Actually they also noted that it helps with easier mapping. By including the logical shard ID in the ID, they don't need to keep a giant index of IDs-to-shards

Actually they also noted that it helps with easier mapping. By including the logical shard ID in the ID, they don't need to keep a giant index of IDs-to-shards to figure out which machine an ID lives on. Just a tiny mapping of logical to physical shards, which every app server instance can cache in memory. One less moving part at the expense of wedding yourself to a prepartitioned logical shard scheme, I guess. (I wonder how painful it would be to rebucket data into a different logical shard should the need arise...)