WebSVN - Blame - Rev 12 - /trunk/liboop.org/why.html

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3	magnus	1	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
		2	<html><head>
		3	<title>liboop: Why?</title>
		4	<link rel="stylesheet" type="text/css" href="style.css">
		5	</head><body>
		6
		7	<h2>Why use liboop?</h2>
		8
		9	<h4>The problem.</h4>
		10
		11	Developers often wish to write applications which serve as a mediator between
		12	several logical interfaces simultaneously; in fact, most applications work
		13	this way. For example, a browser application might wish to maintain a user
		14	interface while also managing a network connection and occasionally exchanging
		15	data with the local filesystem. A server application might be communicating
		16	with several clients at once while also occasionally receiving a signal from
		17	the administrator directing it to reload its configuration. A multiplayer game
		18	might want to maintain several active user interfaces at once.
		19	<p>
		20	Furthermore, each of these interfaces may be quite complex, sufficiently so to
		21	merit shared code modules which specialize in managing the interface.
		22	Widget sets deal with the details of the X protocol and graphical user
		23	interface management; "curses" deals with the arcana of character-based
		24	terminals; WWW libraries offer high-level access to whole families of Internet
		25	transfer protocols; standard I/O and database routines manage filesystem data.
		26	<p>
		27	However, the existing techniques available for multiplexing interface code are
		28	very poor. Most of these libraries work in "blocking" fashion; once
		29	instructed to complete a task (such as downloading a file, or presenting a
		30	dialog to the user), they do not return until the task is complete (or failed),
		31	even though this may mean waiting an arbitrary amount of time for some external
		32	agent (such as the user or the network) to respond. Some of the better systems
		33	are able to manage several concurrent tasks internally, but cannot work with
		34	other components.
		35	<p>
		36	Developers are thus left with several unpalatable choices:
		37	<ol>
		38	<li>Accept "blocking" operation. User interfaces stop functioning while the
		39	application waits for the network; one network client's access is stalled
		40	while another client performs a transaction. As more data moves from local
		41	storage (where access is fast enough that blocking is acceptable) to
		42	delay-prone networked media, this is becoming less and less acceptable.
		43	<li>Use multiple threads for concurrency. While this is a good solution for
		44	some problems, developers who choose this route must struggle with relatively
		45	immature and unportable threading models, and deal with the many libraries
		46	which are not thread-safe; furthermore, threaded programming requires
		47	thought-intensive and error-prone synchronization.
		48	<li>Use multiple processes ("forking") for concurrency. This can also work,
		49	but requires all communication between modules to use some form of
		50	inter-process communication, which increases complexity and decreases
		51	performance. Forking itself is a slow operation, leading to complex
		52	"pre-forking" schemes for better performance. Worst of all, each process
		53	must somehow multiplex IPC from other processes with whatever I/O task it had
		54	to accomplish in the first place; this brings back the very problem forking
		55	was designed to address.
		56	<li>Attempt to multiplex each library's I/O operations directly in a master
		57	"select loop". This requires the developer to understand intimately the
		58	exact details of each library's I/O interactions, thus breaking modularity,
		59	fostering unhealthy dependency and leading to a single central snarl through
		60	which all I/O must pass.
		61	</ol>
		62	The paucity of options is reflected in the quality of applications. How many
		63	programs hang unpleasantly while performing simple network operations like
		64	hostname resolution? How many user interfaces are unnecessarily "modal"?
		65	How many simple servers fork for no good reason? How many network applications
		66	simply don't exist because it's so difficult to write them?
		67
		68	<h4>The solution.</h4>
		69
		70	Liboop offers a single, simple, central event loop. Modules wishing to perform
		71	I/O without blocking request <em>callbacks</em> from the central <em>event
		72	source</em>. These callbacks may be tied to file-descriptor activity, the
		73	system time, or process signals. Liboop is responsible for invoking these
		74	callbacks as appropriate.
		75	<p>
		76	With this system, each module "owns" its own I/O; it can perform arbitrarily
		77	complex operations without blocking anything else in the program. But since
		78	callbacks are executed purely sequentially, there is no complex concurrent code
		79	to manage. From the application developer's point of view, working with liboop
		80	is very simple; the developer simply makes calls to libraries which work their
		81	magic and call the application back when they finish. Applications can easily
		82	manage an arbitrary amount of multiplexed I/O operations using as many
		83	interface libraries as they like without blocking.
		84	<p>
		85	To work with this system, libraries and applications must be liboop-aware.
		86	Development with legacy code uses <em>adapters</em> which translate the I/O
		87	model of an application or library into liboop's model. This does require
		88	knowledge of the code's I/O structure, but can at least keep the modules in
		89	an application independent of each other.
		90	<p>
12	magnus	91	For more about liboop, see the <a href="how.html">documentation</a>.
3	magnus	92
		93	<h4>Q&A</h4>
		94
		95	<dl>
		96	<dt><em>Why don't you just use (favorite widget set), which lets you register
		97	callbacks on file descriptors and all that good stuff?</em>
		98	<dd>Because not everyone might want to be tied to that widget set. In
		99	particular, the developer of a general-purpose I/O library would want to
		100	allow everyone to use it, without requiring a particular widget set.
		101	Liboop lets the library developer write to a standard interface,
		102	which can then be used with most widget sets and other event loops.<p>
		103
		104	<a name="glib"></a>
		105	<dt><em>Doesn't GLib's <a
		106	href="http://developer.gnome.org/doc/API/glib/glib-the-main-event-loop.html">Main
		107	Event Loop</a> do all this, and more?</em>
		108	<dd>Not quite. GLib is a fine implementation of an event loop (with
		109	bells and whistles) that supports some extensibility (such as the ability to
		110	add extra sources). However, I'm doubtful that it extends far enough that
		111	it could run on top of someone else's event loop (such as the Tk event loop).
		112	Furthermore, the GLib event loop doesn't manage signals; synchronous handling
		113	of asynchronous signals is very difficult to do properly and safely in most
		114	existing systems (without kludges like polling).
		115
		116	<p>In any case, we do have a
12	magnus	117	<a href="oop_glib.html">GLib source adapter</a> so you can use the GLib event loop
3	magnus	118	with the liboop interface.</p>
		119
		120	<dt><em>How does liboop compare to Niels Provos' <a
		121	href="http://www.monkey.org/~provos/libevent/">libevent</a>?</em>
		122	<dd>Like GLib, libevent is a concrete implementation of an event loop, not
		123	an abstract interface for many event loops; also like GLib, libevent does not
		124	manage signals. Libevent is smaller and simpler than either liboop or Glib.
		125	While liboop and GLib are both licensed under the
		126	<a href="http://www.fsf.org/copyleft/lesser.html">Lesser GPL</a>, libevent
		127	appears to be licensed under the original BSD license, including the
		128	advertising clause. Note that the advertising clause renders libevent
		129	incompatible with GPL software!
		130
		131	<p>It is entirely possible to imagine a libevent source adapter for liboop.
		132	If anyone is interested in such an adapter, please contact me.</p>
		133
		134	</dl>
		135
		136	<hr><a href="">liboop home</a></body></html>

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