Write slick queries with cats.data.EitherT[Future, DatabaseError, R] (R being result) and replica support on READ queries.

In Snapptrip.com, we were doing pretty cool stuffs:

1. Scaling postgreSQL database by dividing database queries into multiple forms of actions and having READ queries in replication databases. http://danielwestheide.com/blog/2015/06/28/put-your-writes-where-your-master-is-compile-time-restriction-of-slick-effect-types.html
2. A nicer error handling.

1. Scalability You need to declare two different DatabaseHandlers, one for "READ" actions and one for other actions. This might seem naive but since that's all we needed so it is prefect for our goal.

  class ReplicaDB @Inject()(
      @NamedDatabase("replica")
      protected val dbConfigProvider: DatabaseConfigProvider
  )(implicit ec: ExecutionContext)
      extends HasDatabaseConfigProvider[JdbcProfile] with DatabaseHandler {

    override def run[R](query: BaseQueryT[R]) =
      db.run[R](query)
  }
  
    class MasterDB @Inject()(
      @NamedDatabase("master")
      protected val dbConfigProvider: DatabaseConfigProvider
  )(implicit ec: ExecutionContext)
      extends HasDatabaseConfigProvider[JdbcProfile] with DatabaseHandler {

    override def run[R](query: BaseQueryT[R]) =
      db.run[R](query)
  }
  
  class DatabaseIO @Inject()(
    MasterDB: masterDB,
    ReplicaDB: replicaDB
  )(implicit ec: ExecutionContext) extends ReadWriteDB(masterDB, replicaDB)
  
  class UserRepo @Inject()(DatabaseIO: db)(implicit ec: ExecutionContext) {
    def create(user: User) = userTable.insertOrUpdate(user)
    def findBy(id: Int) = userTable.filter(_.id === id).result.head
    
    def createIO = db.run(this.create _)
    def findByIO = db.run(this.findBy _)
  }

2. So far we did not mention any specific approach regarding handling errors. You should have heard about cats library by now if not I don't know what to say. Using EitherT and stacking Either and Future (http://eed3si9n.com/herding-cats/monad-transfomers.html) we can write clean for-comprehensions.

• First wrap your queries around a HandyQuery with toQuery functionality which is going to provide following interface for you:

  class UserRepo @Inject()(DatabaseIO: db)(implicit ec: ExecutionContext) {
    implicit val dbIO: ReadWriteDB = db
    
    def create(user: User) = toQuery { 
      userTable.insertOrUpdate(user)
    }

    def findBy(id: Int) = toQuery { 
      userTable.filter(_.id === id).result.head
    }
  }

And you can use the queries with userRepo.findBy(1).runQuery in order to run the database action and it will either is going to get complete or will return an error which has the reason and the line it was caused.

I haven't touched a single line of this library in two years! Obviously, Today I'd write it in a different way or I'd might not even write it at all. It served its purpose and now I just have it in my resume! Doesn't mean I am proud of it.

MIT - credit to Snapptrip.com