The 'temp' property of collections is a replica-set level concept – it is not sharding-aware. As we prepare to support $out to collections tracked on the sharding catalog (unsplittable collections in the contest of PM-3229, and sharded collections later on PM-1931), $out can no longer use on 'temp: true' on sharded clusters. Instead, on shard servers, $out will write down the temporary collections it uses to a collection, and these will be garbage-collected on step-up.