The production of open source software continues to grow at a good pace. Of course, not all open source software is created equally, particularly when it comes to the license under which it’s offered. Different licenses impose different kinds of restrictions on what one can do with any derivative works created (e.g., can you charge money for something based on open source software?). License choice can, potentially, impact the growth of a community around a particular open source project. New research is now shedding some light on the effect of license choice on the growth of open source projects.
German researchers Gottfried Hoffmann, Dirk Riehle, Carsten Kolassa and Wolfgang Mauerer recently set out to examine the relationship between open source license choice and project growth and their results were recently published in the paper “A Dual Model of Open Source License Growth”. They looked at data obtained from Ohloh.net (now Black Duck Open Hub) on over 5,000 open source projects active between 1995 through June, 2007. Their goal was to explore the effect of choosing more “restrictive” open source licenses versus more “permissive” licenses.
Restrictive licenses were defined as those that required derivative works to be released under the same license, such as the GNU General Public License (AKA GPL, copyleft) and CC-BY-SA. Permissive licenses, on the other hand, were defined as those that let licensees do what they want with the resulting work, including selling it. Permissive licenses include the MIT, BSD, Apache, Python and public domain licenses. The researchers ignored licenses that were in between, in terms of permissiveness, such as LGPL.
The metric used to measure the growth of an open source project was the number of (non-empty, non-comment) source lines of code (SLoC) created per month, ignoring the first month of each project. They then totaled up the SLoC per month across all projects for restrictive and permissive licenses as groups and compared at the growth rates of open source projects for these two types of licenses.
The key findings:
During the time period covered by the study (1995 - 2007), open source projects grew at an exponential rate, regardless of the license type. No shock there.
Prior to 2001, restrictively licensed projects grew a faster rate than permissively licensed projects.
Around 2001, however, things switched: since then permissively licensed projects have grown at a faster rate than restrictively licensed projects.
The authors suggest that the accelerated growth or permissively licensed projects coincided with the rise of commercially sponsored open source projects such as Linux and Apache. Their theory is that, in projects like those, companies will chip in to create something for the common good, but then need to be able to benefit from it by creating “competitively differentiated complementary products” in order for their participation to ultimately make financial sense. The researchers expect the growth of permissively licensed projects to continue to accelerate relative to more restrictively licensed projects.
Be sure to dig into their paper for more details and statistical methods. As always, the authors point out that more research needs to be done to further confirm their findings. But, these results suggest that if you want your open source project to attract a community as quickly as possible, you should consider a more permissive license.
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