The shard where the create coordinator for config.system.sessions is run is the first shard in alphabetical order. This shard may change if a shard is added with a lower alphabetical value than the lowest one or the first shard is removed.

If this shard changes while the config.system.sessions collection is being sharded, a retry of the command or another node issuing another shardCollection command can lead to multiple shards running the create coordinator for the same collection, leading one to hang at commit time. The shard who's coordinator is hung will not be able to accept chunks for the config.system.sessions collection which could lead to problems for sessions collection scaling.