Recently, an article by Rich Felker called Broken by design: systemd has been making the rounds. I have a number of things to say about this article but today I want to talk about one specific issue it brings up, which is systemd's novelty (or lack thereof) and why it is succeeding. To start with, here is the relevant quote from Felker's article:

None of the things systemd "does right" are at all revolutionary. They've been done many times before. DJB's daemontools, runit, and Supervisor, among others, have solved the "legacy init is broken" problem over and over again (though each with some of their own flaws). Their failure to displace legacy sysvinit in major distributions had nothing to do with whether they solved the problem, and everything to do with marketing. [...]

This is wrong on several levels. To start with and as usual, social problems are the real problems. In specific, none of these alternate init systems did the hard work to actually become a replacement init system for anything much. Anyone can write an init system, especially a partial one (I did once, long ago). Getting it adopted by people is the hard part and none of these alternatives tackled that effectively (if they did so at all, and some of them certainly didn't). And as Felker admits, each of these theoretical alternatives have flaws of their own.

(Note that this is not a criticism of those alternate init systems. I don't think any of them have really been developed with replacing SysV init in Linux distributions or elsewhere as a goal. DJB daemontools certainly wasn't; I believe that DJB's attitude towards it, as towards more or less everything he's developed, can be summed up as 'I showed you the way, what you do with it is up to you'.)

The reason systemd has succeeded in becoming an SysV init replacement is simple: it did the work. Not only did it put together a lot of good ideas regardless of their novelty or lack thereof but its developers put in the time and effort to convince people that it was a good idea, the right answer, a good solution to problems and so on. Then they dealt with lots and lots of practical concerns, backwards compatibility, corner cases, endless arguments, and so on and so forth. I want to specifically mention here that one of the things the systemd people did was write extensive documentation on systemd's design, how to configure and operate it, and what sorts of neat things you can do with it. While this documentation is not perfect, most init systems are an order of magnitude less well documented.

(I am sure that in some quarters it's popular to believe that Lennart Poettering bulldozed the Fedora technical people into adopting his new thing. I do not think that the Fedora technical people are that easily overrun (or that impressed by Poettering, especially after PulseAudio), and for that matter at least some of the Debian technical people feel that systemd is the best option despite having looked deeply at the alternatives (cf).)

You can call this marketing if you want, although I don't think that that's a useful label for what is really happening. I call this 'trying' versus 'not trying'. If you don't try hard and work hard to become a replacement init system, it should be no surprise when you don't.

(In particular, note that SysV init is not a particularly bad init system so it should be no surprise when it is not particularly easy to displace.)

Beyond that I have some degree of experience with one of these alternate init systems, specifically DJB daemontools, and I've looked at the documentation for the other two. Speaking as a system administrator, systemd solves my problems better. The authors of systemd have looked at problems that are not solved by SysV init and come up with real solutions to them. Many of these problems are not solved by any of the alternatives that Felker put forward. In specific, often the alternatives assume (or require) cooperative daemon processes in order to fully realize their benefits; systemd is deliberately designed so that it does not and can fully manage even existing obstreperous Unix daemons with their willful backgrounding and other inconvenient behaviors.

(I don't know the field of Linux and Unix init-like systems well enough to say whether or not features like socket activation and clever use of control groups are genuinely novel in systemd or simply the first time I've become aware of them. They do feel novel.)

Since that may not be clear, let me be plain: systemd is a better init system than the alternatives. It does more to solve real problems and it does it better. That alone is a good reason for it to win in the practical world, the one where people care about getting stuff done. That systemd is not necessarily novel or the first to come up with the ideas that it embodies is irrelevant to this. Implementation matters more than ideas.

(Arguably it's an advantage that systemd feels no urge to reinvent different wheels when perfectly decent ones exist.)

PS: Please note that the reason that Unix itself succeeded is not its ideas alone, it is that Unix implemented them very well. A number of Unix's ideas are both great and novel, but a bad implementation would have doomed the whole enterprise. The fate of good ideas with a bad implementation is to be reimplemented elsewhere, cf the Xerox Alto and for that matter the Apple Lisa.

PPS: Also note that the one serious competitor to systemd is Upstart, which is also the product of a great deal of work and polishing.