Sorry, I need to correct that. I meant that shards are basically still primary / secondary with an arbiter for failover. We prefer horizontal nodes instead, like Cloudant, Elastic Search or Cassandra offers.
Ahh, I see. Why do you guys prefer that configuration? Space/resources, reliability?
probably large amounts of writes.
Space/resources was certainly a concern in this case. A disk-based solution with predictable latencies enables storage of far more data than something that requires holding the working set in RAM.