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Day (http://www.day.com
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Sling started as an internal project at Day Software, and has been contributed to the Apache Incubator in September 2007. It can be considered an important part of Day's software stack, and was open-sourced to make the technical innovations contained therein available to the public.
Sling powers Day's entire CQ5 family of content management products (Web Content Management, Social Collaboration, Digital Asset Management), and is included in Day's CRX content repository, which is itself based on Jackrabbit.
Quite a few high profile mission critical websites are powered by CQ5 - Day's customer list mentions some of them. Day's website runs on CQ5/Sling as well.
CQ5 has been very well received by the press and by analysts, and Sling has played a major part in this success, by enabling the CQ5 development team to create robust and higly modularized software very efficiently.
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