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small-programs
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README

vim: ts=2 sw=2 noexpandtab

To Do:

	Develop a rough outline with the order we want to present things.

Topics:

Intro To POE Braindump.
-----------------------

1. Who?
	Who uses POE now?
		Yahoo!
		poe.perl.org lists others
		(Need to update poe.perl.org's list... it's been several months.)
	Who should be using POE?
		Everyone! :)
		A lot of the "what" details cover this.
	Who built POE?
		Rocco Caputo
		Plus a dozen or two named contributors.
		Plus dozens of contributors only named in the changelogs.
		(Need to pull their names out and list them.)
		(Also see Acme::CPANAuthors::POE.)
	Who are the community?
		#poe on irc.perl.org, freenode, efnet, oftc
		poe@perl.org mailing list
		Interwoven throughout the Perl community.
			http://www.flickr.com/photos/franck_/3877930818/sizes/l/
	etc.

2. What?
	What kinds of things does POE do?
		Co-processing.
			Cooperative, event driven co-processing using sessions rather than threads.
		Workers.
			Dispatch work on multiple CPUs or machines, using a central controller.
		Multi- and simultaneous processing.
			Cooperative, if I/O wait is your bottleneck.
		Differences between POE and other forms of concurrency.
			POE multiplexes I/O and related things.
			Single processor.
			Single memory space.
			Can coordinate multiple processes and threads.
				Kind of erlang-like in that regard.
				Not as tightly bound to the idea as erlang.
	What can I do with POE?
		Network clients and servers.
		Watchdogs.
		Process managers.
		Device interfaces.
		Process simulations.
		(Page through CPAN POE components & the cookbook for ideas.)
	What should/shouldn't be done with POE?
		If I/O wait is your bottleneck, then POE will help a lot.
		If I/O bandwidth is your bottleneck, only hardware will help.
			Or altering your protocols to be more compact.
			Consider compressing data on the wire.
				Although this burns more CPU, and could make CPU your bottleneck.
		If CPU is your bottleneck, then POE won't help so much.
			But it can be used to coordinate multiple threads or processes.
		If disk wait is your bottleneck, POE could help.
			Although POE might help even more with IO::AIO.
			Hardware is another option.
			Distribute the work across many drives, a la Hadoop.
			POE can help coordinate this as well.
		If RAM is your bottleneck...
			Add more!
			Shrink your data structures.
			Delegate to disk, although this may make disk your bottleneck.

3. Where?
	Where should I put POE?
		The usual cpan -i stuff.
		Relocatable.
			Untar it anywhere, and begin testing it there.
			Great for regression testing and evaluation.
	Does it play well with others, or does it want to run alone?
		13 event loops supported as of this writing.
		run_one_timeslice() allows it to be driven by external loops.
	Examples of stand-alone POE apps.
		(Aren't they all?)
	Using POE to delegate tasks that need long-running processes and workers.

4. When?
	When should I use POE?
		(See "what" section.)
	When do I start looking at my app and saying "this needs POE!"?
		When it's waiting for external input a lot.
		When it needs to wait for external input from more than one source.
		When it needs to manage multiple timers.
		When it needs concurrency but multiple CPUs aren't really an issue.
		(Those are simplistic, but they're a start.)
	When does an app need POE?
		When it starts doing processing on its own that could be better and more efficiently handled by a dedicated process or app?
		(Kinda redundant question, but maybe I don't understand the nuances?)

5. Why?
	Why should I use POE?
	What advantages does it give me over Coro, Erlang, etc.?
		Speed?
		Size?
		Maturity?
		Good architecture?

6. How?
	How do I actually use POE?
		(See most of the rest of this document.)
	Examples of the aforementioned points, with scenarios.

===== Tutorial =====

Intro to POE.

	What it is (1-2 paragraphs).
	- practical applications in web 
	  0. show how POE provides the event driven/actors paradigm.  Try to provide that "aha" push.
	  1. a web app (shined up paste app), 
	  2. a bot
	  3. an app interfacing with a Catalyst App (ThumbIt perhaps?), 
	  
	What it does (1-2 paragraphs).
	- what its role would be in these situations
	  1. perform as a web framework
	  2. a scalable and multi-process bot for doing x y and z (probably most useful would be an infobot for irc, or a web scraper)
	  3. a job queue and worker application to delegate image resizing to

Hello, World 1

	Count to 10 (Hello, world!)
	Explain what's going on.
	Use all verbose, explicit constructs.
	Explain that this is longer than it needs to be.
	

Hello, World 2

	Replace the verbose bits with shortcuts.
	Explain where things went, and why we did it that way.

Hello, Worlds

	Count to 10 concurrently.
	Multiple sessions.
	Explain how POE's concurrency works.

...

Gradually build upon these with 'branches' involving aforementioned scenarios

===== OLDER BRAINDUMP FOLLOW =====

Basic concepts.

	Initial example and notes:
		anatomy of a program - hello.perl
		looping - looping.perl

	Ideas:
		different session types - inline handlers
		different session types - object handlers
		different session types - class handlers

		wrapping sessions in object interfaces

		event handlers in more detail (event parameters)
		Kernel and session as concepts

		alarms
		multitasking within a session
		multitasking with multiple sessions
		message passing between sessions
		complex request and response between sessions
		managing request state while performing large tasks
		replacing IO::Select (note 1)
		handling signals
		sessions
		the kernel
		different types of event handler (inline, object, class/package)
		anonymous vs. named inline handlers
		repeating alarms
		debugging - TRACE_FOO and ASSERT_FOO
		parent/child session interaction
		... what else?

User interfaces:

	Ideas:
		Wheel::ReadLine
		Wheel::Curses
		Wheel::ReadWrite on STDIN/STDOUT
		Term::Visual
		Tk
		Gtk
		Gtk2
		... what else?

Wheels, filters, drivers.

	Ideas:
		wheels in general
		drivers in general
		filters in general
		how they fit together
		graceful POE::Wheel::ReadWrite shutdown
			shutdown flag
			creating a "flushed" state on the fly
			directing FlushedEvent to a shutdown handler
		complex flow control in POE::Wheel::ReadWrite
		using filters outside POE
		using drivers outside POE

		POE::Wheel::SocketFactory (note 1)
		POE::Wheel::ReadWrite (note 1)
		POE::Wheel::ReadLine
		POE::Wheel::FollowTail
		POE::Wheel::Curses
		POE::Wheel::ListenAccept
		POE::Wheel::Run - one process per session
		POE::Wheel::Run - many processes per session
		POE::Wheel::SocketFactory
		POE::Filter::Block
		POE::Filter::Grep
		POE::Filter::HTTPD
		POE::Filter::Line
		POE::Filter::Map
		POE::Filter::RecordBlock
		POE::Filter::Reference
		POE::Filter::Stackable
		POE::Filter::Stream
		POE::Driver::SysRW
		... what else?

High-level networking:

	Ideas:
		UDP sockets
		UNIX sockets
		FIFOs

IRC programming:

	Ideas:
		Logging to IRC (Randal Schwartz)
		Bot debugging techniques
		Graceful bot shutdown
		Graceful bot reconnecting
		Simple IRC bots
		IRC plugins
		Letting POE::Component::IRC track state

IPC

	Ideas:
		poe server, poe client, low-level via POE::Filter::Reference
		poe server, poe client, POE::Component::IKC
		poe server, light client, via POE::Filter::Reference
		poe server, light client, POE::Component::IKC's light client

Process management:

	Ideas:
		POE::Wheel::Run
		POE::Component::Generic
		Managing multiple forked workers (dynamic)
		Managing multiple forked workers (static pool)
		Managig child processes that require ttys

System administration:

	Ideas:
		POE::Component::Client::Ping
		POE::Component::SNMP?
		Gathering banners from multiple hosts/services.
		Following logs - POE::Wheel::FollowTail
		Following snort logs - POE::Filter::Snort

Extending event flow control:

	Ideas:
		Broadcast groups
		Combining services (telnet chat + web service) ???
		Dynamically creating events from input
		Changing events emitted by wheels
		POE::NFA
		Changing event handlers on the fly

Database interaction:

	Ideas:
		DBIAgent
		EasyDBI
		etc.

Device interfaces:

	Ideas:
		Serial ports

Networking tools:

	Ideas:
		TCP forwarder: client <-> this <-> server (components)
		TCP forwarder: client <-> this <-> server (wheels)
		client1 <-> server1 <-events-> server2 <-> client2

Multiuser servers:

	Ideas:
		Chat server
		Chat client with Term::Visual

Asynchronous DNS:

	Ideas:
		POE::Component::Client::DNS - log file resolving

Fanciful applicotions

	Ideas:
		Neural networks
		Quantum computing
		Filter::Template tricks
		Curses based media player

Web programming:

	Ideas:
		Web client (POE::Component::Client::HTTP)
		Simple web server with POE::Filter::HTTPD
		Complex web server with POE::Component::Server::HTTP
		Complex web server with POE::Component::Server::SimpleHTTP
		Complex web server with POE::Component::Server::HTTPServer?
		A pre-forking web server
		Web server for large media files
		A web proxy with streaming support
		Handling CGI requests

-----

Notes:

1. Some articles will be adapted from the "evolution of a server"
tutorial.  The idea is to break the entire tutorial into separate
examples and discussions, then re-create the original tutorial by
stringing them together.

-----

Random notes pulled from various places.  Not anywhere near complete,
but it's a lot of ideas to extract and place in the above article
buckets.

Combined Concept

This section discusses how things interact in POE.

Kernel and Sessions

The kernel's largest job is watching resources. Not only does it check the resources for new events, but it also tracks their uses. It does this itself, rather than relying on Perl reference counting, because it was written prior to Perl's weak references, and it's still in use by Perl versions as old as 5.004.

The kernel tracks resources by the sessions that watch them. It maintains each resource's reference count, cleaning them up and releasing their memory when they are no longer in use. That is, it will do this if programs don't keep extra resource references. Again, weak references would help here, but they don't exist in all the places where POE is useful.

Sessions are resources themselves, and a session created within another becomes a child of its creator. The kernel tracks these relationships for the purpose of job control-- especially the ability for one session to manage a pool of several others-- and signal propagation within a program.

Sessions and Wheels

While writing sample programs and test cases for POE's early development, the same sorts of algorithms came up again and again. The most common routines involved I/O, either at the socket establishment level or for buffered reading and writing on open files.

These recurring routines are often filehandle watcher callbacks or supporting states. Rewriting them had become tiresome almost as soon as POE was released, so it was decided to make modular units out of them. Wheels were invented, replacing the ones being re-created with every program.

Wheels were invented to encapsulate these bundles of redundant states and provide the glue logic to insert them into sessions. They are state machine fragments that plug themselves into sessions at creation time, giving their owners new capabilities.

Wheels are not resources, and POE's kernel will not keep track of them. It's therefore important that sessions hold onto their wheel references for as long as they're needed. Wheels may be given resources to manage; in this case, the resource is watched internally by the wheel, and destroying it will cascade cleanup to the resource itself. Wheels are implemented so that any circular references are broken at destruction time, ensuring complete destruction and memory reuse.

Wheels bind themselves tightly to the sessions that create them. While it's possible to pass wheel references amongst sessions, their states will remain in the sessions that created them.

Wheels often deal with resources on behalf of their sessions, finally passing back events when something truly notable occurs. On the other hand, sessions usually invoke wheel features through direct method calls.

I/O Wheels, Drivers, and Filters

Many I/O wheels use drivers and filters to abstract away the gory details of raw file I/O and the specifics of low-level data parsing and marshalling. This division allows the I/O wheels themselves to focus on the logic necessary to perform a task.

The ReadWrite wheel, for example, performs simple reading and writing. It can adapt to the natures of several different file types by virtue of using different filters. It can perform HTTP server transactions by using Filter::HTTPD; it can read and write lines by using Filter::Line; or it can use some other filter, either currently available or written in the future.

This style guide is copyright 2001-2002 by Rocco Caputo. All rights reserved. This guide is free text and may be distributed and/or modified under the same terms as Perl itself. 

---

POE - A Perl Object Environment, or Perl Objects for Events.

Introduction - What POE Is

	Perl Object Environment
	Originally the core of a larger project.
	Originally about 1/10 its current size.
	A lot of conveniences added over time.
	At its heart, POE is an event loop.

Event loops

	Watchers and handlers.
	Queue and dispatcher.

POE's event loop

	Kernel (event queue, watcher methods, watchers, dispatcher).
	Session (dispatcher adapter, task concept, handlers).

POE's events

	Just lists of data, provided by various places.
	Kernel includes itself, event name, sender.
	Sessions include themselves, their private data spaces, and other things.
	Watchers include their own fields.
	Programs also include data.

POE's Kernel

	POE::Queue::Array (enqueue, dequeue)
	POE::Kernel (post, select_read, alarm_set)
	POE::Loop::Select (others)
	POE::Resource::Filehandles
	POE::Resource::Alarms

	POE::Session (invoke)

POE's Sessions

	invoke()
	handler lookup
	handler invocation
	handlers call Kernel methods to affect new changes

Sessions as "threads".

	Sessions are instances of POE::Session (or subclasses).
	Each session's stuff (data, watchers, events, etc.) is kept apart from stuff of the others.
	POE::Kernel knows which sessions call it.

	These rules codify the boundaries between session contexts.  POE is
	still just Perl, though.  There are no shotguns to enforce those
	boundaries, but the idea that they're there is strong enough to keep
	most developers from crossing them.

Sessions as interfaces.

	Sessions can act as message-based interfaces.
	Calls and return values can be propagated by events.
	POE::Component::SubWrapper.

Sessions as adapters.

	Session classes customize event dispatch.
	POE::NFA
	POE::Session::MessageBased
	POE::Session::MultiDispatch
	etc.

Event handlers (syntax, examples).

	Inline (syntax, examples)
	Object methods (method name = event name, and method != event)
	Class methods (same as objects, but use class names)
	Can mix and match styles, but the last definition wins any conflict.

Hello, World! program.

Event handler parameters.

	Event fields are members of @_.
	POE::Session exports constants for field positions within @_.
		KERNEL, SESSION, HEAP, ARG0
		etc.
	ARG0 is always towards the end of @_.
		my @stuff = @\_\[ARG0..$#_\].

Enqueuing events.

	yield()
	post()
	call()

	Destinations are very flexible.
	call() is an immediate call, bypassing the queue.  Returns values.

Hello, World! with events passed around.

Tod do is to be.

	Sessions stop when they run out of work to do.
	Events count as work.
	Most event watchers do, as well.
	Signals also count, although they didn't until recently.
	Aliases are special, and will be covered later.

Cooperation

	Each do_count() iteration is triggered by a "count" event.
	The "count" events take turns in the queue.
	A session can juggle multiple events.  Only one is handled at a time.
	POE::Kernel can juggle multiple sessions.

Aliases.

	Allow sessions to be addressed by symbolic names.
	Each may only refer to a single session.
	Although a session can go by many aliases.
	Are useful for message passing.
	Only count as work when something can generate events.

Alias examples.

	alias_set()
	alias_remove()
	alias_resolve()
	alias_list()

Using aliases.

	Example and discussion.

Parents and children.

	A session is the parent of the sessions it creates.
	POE::Kernel is the ancestor of all sessions.
	Child sessions count as work for their parents.
	POE notifies sessions when their parents or children come or go.

Parent notification

	_parent tells a session its parent session has changed.
	Parameters: ARG0 = old parent; ARG1 = new parent

Child notification

	_child tells a session that a child's state has changed.
	It's not related to SIGCHLD, although it performs a similar purpose.
	Parameters: ARG0 is the state transition; ARG1 is the child session
	reference; ARG2 is the child's _start or _stop value.

Worker manager example

Parent and child events matrix

	EVENT - ARG0 - ARG1 - ARG2
	_parent - old parent - new parent - N/A
	_child - "gain" or "lose" - child session - _stop return value
	_child - "create" - child session - _start return value

Watching the clock

	Timers are event watchers.
		They watch for a certain wall clock time.
		They watch for elapsed time.
		They do not repeat.
	POE's queue is kept in due-time order.
	Events created with yield() or post() are due for "now".

Kinds of timers

	Two ways to watch time:
		Absolute timers (alarms)
		Relative timers (delays)

	Two ways to track the watcher:
		Event name
		Unique IDs

	All just ways to manipulate future events.

Named timers
	Keyed by event name
	Only one timer per name (alarm(), delay()), per session.
	Unless you add extras (alarm_add(), delay_add()).
	Clearing by name clears ALL timers with the same name.
	Within the current session, of course.

Identified timers
	Can be set, cleared, or adjusted by ID.
	Duplicate timer names are not a problem.
	Unless you use a named timer function.

Uncooperative timer example
	(uses sleep)

Cooperative timer example
	(uses delay)

	delay() posts the event, which is scheduled to be delivered in the
	future.  It returns right away.  The event will linger in the queue
	until it becomes due.

Timer methods matrix

	... alarms - delays
	named - (functions) - (functions)
	identified - (functions) - (functions)
	either/both - alarm_remove_all() - N/A

I/O watchers

	Select-like
	Input, output, and exceptional (out-of-band) input
	Start, stop, pause, and resume.
	They condition filehandles.
	They do NOT perform I/O.

I/O watchers example(s)

I/O methods matrix
	etc.

Signal watchers

	Watch for OS signals or internal ones.
		Internal signals are created by the program itself.
		No processes are killed in their making.
	Keep sessions alive.
	Safe perl signals not needed with SIGCHLD.

Signal types

	All signals are not created equal.
	Benign: Information only.
		IO, WINCH, INFO, CHLD, PIPE, etc.
	Terminal.  Stop sessions unless handled.
		QUIT, INT, TERM, HUP, "IDLE"
	Non-maskable.  Always stop sessions.
		"ZOMBIE", "UIDESTROY"

Signal dispatch

	Signals sent to a session are also dispatched to its children
	If any of those sessions handles a signal, then it's handled for the
	whole tree.
	Signaling POE::Kernel signals the entire program.

Signal watcher example.

Signal watcher method matrix.

Signal event arguments.

Kernel shutdown.

	Kernel runs until all sessions stop.
	It sends everyone a SIGIDLE, if only aliased sessions remait.
	It sends SIGZOMBIE if SIGIDLE didn't help.
	run() returns when the last session stops.

Wheels

	Objects that encapsulate common watcher and event handler patterns.
	They are not managed by POE.
	They create and destroy watchers and handlers.

Filters and drivers

	Filters translate data formats for wheels
	But they don't understand high-level protocols
	POE::Filter::Line is assumed.
	Drivers perform file I/O.
	But there's only one: POE::Driver::SysRW
	POE::Driver::SysRW is the default

Session and wheels interaction

Wheel interaction with drivers and filters

POE::Wheel::FollowTail

	Watches the end of a file
	Doesn't block the rest of the program
	Can be used to watch many log files
	May be used with AIM/Yahoo/ICQ/IRC components for reporting

Wheel::FollowTail example

POE::Wheel::Run

	Runs programs and interacts with them via standard I/O
	Can use pipes, pseudo terminals, or a mix
	Can change the child's user, priority
	Can close the child's STDIN
	Can send signals to the child program

POE::Wheel::Run example

POE::Wheel::SocketFactory

	Creates sockets.  That's it.  Does NOT read or write.
	Client mode: builds one connection.
	Server mode: Listens, and builds one socket per connection.
	Supports most of the Berkeley sockets API.
	Can create UNIX sockets.
	Supports UDP, but why bother?

POE::Wheel::SocketFactory example

Throttling connections

	pause_accept()
	resume_accept()

Session initialization

	create() args parameter
	create() hash parameter
	closures

POE::Wheel::ReadWrite

	Performs buffered, non-blocking I/O on streams
	Does NOT create filehandles
	Not really appropriate for datagram I/O
	Although people have used it that way

POE::Wheel::ReadWrite example

Separate I/O for input and output

	Can use separate InputHandle and OutputHandle
	Instead of just Handle
	Can use separate InputFilter and OutputFilter
	Instead of just Filter

Water marks

	Tell when a driver's put() buffer fills up.
	Indicates an imminent emptiness in the put() buffer.
	put() returns true if the driver is overfull.
	LowMark and LowEvent indicate when it's ok to send again.
	FlushedEvent tells when output buffer is empty.

Water marks example

Changing watermarks on the fly

	set_high_mark()
	set_low_mark()

FlushedEvent example

Other flow control

	pause_input()
	resume_input()
	shutdown_input()
	shutdown_output()

Querying buffers

	get_driver_out_octets()
	get_driver_out_messages()

Switching filters on the fly

	Changes data formats in mid-stream.
	For example, HTTP streams
	For example, SMTP sessions

Switching filters example

Switching events

	Changes the events a wheel generates
	Re-routes wheel information to new code
	Useful for stateful things, like protocols

Switching events example

POE guts---

POE::Loop
POE::Queue
POE::Resource
POE::API
POE::NFA in detail
Dozens of components on the CPAN

Writing a filter

	Variable-length records, length-prepended
	Filters are plain Perl objects.
	Stream and processed data passed by list reference.
	Filter switching requires access to buffered stream data
	use bytes;

Examples of new() and put()
Examples of get_one_start() and get_pending()
Examples of get_one()

Postbacks

	Create callbacks that post POE events
	Created for graphical toolkits
	Keep their sessions alive until destroyed

	Callbacks (like postbacks, but synchronous)

Graphical toolkits

	POE detects other event loops and uses them internally.
	Tk, Gtk, Gtk2, Event, IO::Poll, WxWindows, possibly others.
	Can support anything that provides I/O watchers and timeouts.

Tk + POE

	Use Tk before using POE::Kernel
	POE APIs work the same
	POE::Loop::Tk uses native Tk facilities
	Postbacks translate Tk callbacks into events
	POE exports Tk's main window as \$poe_main_window.

	Once written, an interface-neutral POE component is portable to
	various event loops with no modification.  Your application can be
	ported to graphical or textual user interfaces by adding new
	interface code.

	Tk's event loop doesn't run without at least one widget.  POE
	creates a main window for the application and uses it as the handle
	into Tk's event loop.  The window is exported so developers can use
	it.

Tk + POE example

Session IDs

	Unique session identifiers
	Originally created for component developers
	\$session->ID (numeric)
	\$kernel->ID (non-numeric uuid)
	\$kernel->ID_id_to_session(session id)
	\$kernel->ID_session_to_id(session ref)

Extra references

	One session can keep itself or another active arbitrarily.
	Often used with message passing.
	Also used in postbacks and callbacks.

Extra references examples

Extra references API matrix

Self-modification

	Add, remove, or replace handlers at runtime
	First implemented for Wheel classes
	Named events let this work smoothly

	POE::Wheel classes use these facilities to add their handlers to
	sessions that create them, then later remove those handlers at
	DESTROY time.

	Yes, POE::Wheel classes build upon the basic POE libraries covered
	here.  It's easy to create new ones, or replace them entirely with
	something you might like better.  Whatever you create will coexist
	with wheels because they all use the same low-level libraries.

Self-modification example

Self-modification API matrix

	Explain state() name.

Session options

	Options change a session's behavior.
	Set at create time with "options" parameter.
	Set at runtime with \$session->option()
	Watch events with the "trace" option.
	Catch duplicate states with "debug".
	Catch unknown events with "default".

Library helpers

	\$poe_kernel - exported by POE::Kernel
	get_active_session()
	\$session->get_heap()

Components

	High level modules, often nearly complete programs
	Facades hiding one or more sessions
	There's more than one way to interface it.

TCP components

	Abstract common TCP patterns
	Use callbacks instead of status events
	Touch \$_[HEAP] (usually forbidden)
	Client and Server work mostly the same.
	Use and expose wheels.

POE::Component::Client::TCP example

POE::Component::Server::TCP example

(... worthy components here ...)


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