Provided by: libberkeleydb-perl_0.54-1_amd64 
NAME
BerkeleyDB - Perl extension for Berkeley DB version 2, 3, 4 or 5
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::Heap [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 ;
$env = $db->Env()
$status = $db->db_get()
$status = $db->db_exists() ;
$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);
$status = $db->get_blob_threshold($t1) ;
$status = $db->get_blob_dir($dir) ;
$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() ;
$stream = $cursor->db_stream() ;
$cursor = $db->db_join() ;
$status = $cursor->c_get() ;
$status = $cursor->c_close() ;
$status = $stream->size($S);
$status = $stream->read($data, $offset, $size);
$status = $stream->write($data, $offset);
$status = $env->txn_checkpoint()
$hash_ref = $env->txn_stat()
$status = $env->set_mutexlocks()
$status = $env->set_flags()
$status = $env->set_timeout()
$status = $env->lock_detect()
$status = $env->lsn_reset()
$status = $env->get_blob_threshold($t1) ;
$status = $env->get_blob_dir($dir) ;
$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 = $txn->set_timeout()
$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, 5 and 6. 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 later 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 (or later). 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, ]
[ -MsgFile => filename, ]
[ -ErrPrefix => "string", ]
[ -Flags => number, ]
[ -SetFlags => bitmask, ]
[ -LockDetect => number, ]
[ -TxMax => number, ]
[ -LogConfig => number, ]
[ -MaxLockers => number, ]
[ -MaxLocks => number, ]
[ -MaxObjects => number, ]
[ -SharedMemKey => number, ]
[ -Verbose => boolean, ]
[ -BlobThreshold=> $number, ]
[ -BlobDir => directory, ]
[ -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 => $env
...
-Server
If present, this parameter should be the hostname of a server that is running the
Berkeley DB RPC server. All databases will be accessed via the RPC server.
-Encrypt
If present, this parameter will enable encryption of all data before it is written
to the database. This parameters must be given a hash reference. The format is shown
below.
-Encrypt => { -Password => "abc", Flags => DB_ENCRYPT_AES }
Valid values for the Flags are 0 or "DB_ENCRYPT_AES".
This option requires Berkeley DB 4.1 or better.
-Cachesize
If present, this parameter sets the size of the environments shared memory buffer
pool.
-TxMax
If present, this parameter sets the number of simultaneous transactions that are
allowed. Default 100. This default is definitely too low for programs using the
MVCC capabilities.
-LogConfig
If present, this parameter is used to configure log options.
-MaxLockers
If present, this parameter is used to configure the maximum number of processes doing
locking on the database. Default 1000.
-MaxLocks
If present, this parameter is used to configure the maximum number of locks on the
database. Default 1000. This is often lower than required.
-MaxObjects
If present, this parameter is used to configure the maximum number of locked objects.
Default 1000. This is often lower than required.
-SharedMemKey
If present, this parameter sets the base segment ID for the shared memory region used
by Berkeley DB.
This option requires Berkeley DB 3.1 or better.
Use "$env->get_shm_key($id)" to find out the base segment ID used once the
environment is open.
-ThreadCount
If present, this parameter declares the approximate number of threads that will be
used in the database environment. This parameter is only necessary when the
$env->failchk method will be used. It does not actually set the maximum number of
threads but rather is used to determine memory sizing.
This option requires Berkeley DB 4.4 or better. It is only supported on Unix/Linux.
-BlobThreshold
Sets the size threshold that will be used to decide when data is stored as a BLOB.
This option must be set for a blobs to be used.
This option requires Berkeley DB 6.0 or better.
-BlobDir
The directory where the BLOB objects are stored.
If not specified blob files are stores in the environment directoy.
This option requires Berkeley DB 6.0 or better.
-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 filehandle. 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
Initialize the shared memory buffer pool subsystem. This subsystem should be used
whenever an application is using any Berkeley DB access method.
DB_INIT_TXN
Initialize the transaction subsystem. This subsystem should be used when recovery and
atomicity of multiple operations are important. The DB_INIT_TXN flag implies the
DB_INIT_LOG flag.
DB_MPOOL_PRIVATE
Create a private memory pool; see memp_open. Ignored unless DB_INIT_MPOOL is also
specified.
DB_INIT_MPOOL is also specified.
DB_NOMMAP
Do not map this database into process memory.
DB_RECOVER
Run normal recovery on this environment before opening it for normal use. If this
flag is set, the DB_CREATE flag must also be set since the regions will be removed
and recreated.
The db_appinit function returns successfully if DB_RECOVER is specified and no log
files exist, so it is necessary to ensure all necessary log files are present before
running recovery.
DB_PRIVATE
DB_RECOVER_FATAL
Run catastrophic recovery on this environment before opening it for normal use. If
this flag is set, the DB_CREATE flag must also be set since the regions will be
removed and recreated.
The db_appinit function returns successfully if DB_RECOVER_FATAL is specified and no
log files exist, so it is necessary to ensure all necessary log files are present
before running recovery.
DB_THREAD
Ensure that handles returned by the Berkeley DB subsystems are useable by multiple
threads within a single process, i.e., that the system is free-threaded.
DB_TXN_NOSYNC
On transaction commit, do not synchronously flush the log; see txn_open. Ignored
unless DB_INIT_TXN is also specified.
DB_USE_ENVIRON
The Berkeley DB process' environment may be permitted to specify information to be
used when naming files; see Berkeley DB File Naming. As permitting users to specify
which files are used can create security problems, environment information will be
used in file naming for all users only if the DB_USE_ENVIRON flag is set.
DB_USE_ENVIRON_ROOT
The Berkeley DB process' environment may be permitted to specify information to be
used when naming files; see Berkeley DB File Naming. As permitting users to specify
which files are used can create security problems, if the DB_USE_ENVIRON_ROOT flag is
set, environment information will be used for file naming only for users with a user-
ID matching that of the superuser (specifically, users for whom the getuid(2) system
call returns the user-ID 0).
-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
Use the default policy as specified by db_deadlock.
DB_LOCK_OLDEST
Abort the oldest transaction.
DB_LOCK_RANDOM
Abort a random transaction involved in the deadlock.
DB_LOCK_YOUNGEST
Abort the youngest transaction.
-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 environmet to set the base segment
ID.
$env->set_isalive()
Set the callback that determines if the thread of control, identified by the pid and
tid arguments, is still running. This method should only be used in combination with
$env->failchk.
This option requires Berkeley DB 4.4 or better.
$env->failchk($flags)
The $env->failchk method checks for threads of control (either a true thread or a
process) that have exited while manipulating Berkeley DB library data structures,
while holding a logical database lock, or with an unresolved transaction (that is, a
transaction that was never aborted or committed).
If $env->failchk determines a thread of control exited while holding database read
locks, it will release those locks. If $env->failchk determines a thread of control
exited with an unresolved transaction, the transaction will be aborted.
Applications calling the $env->failchk method must have already called the
$env->set_isalive method, on the same DB environment, and must have configured their
database environment using the -ThreadCount flag. The ThreadCount flag cannot be used
on an environment that wasn't previously initialized with it.
This option requires Berkeley DB 4.4 or better.
$env->stat_print
Prints statistical information.
If the "MsgFile" option is specified the output will be sent to the file. Otherwise
output is sent to standard output.
This option requires Berkeley DB 4.3 or better.
$env->lock_stat_print
Prints locking subsystem statistics.
If the "MsgFile" option is specified the output will be sent to the file. Otherwise
output is sent to standard output.
This option requires Berkeley DB 4.3 or better.
$env->mutex_stat_print
Prints mutex subsystem statistics.
If the "MsgFile" option is specified the output will be sent to the file. Otherwise
output is sent to standard output.
This option requires Berkeley DB 4.4 or better.
$status = $env->get_blob_threshold($t1) ;
Sets the parameter $t1 to the threshold value (in bytes) that is used to determine
when a data item is stored as a Blob.
$status = $env->get_blob_dir($dir) ;
Sets the $dir parameter to the directory where blob files are stored.
$env->set_timeout($timeout, $flags)
$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::Heap
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 }, ],
[ -BlobThreshold=> $number, ]
[ -BlobDir => directory, ]
# BerkeleyDB::Hash specific
[ -Ffactor => number,]
[ -Nelem => number,]
[ -Hash => code reference,]
[ -DupCompare => code reference,]
and this
[$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 }, ],
[ -BlobThreshold=> $number, ]
[ -BlobDir => directory, ]
# BerkeleyDB::Hash specific
[ -Ffactor => number,]
[ -Nelem => number,]
[ -Hash => code reference,]
[ -DupCompare => code reference,]
When the "tie" interface is used, reading from and writing to the database is achieved via
the tied hash. In this case the database operates like a Perl associative array that
happens to be stored on disk.
In addition to the high-level tied hash interface, it is possible to make use of the
underlying methods provided by Berkeley DB
Options
In addition to the standard set of options (see "COMMON OPTIONS") BerkeleyDB::Hash
supports these options:
-Property
Used to specify extra flags when opening a database. The following flags may be
specified by bitwise OR'ing together one or more of the following values:
DB_DUP
When creating a new database, this flag enables the storing of duplicate keys in the
database. If DB_DUPSORT is not specified as well, the duplicates are stored in the
order they are created in the database.
DB_DUPSORT
Enables the sorting of duplicate keys in the database. Ignored if DB_DUP isn't also
specified.
-Ffactor
-Nelem
See the Berkeley DB documentation for details of these options.
-Hash
Allows you to provide a user defined hash function. If not specified, a default hash
function is used. Here is a template for a user-defined hash function
sub hash
{
my ($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 }, ],
[ -BlobThreshold=> $number, ]
[ -BlobDir => directory, ]
# BerkeleyDB::Btree specific
[ -Minkey => number,]
[ -Compare => code reference,]
[ -DupCompare => code reference,]
[ -Prefix => code reference,]
and this
[$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 }, ],
[ -BlobThreshold=> $number, ]
[ -BlobDir => directory, ]
# BerkeleyDB::Btree specific
[ -Minkey => number,]
[ -Compare => code reference,]
[ -DupCompare => code reference,]
[ -Prefix => code reference,]
Options
In addition to the standard set of options (see "COMMON OPTIONS") BerkeleyDB::Btree
supports these options:
-Property
Used to specify extra flags when opening a database. The following flags may be
specified by bitwise OR'ing together one or more of the following values:
DB_DUP
When creating a new database, this flag enables the storing of duplicate keys in the
database. If DB_DUPSORT is not specified as well, the duplicates are stored in the
order they are created in the database.
DB_DUPSORT
Enables the sorting of duplicate keys in the database. Ignored if DB_DUP isn't also
specified.
Minkey
TODO
Compare
Allow you to override the default sort order used in the database. See "Changing the
sort order" for an example.
sub compare
{
my ($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,
...
set_bt_compress
Enabled compression of the btree data. The callback interface is not supported at
present. Need Berkeley DB 4.8 or better.
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::Heap
Equivalent to calling db_create followed by DB->open with type DB_HEAP in Berkeley DB
5.2.x or greater. This database format isn't available if you use an older version of
Berkeley DB.
One form of constructor is supported:
$db = new BerkeleyDB::Heap
[ -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 }, ],
[ -BlobThreshold=> $number, ]
[ -BlobDir => directory, ]
# BerkeleyDB::Heap specific
[ -HeapSize => number, ]
[ -HeapSizeGb => number, ]
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.
$env = $db->Env();
Returns the environment object the database is associated with or "undef" when no
environment was used when opening the database.
$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
The variant "db_pget" allows you to query a secondary database:
$status = $sdb->db_pget($skey, $pkey, $value);
using the key $skey in the secondary db to lookup $pkey and $value from the primary db.
$status = $db->db_exists($key [, $flags])
This method checks for the existence of the given key ($key), but does not read the value.
If the key is not found, db_exists will return DB_NOTFOUND. Requires BDB 4.6 or better.
$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 = $env->stat_print([$flags])
Prints statistical information.
If the "MsgFile" option is specified the output will be sent to the file. Otherwise output
is sent to standard output.
This option requires Berkeley DB 4.3 or better.
$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
$status = $db->get_blob_threshold($t1) ;
Sets the parameter $t1 to the threshold value (in bytes) that is used to determine when a
data item is stored as a Blob.
$status = $db->get_blob_dir($dir) ;
Sets the $dir parameter to the directory where blob files are stored.
$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 explicitly 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 database 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 unused 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->compact(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.
• compact_fillpercent
• compact_timeout
The following keys, along with associated values, will be created in the hash reference if
the "compact" operation was successful.
• compact_deadlock
• compact_levels
• compact_pages_free
• compact_pages_examine
• compact_pages_truncated
You need to be running Berkeley DB 4.4 or better if you want to make use of "compact".
$status = $db->associate($secondary, \&key_callback)
Associate $db with the secondary DB $secondary
New key/value pairs inserted to the database will be passed to the callback which must set
its third argument to the secondary key to allow lookup. If an array reference is set
multiple keys secondary keys will be associated with the primary database entry.
Data may be retrieved fro the secondary database using "db_pget" to also obtain the
primary key.
Secondary databased are maintained automatically.
$status = $db->associate_foreign($secondary, callback, $flags)
Associate a foreign key database $db with the secondary DB $secondary.
The second parameter must be a reference to a sub or "undef".
The $flags parameter must be either "DB_FOREIGN_CASCADE", "DB_FOREIGN_ABORT" or
"DB_FOREIGN_NULLIFY".
When the flags parameter is "DB_FOREIGN_NULLIFY" the second parameter is a reference to a
sub of the form
sub foreign_cb
{
my $key = \$_[0];
my $value = \$_[1];
my $foreignkey = \$_[2];
my $changed = \$_[3] ;
# for ... set $$value and set $$changed to 1
return 0;
}
$foreign_db->associate_foreign($secondary, \&foreign_cb, DB_FOREIGN_NULLIFY);
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() ;
See "db_pget"
$status = $cursor->c_close()
Closes the cursor $cursor.
$stream = $cursor->db_stream($flags);
Create a BerkeleyDB::DbStream object to read the blob at the current cursor location. See
Blob for details of the the BerkeleyDB::DbStream object.
$flags must be one or more of the following OR'ed together
DB_STREAM_READ DB_STREAM_WRITE DB_STREAM_SYNC_WRITE
For full information on the flags refer to the Berkeley DB Reference Guide.
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
Transactions are created using the "txn_begin" method on BerkeleyDB::Env:
my $txn = $env->txn_begin;
If this is a nested transaction, supply the parent transaction as an argument:
my $child_txn = $env->txn_begin($parent_txn);
Then in order to work with the transaction, you must set it as the current transaction on
the database handles you want to work with:
$db->Txn($txn);
Or for multiple handles:
$txn->Txn(@handles);
The current transaction is given by BerkeleyDB each time to the various BDB operations. In
the C api it is required explicitly as an argument to every operation.
To commit a transaction call the "commit" method on it:
$txn->txn_commit;
and to roll back call abort:
$txn->txn_abort
After committing or aborting a child transaction you need to set the active transaction
again using "Txn".
BerkeleyDBB::DbStream -- support for BLOB
Blob support is available in Berkeley DB starting with version 6.0. Refer to the section
"Blob Support" in the Berkeley DB Programmer Reference for details of how Blob supports
works.
A Blob is access via a BerkeleyDBB::DbStream object. This is created via a cursor object.
# Note - error handling not shown below.
# Set the key we want
my $k = "some key";
# Don't want the value retrieved by the cursor,
# so use partial_set to make sure no data is retrieved.
my $v = '';
$cursor->partial_set(0,0) ;
$cursor->c_get($k, $v, DB_SET) ;
$cursor->partial_clear() ;
# Now create a stream to the blob
my $stream = $cursor->db_stream(DB_STREAM_WRITE) ;
# get the size of the blob
$stream->size(my $s) ;
# Read the first 1k of data from the blob
my $data ;
$stream->read($data, 0, 1024);
A BerkeleyDB::DbStream object has the following methods available:
$status = $stream->size($SIZE);
Outputs the length of the Blob in the $SIZE parameter.
$status = $stream->read($data, $offset, $size);
Read from the blob. $offset is the number of bytes from the start of the blob to read
from. $size if the number of bytes to read.
$status = $stream->write($data, $offset, $flags);
Write $data to the blob, starting at offset $offset.
Example
Below is an example of how to walk through a database when you don't know beforehand which
entries are blobs and which are not.
while (1)
{
my $k = '';
my $v = '';
$cursor->partial_set(0,0) ;
my $status = $cursor->c_get($k, $v, DB_NEXT) ;
$cursor->partial_clear();
last if $status != 0 ;
my $stream = $cursor->db_stream(DB_STREAM_WRITE);
if (defined $stream)
{
# It's a Blob
$stream->size(my $s) ;
}
else
{
# Not a Blob
$cursor->c_get($k, $v, DB_CURRENT) ;
}
}
Berkeley DB Concurrent Data Store (CDS)
The Berkeley DB Concurrent Data Store (CDS) is a lightweight locking mechanism that is
useful in scenarios where transactions are overkill.
What is CDS?
The Berkeley DB CDS interface is a simple lightweight locking mechanism that allows safe
concurrent access to Berkeley DB databases. Your application can have multiple reader and
write processes, but Berkeley DB will arrange it so that only one process can have a write
lock against the database at a time, i.e. multiple processes can read from a database
concurrently, but all write processes will be serialised.
Should I use it?
Whilst this simple locking model is perfectly adequate for some applications, it will be
too restrictive for others. Before deciding on using CDS mode, you need to be sure that it
is suitable for the expected behaviour of your application.
The key features of this model are
• All writes operations are serialised.
• A write operation will block until all reads have finished.
There are a few of the attributes of your application that you need to be aware of before
choosing to use CDS.
Firstly, if you application needs either recoverability or transaction support, then CDS
will not be suitable.
Next what is the ratio of read operation to write operations will your application have?
If it is carrying out mostly read operations, and very few writes, then CDS may be
appropriate.
What is the expected throughput of reads/writes in your application?
If you application does 90% writes and 10% reads, but on average you only have a
transaction every 5 seconds, then the fact that all writes are serialised will not matter,
because there will hardly ever be multiple writes processes blocking.
In summary CDS mode may be appropriate for your application if it performs mostly reads
and very few writes or there is a low throughput. Also, if you do not need to be able to
roll back a series of database operations if an error occurs, then CDS is ok.
If any of these is not the case you will need to use Berkeley DB transactions. That is
outside the scope of this document.
Locking Used
Berkeley DB implements CDS mode using two kinds of lock behind the scenes - namely read
locks and write locks. A read lock allows multiple processes to access the database for
reading at the same time. A write lock will only get access to the database when there are
no read or write locks active. The write lock will block until the process holding the
lock releases it.
Multiple processes with read locks can all access the database at the same time as long as
no process has a write lock. A process with a write lock can only access the database if
there are no other active read or write locks.
The majority of the time the Berkeley DB CDS mode will handle all locking without your
application having to do anything. There are a couple of exceptions you need to be aware
of though - these will be discussed in "Safely Updating Records" and "Implicit Cursors"
below.
A Berkeley DB Cursor (created with "$db->db_cursor") will by hold a lock on the database
until it is either explicitly closed or destroyed. This means the lock has the potential
to be long lived.
By default Berkeley DB cursors create a read lock, but it is possible to create a cursor
that holds a write lock, thus
$cursor = $db->db_cursor(DB_WRITECURSOR);
Whilst either a read or write cursor is active, it will block any other processes that
wants to write to the database.
To avoid blocking problems, only keep cursors open as long as they are needed. The same is
true when you use the "cursor" method or the "cds_lock" method.
For full information on CDS see the "Berkeley DB Concurrent Data Store applications"
section in the Berkeley DB Reference Guide.
Opening a database for CDS
Here is the typical signature that is used when opening a database in CDS mode.
use BerkeleyDB ;
my $env = new BerkeleyDB::Env
-Home => "./home" ,
-Flags => DB_CREATE| DB_INIT_CDB | DB_INIT_MPOOL
or die "cannot open environment: $BerkeleyDB::Error\n";
my $db = new BerkeleyDB::Hash
-Filename => 'test1.db',
-Flags => DB_CREATE,
-Env => $env
or die "cannot open database: $BerkeleyDB::Error\n";
or this, if you use the tied interface
tie %hash, "BerkeleyDB::Hash",
-Filename => 'test2.db',
-Flags => DB_CREATE,
-Env => $env
or die "cannot open database: $BerkeleyDB::Error\n";
The first thing to note is that you MUST always use a Berkeley DB environment if you want
to use locking with Berkeley DB.
Remember, that apart from the actual database files you explicitly create yourself,
Berkeley DB will create a few behind the scenes to handle locking - they usually have
names like "__db.001". It is therefore a good idea to use the "-Home" option, unless you
are happy for all these files to be written in the current directory.
Next, remember to include the "DB_CREATE" flag when opening the environment for the first
time. A common mistake is to forget to add this option and then wonder why the application
doesn't work.
Finally, it is vital that all processes that are going to access the database files use
the same Berkeley DB environment.
Safely Updating a Record
One of the main gotchas when using CDS is if you want to update a record in a database,
i.e. you want to retrieve a record from a database, modify it in some way and put it back
in the database.
For example, say you are writing a web application and you want to keep a record of the
number of times your site is accessed in a Berkeley DB database. So your code will have a
line of code like this (assume, of course, that %hash has been tied to a Berkeley DB
database):
$hash{Counter} ++ ;
That may look innocent enough, but there is a race condition lurking in there. If I
rewrite the line of code using the low-level Berkeley DB API, which is what will actually
be executed, the race condition may be more apparent:
$db->db_get("Counter", $value);
++ $value ;
$db->db_put("Counter", $value);
Consider what happens behind the scenes when you execute the commands above. Firstly, the
existing value for the key "Counter" is fetched from the database using "db_get". A read
lock will be used for this part of the update. The value is then incremented, and the new
value is written back to the database using "db_put". This time a write lock will be used.
Here's the problem - there is nothing to stop two (or more) processes executing the read
part at the same time. Remember multiple processes can hold a read lock on the database at
the same time. So both will fetch the same value, let's say 7, from the database. Both
increment the value to 8 and attempt to write it to the database. Berkeley DB will ensure
that only one of the processes gets a write lock, while the other will be blocked. So the
process that happened to get the write lock will store the value 8 to the database and
release the write lock. Now the other process will be unblocked, and it too will write the
value 8 to the database. The result, in this example, is we have missed a hit in the
counter.
To deal with this kind of scenario, you need to make the update atomic. A convenience
method, called "cds_lock", is supplied with the BerkeleyDB module for this purpose. Using
"cds_lock", the counter update code can now be rewritten thus:
my $lk = $dbh->cds_lock() ;
$hash{Counter} ++ ;
$lk->cds_unlock;
or this, where scoping is used to limit the lifetime of the lock object
{
my $lk = $dbh->cds_lock() ;
$hash{Counter} ++ ;
}
Similarly, "cds_lock" can be used with the native Berkeley DB API
my $lk = $dbh->cds_lock() ;
$db->db_get("Counter", $value);
++ $value ;
$db->db_put("Counter", $value);
$lk->unlock;
The "cds_lock" method will ensure that the current process has exclusive access to the
database until the lock is either explicitly released, via the "$lk->cds_unlock()" or by
the lock object being destroyed.
If you are interested, all that "cds_lock" does is open a "write" cursor. This has the
useful side-effect of holding a write-lock on the database until the cursor is deleted.
This is how you create a write-cursor
$cursor = $db->db_cursor(DB_WRITECURSOR);
If you have instantiated multiple "cds_lock" objects for one database within a single
process, that process will hold a write-lock on the database until ALL "cds_lock" objects
have been destroyed.
As with all write-cursors, you should try to limit the scope of the "cds_lock" to as short
a time as possible. Remember the complete database will be locked to other process whilst
the write lock is in place.
Cannot write with a read cursor while a write cursor is active
This issue is easier to demonstrate with an example, so consider the code below. The
intention of the code is to increment the values of all the elements in a database by one.
# Assume $db is a database opened in a CDS environment.
# Create a write-lock
my $lock = $db->db_cursor(DB_WRITECURSOR);
# or
# my $lock = $db->cds_lock();
my $cursor = $db->db_cursor();
# Now loop through the database, and increment
# each value using c_put.
while ($cursor->c_get($key, $value, DB_NEXT) == 0)
{
$cursor->c_put($key, $value+1, DB_CURRENT) == 0
or die "$BerkeleyDB::Error\n";
}
When this code is run, it will fail on the "c_put" line with this error
Write attempted on read-only cursor
The read cursor has automatically disallowed a write operation to prevent a deadlock.
So the rule is -- you CANNOT carry out a write operation using a read-only cursor (i.e.
you cannot use "c_put" or "c_del") whilst another write-cursor is already active.
The workaround for this issue is to just use "db_put" instead of "c_put", like this
# Assume $db is a database opened in a CDS environment.
# Create a write-lock
my $lock = $db->db_cursor(DB_WRITECURSOR);
# or
# my $lock = $db->cds_lock();
my $cursor = $db->db_cursor();
# Now loop through the database, and increment
# each value using c_put.
while ($cursor->c_get($key, $value, DB_NEXT) == 0)
{
$db->db_put($key, $value+1) == 0
or die "$BerkeleyDB::Error\n";
}
Implicit Cursors
All Berkeley DB cursors will hold either a read lock or a write lock on the database for
the existence of the cursor. In order to prevent blocking of other processes you need to
make sure that they are not long lived.
There are a number of instances where the Perl interface to Berkeley DB will create a
cursor behind the scenes without you being aware of it. Most of these are very short-lived
and will not affect the running of your script, but there are a few notable exceptions.
Consider this snippet of code
while (my ($k, $v) = each %hash)
{
# do something
}
To implement the "each" functionality, a read cursor will be created behind the scenes to
allow you to iterate through the tied hash, %hash. While that cursor is still active, a
read lock will obviously be held against the database. If your application has any other
writing processes, these will be blocked until the read cursor is closed. That won't
happen until the loop terminates.
To avoid blocking problems, only keep cursors open as long as they are needed. The same is
true when you use the "cursor" method or the "cds_lock" method.
The locking behaviour of the "values" or "keys" functions, shown below, is subtly
different.
foreach my $k (keys %hash)
{
# do something
}
foreach my $v (values %hash)
{
# do something
}
Just as in the "each" function, a read cursor will be created to iterate over the database
in both of these cases. Where "keys" and "values" differ is the place where the cursor
carries out the iteration through the database. Whilst "each" carried out a single
iteration every time it was invoked, the "keys" and "values" functions will iterate
through the entire database in one go -- the complete database will be read into memory
before the first iteration of the loop.
Apart from the fact that a read lock will be held for the amount of time required to
iterate through the database, the use of "keys" and "values" is not recommended because it
will result in the complete database being read into memory.
Avoiding Deadlock with multiple databases
If your CDS application uses multiple database files, and you need to write to more than
one of them, you need to be careful you don't create a deadlock.
For example, say you have two databases, D1 and D2, and two processes, P1 and P2. Assume
you want to write a record to each database. If P1 writes the records to the databases in
the order D1, D2 while process P2 writes the records in the order D2, D1, there is the
potential for a deadlock to occur.
This scenario can be avoided by either always acquiring the write locks in exactly the
same order in your application code, or by using the "DB_CDB_ALLDB" flag when opening the
environment. This flag will make a write-lock apply to all the databases in the
environment.
Add example here
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.oracle.com/technology/products/berkeley-db/db/index.html.
COPYRIGHT
Copyright (c) 1997-2013 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.oracle.com/technology/products/berkeley-db/db/index.html) 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>.
SEE ALSO
perl(1), DB_File, Berkeley DB.