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Does launchd Beat cron?
Blamemyparents writes "For those who aren't Appleheads, you may not have heard that with Tiger, Apple swapped out old Unix standby cron for their own creation, launchd (Apple mentions it on their OS X page and has the man page for it up as well). Seems like it's a bit nerdy, but this changes a LOT about how *nix systems have done things for many years. Launchd is Apple's replacement for quite a few utilities, including launching and quitting quite a few different things, and getting info from the system and other running processes. This page from Ars Technica talks a bit more in depth about it. Apple has open sourced the thing, and is apparently hoping all the unix kids will take a look."
Based on what I've read about it, it's more a replacement for xinetd and the sysv init scripts than cron.
B
Simply referring to launchd as a cron replacement is a major understatement. launchd runs as the init process and according to TFA was primarily made to replace the /etc/rc.d scripts during startup.
/etc/rc.d scripts (considering the length of pause an fsck would take--users would certainly assume there system hung during boot).
This is somewhat understandable for something like OS X. Doing something simple like displaying a GUI detailing startup is terribly difficult with
I'm not sure launchd is something you'd want in 99% of Linux installs but if you're looking for a end-to-end user-oriented desktop I can see how a technology like this is necessary.
I'm not sure Apple Gets It though. Why in the world would they use XML configs? Gesh.
the machine boots to a desktop (auto log in and all) in less than 15 seconds.
I am the Alpha and the Omega-3
Launchd provides faster startup through a unified framework for starting, stopping and managing daemons, and incorporates inetd, init, mach_init, System Starter and related services.
Sun came up with (at first glance) a similar thing called Service Management Facility in Solaris 10.
did you RTFA? launchd seems to be a system wide super program launcher. startup, timed, event driven (plug in a usb memory stick and get a pop up window asking what you want to do).
from reading the article, launchd sounds like a viable technology to replace quite a few convoluted system on my linux machines.
Actually, 15 seconds sounds really wrong to me.
To keep things simple, we divide the boot process into two halves: the hardware half and the software half. The hardware half involves power-on self-tests, and the time it takes varies wildly from computer type to computer type, or even from configuration to configuration. It takes longer to test 8 GB of RAM than 512 MB of RAM.
The software half begins with the grey Apple-logo screen turns blue and the progress window appears.
From the time that happens to the time the menu bar appears (via autologin) should be four seconds on a stock Mac OS X install with middle-of-the-road hardware (dual 1 GHz G4).
Uh, the code is licensed under the BSD model. So Apple will get in trouble as well if there is patent trouble. /. will get you modded up automatically, but come on people....
I know making the statement, "boo patents!" on
Monstar L
While confusing, the styling of the article title is correct - it's just the combination of capitalisation as is normal for titles with the names of the commands all being in lower-case. UNIX commands are case-sensitive, thus there is no commad called 'Cron', only 'cron', and it's good journalistic form to spell names for things with correct case, even if (in this example) it does make things a little confusing/AOL-ish
Dealing with lawyers would be a lot less tedious if they all looked like Casey Novak.
if you want to document your xml config file format, you can give a schema (xsd) or dtd for your document. you can give all the information about al l the supported options for the config file in your schema definition.
xml is more robust than its property file counterpart.
.ini has the serious disadvantage of not allowing for more than two levels of properties.
:p
But don't worry. I've once used a mmapped structure to make sure the config gets preserved over crashed -- but fortunately, someone beat me back into the senses. Try to exceed _this_ level of brain-damage
The creatures outside looked from Alt-Right to Antifa; but already it was impossible to say which was which.
Speaking of which, back in about 1995, I think it was, I installed my first Linux distro that I actually used on a daily basis. It was an early version of SuSE linux. Anyway, as I scratched deeper and deeper under the surface, I came to the conclusion that the init system was a mess... the entire system's functionality was implemented as piles upon piles of shell scripts, organized neatly into a whole bunch of directories, and activated according to their name and whatnot. This seemed kind of dumb, actually, because it meant that startup, shutdown, and switching runmodes were a lot less efficient than necessary, and also because you'd have to search through a zillion (that's a big number) scripts to find what you need.
A few years later, in 1999 or so, I tried FreeBSD for the first time. (Here it comes, I'm gonna be modded troll for starting another BSD- vs. SYSV-style init script war...) There, the functionality is still implemented in scripts, but there was a much more sane system. There is one script that is the same across all FreeBSD installs of that version. Then, there's the script you get to customize if you want. There's one more file, and that's where you simply give yes or no answers, or provide other data, in the form of environment variables that influence the running of those two scripts. It's so simple, and works quite quickly. Also, since you really only mess with one file, and two if you modify the script, it's much easier to find where things are. It's more efficient, from many standpoints.
So I don't blame Apple for getting rid of that SYSV stuff. It might have been cool back in the day, but it has lost its luster.
This thread, so far, seems to be filled mostly with "Ah! Someone's doing something differently than UNIX has for 30 years! My knee is jerking!" So I feel I should respond.
launchd is neat. It's not simply a different way of doing the same things, it lets you do different things.
Like automatically evaluating dependencies between daemons, starting them in the right order, and running them in parallel if needed. FreeBSD's the only other OSS system I've ever seen do this; Gentoo does the dependencies but not the parallel startup. (Which is annoying while it's, say, trying to get an address from a nonexistent DHCP server.) Long story short, it dramatically reduces boot time, while eliminating dependency hacks like runlevels and numbered scripts. (Not that BSD had them.)
For those of you who posted without reading the manpage (or administering an OSX system), it also lets you do init-style startup tasks on a per-user basis. You can configure it to start daemons and other processes on the behalf of users as the log in, and shut them down -- gracefully, not by TERM; KILL -- when the user logs out.
Anyone who's ever dealt with the myriad of hacks to get ssh-agent in place will understand why this is good.
There's a lot of resentment these days toward anyone who does something that's perceived as "not the UNIX way." Change is the only way to innovation, people; perhaps the UNIX way is broader than you thought?
Is there a way to make Dashboard widgets sticky on the desktop?
You don't want them to. One of the benefits of the Dashboard layer is the fact that when the Dashboard isn't visible, the widgets are completely suspended, consuming no CPU cycles at all. If you put a widget on the screen outside the Dasboard layer --which of course you can do, if you run Dashboard in debug mode --it'll execute its run loop all the time, sucking up CPU cycles unnecessarily.
There is a method for detaching Dashboard widgets described here.
This lets you keep as many widgets as you want on your desktop, and they don't jump back into the Dashboard every time you display it.
The one remaining annoyance is that they float above all of your other windows. Hopefully there will be a hack for this soon.
daemontools and the free-er clone runit which both give you the advantages of a non-linear system startup process, automatic service restarting, no need to write daemon-ising code in each program etc. Each author also has common tools to use with these service supervision programs to ensure network-based daemons don't need to write network code: ucspi-tcp and ipsvd respectively.
/sbin/runit instead of init from then on; we tend to use it on top of the existing init scripts.
:-)
Brave Deian owners can apt-get install runit-run which will start your system with
And the system that's had such a sensible service supervision system since the year dot? Windows NT and its service control manager. Of course you could argue a centrally-controlled daemon restarter is much more of a priority when you expect your daemons to crash more
Matthew @ Bytemark Hosting
Ye Olde cron is still there and running. My Bashpodder script still runs on Tiger. As for launchd, I am going to have to read up on that. Sounds interesting.
Are there schedulers better then cron? You bet. As a scheduler for home use, cron is fine. We are currently trying to find a ore robust replacement for cron that will attempt to correct things before reattempting a script and with dependency checking. When I used to be a operator on a DOS/VSE based mainframe, our scheduler had a dependcy check that would hold jobs from running at thier assigned time if the previous job did not finish and other things like that. It had very robust logging to. No e-mails to root. I could just run a report. There are schedulers that come close to this on UNIX but cron is a very basic scheduler that is flexible which is a saving grace.
Gorkman
Solaris 10 has something very similar to launchd called SMF: Service Management Framework. It doesn't replace cron but it does replace init.d and inetd. It also provides backwards compatibility so that existing init.d scripts and run and existing inetd entries are migrated to SMF on upgrade.
SMF also does dependancy checks and auto matic restart on failure (or some other conditions). It also uses XML for its configuration but imports that into an SQlite database so that it doesn't need to reparse the XML on every service restart or system boot.
For more information on SMF in Solaris see the links in the main architect/developers blog .
Your reply sounds like it comes from a geek, or perhaps wannabe geek. Well, just check out the source and tell us. http://www.opensource.apple.com/darwinsource/10.4/ launchd-106
There have been several comments along these lines. I don't understand them at all.
The launchd configuration files are property lists, which are serialized Core Foundation data structures. They consist of key-value pairs. They map directly to data structures in memory. The code for parsing them is used all over the place in Mac OS X, and is very old. It dates all the way back to the first development in the late 1990s. It's highly optimized, and it's reused over and over and over and over again.
The second advantage to plists is that they're self-validating. When a program tries to load a plist, if the file doesn't validate against the PropertyList-1.0 DTD, nothing happens. An error is returned, and the program (in this case, launchd) moves on. No chance of a corrupt file producing unexpected results.
The third advantage is that we ship a handy property-list editor with our operating system. This makes it easy for developers to create and modify plists.
Finally, plists are always in UTF-8 format. That's vitally important for us, because our system is fully localized. We have to be able to launch services with names that don't fit into the ASCII/Latin-1 character set. Using plists ensures that we can do that, and gives us that capability for free.
The question here is, why would anybody want to invent their own proprietary ASCII-only, non-validating file format for something as important as system startup control?
MacOSX programs make frequent use of NSDictionary, a Objective C collection class. There's also a C version, known as CFDictionary. It's worth noting that most of the relevant C interfaces, known collectively as "CoreFoundation" are open source
The CoreFoundation interface to MacOSX's built in XML parser is described here.
I'm not an expert, but from what I read of the XML files, launchd seems more of a wrapper program than a replacement for cron. Translated: ProgramArguments = /usr/sbin/cron;
QueueDirectories = /var/cron/tabs;
ps -wwax|grep cron # shows two cron processes in memory;
So launchd takes the place of init, and starts up cron, with the proper arguments. Cron then takes care of its own.
The programmer or administrator is responsible for putting that information in the XML file. That's why they're using XML. It allows for more data to be available. Where it (init) used to basically be a list of programs to launch, it (launchd) is now a system whereby a collection of programs are launched. It is much more flexible, and since it can launch programs concurrently, much faster as well.
Lack of eloquence does not denote lack of intelligence, though they often coincide.
We did open-source launchd. It's part of Darwin 8.
It's part of Darwin. Try this page: http://www.opensource.apple.com/darwinsource/10.4/
Well
The format we're talking about here is what we call a "property list," a serialized data structure that's written out to disk in XML format. Here's an example. This is the launchd config file for KernelEventAgent, an important Mac OS X system service.This files are generated by the serialization functions in Core Foundation. You create a data structure, in this case a CFDictionary. Then you populate it with objects. In this example, that's four key-value pairs. The keys are "Label," "ProgramArguments," "OnDemand" and "ServiceIPC." The values are a CFString, a CFArray of CFStrings (with just one object in this case), and two CFBooleans. Then you issue one function call to serialize that property list to XML and another to write it to disk as a file.
Of course, in this particular case you wouldn't go through that trouble. But the important part is the other part of the process. You make one function call, passing it a path to a file, and you get back a de-serialized Core Foundation data structure, all type-checked and everything. If at any point the property list fails --if it fails in the XML validation or the static type checking --then you get back null and an error. All atomic, no muss, no fuss.
That's what launchd does. All clear now?
And shipping one of those implies there's difficulty in editing or reading the configs manually.
Look at it and you tell me. How daunted are you?
If you're building better solutions, maybe you can architect them to work well for most unixes?
We're releasing launchd, we're releasing Core Foundation, the property list format is both wide-ass open and fully documented in many different places. What else do you want?
You can ensure that the file is a valid property list XML file, but you can't make sure that it contains launchd configuration info
Of course you can. A launchd configuration property list de-serializes to a CFDictionary. If the CFDictionary doesn't include the required key-value pairs, it's not a valid launchd configuration file.
Syntactic and semantic structure happen in two different places. One happens as the file is de-serialized in Core Foundation. The other happens inside the launchd code itself.
DTD/Schema sensitive editors still wont help you out with semantic structure.
Technically this is a valid point, I agree. But property lists are almost always so incredibly simple that this objection is purely academic.
Since XML is an abstraction on top of plain text files, there's absolutely no way it could be simpler than a plain text file to begin with!
Not true. By text files, we tend to mean line-oriented files. Often to be useful, it must also be field-delimited files.. colon-delimited in password files, or worse with cron-files a mixture of space-delimiting and position-dependence. (after first sevearl spaces, remaining spaces are no longer delimiters).
With XML you are no longer rectangular in your data-structure; no longer rows and columns (and potentially sub-colums), but instead you can more naturally represent the data however complexly it needs, and not have to worry about escaping arbitrarily complex characters, because all XML files use the same escaping.
-Michael
Wrong, at least on your first point. launchd is APSL-licensed open source. Oh, and these chaps seem quite happy with the APSL, so it'd be churlish to begrudge Apple that choice of license.
From an educational perspective, I'd be interested to know which Unix design ethics it violates, though.