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=head1 NAME

Coro::Mysql - let other threads run while doing mysql/mariadb requests

=head1 SYNOPSIS

 use Coro::Mysql;

 my $DBH = Coro::Mysql::unblock DBI->connect (...);

=head1 DESCRIPTION

(Note that in this manual, "thread" refers to real threads as implemented
by the L<Coro> module, not to the built-in windows process emulation which
unfortunately is also called "threads").

This module replaces the I/O handlers for a database connection, with the
effect that "patched" database handles no longer block all threads of a
process, but only the thread that does the request. It should work for
both L<DBD::mysql> and L<DBD::MariaDB> connections and a wide range of
mariadb/mysql client libraries.

This can be used to make parallel sql requests using Coro, or to do other
stuff while mariadb is rumbling in the background.

=head2 CAVEAT

Note that this module must be linked against exactly the same (shared,
possibly not working with all OSes) F<libmariadb>/F<libmysqlclient>
library as L<DBD::MariaDB>/L<DBD::mysql>, otherwise it will not work.

Also, while this module makes database handles non-blocking, you still
cannot run multiple requests in parallel on the same database handle. If
you want to run multiple queries in parallel, you have to create multiple
database connections, one for each thread that runs queries. Not doing
so can corrupt your data - use a Coro::Semaphore to protetc access to a
shared database handle when in doubt.

If you make sure that you never run two or more requests in parallel, you
can freely share the database handles between threads, of course.

=head2 SPEED

This module is implemented in XS, and as long as mysqld replies quickly
enough, it adds no overhead to the standard libmysql communication
routines (which are very badly written, btw.). In fact, since it has a
more efficient buffering and allows requests to run in parallel, it often
decreases the actual time to run many queries considerably.

For very fast queries ("select 0"), this module can add noticable overhead
(around 15%, 7% when EV can be used) as it tries to switch to other
coroutines when mysqld doesn't deliver the data immediately, although,
again, when running queries in parallel, they will usually execute faster.

For most types of queries, there will be no extra latency, especially on
multicore systems where your perl process can do other things while mysqld
does its stuff.

=head2 LIMITATIONS

This module only supports "standard" mysql connection handles - this
means unix domain or TCP sockets, and excludes SSL/TLS connections, named
pipes (windows) and shared memory (also windows). No support for these
connection types is planned, either.

=head1 CANCELLATION

Cancelling a thread that is within a mysql query will likely make the
handle unusable. As far as Coro::Mysql is concerned, the handle can be
safely destroyed, but it's not clear how mysql itself will react to a
cancellation.

=head1 FUNCTIONS

Coro::Mysql offers these functions, the only one that oyu usually need is C<unblock>:

=over 4

=cut

package Coro::Mysql;

use strict qw(vars subs);
no warnings;

use Scalar::Util ();
use Carp qw(croak);

use Guard;
use AnyEvent ();
use Coro ();
use Coro::AnyEvent (); # not necessary with newer Coro versions

# we need this extra indirection, as Coro doesn't support
# calling SLF-like functions via call_sv.

sub readable { &Coro::Handle::FH::readable }
sub writable { &Coro::Handle::FH::writable }

BEGIN {
   our $VERSION = '2.1';

   require XSLoader;
   XSLoader::load Coro::Mysql::, $VERSION;
}

=item $DBH = Coro::Mysql::unblock $DBH

This function takes a DBI database handles and "patches" it
so it becomes compatible to Coro threads.

After that, it returns the patched handle - you should always use the