Stream records from DataBase using Akka Stream

Question

I have a system using Akka which currently handles incoming streaming data over message queues. When a record arrives then it is processed, mq is acked and record is passed on for further handling within the system.

Now I would like to add support for using DBs as input.
What would be a way to go for the input source to be able to handle DB (should stream in > 100M records at the pace that the receiver can handle - so I presume reactive/akka-streams?)?

Answer 1

Slick Library

Slick streaming is how this is usually done.

Extending the slick documentation a bit to include akka streams:

//SELECT Name from Coffees
val q = for (c <- coffees) yield c.name

val action = q.result

type Name = String

val databasePublisher : DatabasePublisher[Name] = db stream action

import akka.stream.scaladsl.Source

val akkaSourceFromSlick : Source[Name, _] = Source fromPublisher databasePublisher

Now akkaSourceFromSlick is like any other akka stream Source.

"Old School" ResultSet

It is also possible to use a plain ResultSet, without slick, as the "engine" for an akka stream. We will utilize the fact that a stream Source can be instantiated from an Iterator.

First create the ResultSet using standard jdbc techniques:

import java.sql._

val resultSetGenerator : () => Try[ResultSet] = Try {
  val statement : Statement = ???
  statement executeQuery "SELECT Name from Coffees"
}

Of course all ResultSet instances have to move the cursor before the first row:

val adjustResultSetBeforeFirst : (ResultSet) => Try[ResultSet] = 
  (resultSet) => Try(resultSet.beforeFirst()) map (_ => resultSet)

Once we start iterating through rows we'll have to pull the value from the correct column:

val getNameFromResultSet : ResultSet => Name = _ getString "Name"

And now we can implement the Iterator Interface to create a Iterator[Name] from a ResultSet:

val convertResultSetToNameIterator : ResultSet => Iterator[Name] = 
  (resultSet) => new Iterator[Try[Name]] {
    override def hasNext : Boolean  = resultSet.next
    override def next() : Try[Name] = Try(getNameFromResultSet(resultSet))
   } flatMap (_.toOption)

And finally, glue all the pieces together to create the function we'll need to pass to Source.fromIterator:

val resultSetGenToNameIterator : (() => Try[ResultSet]) => () => Iterator[Name] = 
  (_ : () => Try[ResultSet])
    .andThen(_ flatMap adjustResultSetBeforeFirst) 
    .andThen(_ map convertResultSetToNameIterator) 
    .andThen(_ getOrElse Iterator.empty)

This Iterator can now feed a Source:

val akkaSourceFromResultSet : Source[Name, _] = 
  Source fromIterator resultSetGenToNameIterator(resultSetGenerator)

This implementation is reactive all the way down to the database. Since the ResultSet pre-fetches a limited number of rows at a time, data will only come off the hard drive through the database as the stream Sink signals demand.