I believe an architecture they used was something like eight DB instances with say four engine nodes per DB instance. One pair was clustered for web requests, the other pair for job executor processing.
Given they had eight shards, to route to the correct shard, take something like the email address of the order and hash it down to a number from 0…7. Hence this determines which to route to.
They may have configured all eight engines on each node, as that means any node could handle any request, but Im not 100% clear if they did or didnt do this.
The above is my understanding of their architecture from a few years ago (it may have changed) and I gleaned this from presentation snippets (so I could get a few parts wrong)…
Perhaps an @zalando rep in the forum may comment with more authority…