How Upstart boots Ubuntu-based systems

Ignition

Article from ADMIN 07/2012

By Tim Schürmann

As a replacement for the legacy System V init, Upstart has abandoned many concepts: Instead of runlevels, Upstart has jobs and events, and its ability to start services in parallel considerably accelerates the boot process. Yet, configuration is almost child's play.

Once the Linux kernel takes control of the hardware and loaded all the drivers, it gives control to the small init program, which then sets up and starts the rest of the system. Until a couple of years ago, almost every Linux distribution used the System V init with its either loved or hated runlevel concept. System V tentatively launched one service after another, resulting in fairly lengthy boot times. This prompted Canonical to start developing a faster replacement in 2006. The results, dubbed Upstart [1], have booted Ubuntu distributions and RedHat Enterprise Linux (RHEL) 6 ever since.

Eventful Workplaces

In contrast to the legacy System V init, Upstart tries to boot all the required services and programs in parallel wherever possible. Additionally, Upstart can react to certain events, such as plugging in or replacing a hard disk at run time. It also will monitor the processes it launches, if desired, and restart them in the case of a crash. To do all this, Upstart completely breaks with the legacy runlevel model. If you still need to work with runlevels, forget everything you thought you knew as you read this article.

To boot a Linux system completely, Upstart needs to perform various tasks – such as enabling the network or launching the httpd. Each of these tasks is a job from Upstart's point of view, but it will only take on a job if a matching event has occurred previously. An event of this kind could be "Network enabled" or "Printer daemon started." If multiple jobs exist for an event, Upstart tries to run them in parallel, and all of this happens automatically. A web server simply needs to tell Upstart that it would like to launch with the "Network enabled" event and Upstart takes care of the details.

For each job, a separate configuration file with a

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