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BerkeleyDB(3) User Contributed Perl Documentation BerkeleyDB(3) 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); $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->set_mutexlocks() $status = $env->set_flags() $status = $env->set_timeout() $status = $env->lock_detect() $status = $env->lsn_reset() $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 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, ] [ -MsgFile => filename, ] [ -ErrPrefix => "string", ] [ -Flags => number, ] [ -SetFlags => bitmask, ] [ -LockDetect => number, ] [ -TxMax => number, ] [ -LogConfig => number, ] [ -MaxLockers => number, ] [ -MaxLocks => number, ] [ -MaxObjects => 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 => $env ... -Server If present, this parameter should be the hostname of a server that is running the Berkeley DB RPC server. All databases will be accessed via the RPC server. -Encrypt If present, this parameter will enable encryption of all data before it is written to the database. This parameters must be given a hash reference. The format is shown below. -Encrypt => { -Password => "abc", Flags => DB_ENCRYPT_AES } Valid values for the Flags are 0 or "DB_ENCRYPT_AES". This option requires Berkeley DB 4.1 or better. -Cachesize If present, this parameter sets the size of the environments shared memory buffer pool. -TxMax If present, this parameter sets the number of simultaneous transactions that are allowed. Default 100. This default is definitely too low for programs using the MVCC capabilities. -LogConfig If present, this parameter is used to configure log options. -MaxLockers If present, this parameter is used to configure the maximum number of processes doing locking on the database. Default 1000. -MaxLocks If present, this parameter is used to configure the maximum number of locks on the database. Default 1000. This is often lower than required. -MaxObjects If present, this parameter is used to configure the maximum number of locked objects. Default 1000. This is often lower than required. -SharedMemKey If present, this parameter sets the base segment ID for the shared memory region used by Berkeley DB. This option requires Berkeley DB 3.1 or better. Use "$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. -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 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 environemt 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 environement, 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. $env->set_timeout($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 => $txn,] [ -Encrypt => { Password => "string", Flags => number }, ], # BerkeleyDB::Hash specific [ -Ffactor => number,] [ -Nelem => number,] [ -Hash => code reference,] [ -DupCompare => code reference,] and this [$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, ... 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 }, ], # 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 $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->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. 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". 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 sideeffect 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) 1997-2004 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.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. perl v5.12.5 2011-08-06 BerkeleyDB(3) |
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