That said, we have looked at Druid, which is also a good example of using lambda architecture in practice (http://druid.io/docs/0.8.0/design/design.html -- note the historical vs realtime distinction). They use many of the same design principles as us, and one of our sub-systems is very similar to it. We still believe the pre-aggregation approach is critical for performance in our use case, though. Lastly, when we started building the architecture (mid-2014), Druid was very new, and I'm generally wary of designing everything around a new and potentially unstable piece of software.