we noticed that one of these shards was performing poorly because a disproportionate share of check-ins were being written to it. I'd love to know the root cau

we noticed that one of these shards was performing poorly because a disproportionate share of check-ins were being written to it. I'd love to know the root cause behind this specific issue. Was this a behavioral issue within the user base, or a technical problem that routed check-ins to this specific shard more than others? Since they mentioned they partition their shards by userId that would probably rule out their routing process. I wonder if there was some event that caused a certain sharded subsection of users to start sending so many checkins? And since this was a subset of userIds on the same shard - could this have been a targeted DOS or SPAM event? I'm making a very conscious migration to MongoDB so I'm very interested to hear what the root cause of this was.