Asking for a recommendation letter from an open source repository mentor I wish to apply for an MS in a particular branch (a sub-field) of Computer Science at various US universities.

I have been working with a person on an open source repository for some time (about 5 months). That person is the owner of the repo and the work done in that repo is very much related to the branch I wish to apply for. I have made many significant contributions to the project. I might perhaps be one of the top 3 contributors of the repo. Me and that person have worked closely on several issues from time to time. He has, many times, especially asked my opinions on some of them. That person used to be a professor, but now works in the private sector.

Now, the problem is that, he is a celebrity in that branch of Computer Science. If you have ever ventured even a little in this branch, I am sure you have heard of him.

For my questions,


On the basis of my relationship with him as I have described above, is it alright if I ask him for a recommendation letter?
Is it necessary to have a student-professor or similar relationship to ask for a recommendation letter?
Considering the facts that he is famous, he knows me only through emails and my written code and tons of people would be asking him for recommendations each year, what are the chances that I'll get one from him?
Go for it. 

I had the doubts like you earlier. But changed my mind after talking to one of my mentors. He fits all your above descriptions. He said, he would be happy to give a recommendation letter to his Google Summer of Code (a remote open source summer internship sponsored by Google) students who performed exceptionally.

While you do not have a student-professor relationship, he might be the only professor that ever witnessed you coding in a professional environment. He might have noticed your positive sides that other professors will never have the chance to find (except on academic projects).

Just ask. It won't hurt. If the professor feels he does not know you enough, he will let you know.