NAME
    BDB - Asynchronous Berkeley DB access

SYNOPSIS
     use BDB;

     my $env = db_env_create;

     mkdir "bdtest", 0700;
     db_env_open
        $env,
        "bdtest",
        BDB::INIT_LOCK | BDB::INIT_LOG | BDB::INIT_MPOOL
        | BDB::INIT_TXN | BDB::RECOVER | BDB::USE_ENVIRON | BDB::CREATE,
        0600;

     $env->set_flags (BDB::AUTO_COMMIT | BDB::TXN_NOSYNC, 1);

     my $db = db_create $env;
     db_open $db, undef, "table", undef, BDB::BTREE, BDB::AUTO_COMMIT | BDB::CREATE
                                         | BDB::READ_UNCOMMITTED, 0600;
     db_put $db, undef, "key", "data", 0, sub {
        db_del $db, undef, "key";
     };
     db_sync $db;

     # when you also use Coro, management is easy:
     use Coro::BDB;

     # automatic event loop integration with AnyEvent:
     use AnyEvent::BDB;

     # automatic result processing with EV:
     my $WATCHER = EV::io BDB::poll_fileno, EV::READ, \&BDB::poll_cb;

     # with Glib:
     add_watch Glib::IO BDB::poll_fileno,
               in => sub { BDB::poll_cb; 1 };

     # or simply flush manually
     BDB::flush;

DESCRIPTION
    See the BerkeleyDB documentation
    (<http://www.oracle.com/technology/documentation/berkeley-db/db/index.ht
    ml>). The BDB API is very similar to the C API (the translation has been
    very faithful).

    See also the example sections in the document below and possibly the eg/
    subdirectory of the BDB distribution. Last not least see the IO::AIO
    documentation, as that module uses almost the same asynchronous request
    model as this module.

    I know this is woefully inadequate documentation. Send a patch!

REQUEST ANATOMY AND LIFETIME
    Every request method creates a request. which is a C data structure not
    directly visible to Perl.

    During their existance, bdb requests travel through the following
    states, in order:

    ready
        Immediately after a request is created it is put into the ready
        state, waiting for a thread to execute it.

    execute
        A thread has accepted the request for processing and is currently
        executing it (e.g. blocking in read).

    pending
        The request has been executed and is waiting for result processing.

        While request submission and execution is fully asynchronous, result
        processing is not and relies on the perl interpreter calling
        "poll_cb" (or another function with the same effect).

    result
        The request results are processed synchronously by "poll_cb".

        The "poll_cb" function will process all outstanding aio requests by
        calling their callbacks, freeing memory associated with them and
        managing any groups they are contained in.

    done
        Request has reached the end of its lifetime and holds no resources
        anymore (except possibly for the Perl object, but its connection to
        the actual aio request is severed and calling its methods will
        either do nothing or result in a runtime error).

BERKELEYDB FUNCTIONS
    All of these are functions. The create functions simply return a new
    object and never block. All the remaining functions take an optional
    callback as last argument. If it is missing, then the function will be
    executed synchronously. In both cases, $! will reflect the return value
    of the function.

    BDB functions that cannot block (mostly functions that manipulate
    settings) are method calls on the relevant objects, so the rule of thumb
    is: if it's a method, it's not blocking, if it's a function, it takes a
    callback as last argument.

    In the following, $int signifies an integer return value, "bdb_filename"
    is a "filename" (octets on unix, madness on windows), "U32" is an
    unsigned 32 bit integer, "int" is some integer, "NV" is a floating point
    value.

    Most "SV *" types are generic perl scalars (for input and output of data
    values).

    The various "DB_ENV" etc. arguments are handles return by
    "db_env_create", "db_create", "txn_begin" and so on. If they have an
    appended "_ornull" this means they are optional and you can pass "undef"
    for them, resulting a NULL pointer on the C level.

    The "SV *callback" is the optional callback function to call when the
    request is completed. This last callback argument is special: the
    callback is simply the last argument passed. If there are "optional"
    arguments before the callback they can be left out. The callback itself
    can be left out or specified as "undef", in which case the function will
    be executed synchronously.

    For example, "db_env_txn_checkpoint" usually is called with all integer
    arguments zero. These can be left out, so all of these specify a call to
    "DB_ENV->txn_checkpoint", to be executed asynchronously with a callback
    to be called:

       db_env_txn_checkpoint $db_env, 0, 0, 0, sub { };
       db_env_txn_checkpoint $db_env, 0, 0, sub { };
       db_env_txn_checkpoint $db_env, sub { };

    While these all specify a call to "DB_ENV->txn_checkpoint" to be
    executed synchronously:

       db_env_txn_checkpoint $db_env, 0, 0, 0, undef;
       db_env_txn_checkpoint $db_env, 0, 0, 0;
       db_env_txn_checkpoint $db_env, 0;

  BDB functions
    Functions in the BDB namespace, exported by default:

       $env = db_env_create (U32 env_flags = 0)
          flags: RPCCLIENT

       db_env_open (DB_ENV *env, bdb_filename db_home, U32 open_flags, int mode, SV *callback = 0)
          open_flags: INIT_CDB INIT_LOCK INIT_LOG INIT_MPOOL INIT_REP INIT_TXN RECOVER RECOVER_FATAL USE_ENVIRON USE_ENVIRON_ROOT CREATE LOCKDOWN PRIVATE REGISTER SYSTEM_MEM
       db_env_close (DB_ENV *env, U32 flags = 0, SV *callback = 0)
       db_env_txn_checkpoint (DB_ENV *env, U32 kbyte = 0, U32 min = 0, U32 flags = 0, SV *callback = 0)
          flags: FORCE
       db_env_lock_detect (DB_ENV *env, U32 flags = 0, U32 atype = DB_LOCK_DEFAULT, SV *dummy = 0, SV *callback = 0)
          atype: LOCK_DEFAULT LOCK_EXPIRE LOCK_MAXLOCKS LOCK_MAXWRITE LOCK_MINLOCKS LOCK_MINWRITE LOCK_OLDEST LOCK_RANDOM LOCK_YOUNGEST
       db_env_memp_sync (DB_ENV *env, SV *dummy = 0, SV *callback = 0)
       db_env_memp_trickle (DB_ENV *env, int percent, SV *dummy = 0, SV *callback = 0)
       db_env_dbremove (DB_ENV *env, DB_TXN_ornull *txnid, bdb_filename file, bdb_filename database, U32 flags = 0, SV *callback = 0)
       db_env_dbrename (DB_ENV *env, DB_TXN_ornull *txnid, bdb_filename file, bdb_filename database, bdb_filename newname, U32 flags = 0, SV *callback = 0)
       db_env_log_archive (DB_ENV *env, SV *listp, U32 flags = 0, SV *callback = 0)
       db_env_lsn_reset (DB_ENV *env, bdb_filename db, U32 flags = 0, SV *callback = 0)
       db_env_fileid_reset (DB_ENV *env, bdb_filename db, U32 flags = 0, SV *callback = 0)

       $db = db_create (DB_ENV *env = 0, U32 flags = 0)
          flags: XA_CREATE

       db_open (DB *db, DB_TXN_ornull *txnid, bdb_filename file, bdb_filename database, int type, U32 flags, int mode, SV *callback = 0)
          flags: AUTO_COMMIT CREATE EXCL MULTIVERSION NOMMAP RDONLY READ_UNCOMMITTED THREAD TRUNCATE
       db_close (DB *db, U32 flags = 0, SV *callback = 0)
          flags: DB_NOSYNC
       db_verify (DB *db, bdb_filename file, bdb_filename database = 0, SV *dummy = 0, U32 flags = 0, SV *callback = 0)
       db_upgrade (DB *db, bdb_filename file, U32 flags = 0, SV *callback = 0)
       db_compact (DB *db, DB_TXN_ornull *txn = 0, SV *start = 0, SV *stop = 0, SV *unused1 = 0, U32 flags = DB_FREE_SPACE, SV *unused2 = 0, SV *callback = 0)
          flags: FREELIST_ONLY FREE_SPACE
       db_sync (DB *db, U32 flags = 0, SV *callback = 0)
       db_key_range (DB *db, DB_TXN_ornull *txn, SV *key, SV *key_range, U32 flags = 0, SV *callback = 0)
       db_put (DB *db, DB_TXN_ornull *txn, SV *key, SV *data, U32 flags = 0, SV *callback = 0)
          flags: APPEND NODUPDATA NOOVERWRITE
       db_exists (DB *db, DB_TXN_ornull *txn, SV *key, U32 flags = 0, SV *callback = 0) (v4.6)
       db_get (DB *db, DB_TXN_ornull *txn, SV *key, SV *data, U32 flags = 0, SV *callback = 0)
          flags: CONSUME CONSUME_WAIT GET_BOTH SET_RECNO MULTIPLE READ_COMMITTED READ_UNCOMMITTED RMW
       db_pget (DB *db, DB_TXN_ornull *txn, SV *key, SV *pkey, SV *data, U32 flags = 0, SV *callback = 0)
          flags: CONSUME CONSUME_WAIT GET_BOTH SET_RECNO MULTIPLE READ_COMMITTED READ_UNCOMMITTED RMW
       db_del (DB *db, DB_TXN_ornull *txn, SV *key, U32 flags = 0, SV *callback = 0)
       db_txn_commit (DB_TXN *txn, U32 flags = 0, SV *callback = 0)
          flags: TXN_NOSYNC TXN_SYNC
       db_txn_abort (DB_TXN *txn, SV *callback = 0)

       db_c_close (DBC *dbc, SV *callback = 0)
       db_c_count (DBC *dbc, SV *count, U32 flags = 0, SV *callback = 0)
       db_c_put (DBC *dbc, SV *key, SV *data, U32 flags = 0, SV *callback = 0)
          flags: AFTER BEFORE CURRENT KEYFIRST KEYLAST NODUPDATA
       db_c_get (DBC *dbc, SV *key, SV *data, U32 flags = 0, SV *callback = 0)
          flags: CURRENT FIRST GET_BOTH GET_BOTH_RANGE GET_RECNO JOIN_ITEM LAST NEXT NEXT_DUP NEXT_NODUP PREV PREV_DUP PREV_NODUP SET SET_RANGE SET_RECNO READ_UNCOMMITTED MULTIPLE MULTIPLE_KEY RMW
       db_c_pget (DBC *dbc, SV *key, SV *pkey, SV *data, U32 flags = 0, SV *callback = 0)
       db_c_del (DBC *dbc, U32 flags = 0, SV *callback = 0)

       db_sequence_open (DB_SEQUENCE *seq, DB_TXN_ornull *txnid, SV *key, U32 flags = 0, SV *callback = 0)
          flags: CREATE EXCL
       db_sequence_close (DB_SEQUENCE *seq, U32 flags = 0, SV *callback = 0)
       db_sequence_get (DB_SEQUENCE *seq, DB_TXN_ornull *txnid, int delta, SV *seq_value, U32 flags = DB_TXN_NOSYNC, SV *callback = 0)
          flags: TXN_NOSYNC
       db_sequence_remove (DB_SEQUENCE *seq, DB_TXN_ornull *txnid = 0, U32 flags = 0, SV *callback = 0)
          flags: TXN_NOSYNC

   db_txn_finish (DB_TXN *txn, U32 flags = 0, SV *callback = 0)
    This is not actually a Berkeley DB function but a BDB module extension.
    The background for this exytension is: It is very annoying to have to
    check every single BDB function for error returns and provide a codepath
    out of your transaction. While the BDB module still makes this possible,
    it contains the following extensions:

    When a transaction-protected function returns any operating system error
    (errno > 0), BDB will set the "TXN_DEADLOCK" flag on the transaction.
    This flag is also set by Berkeley DB functions themselves when an
    operation fails with LOCK_DEADLOCK, and it causes all further operations
    on that transaction (including "db_txn_commit") to fail.

    The "db_txn_finish" request will look at this flag, and, if it is set,
    will automatically call "db_txn_abort" (setting errno to "LOCK_DEADLOCK"
    if it isn't set to something else yet). If it isn't set, it will call
    "db_txn_commit" and return the error normally.

    How to use this? Easy: just write your transaction normally:

       my $txn = $db_env->txn_begin;
       db_get $db, $txn, "key", my $data;
       db_put $db, $txn, "key", $data + 1 unless $! == BDB::NOTFOUND;
       db_txn_finish $txn;
       die "transaction failed" if $!;

    That is, handle only the expected errors. If something unexpected
    happens (EIO, LOCK_NOTGRANTED or a deadlock in either db_get or db_put),
    then the remaining requests (db_put in this case) will simply be skipped
    (they will fail with LOCK_DEADLOCK) and the transaction will be aborted.

    You can use the "$txn->failed" method to check wether a transaction has
    failed in this way and abort further processing (excluding
    "db_txn_finish").

  DB_ENV/database environment methods
    Methods available on DB_ENV/$env handles:

       DESTROY (DB_ENV_ornull *env)
               CODE:
               if (env)
                 env->close (env, 0);

       $int = $env->set_data_dir (const char *dir)
       $int = $env->set_tmp_dir (const char *dir)
       $int = $env->set_lg_dir (const char *dir)
       $int = $env->set_shm_key (long shm_key)
       $int = $env->set_cachesize (U32 gbytes, U32 bytes, int ncache = 0)
       $int = $env->set_flags (U32 flags, int onoff = 1)
       $int = $env->log_set_config (U32 flags, int onoff = 1) (v4.7)
       $int = $env->set_intermediate_dir_mode (const char *modestring) (v4.7)
       $env->set_errfile (FILE *errfile = 0)
       $env->set_msgfile (FILE *msgfile = 0)
       $int = $env->set_verbose (U32 which, int onoff = 1)
       $int = $env->set_encrypt (const char *password, U32 flags = 0)
       $int = $env->set_timeout (NV timeout_seconds, U32 flags = SET_TXN_TIMEOUT)
       $int = $env->set_mp_max_openfd (int maxopenfd);
       $int = $env->set_mp_max_write (int maxwrite, int maxwrite_sleep);
       $int = $env->set_mp_mmapsize (int mmapsize_mb)
       $int = $env->set_lk_detect (U32 detect = DB_LOCK_DEFAULT)
       $int = $env->set_lk_max_lockers (U32 max)
       $int = $env->set_lk_max_locks (U32 max)
       $int = $env->set_lk_max_objects (U32 max)
       $int = $env->set_lg_bsize (U32 max)
       $int = $env->set_lg_max (U32 max)
       $int = $env->mutex_set_increment (U32 increment)
       $int = $env->mutex_set_tas_spins (U32 tas_spins)
       $int = $env->mutex_set_max (U32 max)
       $int = $env->mutex_set_align (U32 align)

       $txn = $env->txn_begin (DB_TXN_ornull *parent = 0, U32 flags = 0)
          flags: READ_COMMITTED READ_UNCOMMITTED TXN_NOSYNC TXN_NOWAIT TXN_SNAPSHOT TXN_SYNC TXN_WAIT TXN_WRITE_NOSYNC
       $txn = $env->cdsgroup_begin; (v4.5)

   Example:
       use AnyEvent;
       use BDB;

       our $FH; open $FH, "<&=" . BDB::poll_fileno;
       our $WATCHER = AnyEvent->io (fh => $FH, poll => 'r', cb => \&BDB::poll_cb);

       BDB::min_parallel 8;

       my $env = db_env_create;

       mkdir "bdtest", 0700;
       db_env_open
          $env,
          "bdtest",
          BDB::INIT_LOCK | BDB::INIT_LOG | BDB::INIT_MPOOL | BDB::INIT_TXN | BDB::RECOVER | BDB::USE_ENVIRON | BDB::CREATE,
          0600;

       $env->set_flags (BDB::AUTO_COMMIT | BDB::TXN_NOSYNC, 1);

  DB/database methods
    Methods available on DB/$db handles:

       DESTROY (DB_ornull *db)
               CODE:
               if (db)
                 {
                   SV *env = (SV *)db->app_private;
                   db->close (db, 0);
                   SvREFCNT_dec (env);
                 }

       $int = $db->set_cachesize (U32 gbytes, U32 bytes, int ncache = 0)
       $int = $db->set_flags (U32 flags)
          flags: CHKSUM ENCRYPT TXN_NOT_DURABLE
                 Btree: DUP DUPSORT RECNUM REVSPLITOFF
                 Hash:  DUP DUPSORT
                 Queue: INORDER
                 Recno: RENUMBER SNAPSHOT

       $int = $db->set_encrypt (const char *password, U32 flags)
       $int = $db->set_lorder (int lorder)
       $int = $db->set_bt_minkey (U32 minkey)
       $int = $db->set_re_delim (int delim)
       $int = $db->set_re_pad (int re_pad)
       $int = $db->set_re_source (char *source)
       $int = $db->set_re_len (U32 re_len)
       $int = $db->set_h_ffactor (U32 h_ffactor)
       $int = $db->set_h_nelem (U32 h_nelem)
       $int = $db->set_q_extentsize (U32 extentsize)

       $dbc = $db->cursor (DB_TXN_ornull *txn = 0, U32 flags = 0)
          flags: READ_COMMITTED READ_UNCOMMITTED WRITECURSOR TXN_SNAPSHOT
       $seq = $db->sequence (U32 flags = 0)

   Example:
       my $db = db_create $env;
       db_open $db, undef, "table", undef, BDB::BTREE, BDB::AUTO_COMMIT | BDB::CREATE | BDB::READ_UNCOMMITTED, 0600;

       for (1..1000) {
          db_put $db, undef, "key $_", "data $_";

          db_key_range $db, undef, "key $_", my $keyrange;
          my ($lt, $eq, $gt) = @$keyrange;
       }

       db_del $db, undef, "key $_" for 1..1000;

       db_sync $db;

  DB_TXN/transaction methods
    Methods available on DB_TXN/$txn handles:

       DESTROY (DB_TXN_ornull *txn)
               CODE:
               if (txn)
                 txn->abort (txn);

       $int = $txn->set_timeout (NV timeout_seconds, U32 flags = SET_TXN_TIMEOUT)
          flags: SET_LOCK_TIMEOUT SET_TXN_TIMEOUT

       $bool = $txn->failed
       # see db_txn_finish documentation, above

  DBC/cursor methods
    Methods available on DBC/$dbc handles:

       DESTROY (DBC_ornull *dbc)
               CODE:
               if (dbc)
                 dbc->c_close (dbc);

       $int = $cursor->set_priority ($priority = PRIORITY_*) (v4.6)

   Example:
       my $c = $db->cursor;

       for (;;) {
          db_c_get $c, my $key, my $data, BDB::NEXT;
          warn "<$!,$key,$data>";
          last if $!;
       }

       db_c_close $c;

  DB_SEQUENCE/sequence methods
    Methods available on DB_SEQUENCE/$seq handles:

       DESTROY (DB_SEQUENCE_ornull *seq)
               CODE:
               if (seq)
                 seq->close (seq, 0);

       $int = $seq->initial_value (db_seq_t value)
       $int = $seq->set_cachesize (U32 size)
       $int = $seq->set_flags (U32 flags)
          flags: SEQ_DEC SEQ_INC SEQ_WRAP
       $int = $seq->set_range (db_seq_t min, db_seq_t max)

   Example:
       my $seq = $db->sequence;
      
       db_sequence_open $seq, undef, "seq", BDB::CREATE;
       db_sequence_get $seq, undef, 1, my $value;

SUPPORT FUNCTIONS
  EVENT PROCESSING AND EVENT LOOP INTEGRATION
    $msg = BDB::strerror [$errno]
        Returns the string corresponding to the given errno value. If no
        argument is given, use $!.

        Note that the BDB module also patches the $! variable directly, so
        you should be able to get a bdb error string by simply stringifying
        $!.

    $fileno = BDB::poll_fileno
        Return the *request result pipe file descriptor*. This filehandle
        must be polled for reading by some mechanism outside this module
        (e.g. Event or select, see below or the SYNOPSIS). If the pipe
        becomes readable you have to call "poll_cb" to check the results.

        See "poll_cb" for an example.

    BDB::poll_cb
        Process some outstanding events on the result pipe. You have to call
        this regularly. Returns the number of events processed. Returns
        immediately when no events are outstanding. The amount of events
        processed depends on the settings of "BDB::max_poll_req" and
        "BDB::max_poll_time".

        If not all requests were processed for whatever reason, the
        filehandle will still be ready when "poll_cb" returns.

        Example: Install an Event watcher that automatically calls
        BDB::poll_cb with high priority:

           Event->io (fd => BDB::poll_fileno,
                      poll => 'r', async => 1,
                      cb => \&BDB::poll_cb);

    BDB::max_poll_reqs $nreqs
    BDB::max_poll_time $seconds
        These set the maximum number of requests (default 0, meaning
        infinity) that are being processed by "BDB::poll_cb" in one call,
        respectively the maximum amount of time (default 0, meaning
        infinity) spent in "BDB::poll_cb" to process requests (more
        correctly the mininum amount of time "poll_cb" is allowed to use).

        Setting "max_poll_time" to a non-zero value creates an overhead of
        one syscall per request processed, which is not normally a problem
        unless your callbacks are really really fast or your OS is really
        really slow (I am not mentioning Solaris here). Using
        "max_poll_reqs" incurs no overhead.

        Setting these is useful if you want to ensure some level of
        interactiveness when perl is not fast enough to process all requests
        in time.

        For interactive programs, values such as 0.01 to 0.1 should be fine.

        Example: Install an EV watcher that automatically calls BDB::poll_cb
        with low priority, to ensure that other parts of the program get the
        CPU sometimes even under high load.

           # try not to spend much more than 0.1s in poll_cb
           BDB::max_poll_time 0.1;

           my $bdb_poll = EV::io BDB::poll_fileno, EV::READ, \&BDB::poll_cb);

    BDB::poll_wait
        If there are any outstanding requests and none of them in the result
        phase, wait till the result filehandle becomes ready for reading
        (simply does a "select" on the filehandle. This is useful if you
        want to synchronously wait for some requests to finish).

        See "nreqs" for an example.

    BDB::poll
        Waits until some requests have been handled.

        Returns the number of requests processed, but is otherwise strictly
        equivalent to:

           BDB::poll_wait, BDB::poll_cb

    BDB::flush
        Wait till all outstanding BDB requests have been handled.

        Strictly equivalent to:

           BDB::poll_wait, BDB::poll_cb
              while BDB::nreqs;

  VERSION CHECKING
    BerkeleyDB comes in various versions, many of them have minor
    incompatibilities. This means that traditional "at least version x.x"
    checks are often not sufficient.

    Example: set the log_autoremove option in a way compatible with <v4.7
    and v4.7. Note the use of & on the constants to avoid triggering a
    compiletime bug when the symbol isn't available.

       $DB_ENV->set_flags      (&BDB::LOG_AUTOREMOVE ) if BDB::VERSION v0, v4.7;
       $DB_ENV->log_set_config (&BDB::LOG_AUTO_REMOVE) if BDB::VERSION v4.7;

    BDB::VERSION
        The "BDB::VERSION" function, when called without arguments, returns
        the Berkeley DB version as a v-string (usually with 3 components).
        You should use "lt" and "ge" operators exclusively to make
        comparisons.

        Example: check for at least version 4.7.

           BDB::VERSION ge v4.7 or die;

    BDB::VERSION min-version
        Returns true if the BDB version is at least the given version
        (specified as a v-string), false otherwise.

        Example: check for at least version 4.5.

           BDB::VERSION v4.7 or die;

    BDB::VERSION min-version, max-version
        Returns true of the BDB version is at least version "min-version"
        (specify "undef" or "v0" for any minimum version) and less then
        "max-version".

        Example: check wether version is strictly less then v4.7.

           BDB::VERSION v0, v4.7
              or die "version 4.7 is not yet supported";

  CONTROLLING THE NUMBER OF THREADS
    BDB::min_parallel $nthreads
        Set the minimum number of BDB threads to $nthreads. The current
        default is 8, which means eight asynchronous operations can execute
        concurrently at any one time (the number of outstanding requests,
        however, is unlimited).

        BDB starts threads only on demand, when an BDB request is queued and
        no free thread exists. Please note that queueing up a hundred
        requests can create demand for a hundred threads, even if it turns
        out that everything is in the cache and could have been processed
        faster by a single thread.

        It is recommended to keep the number of threads relatively low, as
        some Linux kernel versions will scale negatively with the number of
        threads (higher parallelity => MUCH higher latency). With current
        Linux 2.6 versions, 4-32 threads should be fine.

        Under most circumstances you don't need to call this function, as
        the module selects a default that is suitable for low to moderate
        load.

    BDB::max_parallel $nthreads
        Sets the maximum number of BDB threads to $nthreads. If more than
        the specified number of threads are currently running, this function
        kills them. This function blocks until the limit is reached.

        While $nthreads are zero, aio requests get queued but not executed
        until the number of threads has been increased again.

        This module automatically runs "max_parallel 0" at program end, to
        ensure that all threads are killed and that there are no outstanding
        requests.

        Under normal circumstances you don't need to call this function.

    BDB::max_idle $nthreads
        Limit the number of threads (default: 4) that are allowed to idle
        (i.e., threads that did not get a request to process within 10
        seconds). That means if a thread becomes idle while $nthreads other
        threads are also idle, it will free its resources and exit.

        This is useful when you allow a large number of threads (e.g. 100 or
        1000) to allow for extremely high load situations, but want to free
        resources under normal circumstances (1000 threads can easily
        consume 30MB of RAM).

        The default is probably ok in most situations, especially if thread
        creation is fast. If thread creation is very slow on your system you
        might want to use larger values.

    $oldmaxreqs = BDB::max_outstanding $maxreqs
        This is a very bad function to use in interactive programs because
        it blocks, and a bad way to reduce concurrency because it is
        inexact: Better use an "aio_group" together with a feed callback.

        Sets the maximum number of outstanding requests to $nreqs. If you to
        queue up more than this number of requests, the next call to the
        "poll_cb" (and "poll_some" and other functions calling "poll_cb")
        function will block until the limit is no longer exceeded.

        The default value is very large, so there is no practical limit on
        the number of outstanding requests.

        You can still queue as many requests as you want. Therefore,
        "max_oustsanding" is mainly useful in simple scripts (with low
        values) or as a stop gap to shield against fatal memory overflow
        (with large values).

    $old_cb = BDB::set_sync_prepare $cb
        Sets a callback that is called whenever a request is created without
        an explicit callback. It has to return two code references. The
        first is used as the request callback (it should save the return
        status), and the second is called to wait until the first callback
        has been called (it must set $! to the return status).

        This mechanism can be used to include BDB into other event
        mechanisms, such as Coro::BDB.

        To allow other, callback-based, events to be executed while
        callback-less ones are run, you could use this sync prepare
        function:

           sub {
              my $status;
              (
                 sub { $status = $! },
                 sub { BDB::poll while !defined $status; $! = $status },
              )
           }

        It works by polling for results till the request has finished and
        then sets $! to the return value. This means that if you don't use a
        callback, BDB would simply fall back to synchronous operations.

        By default, or if the sync prepare function is set to "undef", is to
        execute callback-less BDB requests in the foreground thread, setting
        $! to the return value, without polling for other events.

  STATISTICAL INFORMATION
    BDB::nreqs
        Returns the number of requests currently in the ready, execute or
        pending states (i.e. for which their callback has not been invoked
        yet).

        Example: wait till there are no outstanding requests anymore:

           BDB::poll_wait, BDB::poll_cb
              while BDB::nreqs;

    BDB::nready
        Returns the number of requests currently in the ready state (not yet
        executed).

    BDB::npending
        Returns the number of requests currently in the pending state
        (executed, but not yet processed by poll_cb).

COMMON PITFALLS
  Unexpected Crashes
    Remember that, by default, BDB will execute requests in parallel, in
    somewhat random order. That means that it is easy to run a "db_get"
    request on the same database as a concurrent "db_close" request, leading
    to a crash, silent data corruption, eventually the next world war on
    terrorism.

    If you only ever use foreground requests (without a callback), this will
    not be an issue (unless you use threads).

  Unexpected Freezes or Deadlocks
    Remember that, by default, BDB will execute requests in parallel, which
    easily leads to deadlocks (even concurrent put's on the same database
    can deadlock).

    You either need to run deadlock detection (and handle the resulting
    errors), or make sure only one process ever updates the database, ine
    one thread, e.g. by using only foreground requests (without a callback).

FORK BEHAVIOUR
    This module should do "the right thing" when the process using it forks:

    Before the fork, BDB enters a quiescent state where no requests can be
    added in other threads and no results will be processed. After the fork
    the parent simply leaves the quiescent state and continues
    request/result processing, while the child frees the request/result
    queue (so that the requests started before the fork will only be handled
    in the parent). Threads will be started on demand until the limit set in
    the parent process has been reached again.

    In short: the parent will, after a short pause, continue as if fork had
    not been called, while the child will act as if BDB has not been used
    yet.

    Win32 note: there is no fork on win32, and perls emulation of it is too
    broken to be supported, so do not use BDB in a windows pseudo-fork,
    better yet, switch to a more capable platform.

MEMORY USAGE
    Per-request usage:

    Each aio request uses - depending on your architecture - around 100-200
    bytes of memory. In addition, stat requests need a stat buffer (possibly
    a few hundred bytes), readdir requires a result buffer and so on. Perl
    scalars and other data passed into aio requests will also be locked and
    will consume memory till the request has entered the done state.

    This is not awfully much, so queuing lots of requests is not usually a
    problem.

    Per-thread usage:

    In the execution phase, some aio requests require more memory for
    temporary buffers, and each thread requires a stack and other data
    structures (usually around 16k-128k, depending on the OS).

WIN32 FILENAMES/DATABASE NAME MESS
    Perl on Win32 supports only ASCII filenames (the reason is that it
    abuses an internal flag to store wether a filename is Unicode or ANSI,
    but that flag is used for somethign else in the perl core, so there is
    no way to detect wether a filename is ANSI or Unicode-encoded). The BDB
    module tries to work around this issue by assuming that the filename is
    an ANSI filename and BDB was built for unicode support.

KNOWN BUGS
    Known bugs will be fixed in the next release, except:

       If you use a transaction in any request, and the request returns
       with an operating system error or DB_LOCK_NOTGRANTED, the internal
       TXN_DEADLOCK flag will be set on the transaction. See C<db_txn_finish>,
       above.

SEE ALSO
    AnyEvent::BDB (event loop integration), Coro::BDB (more natural syntax),
    IO::AIO (nice to have).

AUTHOR
     Marc Lehmann <schmorp@schmorp.de>
     http://home.schmorp.de/