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|Stable release||4.0.1 / August 3, 2009</td></tr>|
|License||GNU General Public License</td></tr>|
Resin Open Source, with reduced optimization and other features, is essentially a crippleware version of the Professional version. This is a common practice in the commercial software industry; see for example the one-user and two-user limits of Microsoft Remote Desktop Services as included in the Windows operating system. However Resin is notable in that unlike the crippleware versions of many products such as Remote Desktop the source code of its Open Source version has been made available and licensed in a manner that donates it to the software community at large for partial or complete re-use.
According to Caucho Technology marketing material, Resin Open Source is suitable for hobbyists, developers, and low traffic websites that do not need the performance and reliability enhancements of Resin Professional. Resin Professional would add features and enhancements commonly needed in a professional production environment.
Quercus is a Java-based implementation of the PHP language that is included with Resin. According to a slideshow presented by Emil Ong (from Caucho), to a San Francisco Java Meetup Group in April of 2008 pertaining to Resin 3.1, an essential difference in the operation of Quercus between the Resin Open Source and the Resin Professional editions is that in Resin Professional the PHP is compiled to Java bytecode whereas in the open source version PHP is executed by an interpreter. 
Early versions of the Resin Open Source product were released with problematic licensing but more recent versions are available under a much more conventional, standard open source license.
The software license under which Resin was initially released deviated significantly from the conventional definition of open source software. Initially Resin was non-redistributable, and the copyright of any improvements made to the code of the application, if communicated to others, became the property of Caucho Technology. Furthermore, the license stipulated that if any legal action arose out of breach of the licensing terms and a court decision was granted in favor of Caucho Technology all of Caucho's legal expenses must be paid by the licensee. Hence even firms and individuals simply using the software may have been exposed to substantial risk had Caucho found them in breach of any term of the license.
Previous licensing terms for the product included the following, which may have introduced something like a copyleft legal status into the products and services offered by earlier users of Resin:
Since version 3.0.9, Resin Open Source has been licensed under the GPL License (version 2 or later), a license which has passed through the Open Source Initiative's License Review Process. Consequently current versions of Resin are significantly less hazardous to use or incorporate into software systems than were previous versions.