How does an application know which shard it should use in any particular situation? This answer requires one more modifications to the table structure our application uses. Our last decision involves adding a shard_id column to one table. This table can be anything but should be some central value that all data can eventually be traced to.
A good choice for this is a customer table. In an order system, all interaction is eventually driven by customer activity. If we assign a customer a specific shard ID, all of their order data will be stored in that shard. As the application likely has the customer row information available at all times, it should also know the associated shard and, hence, ...
