The argument is that without stronger consistency guarantees you can't do joins between two streams (or even something like argmax over a single stream, since i

The argument is that without stronger consistency guarantees you can't do joins between two streams (or even something like argmax over a single stream, since it splits the stream into two subcomputations, which then have to be joined back together). I think when folks say that eventual consistency is okay, they're thinking about simple aggregates - where transient incorrectness in the result is indistinguishable from noise. But if you want to do joins, you really want to be able to reason about your unbounded streams causally - Flink, Beam, (and as another commenter points out, Firebase as well) provide stronger consistency guarantees on computations over unbounded streams.