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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html><head>
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<title>liboop: Why?</title>
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<link rel="stylesheet" type="text/css" href="style.css">
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</head><body>
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<h2>Why use liboop?</h2>
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<h4>The problem.</h4>
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Developers often wish to write applications which serve as a mediator between
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several logical interfaces simultaneously; in fact, most applications work
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this way.  For example, a browser application might wish to maintain a user
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interface while also managing a network connection and occasionally exchanging
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data with the local filesystem.  A server application might be communicating
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with several clients at once while also occasionally receiving a signal from
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the administrator directing it to reload its configuration.  A multiplayer game
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might want to maintain several active user interfaces at once.
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<p>
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Furthermore, each of these interfaces may be quite complex, sufficiently so to
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merit shared code modules which specialize in managing the interface.
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Widget sets deal with the details of the X protocol and graphical user
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interface management; "curses" deals with the arcana of character-based
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terminals; WWW libraries offer high-level access to whole families of Internet
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transfer protocols; standard I/O and database routines manage filesystem data.
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<p>
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However, the existing techniques available for multiplexing interface code are
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very poor.  Most of these libraries work in "blocking" fashion; once
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instructed to complete a task (such as downloading a file, or presenting a
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dialog to the user), they do not return until the task is complete (or failed),
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even though this may mean waiting an arbitrary amount of time for some external
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agent (such as the user or the network) to respond.  Some of the better systems
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are able to manage several concurrent tasks internally, but cannot work with
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other components.
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<p>
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Developers are thus left with several unpalatable choices:
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<ol>
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<li>Accept "blocking" operation.  User interfaces stop functioning while the
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application waits for the network; one network client's access is stalled
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while another client performs a transaction.  As more data moves from local
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storage (where access is fast enough that blocking is acceptable) to
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delay-prone networked media, this is becoming less and less acceptable.
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<li>Use multiple threads for concurrency.  While this is a good solution for
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some problems, developers who choose this route must struggle with relatively
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immature and unportable threading models, and deal with the many libraries
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which are not thread-safe; furthermore, threaded programming requires
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thought-intensive and error-prone synchronization.
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<li>Use multiple processes ("forking") for concurrency.  This can also work,
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but requires all communication between modules to use some form of
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inter-process communication, which increases complexity and decreases
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performance.  Forking itself is a slow operation, leading to complex
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"pre-forking" schemes for better performance.  Worst of all, each process
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must somehow multiplex IPC from other processes with whatever I/O task it had
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to accomplish in the first place; this brings back the very problem forking
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was designed to address.
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<li>Attempt to multiplex each library's I/O operations directly in a master
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"select loop".  This requires the developer to understand intimately the
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exact details of each library's I/O interactions, thus breaking modularity,
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fostering unhealthy dependency and leading to a single central snarl through
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which all I/O must pass.
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</ol>
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The paucity of options is reflected in the quality of applications.  How many
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programs hang unpleasantly while performing simple network operations like
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hostname resolution?  How many user interfaces are unnecessarily "modal"?  
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How many simple servers fork for no good reason?  How many network applications
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simply don't exist because it's so difficult to write them?
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<h4>The solution.</h4>
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Liboop offers a single, simple, central event loop.  Modules wishing to perform
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I/O without blocking request <em>callbacks</em> from the central <em>event
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source</em>.  These callbacks may be tied to file-descriptor activity, the
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system time, or process signals.  Liboop is responsible for invoking these
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callbacks as appropriate.
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<p>
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With this system, each module "owns" its own I/O; it can perform arbitrarily
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complex operations without blocking anything else in the program.  But since
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callbacks are executed purely sequentially, there is no complex concurrent code
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to manage.  From the application developer's point of view, working with liboop
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is very simple; the developer simply makes calls to libraries which work their
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magic and call the application back when they finish.  Applications can easily
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manage an arbitrary amount of multiplexed I/O operations using as many
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interface libraries as they like without blocking.
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<p>
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To work with this system, libraries and applications must be liboop-aware.
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Development with legacy code uses <em>adapters</em> which translate the I/O
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model of an application or library into liboop's model.  This does require
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knowledge of the code's I/O structure, but can at least keep the modules in
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an application independent of each other.
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<p>
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For more about liboop, see the <a href="how">documentation</a>.
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<h4>Q&amp;A</h4>
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<dl>
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<dt><em>Why don't you just use (favorite widget set), which lets you register
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callbacks on file descriptors and all that good stuff?</em>
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<dd>Because not everyone might want to be tied to that widget set.  In
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particular, the developer of a general-purpose I/O library would want to
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allow everyone to use it, without requiring a particular widget set.  
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Liboop lets the library developer write to a standard interface,
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which can then be used with most widget sets and other event loops.<p>
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<a name="glib"></a>
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<dt><em>Doesn't GLib's <a
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href="http://developer.gnome.org/doc/API/glib/glib-the-main-event-loop.html">Main
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Event Loop</a> do all this, and more?</em>
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<dd>Not quite.  GLib is a fine implementation of an event loop (with
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bells and whistles) that supports some extensibility (such as the ability to
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add extra sources).  However, I'm doubtful that it extends far enough that
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it could run on top of someone else's event loop (such as the Tk event loop).  
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Furthermore, the GLib event loop doesn't manage signals; synchronous handling
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of asynchronous signals is very difficult to do properly and safely in most
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existing systems (without kludges like polling).
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<p>In any case, we do have a
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<a href="oop_glib">GLib source adapter</a> so you can use the GLib event loop
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with the liboop interface.</p>
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<dt><em>How does liboop compare to Niels Provos' <a
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href="http://www.monkey.org/~provos/libevent/">libevent</a>?</em>
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<dd>Like GLib, libevent is a concrete implementation of an event loop, not
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an abstract interface for many event loops; also like GLib, libevent does not
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manage signals.  Libevent is smaller and simpler than either liboop or Glib.  
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While liboop and GLib are both licensed under the
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<a href="http://www.fsf.org/copyleft/lesser.html">Lesser GPL</a>, libevent
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appears to be licensed under the original BSD license, including the
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advertising clause.  Note that the advertising clause renders libevent
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incompatible with GPL software!
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<p>It is entirely possible to imagine a libevent source adapter for liboop.
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If anyone is interested in such an adapter, please contact me.</p>
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</dl>
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