In modern applications, it is becoming increasingly common to inject a secondary cache layer between the application and database. This layer stores commonly retrieved data in memory for immediate use. This layer might be composed of memcached or a NoSQL database such as CouchDB, MongoDB, or Redis.
Once such a layer exists, it's important that the shard_map table is one of the first tables copied there. It has very few rows, and storing it in memory removes the relatively expensive round-trip to the database. With this mapping in memory, the application will always know which physical server it should be connected to as long as it also knows which shard it is using.
