(disclaimer: i work at Google and was one of the founders of the project)
when we were looking at building k8s our mission was to help the world move forwards to a more cloud native approach to development. by cloud native i mean container packaged, dynamically scheduled, micro-services oriented. we figured that in the end our data centers are going to be well suited to run cloud native apps, since they were designed from the ground up for this approach to management, and will offer performance and efficiency advantages over the alternatives. we also however recognized that no matter how cheap, fast and reliable the hosting offering is, most folks don't want to be locked into a single provider and Google in particular. we needed to do what we were doing in the open, and the thing that we built needed to be pattern compatible with our approach to management and quite frankly address some of the mistakes we had in previous frameworks (Borg mostly as a first system).
we looked really closely at Apache Mesos and liked a lot of what we saw, but there were a couple of things that stopped us just jumping on it. (1) it was written in C++ and the containers world was moving to Go -- we knew we planned to make a sustained and considerable investment in this and knew first hand that Go was more productive (2) we wanted something incredibly simple to showcase the critical constructs (pods, labels, label selectors, replication controllers, etc) and to build it directly with the communities support and mesos was pretty large and somewhat monolithic (3) we needed what Joe Beda dubbed 'over-modularity' because we wanted a whole ecosystem to emerge, (4) we wanted 'cluster environment' to be lightweight and something you could easily turn up or turn down, kinda like a VM; the systems integrators i knew who worked with mesos felt that it was powerful but heavy and hard to setup (though i will note our friends at Mesosphere are helping to change this).
so we figured doing something simple to create a first class cluster environment for native app management, 'but this time done right' as Tim Hockin likes to say everyday.
now we really like the guys at Mesosphere and we respect the fact that Mesos runs the vast majority of existing data processing frameworks. by adding k8s on mesos you get the next-generation cloud native scheduler and the ability to run existing workloads. by running k8s by itself you get a lightweight cluster environment for running next gen cloud native apps.
-- craig