POE::Stage - a base class for message-driven objects
#!/usr/bin/env perl { package App; use POE::Stage::App qw(:base); sub on_run { print "hello, ", my $arg_whom, "!\n"; } } App->new()->run( whom => "world" ); exit;
POE::Stage is a set of base classes for message-driven objects. It cleanly implements standard patterns that have emerged from years of working with POE and POE::Component modules.
As I hope the name implies, POE::Stage objects encapsulate discrete steps, or stages, of a larger task. Eventually they come together to implement programs.
For example, HTTP requests are performed in four or so distinct stages: 1. The server's address is resolved. 2. The client establishes a connection to the server. 3. The client transmits a request. 4. The client receives a response.
By design, POE::Stage promotes the decomposition of tasks into multiple, smaller stages. If these stages are generic enough, new tasks may be handled by reusing them in different configurations.
The hypothetical HTTP client might be a single stage composed of three smaller ones: A DNS resolver stage, which accepts DNS requests and returns DNS responses. A TCP client connection factory, which takes socket endpoint descriptions and other parameters, and eventually returns established connections. Finally, there would be an HTTP protocol stage that uses established connections to send requests and parse responses.
These stages would be encapsulated by a higher-level HTTP client stage. This would accept HTTP requests and return HTTP responses after performing the necessary steps to gather them.
This will sound familiar to anyone working with objects.
These objects are asynchronous and message-driven, however. The base message class, POE::Request, and its subclasses, implement a standard request/response interface between POE::Stage objects. Where possible, these messages attempt to mimic simpler, more direct call/return syntax, albeit asynchronously. POE::Stage also provides a powerful closure-based system for maintaining request and response state, so you don't have to.
To do its job, POE::Stage requires some methods for its own. To be extensible, it reserves other methods for standard purposes. To remain useful, it reserves the least number of methods possible.
new() creates and returns a new POE::Stage object. An optional set of named ARGUMENT_PAIRS will be passed to the object's init() callback before new() returns.
Subclasses should not override new() unless they're careful to call the base POE::Stage's constructor. Object construction is customized through the init() callback instead.
init() is a callback used to initialize POE::Stage objects after they are constructed. POE::Stage's new() constructor passes its named ARGUMENT_PAIRS to init() prior to returning the new object. The values of these arguments will be available as $arg_name lexicals within the init() callback:
my $object = POE::Stage::Something->new( foo => 123 ); package POE::Stage::Something; sub init { print my $arg_foo, "\n"; # displays "123\n". }
The init() callback is optional.
The Handler method implements an attribute handler that defines which methods handle messages. Only message handlers have access to the closures that maintain state between messages.
The Handler method is used as a subroutine attribute:
sub some_method :Handler { # Lexical magic occurs here. } sub not_a_handler { # No lexical magic happens in this one. }
Methods with names beginning with "on_" acquire Handler magic automatically.
sub on_event { # Lexical magic occurs here. No :Handler necessary. }
expose() is a function (not a method) that allows handlers to expose members of specific request or response OBJECT. Each member will be exposed as a particular LEXICAL variable. OBJECTs must inherit from POE::Request.
The LEXICAL's name is significant. The part of the variable name up to the leading underscore is treated as a prefix and ignored. The remainder of the variable name must match one of the OBJECT's member names. The sigil is also significant, and it is treated as part of the member name.
The following example exposes the '$cookie' member of a POE::Request object as the '$sub_cookie' lexical variable. The exposed variable is then initialized. In doing so, the value stored into it is saved within the request's closure. It will be available whenever that request (or a response to it) is visible.
use POE::Stage qw(expose); sub do_request :Handler { my $req_subrequest = POE::Request->new( ... ); expose $req_subrequest, my $sub_cookie; $sub_cookie = "stored in the subrequest"; }
LEXICAL prefixes are useful for exposing the same member name from multiple OBJECTs within the same lexical scope. Otherwise the variable names would clash.
TODO - Describe how POE::Stage is used. Outline the general pattern for designing and subclassing.
POE::Stage implements the most important and common design patterns for POE programs in a consistent and convenient way.
POE::Stage hides nearly all of POE, including the need to create POE::Session objects and explicitly define event names and their handlers. The :Handler subroutine attribute defines which methods handle messages. There's never a need to guess which message types they handle:
# Handle the "foo" message. sub foo :Handler { ... }
POE::Stage simplifies message passing and response handling in at least three ways. Consider:
my $request = POE::Request->new( stage => $target_stage, method => $target_method, args => \%arguments, on_response_x => "handler_x", on_response_y => "handler_y", on_response_z => "handler_z", );
First, it provides standard message clasess. Developers don't need to roll their own, potentially non-interoperable message-passing schemes. The named \%arguments are supplied and are available to each handler in a standard way, which is described later in the MAGICAL LEXICAL TOUR.
Second, POE::Stage provides request-scoped closures via $req_foo, $rsp_foo, and expose(). Stages use these mechanisms to save and access data in specific request and response contexts, eliminating the need to do it explicitly.
Third, response destinations are tied to the requests themselves. In the above example, responses of type "response_x" will be handled by "handler_x". The logic flow of a complex program is more readily apparent. It gets better, too. See HANDLER NAMING CONVENTIONS.
The mechanisms of message passing and context management become implicit, allowing them to be extended transparently. This will be extended across processes, hopefully with few or no seams.
POE::Stage includes object-oriented classes for low-level event watchers. They simplify and standardize POE::Kernel's interface, and they allow watchers to be extended cleanly through normal OO techniques. The lifespan of each resource is tightly coupled to the lifespan of each object, so ownership and relevance are clearly indicated.
POE::Stage standardizes shutdown semantics for requests and stages. Requests are canceled by destroying their objects, and stages are shut down the same way.
POE::Stage simplifies the cleanup of complex, multi-stage activity. Resources for a particular request should be stored within its closure. Canceling the request triggers destruction of that closure and its contents, which in turn triggers the destruction of the resources allocated to that request. These resources include stages and requests created during the lifetime of the request. They too are canceled and freedm
POE::Stage uses lexical aliasing to expose state data to message handlers, which are specified by either the :Handler method attribute or the use of an on_ prefix in the method's name.
Lexical variable prefixes indicate the data's origin. For example, $arg_name is the "name" argument included with a message:
my $request = POE::Request->new( method => "something", args => { name => "ralph" }, ..., ); sub something :Handler { my $arg_name; # already contains "ralph" }
The full list of prefixes and data sources:
Argument (parameter) "xyz". If an "args" parameter is passed to a POE::Request constructor, its value must be a reference to a hash. Usually it's an anonymous hashref. Anyway, the hash's members are named arguments to the message handler. See above for an example.
An incoming request may trigger more than one handler, especially if a POE::Stage object calls itself, or sends sub-requests to a helper stage. The "req_" lexical prefix refers to data members within the current request's scope. Their values will magically reflect the proper request scope, regardless what that is.
TODO - Example.
The "self" scope refers to the currently active POE::Stage object. Data may be stored there, in which case it's available from any and all requests handled by that object. This scope is useful for "singleton" or static data that must be shared between or persistent between all requests.
TODO - Example
The "rsp" scope refers to data stored in a sub-request's scope, but from the response handler's point of view. That is, when persisting data between a request to a substage and its response, one should store the data in the substage's request, then retrieve it later from the corresponding "rsp" variable.
Certain variables are standard: $self refers to the current object; it need not be initialized from @_. $req refers to the higher-level request we're currently handling. When handling responses from substages, $rsp refers to those responses.
All three variables are intended as invocatnts for method calls. Other prefixes exist to access data members within each object's scope.
The techniques used here have been abstracted and released as Lexical::Persistence.
Message handlers are defined in one of two ways. They may be named anything as long as they have a :Handler attribute, or they may be prefixed with "on_". In both cases, they gain lexical persistence magic, as discussed previously.
# Handle the "foo" message. sub foo :Handler { ... } # Handle the "on_foo" and "foo" messages. sub on_foo { ... }
The on_foo() method above handles both "on_foo" and "foo" messages. Given both a foo() and an on_foo(), however, on_foo() will take precedence.
Requests include on_* parameters that map response types to response handlers. For example, this request expects two return types, "success" and "failure". On success, the handle_success() method is called. On failure, handle_failure() is called.
my $req_subrequest = POE::Request->new( ..., on_success => "handle_success", on_failure => "handle_failure", );
Response types are specified by the "type" parameter to $req->emit() and $req->return(). "emit" and "return" are the default types for emit() and return(), respectively.
Requests can also have roles, which are usually descriptive of the transaction. For example, consider a DNS request for a web client component:
my $req_resolve = POE::Request->new( ..., role => "resolver", );
This is the role of the request, not of the stage that will handle it. In this case, there are no on_* parameters. Success and failure come back to methods named "on_" . $request_role . "_" . $response_type. In the previous example, they are:
sub on_resolver_success { ... } sub on_resolver_failure { ... }
When subclassing a POE::Stage class, it's sometimes useful to intercept emit() and return() messages. The subclass may implement handlers directly, or it may override or extend the response. This is done by defining "on_my_" . $response_type methdos in the subclass. For example, a TCP connection stage might emit an "input" event, like so:
sub on_socket_readable { ...; $req->emit( type => "input", input => $data ); }
A subclass might implement the code to handle the input. It can do so by defining on_my_input():
sub on_my_input { # send a response here }
Messages intercepted like this will not be rethrown automatically to the caller. If that's desired, on_my_input() will need to emit() or
TODO - Make a better example. Something that can tie all these things together conceptually.
POE::Stage is not ready for production. Check back here early and often to find out when it will be. Please contact the author if you would like to see POE::Stage production-ready sooner.
https://rt.cpan.org/Dist/Display.html?Status=Active&Queue=POE-Stage
http://thirdlobe.com/svn/poe-stage/
http://search.cpan.org/dist/POE-Stage/
POE::Stage is the base class for message-driven objects. POE::Request is the base class for POE::Stage messages. POE::Watcher is the base class for event watchers.
http://thirdlobe.com/projects/poe-stage/ - POE::Stage is hosted here.
http://www.eecs.harvard.edu/~mdw/proj/seda/ - SEDA, the Staged Event Driven Architecture. It's Java, though.
Rocco Caputo.
POE::Stage is Copyright 2005-2009 by Rocco Caputo. All rights are reserved. You may use, modify, and/or distribute this module under the same terms as Perl itself.
To install POE::Stage, copy and paste the appropriate command in to your terminal.
cpanm
cpanm POE::Stage
CPAN shell
perl -MCPAN -e shell install POE::Stage
For more information on module installation, please visit the detailed CPAN module installation guide.