I addressed this in another answer here but happy to do it again. The way shard splitting works is that the initial shard keeps working while the new shard is b

I addressed this in another answer here but happy to do it again. The way shard splitting works is that the initial shard keeps working while the new shard is being built. During this time, all messages to the old shard are written to the old shard and the new shard, so, unless I don't understand your question I dont see there being a backlog. Regarding EC2, latency isnt really an issue because Twitter is far from real-time. That said, haven't really noticed any problems with EC2 connection latency, but we are not live yet so I would defer to you on that.