The sharding is implemented with the existing connection switching API using a custom Resolver here with its context class here: here
bin/rake db:create && bin/rake db:migrate && bin/rails server
When making a request we need to send the X-SHARD-ID header with the ID corresponding to a database shard, if we omit the X-SHARD-ID we default to connecting to the :master database.
To easily change headers in the browser I use this chrome extension
Note: trying to access a record that lives on the master database with something a X-SHARD-ID set will result in an error, likewise for using a master connection to access a resource outside of master database, like hitting the localhost:3000/shops route without setting a X-SHARD-ID header.
A thread is started in this initializer and it copies over the primary to replica database as a way to mock replication.
This means that shortly after creating a record through the web form the redirect to that records show page will likely 404 until the replication process finished and copies the record over to the replica. This is fairly fast so a retry should suffice.