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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-05 22:26:11 +0000 |
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| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-05 22:26:11 +0000 |
| commit | 90b12bd71bb9fc79a4640b9112c13ef529d0196a (patch) | |
| tree | 523b3f4ee2969e7a729bab2ba749c4b924ae62af /doc/libftrig.html | |
| download | s6-90b12bd71bb9fc79a4640b9112c13ef529d0196a.tar.xz | |
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diff --git a/doc/libftrig.html b/doc/libftrig.html new file mode 100644 index 0000000..da4c25b --- /dev/null +++ b/doc/libftrig.html @@ -0,0 +1,184 @@ +<html> + <head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>s6: libftrig</title> + <meta name="Description" content="s6 libftrig" /> + <meta name="Keywords" content="s6 libftrig" /> + <!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> --> + </head> +<body> + +<p> +<a href="index.html">s6</a><br /> +<a href="http://skarnet.org/software/">Software</a><br /> +<a href="http://skarnet.org/">skarnet.org</a> +</p> + +<h1> libftrig </h1> + +<p> +<t>libftrig</t> is a portable Unix C programming interface allowing a +process (the <em>subscriber</em> or <em>listener</em>) to be instantly +notified when another process (the <em>notifier</em> or <em>writer</em>) +signals an event. +</p> + +<a name="notification"> +<h2> What is notification ? </h2> +</a> + +<h3> Notification vs. polling </h3> + +<p> + Process A is <em>notified</em> of an event E when it gets a instant +notice that event E has happened; the notice may disrupt A's execution +flow. Notification is the opposite of <em>polling</em>, where A has to +periodically (every T milliseconds) check whether E happened and has no +other way to be informed of it. +</p> + +<p> + Polling is generally considered bad practice - and is inferior to +notification in practically every case - for three reasons: +</p> + +<ul> + <li> Reaction time. When event E happens, process A does not know it +instantly. It will only learn of E, and be able to react to it, when +it explicitly checks for E; and if E happened right after A performed +the check, this can take as long as T milliseconds (the <em>polling +period</em>). Polling processes have reaction delays due to the polling +periods. </li> + <li> Resource consumption. Even if <em>no</em> event ever happens, process A +will still wake up needlessly every T milliseconds. This might not seem like +a problem, but it is a serious one in energy-critical environments. Polling +processes use more CPU time than is necessary and are not energy-friendly. </li> + <li> Conflict between the two above reasons. The longer the polling period, +the more energy-friendly the process, but the longer the reaction time. The +shorter the polling period, the shorter the reaction time, but the more +resource-consuming the process. A delicate balance has to be found, and +acceptable behaviour is different in every case, so there's no general rule +of optimization. </li> +</ul> + +<p> + Notification, on the other hand, is generally optimal: reaction time is +zero, and resource consumption is minimal - a process can sleep as soon as +it's not handling an event, and only wake up when needed. +</p> + +<p> + Of course, the problem of notification is that it's often more difficult +to implement. Notification frameworks are generally more complex, involving +lots of asynchronism; polling is widely used +<a href="http://lib.store.yahoo.net/lib/demotivators/mediocritydemotivationalposter.jpg">because +it's easy.</a> +</p> + +<h3> Notifications and Unix </h3> + +<p> + Unix provides several frameworks so that a process B (or the kernel) can +notify process A. +</p> + +<ul> + <li> Signals. The simplest Unix notification mechanism. Sending events amounts +to a <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/kill.html">kill()</a> +call, and receiving events amounts to installing a signal handler (preferrably +using a <a href="http://skarnet.org/software/skalibs/libstddjb/selfpipe.html">self-pipe</a> +if mixing signals with an event loop). Unfortunately, Unix signals, even the more +recent and powerful real-time Posix signals, have important limitations when it's +about generic notification: + <ul> + <li> non-root processes can only send signals to a very restricted and +implementation-dependent set of processes (roughly, processes with the same UID). This is a problem when +designing secure programs that make use of the Unix privilege separation. </li> + <li> you need to know the PID of a process to send it signals. This is generally +impractical; process management systems that do not use supervisor processes have +to do exactly that, and they resort to unreliable, ugly hacks (.pid files) to track +down process PIDs. </li> + </ul> </li> + <li> BSD-style IPCs, i.e. file descriptors to perform select()/poll() system +calls on, in an <em>asynchronous event loop</em>. This mechanism is very widely used, +and rightly so, because it's extremely generic and works in every ordinary situation; +you have to be doing <a href="http://www.kegel.com/c10k.html">very specific stuff</a> +to reach its limits. If process A is reading on +fd <em>f</em>, it is notified everytime another process makes <em>f</em> readable - +for instance by writing a byte to the other end if <em>f</em> is the reading end +of a pipe. And indeed, this is how libftrig works internally; but libftrig is needed +because direct use of BSD-style IPCs also has limitations. + <ul> + <li> Anonymous pipes are the simplest and most common BSD-style IPC. If there is a +pipe from process B to process A, then B can notify A by writing to the pipe. The +limitation is that A and B must have a common ancestor that created the pipe; two +unrelated processes cannot communicate this way. </li> + <li> Sockets are a good many-to-one notification system: once a server is up, it +can be notified by any client, and notify all its clients. The limitation of sockets +is that the server must be up before the client, which prevents us from using them +in a general notification scheme. </li> + </ul> </li> + <li> System V IPCs, i.e. message queues and semaphores. The interfaces to those IPCs +are quite specific and can't mix with select/poll loops, that's why nobody in their +right mind uses them. </li> +</ul> + +<h3> What we want </h3> + +<p> + We need a general framework to: +</p> + +<ul> + <li> Allow an event-generating process to broadcast notifications to every process +that asked for one, without having to know their PIDs </li> + <li> Allow a process to subscribe to a "notification channel" and be instantly, +asynchronously notified when an event occurs on this channel. </li> +</ul> + +<p> + This requires a many-to-many approach that Unix does not provide natively, and +that is what libftrig does. +</p> + +<a name="bus"> +<h2> That's what a bus is for. D-Bus already does all this. </h2> +</a> + +<p> + Yes, a bus is a good many-to-many notification mechanism indeed. However, +a Unix bus can only be implemented via a daemon - you need a long-running +process, i.e. a <em>service</em>, to implement a bus. And s6 is a +<em>supervision suite</em>, i.e. a set of programs designed to manage +services; we would like to be able to use notifications in the supervision +suite, to be able to wait for a service to be up or down... <em>without</em> +relying on a particular service to be up. libftrig provides a notification +mechanism that <em>does not need</em> a bus service to be up, that's its +main advantage over a bus. +</p> + +<p> + If you are not concerned with supervision and can depend on a bus service, +though, then yes, by all means, use a bus for your notification needs. +There is a <a href="http://skarnet.org/software/skabus/">skabus</a> +project in the making, which aims to be simpler, smaller and more +maintainable than D-Bus. +</p> + +<h2> How to use libftrig </h2> + +<p> + <tt>libftrig</tt> is really a part of <tt>libs6</tt>: all the functions +are implemented in the <tt>libs6.a</tt> archive, or the <tt>libs6.so</tt> +dynamic shared object. However, the interfaces are different for notifiers +and listeners: +</p> + +<ul> +<li> Notifiers use the <a href="libftrigw.html">libftrigw</a> interface. </li> +<li> Listeners use the <a href="libftrigr.html">libftrigr</a> interface. </li> +</ul> + +</body> +</html> |