Contributors and Modifications

I described in the first chapters of this book how open source development is a continuous process. Contributors and distributors enhance and improve software at each step by creating collective and derivative works. That explanation was necessary because the BSD and earlier licenses weren't explicit about it. Collaborative open source development progressed under those licenses without the licenses mentioning the process at all.

The MPL defines this process much more precisely in sections 2 and 3. Open source development starts with Original Code supplied by an Initial Developer (in the first instance Netscape Corporation, although the MPL is a template license ) who licenses all relevant open source rights to You. You make Modifications and become a Contributor . As a Contributor, You are required by the reciprocity terms of the MPL to license all relevant open source rights for Your Modifications to everyone under the MPL, and to provide Source Code. You also received, by that recursive MPL license, a license to all the Modifications made by earlier Contributors . (Notice the MPL-defined terms are the italicized nouns with capital letters throughout this paragraph.)

Once again, because of the MPL's definitions, this continuous enhancement process applies not to derivative works as a whole but separately to each file containing Modifications . (The word file is not defined in the MPL. This word is a term of art from the computer field; we must rely on experts to tell a court what that word means; I'm confident, though, that every reader of this book has a clear idea what the word file means and would recognize a modified file even without an MPL definition for it.)

The license grant under the MPL was structured so as to apply more narrowly than the GPL and other previous licenses. It licenses files to be modified, not programs to be turned into derivative works . This has one major consequence for those who create Larger Works , works that combine Covered Code with code not governed by the terms of the MPL License. Under copyright law, such Larger Works might be derivative works , depending upon the nature of the combination of software being created; but under the MPL's definitions, a Larger Work has more limited implications:

You may create a Larger Work by combining Covered Code with other code not governed by the terms of this License and distribute the Larger Work as a single product. In such a case, You must make sure the requirements of this License are fulfilled for the Covered Code. ( MPL section 3.7.)

You are only obligated to apply the MPL's license restrictions to files containing Original Code or Modifications . The rest of the files ”your own files ”are not affected by the reciprocity obligation, even if you have created a derivative work by adding your own files.

Any licensee intending to create Larger Works would be wise to consult an attorney to decide whether such a work is considered a Modification that must be contributed back under the MPL, or whether there are any other license obligations still to honor for the Covered Code .

In a way, the MPL is a kind of compromise between the academic and reciprocal license models. Reciprocity under the MPL is defined narrowly so as to encourage the use of open source software as building blocks to create Larger Works . (Those Larger Works may even be derivative works under copyright law; that doesn't matter.) Those Larger Works may be open or proprietary; with respect to them, the MPL acts like an academic license. But the individual building blocks are licensed with reciprocity obligations. If you distribute improvements to those building blocks, you must license those improvements under the MPL as open source software.