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• upstart

The Upstart system was Canonical's attempt to replace the aging init system. It provides compatibility with the init system, but also provides additional features.
Support for "events" allow it to react to changes in the system, such as plugging in new hardware. In addition, Upstart can work alongside the older init configurations, providing backwards support for older packages and software.

However once Debian (the upstream source for Ubuntu packages) made the switch to systemd, Canonical decided to do the same. The release of Ubuntu 15.04 (Vivid Vervet) was the first to feature the new start-up system by default.