Apache 2.0 since day 0, since it came out of a non-profit central bank switch by the Gates Foundation, which is also Apache 2.0.

Our view is that this kind of infrastructure is simply too valuable, too critical, not to be open source.

For more about our open source and business philosophies, which we see as orthogonal, see this interview we did with Jerod Santo: https://www.youtube.com/watch?v=Z0kzYlCTUuc