From 90b12bd71bb9fc79a4640b9112c13ef529d0196a Mon Sep 17 00:00:00 2001 From: Laurent Bercot Date: Fri, 5 Dec 2014 22:26:11 +0000 Subject: Initial commit --- doc/libftrig.html | 184 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 184 insertions(+) create mode 100644 doc/libftrig.html (limited to 'doc/libftrig.html') 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 @@ + + + + + s6: libftrig + + + + + + +

+s6
+Software
+skarnet.org +

+ +

libftrig

+ +

+libftrig is a portable Unix C programming interface allowing a +process (the subscriber or listener) to be instantly +notified when another process (the notifier or writer) +signals an event. +

+ + +

What is notification ?

+ + +

Notification vs. polling

+ +

+ Process A is notified 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 polling, where A has to +periodically (every T milliseconds) check whether E happened and has no +other way to be informed of it. +

+ +

+ Polling is generally considered bad practice - and is inferior to +notification in practically every case - for three reasons: +

+ +

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 polling +period). Polling processes have reaction delays due to the polling +periods.
Resource consumption. Even if no 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.
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.

+ +

+ 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. +

+ +

+ 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 +because +it's easy. +

+ +

Notifications and Unix

+ +

+ Unix provides several frameworks so that a process B (or the kernel) can +notify process A. +

+ +

Signals. The simplest Unix notification mechanism. Sending events amounts +to a kill() +call, and receiving events amounts to installing a signal handler (preferrably +using a self-pipe +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: +
- 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.
- 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.
BSD-style IPCs, i.e. file descriptors to perform select()/poll() system +calls on, in an asynchronous event loop. 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 very specific stuff +to reach its limits. If process A is reading on +fd f, it is notified everytime another process makes f readable - +for instance by writing a byte to the other end if f 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. +
- 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.
- 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.
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.

+ +

What we want

+ +

+ We need a general framework to: +

+ +

Allow an event-generating process to broadcast notifications to every process +that asked for one, without having to know their PIDs
Allow a process to subscribe to a "notification channel" and be instantly, +asynchronously notified when an event occurs on this channel.

+ +

+ This requires a many-to-many approach that Unix does not provide natively, and +that is what libftrig does. +

+ + +

That's what a bus is for. D-Bus already does all this.

+ + +

+ 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 service, to implement a bus. And s6 is a +supervision suite, 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... without +relying on a particular service to be up. libftrig provides a notification +mechanism that does not need a bus service to be up, that's its +main advantage over a bus. +

+ +

+ 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 skabus +project in the making, which aims to be simpler, smaller and more +maintainable than D-Bus. +

+ +

How to use libftrig

+ +

+ libftrig is really a part of libs6: all the functions +are implemented in the libs6.a archive, or the libs6.so +dynamic shared object. However, the interfaces are different for notifiers +and listeners: +

+ +

Notifiers use the libftrigw interface.
Listeners use the libftrigr interface.

+ + + -- cgit v1.2.3