NAME

DBIx::TxnPool - Massive SQL updates by means of transactions with the deadlock & signal solution

SYNOPSIS

This module will help to you to make quickly DML statements of InnoDB engine. You can forget about deadlocks ;-)

    use DBIx::TxnPool;

    my $pool = txn_item {
        my ( $pool, $item ) = @_;

        $pool->dbh->do( "UPDATE table SET val=? WHERE key=?", undef, $_->{val}, $_->{key} );
        # or
        $dbh->do("INSERT INTO table SET val=?, key=?", undef, $_->{val}, $_->{key} );
    }
    txn_post_item {
        my ( $pool, $item ) = @_;

        # Here we are if transaction is successful
        unlink( 'some_file_' . $_->{key} );
        # or
        unlink( 'some_file_' . $item->{key} );
    }
    txn_commit {
        my $pool = shift;
        log( 'The commit was here...' );
    } dbh => $dbh, size => 100;

    # Here can be deadlocks but they will be resolved by module
    # and repeated (to see example in xt/03_deadlock_solution.t)
    $pool->add( { key => int( rand(100) ), val => $_ } ) for ( 0 .. 300 );
    $pool->finish;

Or other way:

    my $pool = txn_item {
        $dbh->do( "UPDATE table SET val=? WHERE key=?", undef, $_->{val}, $_->{key} );
    }
    txn_sort {
        $a->{key} <=> $b->{key}
    }
    dbh => $dbh, size => 100;

    # Here no deadlocks because all keys are sorted before transaction:
    # circle blocks inside the InnoDB not occur
    $pool->add( { key => int( rand(100) ), val => $_ } ) for ( 0 .. 300 );
    $pool->finish;

DESCRIPTION

If you need massive quickly updates or inserts into InnoDB database - this module for you! It helps to wrap some SQL manipulation statements to one transaction and has the deadlock and signal solution.

DETAILS

If you make alone insert/delete/update statement in the InnoDB engine, MySQL server does fsync (data flushing to disk) after each statement. It can be very slow for many updates. The best solution can be to wrap some insert/delete/update statements in one transaction for example. But this raises a new problem - deadlocks. If a deadlock occurs a DBI module throws exceptions and ideal way to repeat SQL statements again. This module helps to make it. It has a pool inside for data (FIFO buffer) and calls your callbacks for each pushed item. When your pool to be fed by your data, it wraps data in one transaction up to the maximum defined size or up to the finish method. If deadlock occurs a pool repeats your callbacks for every item again. You can define a second callback which will be executed for every item after wrapped transaction. For example there can be non-SQL statements, for example a deleting files, cleanups and etc.

CONSTRUCTOR

Please to see "SYNOPSIS" section

Shortcuts:

The txn_item should be first. Other sortcuts can follow in any order. Parameters should be the last.

txn_item (Required)

The transaction item callback. There should be SQL statements and code should be safe for repeating (when a deadlock occurs). The $_ consists a current item. You can modify it if one is hashref for example. Passing arguments will be DBIx::TxnPool object and current item respectively. Please don't catch exceptions here (by try{} or eval{} for example) - by this way deadlocks are defined outside under the hood!

txn_sort (Optional)

Here you can define sort function for your data before a transaction will be made. If you have only one type SQL statement in txn_item but you didn't sort keys before transaction you can have deadlocks (they will be resolved and transaction will be repeated but you will lose a processing time) unless you define this function. This method minimize deadlock events!

txn_post_item (Optional)

The post transaction item callback. This code will be executed once for each item (defined in $_). It is located outside of the transaction. And it will be called if whole transaction was successful. Passing arguments are DBIx::TxnPool object and current item respectively. You can do here your own error handling in callback. If your code here will throw an excetption it will be propagated above.

txn_commit (Optional)

This callback will be called after each SQL commit statement. Here you can put code for logging for example. The first argument is DBIx::TxnPool object

Parameters:

dbh (Required)

The dbh to be needed for begin_work & commit method (wrap in a transaction).

size (Optional)

The size of pool when a commit method will be called when feeding reaches the same size.

block_signals (Optional)

An arrayref of signals (strings) which should be blocked in slippery places for this pool. Defaults are [ qw( TERM INT ) ]. You can change globaly this list by setting: $DBIx::TxnPool::BlockSignals = [ qw( TERM INT ALARM ... ) ]. For details to see here "SIGNAL HANDLING"

max_repeated_deadlocks (Optional)

The limit of consecutive deadlocks. The default is 5. After limit to be reached the "add" throws exception.

METHODS

add

You can add item of data to the pool. This method makes a wrap to transaction. It can finish transaction if pool reaches up to size or can repeat a whole transaction again if deadlock exception was thrown. The size of transaction may be less than your defined size!

dbh

The accessor of dbh. It's readonly.

finish

It makes a final transaction if pool is not empty.

amount_deadlocks

The amount of deadlocks (repeated transactions)

SIGNAL HANDLING

In DBD::mysql and may be in other DB drivers there is a some bad behavior the bug as i think. If a some signal will arrive (TERM, INT and other) in your program during a some SQL socket work this driver throws an exception like "MySQL lost connection". It happens because the recv or read system calls into MySQL driver return with error code EINTR if signal arrives inside this system call. A right written software should recall a system call again because the EINTR is not fatal error. But i think MySQL driver decides this error as lost connection error. "Deferred Signals" (or Safe Signals) of perl don't help because the MySQL driver uses direct system calls.

Workaround is to use Signal::Mask module for example and to block these signals (TERM / INT) during working with DBI subroutines. The version 0.09 of DBIx::TxnPool has helpers for this. The DBIx::TxnPool wraps all slippery places by blocking your preferred signals (defaults are TERM & INT ones) before entering and by unblocking after (for example the callback handler txn_item and transaction code). This should minimize raised errors like the "MySQL lost connection".

AUTHOR

This module has been written by Perlover <perlover@perlover.com>

LICENSE

This module is free software and is published under the same terms as Perl itself.

SEE ALSO

DBI, Deadlock Detection and Rollback

TODO

A supporting DBIx::Connector object instead DBI