NAME
Event::Distributor - a simple in-process pub/sub mechanism
SYNOPSIS
use Event::Distributor;
my $dist = Event::Distributor->new;
$dist->declare_signal( "announce" );
$dist->subscribe_sync( announce => sub {
my ( $dist, $message ) = @_;
say $message;
});
$dist->subscribe_async( announce => sub {
my ( $dist, $message ) = @_;
return $async_http->POST( "http://server/message", $message );
});
$dist->fire_sync( announce => "Hello, world!" );
DESCRIPTION
Instances of this class provide a simple publish/subscribe mechanism
within a single process, for either synchronous or Future-based
asynchronous use.
A given instance has a set of named events. Subscribers are CODE
references attached to a named event. Publishers can declare the
existence of a named event, and then later invoke it by passing in
arguments, which are distributed to all of the subscribers of that
named event.
It is specifically not an error to request to subscribe an event that
has not yet been declared, in order to allow multiple modules of code
to be loaded and subscribe events the others publish, without
introducing loading order dependencies. An event only needs to be
declared by the time it is fired.
Natively all of the events provided by the distributor are
fully-asynchronous in nature. Each subscriber is expected to return a
Future instance which will indicate its completion; the results of
these are merged into a single future returned by the fire method
itself. However, to support synchronous or semi-synchronous programs
using it, both the observe and invoke methods also have a synchronous
variant. Note however, that this module does not provide any kind of
asynchronous detachment of synchronous functions; using the
"subscribe_sync" method to subscribe a long-running blocking function
will cause the fire_* methods to block until that method returns. To
achieve a truely-asynchronous experience the attached code will need to
use some kind of asynchronous event system.
This module is very-much a work-in-progress, and many ideas may still
be added or changed about it. It is the start of a concrete
implementaion of some of the ideas in my "Event-Reflexive Programming"
series of blog posts. See the "TODO" and "SEE ALSO" sections for more
detail.
EVENTS
Each of the events known by a distributor has a name. Conceptually each
also has a type. Currently there are three types of event, a "signal",
an "action", and a "query".
* A signal event simply informs subscribers that some event or
condition has occurred. Additional arguments can be passed from the
invoker to the subscribers, but subscriptions are not expected to
return a meaningful value, nor does firing this event return a value.
All subscriber functions are invoked sequentually and synchronously
by a fire_* method (though, of course, asynchronous subscribers
synchronously return a future instance, which allows them to continue
working asynchronously).
* An action event requires a single subscriber, and represents a
request from the invoker to the subscriber to perform some activity.
This behaves much like a regular (Future-returning) method call,
except that the indirection mechanism of the distributor allows a
more flexible method of connection between the two sides.
* A query event invokes subscriber code expecting a successful
return, returning the first result that is successful. If a
synchronous subscriber returns a result, or if an asynchronous one
returns a successful immediate Future, then no further subscribers
are invoked, and that result is taken immediately. Any other pending
Futures are then cancelled.
METHODS
declare_signal
$distributor->declare_signal( $name )
Declares a new "signal" event of the given name.
declare_action
$distributor->declare_action( $name )
Since version 0.04.
Declares a new "action" event of the given name.
declare_query
$distributor->declare_query( $name )
Since version 0.02.
Declares a new "query" event of the given name.
subscribe_async
$distributor->subscribe_async( $name, $code )
Adds a new CODE reference to the list of subscribers for the named
event. This subscriber is expected to return a Future that will
eventually yield its result.
When invoked the code will be passed the distributor object itself and
the list of arguments, and is expected to return a Future.
$f = $code->( $distributor, @args )
subscribe_sync
$distributor->subscribe_sync( $name, $code )
Adds a new CODE reference to the list of subscribers for the named
event. This subscriber is expected to perform its work synchronously
and return its result immediately.
In non-blocking or asynchronous applications, this method should only
be used for simple subscribers which can immediately return having
completed their work. If the work is likely to take some time by
blocking on external factors, consider instead using the
"subscribe_async" method.
When invoked the code will be passed the distributor object itself and
the list of arguments.
$code->( $distributor, @args )
fire_async
$f = $distributor->fire_async( $name, @args )
Invokes the named event, passing the arguments to the subscriber
functions. This function returns as soon as all the subscriber
functions have been invoked, returning a Future that will eventually
complete when all the futures returned by the subscriber functions have
completed.
fire_sync
$distributor->fire_sync( $name, @args )
Invokes the named event, passing the arguments to the subscriber
functions. This function synchronously waits until all the subscriber
futures have completed, and will return once they have all done so.
Note that since this method calls the get method on the Future instance
returned by "fire_async", it is required that this either be an
immediate, or be some subclass that can actually perform the await
operation. This should be the case if it is provided by an event
framework or similar, or custom application logic.
TODO
Some of these ideas appear in the "Event-Reflexive Progamming" series
of blog posts, and may be suitable for implementation here. All of
these ideas are simply for consideration; there is no explicit promise
that any of these will actually be implemented.
* Unsubscription from events.
* Define (or document the lack of) ordering between subscriptions of
a given event.
* Refine the failure-handling semantics of signals.
* Ability to invoke signals after the current one is finished, by
deferring the fire method. Should this be a new fire_* method, or a
property of the signal itself?
* More control over the semantics of value-returning events -
scatter/map/gather pattern.
* Sub-heirarchies of events.
* Subclasses for specific event frameworks (IO::Async).
* Subclasses (or other behaviours) for out-of-process event
serialisation and subscribers.
* Event parameter filtering mechanics - allows parametric
heirarchies, instrumentation logging, efficient out-of-process
subscribers.
SEE ALSO
Event-Reflexive Programming
<http://leonerds-code.blogspot.co.uk/search/label/event-reflexive>
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>