Choosing shard keys, hot spots and resharding

A good shard key has many different values, spreads load evenly and matches how data is read. Even so, one shard can run hot. A celebrity account or a trending hashtag sends all its traffic to one key.

You can split a hot key across several partitions by adding a random suffix to it, or cache its reads. Data also grows unevenly, so plan for resharding, moving data between shards, from day one. With hash(key) mod N, changing the number of shards N moves almost every key.

Two layouts avoid that. You can create many small logical partitions and assign them to physical servers, or you can use consistent hashing. Either way, you move only a slice of the data.

Resharding a live system usually means dual writes (writing to the old and new places at once) or copying with change data capture, then switching over.