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NAME
    AnyEvent::DBI - asynchronous DBI access

SYNOPSIS
       use AnyEvent::DBI;

       my $cv = AnyEvent->condvar;

       my $dbh = new AnyEvent::DBI "DBI:SQLite:dbname=test.db", "", "";

       $dbh->exec ("select * from test where num=?", 10, sub {
          my ($dbh, $rows, $rv) = @_;

          $#_ or die "failure: $@";

          print "@$_\n"
             for @$rows;

          $cv->broadcast;
       });

       # asynchronously do sth. else here

       $cv->wait;

DESCRIPTION
    This module is an AnyEvent user, you need to make sure that you use and
    run a supported event loop.

    This module implements asynchronous DBI access by forking or executing
    separate "DBI-Server" processes and sending them requests.

    It means that you can run DBI requests in parallel to other tasks.

    With DBD::mysql, the overhead for very simple statements ("select 0") is
    somewhere around 50% compared to an explicit
    prepare_cached/execute/fetchrow_arrayref/finish combination. With
    DBD::SQlite3, it's more like a factor of 8 for this trivial statement.

  ERROR HANDLING
    This module defines a number of functions that accept a callback
    argument. All callbacks used by this module get their AnyEvent::DBI
    handle object passed as first argument.

    If the request was successful, then there will be more arguments,
    otherwise there will only be the $dbh argument and $@ contains an error
    message.

    A convenient way to check whether an error occurred is to check $#_ - if
    that is true, then the function was successful, otherwise there was an
    error.

  METHODS
    $dbh = new AnyEvent::DBI $database, $user, $pass, [key => value]...
        Returns a database handle for the given database. Each database
        handle has an associated server process that executes statements in
        order. If you want to run more than one statement in parallel, you
        need to create additional database handles.

        The advantage of this approach is that transactions work as state is
        preserved.

        Example:

           $dbh = new AnyEvent::DBI
                     "DBI:mysql:test;mysql_read_default_file=/root/.my.cnf", "", "";

        Additional key-value pairs can be used to adjust behaviour:

        on_error => $callback->($dbh, $filename, $line, $fatal)
            When an error occurs, then this callback will be invoked. On
            entry, $@ is set to the error message. $filename and $line is
            where the original request was submitted.

            If the fatal argument is true then the database connection is
            shut down and your database handle became invalid. In addition
            to invoking the "on_error" callback, all of your queued request
            callbacks are called without only the $dbh argument.

            If omitted, then "die" will be called on any errors, fatal or
            not.

        on_connect => $callback->($dbh[, $success])
            If you supply an "on_connect" callback, then this callback will
            be invoked after the database connect attempt. If the connection
            succeeds, $success is true, otherwise it is missing and $@
            contains the $DBI::errstr.

            Regardless of whether "on_connect" is supplied, connect errors
            will result in "on_error" being called. However, if no
            "on_connect" callback is supplied, then connection errors are
            considered fatal. The client will "die" and the "on_error"
            callback will be called with $fatal true.

            When on_connect is supplied, connect error are not fatal and
            AnyEvent::DBI will not "die". You still cannot, however, use the
            $dbh object you received from "new" to make requests.

        fork_template => $AnyEvent::Fork-object
            "AnyEvent::DBI" uses "AnyEvent::Fork->new" to create the
            database slave, which in turn either "exec"'s a new process
            (similar to the old "exec_server" constructor argument) or uses
            a process forked early (see AnyEvent::Fork::Early).

            With this argument you can provide your own fork template. This
            can be useful if you create a lot of "AnyEvent::DBI" handles and
            want to save memory (And speed up startup) by not having to load
            "AnyEvent::DBI" again and again into your child processes:

               my $template = AnyEvent::Fork
                  ->new                               # create new template
                  ->require ("AnyEvent::DBI::Slave"); # preload AnyEvent::DBI::Slave module

               for (...) {
                  $dbh = new AnyEvent::DBI ...
                     fork_template => $template;

        timeout => seconds
            If you supply a timeout parameter (fractional values are
            supported), then a timer is started any time the DBI handle
            expects a response from the server. This includes connection
            setup as well as requests made to the backend. The timeout spans
            the duration from the moment the first data is written (or
            queued to be written) until all expected responses are returned,
            but is postponed for "timeout" seconds each time more data is
            returned from the server. If the timer ever goes off then a
            fatal error is generated. If you have an "on_error" handler
            installed, then it will be called, otherwise your program will
            die().

            When altering your databases with timeouts it is wise to use
            transactions. If you quit due to timeout while performing
            insert, update or schema-altering commands you can end up not
            knowing if the action was submitted to the database,
            complicating recovery.

            Timeout errors are always fatal.