Saturday, August 28, 2010

The Upstart Event-Based init Daemon (FEDORA)

The Upstart Event-Based init Daemon Because the traditional System V init daemon (SysVinit) does not deal well with modern hardware, including hotplug devices, USB hard and flash drives, and network-mounted filesystems, Fedora replaced it with the Upstart init daemon ( RHEL still uses xinetd, a successor to the init daemon.

Several other replacements for SysVinit are also available. One of the most prominent, initng (, is available for Debian and runs on Ubuntu. In addition, Solaris uses SMF (Service Management Facility) and MacOS uses launchd. Over time, Upstart will likely incorporate features of each of these systems.

The runlevel-based SysVinit daemon (sysvinit package) uses runlevels (single-user, multiuser, and more) and links from the /etc/rc?.d directories to the init scripts in /etc/init.d to start and stop system services. The event-based Upstart init daemon uses events to start and stop system services. With version 9, Fedora switched to the Upstart init daemon and began the transition from the SysVinit setup to the Upstart setup. This section discusses Upstart and the parts of SysVinit that remain: the /etc/rc?.d and /etc/init.d directories and the concept of runlevels.

The Upstart init daemon is event based and runs specified programs when something on the system changes. These programs, which are frequently scripts, start and stop services. This setup is similar in concept to the links to init scripts that Sys-Vinit calls as a system enters runlevels, except Upstart is more flexible. Instead of starting and stopping services only when the runlevel changes, Upstart can start and stop services upon receiving information that something on the system has changed. Such a change is called an event. For example, Upstart can take action when it learns from udev that a filesystem, printer, or other device has been added or removed from the running system. It can also start and stop services when the system boots, when the system is shut down, or when a job changes state.

Future of Upstart. Changing from SysVinit to Upstart involves many parts of the Linux system. To make the switch smoothly and to introduce as few errors as possible, the Upstart team elected to make this transition over several releases.

Over time, Fedora will move away from the SysVinit setup and toward the cleaner, more flexible Upstart setup. As more system services are put under the control of Upstart, entries in the /etc/event.d directory will replace the contents of the /etc/init.d and /etc/rc?.d directories. Runlevels will no longer be a formal feature of Fedora, although they will be maintained for compatibility with third-party software. Eventually Upstart will also replace crond.

Software package. The Upstart system uses the upstart package, which is installed by default. Some of the files Upstart uses are found in the initscripts package, which is also installed by default.

Source of Information : Prentice Hall A Practical Guide to Fedora and Red Hat Enterprise Linux 5th Edition

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