for $data
return $hash ;
}
tie %h, "BerkeleyDB::Hash",
-Filename => $filename,
-Hash => \&hash,
...
See "" for an example.
-DupCompare
Used in conjunction with the DB_DUPOSRT flag.
sub compare
{
my ($key, $key2) = @_ ;
...
# return 0 if $key1 eq $key2
# -1 if $key1 lt $key2
# 1 if $key1 gt $key2
return (-1 , 0 or 1) ;
}
tie %h, "BerkeleyDB::Hash",
-Filename => $filename,
-Property => DB_DUP|DB_DUPSORT,
-DupCompare => \&compare,
...
Methods
BerkeleyDB::Hash only supports the standard database methods. See "COMMON DATABASE METHODS".
A Simple Tied Hash Example
use strict ;
use BerkeleyDB ;
use vars qw( %h $k $v ) ;
my $filename = "fruit" ;
unlink $filename ;
tie %h, "BerkeleyDB::Hash",
-Filename => $filename,
-Flags => DB_CREATE
or die "Cannot open file $filename: $! $BerkeleyDB::Error\n" ;
# Add a few key/value pairs to the file
$h{"apple"} = "red" ;
$h{"orange"} = "orange" ;
$h{"banana"} = "yellow" ;
$h{"tomato"} = "red" ;
# Check for existence of a key
print "Banana Exists\n\n" if $h{"banana"} ;
# Delete a key/value pair.
delete $h{"apple"} ;
# print the contents of the file
while (($k, $v) = each %h)
{ print "$k -> $v\n" }
untie %h ;
here is the output:
Banana Exists
orange -> orange
tomato -> red
banana -> yellow
Note that the like ordinary associative arrays, the order of the keys retrieved from a Hash database
are in an apparently random order.
Another Simple Hash Example
Do the same as the previous example but not using tie.
use strict ;
use BerkeleyDB ;
my $filename = "fruit" ;
unlink $filename ;
my $db = new BerkeleyDB::Hash
-Filename => $filename,
-Flags => DB_CREATE
or die "Cannot open file $filename: $! $BerkeleyDB::Error\n" ;
# Add a few key/value pairs to the file
$db->db_put("apple", "red") ;
$db->db_put("orange", "orange") ;
$db->db_put("banana", "yellow") ;
$db->db_put("tomato", "red") ;
# Check for existence of a key
print "Banana Exists\n\n" if $db->db_get("banana", $v) == 0;
# Delete a key/value pair.
$db->db_del("apple") ;
# print the contents of the file
my ($k, $v) = ("", "") ;
my $cursor = $db->db_cursor() ;
while ($cursor->c_get($k, $v, DB_NEXT) == 0)
{ print "$k -> $v\n" }
undef $cursor ;
undef $db ;
Duplicate keys
The code below is a variation on the examples above. This time the hash has been inverted. The key
this time is colour and the value is the fruit name. The DB_DUP flag has been specified to allow
duplicates.
use strict ;
use BerkeleyDB ;
my $filename = "fruit" ;
unlink $filename ;
my $db = new BerkeleyDB::Hash
-Filename => $filename,
-Flags => DB_CREATE,
-Property => DB_DUP
or die "Cannot open file $filename: $! $BerkeleyDB::Error\n" ;
# Add a few key/value pairs to the file
$db->db_put("red", "apple") ;
$db->db_put("orange", "orange") ;
$db->db_put("green", "banana") ;
$db->db_put("yellow", "banana") ;
$db->db_put("red", "tomato") ;
$db->db_put("green", "apple") ;
# print the contents of the file
my ($k, $v) = ("", "") ;
my $cursor = $db->db_cursor() ;
while ($cursor->c_get($k, $v, DB_NEXT) == 0)
{ print "$k -> $v\n" }
undef $cursor ;
undef $db ;
here is the output:
orange -> orange
yellow -> banana
red -> apple
red -> tomato
green -> banana
green -> apple
Sorting Duplicate Keys
In the previous example, when there were duplicate keys, the values are sorted in the order they are
stored in. The code below is identical to the previous example except the DB_DUPSORT flag is
specified.
use strict ;
use BerkeleyDB ;
my $filename = "fruit" ;
unlink $filename ;
my $db = new BerkeleyDB::Hash
-Filename => $filename,
-Flags => DB_CREATE,
-Property => DB_DUP | DB_DUPSORT
or die "Cannot open file $filename: $! $BerkeleyDB::Error\n" ;
# Add a few key/value pairs to the file
$db->db_put("red", "apple") ;
$db->db_put("orange", "orange") ;
$db->db_put("green", "banana") ;
$db->db_put("yellow", "banana") ;
$db->db_put("red", "tomato") ;
$db->db_put("green", "apple") ;
# print the contents of the file
my ($k, $v) = ("", "") ;
my $cursor = $db->db_cursor() ;
while ($cursor->c_get($k, $v, DB_NEXT) == 0)
{ print "$k -> $v\n" }
undef $cursor ;
undef $db ;
Notice that in the output below the duplicate values are sorted.
orange -> orange
yellow -> banana
red -> apple
red -> tomato
green -> apple
green -> banana
Custom Sorting Duplicate Keys
Another variation
TODO
Changing the hash
TODO
Using db_stat
TODO
BerkeleyDB::Btree
Equivalent to calling db_open with type DB_BTREE in Berkeley DB 2.x and calling db_create followed by
DB->open with type DB_BTREE in Berkeley DB 3.x or greater.
Two forms of constructor are supported:
$db = new BerkeleyDB::Btree
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
# BerkeleyDB::Btree specific
[ -Minkey => number,]
[ -Compare => code reference,]
[ -DupCompare => code reference,]
[ -Prefix => code reference,]
and this
[$db =] tie %hash, 'BerkeleyDB::Btree',
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
# BerkeleyDB::Btree specific
[ -Minkey => number,]
[ -Compare => code reference,]
[ -DupCompare => code reference,]
[ -Prefix => code reference,]
Options
In addition to the standard set of options (see "COMMON OPTIONS") BerkeleyDB::Btree supports these
options:
-Property
Used to specify extra flags when opening a database. The following flags may be specified by
bitwise OR'ing together one or more of the following values:
DB_DUP
When creating a new database, this flag enables the storing of duplicate keys in the database.
If DB_DUPSORT is not specified as well, the duplicates are stored in the order they are created
in the database.
DB_DUPSORT
Enables the sorting of duplicate keys in the database. Ignored if DB_DUP isn't also specified.
Minkey
TODO
Compare
Allow you to override the default sort order used in the database. See "Changing the sort order"
for an example.
sub compare
{
my ($key, $key2) = @_ ;
...
# return 0 if $key1 eq $key2
# -1 if $key1 lt $key2
# 1 if $key1 gt $key2
return (-1 , 0 or 1) ;
}
tie %h, "BerkeleyDB::Hash",
-Filename => $filename,
-Compare => \&compare,
...
Prefix
sub prefix
{
my ($key, $key2) = @_ ;
...
# return number of bytes of $key2 which are
# necessary to determine that it is greater than $key1
return $bytes ;
}
tie %h, "BerkeleyDB::Hash",
-Filename => $filename,
-Prefix => \&prefix,
...
=item DupCompare
sub compare
{
my ($key, $key2) = @_ ;
...
# return 0 if $key1 eq $key2
# -1 if $key1 lt $key2
# 1 if $key1 gt $key2
return (-1 , 0 or 1) ;
}
tie %h, "BerkeleyDB::Hash",
-Filename => $filename,
-DupCompare => \&compare,
...
Methods
BerkeleyDB::Btree supports the following database methods. See also "COMMON DATABASE METHODS".
All the methods below return 0 to indicate success.
$status = $db->db_key_range($key, $less, $equal, $greater [, $flags])
Given a key, $key, this method returns the proportion of keys less than $key in $less, the
proportion equal to $key in $equal and the proportion greater than $key in $greater.
The proportion is returned as a double in the range 0.0 to 1.0.
A Simple Btree Example
The code below is a simple example of using a btree database.
use strict ;
use BerkeleyDB ;
my $filename = "tree" ;
unlink $filename ;
my %h ;
tie %h, 'BerkeleyDB::Btree',
-Filename => $filename,
-Flags => DB_CREATE
or die "Cannot open $filename: $! $BerkeleyDB::Error\n" ;
# Add a key/value pair to the file
$h{'Wall'} = 'Larry' ;
$h{'Smith'} = 'John' ;
$h{'mouse'} = 'mickey' ;
$h{'duck'} = 'donald' ;
# Delete
delete $h{"duck"} ;
# Cycle through the keys printing them in order.
# Note it is not necessary to sort the keys as
# the btree will have kept them in order automatically.
foreach (keys %h)
{ print "$_\n" }
untie %h ;
Here is the output from the code above. The keys have been sorted using Berkeley DB's default sorting
algorithm.
Smith
Wall
mouse
Changing the sort order
It is possible to supply your own sorting algorithm if the one that Berkeley DB used isn't suitable.
The code below is identical to the previous example except for the case insensitive compare function.
use strict ;
use BerkeleyDB ;
my $filename = "tree" ;
unlink $filename ;
my %h ;
tie %h, 'BerkeleyDB::Btree',
-Filename => $filename,
-Flags => DB_CREATE,
-Compare => sub { lc $_[0] cmp lc $_[1] }
or die "Cannot open $filename: $!\n" ;
# Add a key/value pair to the file
$h{'Wall'} = 'Larry' ;
$h{'Smith'} = 'John' ;
$h{'mouse'} = 'mickey' ;
$h{'duck'} = 'donald' ;
# Delete
delete $h{"duck"} ;
# Cycle through the keys printing them in order.
# Note it is not necessary to sort the keys as
# the btree will have kept them in order automatically.
foreach (keys %h)
{ print "$_\n" }
untie %h ;
Here is the output from the code above.
mouse
Smith
Wall
There are a few point to bear in mind if you want to change the ordering in a BTREE database:
1. The new compare function must be specified when you create the database.
2. You cannot change the ordering once the database has been created. Thus you must use the same
compare function every time you access the database.
Using db_stat
TODO
BerkeleyDB::Recno
Equivalent to calling db_open with type DB_RECNO in Berkeley DB 2.x and calling db_create followed by
DB->open with type DB_RECNO in Berkeley DB 3.x or greater.
Two forms of constructor are supported:
$db = new BerkeleyDB::Recno
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
# BerkeleyDB::Recno specific
[ -Delim => byte,]
[ -Len => number,]
[ -Pad => byte,]
[ -Source => filename,]
and this
[$db =] tie @arry, 'BerkeleyDB::Recno',
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
# BerkeleyDB::Recno specific
[ -Delim => byte,]
[ -Len => number,]
[ -Pad => byte,]
[ -Source => filename,]
A Recno Example
Here is a simple example that uses RECNO (if you are using a version of Perl earlier than 5.004_57
this example won't work -- see "Extra RECNO Methods" for a workaround).
use strict ;
use BerkeleyDB ;
my $filename = "text" ;
unlink $filename ;
my @h ;
tie @h, 'BerkeleyDB::Recno',
-Filename => $filename,
-Flags => DB_CREATE,
-Property => DB_RENUMBER
or die "Cannot open $filename: $!\n" ;
# Add a few key/value pairs to the file
$h[0] = "orange" ;
$h[1] = "blue" ;
$h[2] = "yellow" ;
push @h, "green", "black" ;
my $elements = scalar @h ;
print "The array contains $elements entries\n" ;
my $last = pop @h ;
print "popped $last\n" ;
unshift @h, "white" ;
my $first = shift @h ;
print "shifted $first\n" ;
# Check for existence of a key
print "Element 1 Exists with value $h[1]\n" if $h[1] ;
untie @h ;
Here is the output from the script:
The array contains 5 entries
popped black
shifted white
Element 1 Exists with value blue
The last element is green
The 2nd last element is yellow
BerkeleyDB::Queue
Equivalent to calling db_create followed by DB->open with type DB_QUEUE in Berkeley DB 3.x or
greater. This database format isn't available if you use Berkeley DB 2.x.
Two forms of constructor are supported:
$db = new BerkeleyDB::Queue
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
# BerkeleyDB::Queue specific
[ -Len => number,]
[ -Pad => byte,]
[ -ExtentSize => number, ]
and this
[$db =] tie @arry, 'BerkeleyDB::Queue',
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
# BerkeleyDB::Queue specific
[ -Len => number,]
[ -Pad => byte,]
BerkeleyDB::Unknown
This class is used to open an existing database.
Equivalent to calling db_open with type DB_UNKNOWN in Berkeley DB 2.x and calling db_create followed
by DB->open with type DB_UNKNOWN in Berkeley DB 3.x or greater.
The constructor looks like this:
$db = new BerkeleyDB::Unknown
[ -Filename => "filename", ]
[ -Subname => "sub-database name", ]
[ -Flags => flags,]
[ -Property => flags,]
[ -Mode => number,]
[ -Cachesize => number,]
[ -Lorder => number,]
[ -Pagesize => number,]
[ -Env => $env,]
[ -Txn => $txn,]
[ -Encrypt => { Password => "string",
Flags => number }, ],
An example
COMMON OPTIONS
All database access class constructors support the common set of options defined below. All are
optional.
-Filename
The database filename. If no filename is specified, a temporary file will be created and removed
once the program terminates.
-Subname
Specifies the name of the sub-database to open. This option is only valid if you are using
Berkeley DB 3.x or greater.
-Flags
Specify how the database will be opened/created. The valid flags are:
DB_CREATE
Create any underlying files, as necessary. If the files do not already exist and the DB_CREATE
flag is not specified, the call will fail.
DB_NOMMAP
Not supported by BerkeleyDB.
DB_RDONLY
Opens the database in read-only mode.
DB_THREAD
Not supported by BerkeleyDB.
DB_TRUNCATE
If the database file already exists, remove all the data before opening it.
-Mode
Determines the file protection when the database is created. Defaults to 0666.
-Cachesize
-Lorder
-Pagesize
-Env When working under a Berkeley DB environment, this parameter
Defaults to no environment.
-Encrypt
If present, this parameter will enable encryption of all data before it is written to the
database. This parameters must be given a hash reference. The format is shown below.
-Encrypt => { -Password => "abc", Flags => DB_ENCRYPT_AES }
Valid values for the Flags are 0 or "DB_ENCRYPT_AES".
This option requires Berkeley DB 4.1 or better.
-Txn TODO.
COMMON DATABASE METHODS
All the database interfaces support the common set of methods defined below.
All the methods below return 0 to indicate success.
$status = $db->db_get($key, $value [, $flags])
Given a key ($key) this method reads the value associated with it from the database. If it exists,
the value read from the database is returned in the $value parameter.
The $flags parameter is optional. If present, it must be set to one of the following values:
DB_GET_BOTH
When the DB_GET_BOTH flag is specified, db_get checks for the existence of both the $key and
$value in the database.
DB_SET_RECNO
TODO.
In addition, the following value may be set by bitwise OR'ing it into the $flags parameter:
DB_RMW
TODO
$status = $db->db_put($key, $value [, $flags])
Stores a key/value pair in the database.
The $flags parameter is optional. If present it must be set to one of the following values:
DB_APPEND
This flag is only applicable when accessing a BerkeleyDB::Recno database.
TODO.
DB_NOOVERWRITE
If this flag is specified and $key already exists in the database, the call to db_put will
return DB_KEYEXIST.
$status = $db->db_del($key [, $flags])
Deletes a key/value pair in the database associated with $key. If duplicate keys are enabled in the
database, db_del will delete all key/value pairs with key $key.
The $flags parameter is optional and is currently unused.
$status = $db->db_sync()
If any parts of the database are in memory, write them to the database.
$cursor = $db->db_cursor([$flags])
Creates a cursor object. This is used to access the contents of the database sequentially. See
CURSORS for details of the methods available when working with cursors.
The $flags parameter is optional. If present it must be set to one of the following values:
DB_RMW
TODO.
($flag, $old_offset, $old_length) = $db->partial_set($offset, $length) ;
TODO
($flag, $old_offset, $old_length) = $db->partial_clear() ;
TODO
$db->byteswapped()
TODO
$db->type()
Returns the type of the database. The possible return code are DB_HASH for a BerkeleyDB::Hash
database, DB_BTREE for a BerkeleyDB::Btree database and DB_RECNO for a BerkeleyDB::Recno database.
This method is typically used when a database has been opened with BerkeleyDB::Unknown.
$bool = $env->cds_enabled();
Returns true if the Berkeley DB environment $env has been opened on CDS mode.
$bool = $db->cds_enabled();
Returns true if the database $db has been opened on CDS mode.
$lock = $db->cds_lock();
Creates a CDS write lock object $lock.
It is a fatal error to attempt to create a cds_lock if the Berkeley DB environment has not been
opened in CDS mode.
$lock->cds_unlock();
Removes a CDS lock. The destruction of the CDS lock object automatically calls this method.
Note that if multiple CDS lock objects are created, the underlying write lock will not be released
until all CDS lock objects are either explictly unlocked with this method, or the CDS lock objects
have been destroyed.
$ref = $db->db_stat()
Returns a reference to an associative array containing information about the database. The keys of
the associative array correspond directly to the names of the fields defined in the Berkeley DB
documentation. For example, in the DB documentation, the field bt_version stores the version of the
Btree database. Assuming you called db_stat on a Btree database the equivalent field would be
accessed as follows:
$version = $ref->{'bt_version'} ;
If you are using Berkeley DB 3.x or better, this method will work will all database formats. When DB
2.x is used, it only works with BerkeleyDB::Btree.
$status = $db->status()
Returns the status of the last $db method called.
$status = $db->truncate($count)
Truncates the datatabase and returns the number or records deleted in $count.
$status = $db->compact($start, $stop, $c_data, $flags, $end);
Compacts the database $db.
All the parameters are optional - if only want to make use of some of them, use "undef" for those you
don't want. Trailing unusused parameters can be omitted. For example, if you only want to use the
$c_data parameter to set the "compact_fillpercent", write you code like this
my %hash;
$hash{compact_fillpercent} = 50;
$db->commit(undef, undef, \%hash);
The parameters operate identically to the C equivalent of this method. The $c_data needs a bit of
explanation - it must be a hash reference. The values of the following keys can be set before
calling "compact" and will affect the operation of the compaction.
The following keys, along with associated values, will be created in the hash reference if the
"compact" operation was successful.
compact_deadlock =item * compact_levels =item * compact_pages_free =item * compact_pages_examine
=item * compact_pages_truncated
You need to be running Berkeley DB 4.4 or better if you wan to make use of "compact".
CURSORS
A cursor is used whenever you want to access the contents of a database in sequential order. A
cursor object is created with the "db_cursor"
A cursor object has the following methods available:
$newcursor = $cursor->c_dup($flags)
Creates a duplicate of $cursor. This method needs Berkeley DB 3.0.x or better.
The $flags parameter is optional and can take the following value:
DB_POSITION
When present this flag will position the new cursor at the same place as the existing cursor.
$status = $cursor->c_get($key, $value, $flags)
Reads a key/value pair from the database, returning the data in $key and $value. The key/value pair
actually read is controlled by the $flags parameter, which can take one of the following values:
DB_FIRST
Set the cursor to point to the first key/value pair in the database. Return the key/value pair
in $key and $value.
DB_LAST
Set the cursor to point to the last key/value pair in the database. Return the key/value pair in
$key and $value.
DB_NEXT
If the cursor is already pointing to a key/value pair, it will be incremented to point to the
next key/value pair and return its contents.
If the cursor isn't initialised, DB_NEXT works just like DB_FIRST.
If the cursor is already positioned at the last key/value pair, c_get will return DB_NOTFOUND.
DB_NEXT_DUP
This flag is only valid when duplicate keys have been enabled in a database. If the cursor is
already pointing to a key/value pair and the key of the next key/value pair is identical, the
cursor will be incremented to point to it and their contents returned.
DB_PREV
If the cursor is already pointing to a key/value pair, it will be decremented to point to the
previous key/value pair and return its contents.
If the cursor isn't initialised, DB_PREV works just like DB_LAST.
If the cursor is already positioned at the first key/value pair, c_get will return DB_NOTFOUND.
DB_CURRENT
If the cursor has been set to point to a key/value pair, return their contents. If the
key/value pair referenced by the cursor has been deleted, c_get will return DB_KEYEMPTY.
DB_SET
Set the cursor to point to the key/value pair referenced by $key and return the value in $value.
DB_SET_RANGE
This flag is a variation on the DB_SET flag. As well as returning the value, it also returns the
key, via $key. When used with a BerkeleyDB::Btree database the key matched by c_get will be the
shortest key (in length) which is greater than or equal to the key supplied, via $key. This
allows partial key searches. See ??? for an example of how to use this flag.
DB_GET_BOTH
Another variation on DB_SET. This one returns both the key and the value.
DB_SET_RECNO
TODO.
DB_GET_RECNO
TODO.
In addition, the following value may be set by bitwise OR'ing it into the $flags parameter:
DB_RMW
TODO.
$status = $cursor->c_put($key, $value, $flags)
Stores the key/value pair in the database. The position that the data is stored in the database is
controlled by the $flags parameter, which must take one of the following values:
DB_AFTER
When used with a Btree or Hash database, a duplicate of the key referenced by the current cursor
position will be created and the contents of $value will be associated with it - $key is
ignored. The new key/value pair will be stored immediately after the current cursor position.
Obviously the database has to have been opened with DB_DUP.
When used with a Recno ... TODO
DB_BEFORE
When used with a Btree or Hash database, a duplicate of the key referenced by the current cursor
position will be created and the contents of $value will be associated with it - $key is
ignored. The new key/value pair will be stored immediately before the current cursor position.
Obviously the database has to have been opened with DB_DUP.
When used with a Recno ... TODO
DB_CURRENT
If the cursor has been initialised, replace the value of the key/value pair stored in the
database with the contents of $value.
DB_KEYFIRST
Only valid with a Btree or Hash database. This flag is only really used when duplicates are
enabled in the database and sorted duplicates haven't been specified. In this case the
key/value pair will be inserted as the first entry in the duplicates for the particular key.
DB_KEYLAST
Only valid with a Btree or Hash database. This flag is only really used when duplicates are
enabled in the database and sorted duplicates haven't been specified. In this case the
key/value pair will be inserted as the last entry in the duplicates for the particular key.
$status = $cursor->c_del([$flags])
This method deletes the key/value pair associated with the current cursor position. The cursor
position will not be changed by this operation, so any subsequent cursor operation must first
initialise the cursor to point to a valid key/value pair.
If the key/value pair associated with the cursor have already been deleted, c_del will return
DB_KEYEMPTY.
The $flags parameter is not used at present.
$status = $cursor->c_count($cnt [, $flags])
Stores the number of duplicates at the current cursor position in $cnt.
The $flags parameter is not used at present. This method needs Berkeley DB 3.1 or better.
$status = $cursor->status()
Returns the status of the last cursor method as a dual type.
$status = $cursor->c_pget() ;
TODO
$status = $cursor->c_close()
Closes the cursor $cursor.
Cursor Examples
TODO
Iterating from first to last, then in reverse.
examples of each of the flags.
JOIN
Join support for BerkeleyDB is in progress. Watch this space.
TODO
TRANSACTIONS
TODO.
CDS Mode
The Berkeley Db Concurrent Data Store is a lightweight locking mechanism that is useful in scenarios
where transactions are overkill. See the accompanying document .. for details of using this module in
CDS mode.
DBM Filters
A DBM Filter is a piece of code that is be used when you always want to make the same transformation
to all keys and/or values in a DBM database. All of the database classes (BerkeleyDB::Hash,
BerkeleyDB::Btree and BerkeleyDB::Recno) support DBM Filters.
There are four methods associated with DBM Filters. All work identically, and each is used to install
(or uninstall) a single DBM Filter. Each expects a single parameter, namely a reference to a sub.
The only difference between them is the place that the filter is installed.
To summarise:
filter_store_key
If a filter has been installed with this method, it will be invoked every time you write a key
to a DBM database.
filter_store_value
If a filter has been installed with this method, it will be invoked every time you write a value
to a DBM database.
filter_fetch_key
If a filter has been installed with this method, it will be invoked every time you read a key
from a DBM database.
filter_fetch_value
If a filter has been installed with this method, it will be invoked every time you read a value
from a DBM database.
You can use any combination of the methods, from none, to all four.
All filter methods return the existing filter, if present, or "undef" in not.
To delete a filter pass "undef" to it.
The Filter
When each filter is called by Perl, a local copy of $_ will contain the key or value to be filtered.
Filtering is achieved by modifying the contents of $_. The return code from the filter is ignored.
An Example -- the NULL termination problem.
Consider the following scenario. You have a DBM database that you need to share with a third-party C
application. The C application assumes that all keys and values are NULL terminated. Unfortunately
when Perl writes to DBM databases it doesn't use NULL termination, so your Perl application will have
to manage NULL termination itself. When you write to the database you will have to use something like
this:
$hash{"$key\0"} = "$value\0" ;
Similarly the NULL needs to be taken into account when you are considering the length of existing
keys/values.
It would be much better if you could ignore the NULL terminations issue in the main application code
and have a mechanism that automatically added the terminating NULL to all keys and values whenever
you write to the database and have them removed when you read from the database. As I'm sure you have
already guessed, this is a problem that DBM Filters can fix very easily.
use strict ;
use BerkeleyDB ;
my %hash ;
my $filename = "filt.db" ;
unlink $filename ;
my $db = tie %hash, 'BerkeleyDB::Hash',
-Filename => $filename,
-Flags => DB_CREATE
or die "Cannot open $filename: $!\n" ;
# Install DBM Filters
$db->filter_fetch_key ( sub { s/\0$// } ) ;
$db->filter_store_key ( sub { $_ .= "\0" } ) ;
$db->filter_fetch_value( sub { s/\0$// } ) ;
$db->filter_store_value( sub { $_ .= "\0" } ) ;
$hash{"abc"} = "def" ;
my $a = $hash{"ABC"} ;
# ...
undef $db ;
untie %hash ;
Hopefully the contents of each of the filters should be self-explanatory. Both "fetch" filters remove
the terminating NULL, and both "store" filters add a terminating NULL.
Another Example -- Key is a C int.
Here is another real-life example. By default, whenever Perl writes to a DBM database it always
writes the key and value as strings. So when you use this:
$hash{12345} = "something" ;
the key 12345 will get stored in the DBM database as the 5 byte string "12345". If you actually want
the key to be stored in the DBM database as a C int, you will have to use "pack" when writing, and
"unpack" when reading.
Here is a DBM Filter that does it:
use strict ;
use BerkeleyDB ;
my %hash ;
my $filename = "filt.db" ;
unlink $filename ;
my $db = tie %hash, 'BerkeleyDB::Btree',
-Filename => $filename,
-Flags => DB_CREATE
or die "Cannot open $filename: $!\n" ;
$db->filter_fetch_key ( sub { $_ = unpack("i", $_) } ) ;
$db->filter_store_key ( sub { $_ = pack ("i", $_) } ) ;
$hash{123} = "def" ;
# ...
undef $db ;
untie %hash ;
This time only two filters have been used -- we only need to manipulate the contents of the key, so
it wasn't necessary to install any value filters.
Using BerkeleyDB with MLDBM
Both BerkeleyDB::Hash and BerkeleyDB::Btree can be used with the MLDBM module. The code fragment
below shows how to open associate MLDBM with BerkeleyDB::Btree. To use BerkeleyDB::Hash just replace
BerkeleyDB::Btree with BerkeleyDB::Hash.
use strict ;
use BerkeleyDB ;
use MLDBM qw(BerkeleyDB::Btree) ;
use Data::Dumper;
my $filename = 'testmldbm' ;
my %o ;
unlink $filename ;
tie %o, 'MLDBM', -Filename => $filename,
-Flags => DB_CREATE
or die "Cannot open database '$filename: $!\n";
See the MLDBM documentation for information on how to use the module and for details of its
limitations.
EXAMPLES
TODO.
HINTS & TIPS
Sharing Databases With C Applications
There is no technical reason why a Berkeley DB database cannot be shared by both a Perl and a C
application.
The vast majority of problems that are reported in this area boil down to the fact that C strings are
NULL terminated, whilst Perl strings are not. See "An Example -- the NULL termination problem." in
the DBM FILTERS section for a generic way to work around this problem.
The untie Gotcha
TODO
COMMON QUESTIONS
This section attempts to answer some of the more common questions that I get asked.
Relationship with DB_File
Before Berkeley DB 2.x was written there was only one Perl module that interfaced to Berkeley DB.
That module is called DB_File. Although DB_File can be build with Berkeley DB 1.x, 2.x, 3.x or 4.x,
it only provides an interface to the functionality available in Berkeley DB 1.x. That means that it
doesn't support transactions, locking or any of the other new features available in DB 2.x or better.
How do I store Perl data structures with BerkeleyDB?
See "Using BerkeleyDB with MLDBM".
HISTORY
See the Changes file.
AVAILABILITY
The most recent version of BerkeleyDB can always be found on CPAN (see "CPAN" in perlmod for
details), in the directory modules/by-module/BerkeleyDB.
The official web site for Berkeley DB is http://www.sleepycat.com
COPYRIGHT
Copyright (c) 1997-2004 Paul Marquess. All rights reserved. This program is free software; you can
redistribute it and/or modify it under the same terms as Perl itself.
Although BerkeleyDB is covered by the Perl license, the library it makes use of, namely Berkeley DB,
is not. Berkeley DB has its own copyright and its own license. Please take the time to read it.
Here are few words taken from the Berkeley DB FAQ (at http://www.sleepycat.com) regarding the
license:
Do I have to license DB to use it in Perl scripts?
No. The Berkeley DB license requires that software that uses
Berkeley DB be freely redistributable. In the case of Perl, that
software is Perl, and not your scripts. Any Perl scripts that you
write are your property, including scripts that make use of Berkeley
DB. Neither the Perl license nor the Berkeley DB license
place any restriction on what you may do with them.
If you are in any doubt about the license situation, contact either the Berkeley DB authors or the
author of BerkeleyDB. See "AUTHOR" for details.
AUTHOR
Paul Marquess <pmqs@cpan.org>.
Questions about Berkeley DB may be addressed to <db@sleepycat.com>.
SEE ALSO
perl(1), DB_File, Berkeley DB.
perl v5.8.8 2006-07-02 BerkeleyDB(3)
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