[fauigerzigerk]

That's a good argument for a plain web app fronting a DB server. But what if you want to implement a server that keeps a lot of data in memory? What if you're not the user of Cassandra, Redis, Lucene or a specialized analytics engine but its creator?

For request/response type of scenarios (unlike batch processing) you basically have two options: (a) Use a multi-process shared memory design (or even just the file cache), which is very cumbersome to implement because you cannot use pointers and hence none of the data structures in your favorite language's library. (b) Use shared in-process state with powerful concurrency primitives. That's what clojure helps you do.

You cannot avoid to choose between these two architectures by using distribution across machines. Distribution is built on top of whatever you do on a single machine.