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

     use Coro::Mysql;

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

    (Note that in this manual, "thread" refers to real threads as
    implemented by the 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 DBD::mysql and 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.

    Note that this module must be linked against exactly the same (shared,
    possibly not working with all OSes) libmariadb/libmysqlclient library as
    DBD::MariaDB/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.

    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.

    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.

    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

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

    $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 newly returned database handle.

        It is safe to call this function on any database handle (or just
        about any value), but it will only do anything to DBD::mysql
        handles, others are returned unchanged. That means it is harmless
        when applied to database handles of other databases.

        It is also safe to pass "undef", so code like this is works as

           my $dbh = DBI->connect ($database, $user, $pass)->Coro::Mysql::unblock
              or die $DBI::errstr;

    $bool = Coro::Mysql::is_unblocked $DBH
        Returns true iff the database handle was successfully patched for
        non-blocking operations.

    $bool = Coro::Mysql::have_ev
        Returns true if this Coro::Mysql installation is compiled with
        special support for EV or not.

        Even if compiled in, it will only be used if EV is actually the
        AnyEvent event backend.

    This example uses PApp::SQL and Coro::on_enter to implement a function
    "with_db", that connects to a database, uses "unblock" on the resulting
    handle and then makes sure that $PApp::SQL::DBH is set to the
    (per-thread) database handle when the given thread is running (it does
    not restore any previous value of $PApp::SQL::DBH, however):

       use Coro;
       use Coro::Mysql;
       use PApp::SQL;

       sub with_db($$$&) {
          my ($database, $user, $pass, $cb) = @_;

          my $dbh = DBI->connect ($database, $user, $pass)->Coro::Mysql::unblock
             or die $DBI::errstr;

          Coro::on_enter { $PApp::SQL::DBH = $dbh };


    This function makes it possible to easily use PApp::SQL with
    Coro::Mysql, without worrying about database handles.

       # now start 10 threads doing stuff
       async {

          with_db "DBI:mysql:test", "", "", sub {
             sql_exec "update table set col = 5 where id = 7";

             my $st = sql_exec \my ($id, $name),
                               "select id, name from table where name like ?",

             while ($st->fetch) {

             my $id = sql_insertid sql_exec "insert into table values (1,2,3)";
             # etc.

       } for 1..10;

    Coro, PApp::SQL (a user friendly but efficient wrapper around DBI).

    This module was initially hacked together within a few hours on a long
    flight to Malaysia, and seems to have worked ever since, with minor
    adjustments for newer libmysqlclient libraries.

    Well, at least until mariadb introduced the new Pluggable Virtual IO API
    in mariadb 10.3, which changed and broke everything. On the positive
    side, the old system was horrible to use, as many GNU/Linux
    distributions forgot to include the required heaqder files and there
    were frequent small changes, while the new PVIO system seems to be
    "official" and hopefully better supported.

     Marc Lehmann <>