An important licensing and business strategy in commercial Open Source Software industry.
"With a dual-licensing approach, the company is protected by a GPL (or similar) license, because both competitors and potential customers who wish to embed/link with the GPL software must also GPL their own code. Since most competitors/customers don’t wish to do so, they are willing instead to pay for a commercial license. This simple yet subtle point is at the heart of the success of nearly every commercial open source organization." (http://robertogaloppini.net/2007/06/02/open-source-firms-enterprisedb-business-model/)
The EntrepriseDB strategy:
"First, we created a superset of PostgreSQL called EnterpriseDB Advanced Server, and closed-sourced the code. In other words, atop base PostgreSQL, we added deep Oracle-compatibility, dynamic performance tuning, and world-class tools, including replication, debuggers, browsers, and more. Then we closed-sourced the whole package. In this manner, we have crisply defined a set of value-added features for which we charge, much like SugarCRM’s professional edition. If you want the free-and-open-source version version of the software, though, it’s easily available…and it’s called PostgreSQL.
The second — and equally important — part of our business strategy is to be an excellent citizen in the PostgreSQL open source community. We are building a successful company on the shoulders of one of the world’s most successful open source projects, and we have a responsibility to give back to that community to the maximum extent possible, while still protecting our ability to generate revenue. In addition to our ethical responsibility, we also “do well by doing good” because we promote the wider spread of PostgreSQL, the world’s most advanced and enterprise-class open source database (albeit only the second most popular)." (http://robertogaloppini.net/2007/06/02/open-source-firms-enterprisedb-business-model/)