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."

Comments

Not a cron replacement, a init replacement

[Anthony Liguori]

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.

This is somewhat understandable for something like OS X. Doing something simple like displaying a GUI detailing startup is terribly difficult with /etc/rc.d scripts (considering the length of pause an fsck would take--users would certainly assume there system hung during boot).

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.

Re:Not a cron replacement, a init replacement

[As Seen On TV]

Is there any reason why a daemon is necessary to improve startup time?

The launchd service isn't a daemon. It's a replacement for init. And yes, it speeds things up considerably. Previously, every startup script required its own instance of a shell to be fired off. Now that middleman is removed. And it made things considerably faster and simpler.

But why XML over say something like INI or a much simplier format?

Because XML is self-validating, like I said. The contents of the file can be checked for consistency before being used for anything.

It also quickly becomes unreadable.

You don know that we're talking about property lists here, right? It's an XML-based format, but it's not totally free-form XML. You can't just plug in any valid XML and expect launchd to handle it. If the file doesn't validate against the property list schema, launchd will just ignore it.

Besides, we've been using property lists everywhere for five years now. Nobody's complained yet. ;-)

Re:Not a cron replacement, a init replacement

[JQuick]

The launchd source code is a collection of programs which span several related areas. In the broadest terms the contents of the distribution cover init, SystemStarter, and launchd.

Most people talking about launchd in the context of parallel system startup are actually talking about SystemStarter. Also note that the plist files which drive SystemStarter can be in one of two forms. The most common of these is the NeXT style textual plist format which is very human friendly and is not XML-like in structure or appearance.

As a replacement for aither BSD style or System 5 style rc scripts, SystemStarter is a definite improvement, both in terms of ease of use, speed, and reliability. The launchd subsystem uses XML formatted property lists. I am unsure whether NeXT style plists are supported for these.

Regardless, you were dismissive of launchd claiming that it was mere init replacement. You thought you were speaking about launchd but sound like you had only heard about SystemStarter.

launchd itself it a super-server which reads process definitions. These definitions can describe processes to be run using a set of predefined keys. Some examples of key value pairs are: Program/ (path to executable), or ProgramArguments/ (The array of strings to pass as arguments to the program). The interesting thing about launchd is that the predefined key/value space is so rich, and so well considered.

The keys span several related domains which Unix users associate with a number of different tools and data sources. Here are a list of some interesting areas covered.

like sudo or vixie cron the user and group under which to launch the program can be specified using any of { UserName, UID, GroupName, GID }

WorkingDirectory causes a chdir(2) before executing the job.
RootDirectory specieis a chroot(2) for the child process.

StdOutPath and StdErrPath can specify where to redirect output of the program.

OnDemand specifies whether to run only as needed or ensure that a server process should always be available, thus respawned.

Labels and Groups provide a way to refer to processes with greater specificity or to group related processes together for reporting or action using launchctl.

A set of time specific keywords cover a superset of at and cron time specification.

Soft and hard resource limits for the job can be tuned using keys that provide acces to all ulimit functionality (file descriptor file size, memory footprint, core size, etc.. Similarly the nice value can be specified to constrain CPU usage, and the *PriorityIO keys specify what priority the kernel should use for scheduling disk IO for this task.

The Socket Key defines an optional dictionary defining IPC behaviors controlling combinations of open/bind/connect/listen. (think inetd and xinetd). Beyond this you can also specify other things here like "Bonjour" to register this service with mDNSResponder. If a SockPathName is specified attempts to open a named unix domain socket will cause this program to launch.

If these are not enough, a launchd plist can start/stop a program when a watched file changes. When passed the path to a directory, the program can be started when the program is not running and the directory is not empty. Think about this one... A robust queue processing program might be simply:
#!/bin/sh
lockfile $SOMELOCKFILE
for file in *
do
process "$file"
done

WorkingDirectory and a watched directory path, along with whatever CPU/Disk IO/Memory/user/group are all handled by launchd. Similarly launchd ensures that the process runs when there is work to do, and is never launched unless there is work to do.

Finally, if the predefined keys and actions are insufficient you can call a small number of stubs implemented in Tcl to set either one shot timers or interval timers at which point the scripts is re-interpreted. You can pass messages to syslogd, start or stop jobs. A form of system introspection is also pr

Re:Not a cron replacement, a init replacement

[As Seen On TV]

There is no access layer that surrounds a plain text file.

In fact, there is. With Core Foundation, the process of unserializing a property list is atomic. The bytes are read from disk and validated. If and only if the file validates, the data structure is populated and returned by reference, all in one function call.

So yeah, property lists are self-validating.

You have to be DAMN sure that the file is easy to understand and edit by an admin, even if he only has "ed" over a serial connection available.

You mean like the universal, thoroughly documented and self-validating property list format, for example?

I'd much rather work with: TAG:VALUE

Great ... right up to the point where "TAG" has a colon in it, or whatever your delimiter. Or where "TAG" or "VALUE" needs to be in Japanese. Or where "VALUE" needs to be a container like a CFArray or a CFDictionary.

Or, for that matter, where "VALUE" needs to be of a specific type. If you're stuck with "TAG:VALUE" then you're kinda screwed when it comes to explicit type-checking, aren't you?

I can confidently write a program to validate and parse inittabs, but I don't know how to that with XML

You aren't expected to. Core Foundation does it for you. There's one function call to serialize a data structure and one function call to un-serialize it.

XML files, are difficult to read, difficult to edit, difficult to edit with programs, and poorly-defined.

Come on, now. Isn't that kind of a stupid statement? XML files are spectacularly easy to read, especially compared to the random mish-mash of configuration-file formats available on Unix. Editing is trivial, of course, because it's just text. More importantly, editing is trivial wherever you are, because the files are UTF-8 and not just ASCII. And calling a document that includes a machine-readable DTD for automatic validation inside it "poorly defined" borders on the farcical.

Some of us have been complaining since the NeXT days. :\

That's funny. Because there were no XML property lists in NEXTSTEP. They were added for Mac OS X.

Makes booting DAMN fast

[the_2nd_coming]

the machine boots to a desktop (auto log in and all) in less than 15 seconds.

Also check out Solaris SMF

[Wesley Felter]

Sun came up with (at first glance) a similar thing called Service Management Facility in Solaris 10.

Re:Submitter is confused

[As Seen On TV]

The launchd service replaces (takes a deep breath) init, rc, the init.d and rc.d scripts, SystemStarter, inetd and xinetd, atd, crond, and watchdog.

Basically any task the needs to run on its own, rather than being manually started by a human being, can be handled by launchd.

Re:From the apple page:

[netsrek]

So nothing like cron what-so-ever. Another great piece of Timothy's fact checking.

Before you jump the gun:

netstrek@computer: ~ $ cat /etc/crontab
# The periodic and atrun jobs have moved to launchd jobs
# See /System/Library/LaunchDaemons
#
# minute hour mday month wday who command

So yes, it is a cron replacement.

*sigh*

[cbiffle]

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?

Re:*sigh*

[enthused i swear]

Gentoo will do parallel startup, though it is not set to by default:

Modify your /etc/conf.d/rc file thusly:

# Set to "yes" if you want the rc system to try and start services
# in parallel for slight speed improvement.

RC_PARALLEL_STARTUP="yes"

Re:From the apple page:

[De Lemming]

Parent did not state it was no replacement for cron, but it is "nothing like cron what-so-ever." It's actually a replacement for "init, rc, the init.d and rc.d scripts, SystemStarter, inetd and xinetd, atd, crond, and watchdog" (copy/paste from another post, this is all explained thoroughly in the Ars Technica article).

Re:how can it automatically know dependencies?

[sean23007]

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.

Re:Open Source? Where's the source?

[Eridius]

It's part of Darwin. Try this page: http://www.opensource.apple.com/darwinsource/10.4/