Adds basic automatic database switching to Rails · rails/rails@0abcec4

The following PR adds behavior to Rails to allow an application to automatically switch it's connection from the primary to the replica. A request will be sent to the replica if: * The request is a read request (`GET` or `HEAD`) * AND It's been 2 seconds since the last write to the database (because we don't want to send a user to a replica if the write hasn't made it to the replica yet) A request

[tikkss]

HTTP リクエストの種類によって DB の接続先（プライマリ、レプリカ）を切り替える。レプリケーション遅延も考慮してあるし、すごいなぁ。