When looking at Upstart it comes off as very simple. Almost too much so. It's just events. And jobs that take those events. And not much else. You're so used to configuring every last detail of a system to ensure that it's tightly tuned for every possible scenario, certainly you need more tools than just events! When you let go of that preconception and really start to understand it, you realize that events are not only enough, but they're exactly what you need.
One might have thought – as at first I certainly did – that if the rules for a program were simple then this would mean that its behavior must also be correspondingly simple. For our everyday experience in building things tends to give us the intuition that creating complexity is somehow difficult, and requires rules or plans that are themselves complex. But the pivotal discovery that I made some eighteen years ago is that in the world of programs such intuition is not even close to correct. — Stephen Wolfram, A New Kind of Science.
And so we take these simple jobs that we've built and we start to build a system. They each just wait on a set of events, and some emit events as they go along, and the system starts to take form. But where are our initialization phases and guarantees and complex dependencies? (writing code to solve them is fun!) They still exist, but as an emergent behavior of the system. Let's look at Graphviz diagram of the Ubuntu Saucy system init:
(Graphviz | Full SVG)
Without zooming in and just looking at the shapes that emerge you start to see a natural grouping of the jobs. And there are stages of the boot. There are types of jobs that are gathered together. There'd be even more if initctl2dot could break down the runlevel job into its various values. What we see is a the complex boot of a modern operating system broken down into pieces for analysis. What we see is a model of the behavior of the system, but that model is the only place that the complexity actually exists, it becomes the emergent behavior of the system. And that, that is why Upstart can be so simple and yet be powerful enough to boot a modern Linux system.
posted on Sun, 04 Aug 2013 at 00:06 | permanent link