Thoughts and rambling on the X protocol
Two years ago, while working on awesome, I joined the Freedesktop initiative to work on XCB. I had to learn the arcane of the X11 protocol and all the mysterious and old world that goes with it.
Now that I've swum all this months in this mud, I just feel like I need to share my thoughts about what become a mess over the decades.
When I was unborn…
…the Toto band were releasing their song "Africa" and some smart guys were working on a windowing system: the X Window System (this is its full name) which therefore has a (too) long history. The latest version of its protocol, the 11th one, has been designed in the 80's. You can learn more about the history in the Wikipedia article about X.
In 2010, we still listen disco music and we still use various protocols designed in the 80's and even before X. Music have evolved, protocols have evolved, and so did X11.
The problem is that X11 did not evolve that well. The guys at MIT-and-some other-places-with-very-smart-people-in-it created X version 1 in 1984, and updated it until X version 11 (the one we're still using) in 1987. Eleven version in 3 years, that was following the "release early, release often" model. But I don't know why, it just stopped to happen for the last 23 years (that's not totally true: they added (and then deprecated) many extensions.)
I don't know what changes have been made in the first 11 major versions of the X protocol, but I'm rather sure we should have deserve a couple of major version updates this last 2 decades.
In my humble opinion, X11 was not designed to live 23 years. But hey, I'm not blaming anyone here: I was 4 years old and playing Lego® when they released this latest version of the X protocol, so there is little chance I'd have done something better.
We won't fix. We'll work-around.
That is probably one of the guideline of the X protocol for the last years. And don't misread me: I'm not bashing anyone thereafter.
Since the X11 protocol was aging, the X guys started to add extensions. They added tons of them over the years. This, in application of one of the early principles of X:
It is as important to decide what a system is not as to decide what it
is. Do not serve all the world's needs; rather, make the system extensible
so that additional needs can be met in an upwardly compatible fashion.
All of them with no exception were added because, bad luck, the X11 protocol did not anticipated the things that happened in the last 23 years, like video, OpenGL, multiple monitors, or the pleasure to draw oval windows. Some of this extensions are still in use, while some of them have been dropped.
While this is not a bad thing to extends the protocol, it seems like a bad thing to try to fix the protocol with for example the XFixes extension, even with all the good intentions Keith Packard might have in his greatness.
Actually it's even worst than you think
The X11 protocol (without extensions) defines about 120 types of requests: create a window, move a window, etc.
Nowadays, there's at least 25 % of them which are useless: usage of server-side font, or the drawing of squares and polygon, are unused by any modern application or toolkit. All of this is superseded by requests from extensions, like the XRender one.
The handling of multiple monitors displays has totally been screwed up. X11 has been designed to work in Zaphod mode (independent monitors). But Xinerama, and nowadays XRandR have replaced it up: recent X servers (released after ~2007) does not support Zaphod mode anymore, even if it's a core piece of the X11 protocol.
Worst: on many requests, there's limitation or design flaws, like described in this document: Why X Is Not Our Ideal Window System by DEC researchers.
We'll add more broken standard on top of that
Following its early principle, X does not define policies but only mechanisms, which seems like a good thing,
Consequently, people started writing specifications to determine a number of stuff and dogmas: ICCCM. That was 22 years ago in 1988. It's useless to add that many things in this specification are now obsolete, useless, or that it misses many modern stuff.
I was not the only one to think that. The people from what will be the major desktop environments, KDE and GNOME, saw that too in the 90's while I was learning to count. So they wrote EWMH, another standard that comes on top of ICCCM and extends it with nifty features like maximization, full screen mode, etc.
The problem is that this standard has also been written by narrow-minded people who at that time, were working on GNOME or KDE (and maybe others). This desktop environments were having and still have some strong concepts of how should work a desktop: "it should have work-spaces", "a window is only on one workspace", "we only see a workspace at a time", "you do not have multiple screens", etc.
Dude, we don't care: we have toolkits!
This vision of how the desktop should work have now been written in marble in all applications and libraries implementing EWMH, like GTK+ or Qt.
Nowadays, everybody forgot about all of this standards. Toolkits have implemented this, circumvented the X11 protocol limitation and flaws, and nobody wants to look back.
Like all standards, obviously some people implemented them badly. This had some side effects, like OpenOffice acting like a pager.
We don't look back? Worst, we forgot where we came from!
With all these layers of bad designed standards, the desktop continued to evolve for more than a decade. They continued to add more standard, the more recent ones being based on D-Bus like the Desktop Notification Specification or the latest Status Notifier Specification developed by KDE.
The Status Notifier is a new implementation of the good old system tray based on D-Bus and XEmbed instead of the X11 mechanisms, and adding the possibility to show the system tray with something else than icons.
This specification draft saw an important issue and design flaw raised by Wolfgang Draxinger in this thread on the XDG mailing-list. What Wolfgang points out, is that X is network-oriented, and D-Bus is not. Therefore, making the Status Notifier specification to use D-Bus to pass system tray messages around is a bad idea, since running a X application from host A on host B will draw the system tray on the wrong host!
Apparently, reading the thread, this does not fear some of the KDE people:
of course this is a bizarre corner case not worth much thought. at least
that's what you'll think until you actually run into it yourself (be it
because you are testing something or because you are setting up some
weird kiosk environment).
What Oswald describes as a corner case is an actual common use case for many of us. Of course, YMMV.
From my point of view, this is a step back in the wrong direction. But we can conclude that the network part of X is now worthless, to at least KDE.
I used to believe in XCB
When I joined Freedesktop, it was to work on XCB, the X C Binding. XCB is a nice, clean, 21st century technology based API to play with the X11 protocol. Its code is auto generated based on XML file describing the protocol.
In comparison, Xlib is 80's obfuscated code with almost no comments and hard-coded things. Only a few people can understand some of its corner like its i18n or XKB implementations. And all its code is synchronous.
For people not knowing it yet, X is a network protocol where you send request (like a GET in HTTP) and then get a response. Xlib forces the application to wait for the reply to its request, so the application is blocked until the X server sends the reply to the request. XCB on the other hand does not block and allows the application to send a batch of requests, do some other stuff in the mean time, and then gets the replies.
It's like your Web browser would send one request at a time to a Web server, and would wait until you downloaded all the images one by one to display the page.
In cases where X and all its clients are on the same host, the latency is small and not really visible, therefore the gain for XCB to be asynchronous is small. On slow network however, the gain can be huge, as proved in the rewrite of xlsclients with XCB by Peter Harris.
One of the long standing goal of the XCB folks is to kick-out Xlib, to increase speed and hides latency in X11 applications. That requires to port many libraries, because almost none of them (Cairo being an exception) supports XCB.
From where I stand, I don't really see if the work is worth it now. The desktop world is trusted by GNOME and KDE, meaning GTK+ and Qt. It seems none of this toolkits are interested to work on XCB, neither on the X protocol. They probably put hard effort in bypassing X limitation and flaws, and they now sit on top of crap of workarounds and broken-by-design-standard implementation. It seems to me they don't want to go back in the layers and improves things.
They're too high to go back down and they don't see what the gain would be.
Enlightenment with its EFL was the first toolkit to have an XCB back-end with the work of Vincent Torri. Unfortunately, the back-end is not maintained and nobody cares about it. Last time I tried it, it did not compile at all.
There's a page called X12 on the Freedesktop wiki, listing all the things that should be fixed some days. Unfortunately, the list continues to grow up an no one talks about working on X12.
On the other hand, there's a handset of people trying to work when they will have time on XKB2, the second version of the "let's-try-to-fix-up-the-keyboard-part-of-the-protocol-we-wrote-23-years-ago-a-second-time" extension.
To me, it does not seem X12 will happen in the next decade neither.
Do we got alternative to X? There's Wayland, but it's far from being usable. There's DirectFB, but that's not very portable. None seems candidate to replace X some days to me.
Anyhow, none of the main toolkits around support this alternative. GTK+ once supported DirectFB, but as far as I know, it is not supported nor works nowadays, as stated by Josselin Mouette. This is why recent versions of the Debian installer have migrated to X for the graphic part, thanks to Cyril Brulebois work.
XCB has been around for more than half-a-decade, and very few people showed interested in it. As far as I can see, nobody is interested to use the X protocol and everybody tries to encapsulate it in some higher-level API as soon as possible to stop seeing it. This leads to poorly written application and toolkits, with a lot of ugly hack.
All of that also means that starting to write applications and graphical toolkits based on XCB would be a very interesting project, but that would lead to spend too much time learning to circumvent the X protocol flaws, things that have been done in years by predecessors like Qt and GTK+.
Major toolkits implementations have almost nothing to win in going back in the dark water of X. I guess most of their folks prefer to work on shiny 3D effects based on your GPS location, rather than redefining better basis for everyone.
The manpower available in the X world is very small. Debian lacking of X maintainers is just the summit of that. There is very smart and very competent and skilled guys in the X world, as you can see by simply reading blog posts on Planet Freedesktop for example (me excluded). Unfortunately, there's not enough of them to cover all the things involved in X: input devices, graphics devices, new protocol extension specification and so on. The X server is really late, and it seems most of the developers prefers to work on the server itself than on the protocol behalf. Which is understandable.
I'm curious to see where all of that will lead in the upcoming years. I've been walking in the X world hallways for about 3 years now, and I feel desktop alternatives to KDE and GNOME will all die sooner or later. The time were you could choose between a dozen "modern" window managers has passed away.
After all, maybe that is simply Darwinism applied to computer software.