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Paul Evans


IO::Async::MergePoint - resynchronise diverged control flow


This module as now been moved to its own dist of Async::MergePoint.

It is kept here as a trivial subclass for backward compatibility. Eventually this subclass may be removed. Any code using IO::Async::MergePoint should instead use Async::MergePoint.

 use Async::MergePoint;

 my $merge = Async::MergePoint->new(
    needs => [ "leaves", "water" ],

    on_finished => sub {
       my %items = @_;
       # Make tea using $items{leaves} and $items{water}

    on_boiled => sub { $merge->done( "water", $_[0] ) }

    on_fetched => sub { $merge->done( "leaves", $_[0] ) }


Often in program logic, multiple different steps need to be taken that are independent of each other, but their total result is needed before the next step can be taken. In synchonous code, the usual approach is to do them sequentially.

An IO::Async-based program could do this, but if each step involves some IO idle time, better overall performance can often be gained by running the steps in parallel. A Async::MergePoint object can then be used to wait for all of the steps to complete, before passing the combined result of each step on to the next stage.

A merge point maintains a set of outstanding operations it is waiting on; these are arbitrary string values provided at the object's construction. Each time the done method is called, the named item is marked as being complete. When all of the required items are so marked, the on_finished continuation is invoked.

When an item is marked as complete, a value can also be provided, which would contain the results of that step. The on_finished callback is passed a hash (in list form, rather than by reference) of the collected item values.


Paul Evans <leonerd@leonerd.org.uk>