Let me start by saying what's not happening. First, there is zero, zilch, chance of Microsoft open-sourcing Windows or Microsoft Office. On the desktop, Microsoft will remain as proprietary as ever for the foreseeable future.
However, Microsoft continues to transform itself from a software sales company to a software service rental business with Windows as a Service. To power that, Office 365, and its other service offerings such as its Cosmos big-data service, Microsoft is relying on the cloud and it's in that hidden engine behind its services that Microsoft is embracing open-source software.
This isn't new. Microsoft has been moving towards working Linux and open-source for years now. Almost ten-years ago Microsoft partnered with Novell, now SUSE, to bring Windows Server and SUSE Linux Enterprise Server. At the time, this was more of a move to integrate Windows and Linux servers, but it became Microsoft's first steps to bring Linux into what would become Azure and its other cloud programs.
Don't believe me? Look at what Microsoft has been doing with open-source software and the cloud. That early work with SUSE made it possible for Linux to run seamlessly on Azure. Today, 20 percent of Azure's operating systems instances are Linux. True, Azure doesn't support the top business Linux, Red Hat Enterprise Linux (RHEL), but it does support CoreOS Linux, CentOS, Oracle Linux, SUSE, and Ubuntu.
Microsoft isn't even putting all its cloud eggs into its proprietary Azure cloud. Microsoft is working with Canonical, Ubuntu Linux's parent company, to bring Windows Server to OpenStack.
What all these have in common is that while they don't have much to do with the public, end-user face of Microsoft's offerings, they are all vital to increasingly more important Microsoft server and cloud infrastructure. In short, most people won't see it, but Microsoft -- yes Microsoft -- has become an open-source company.