Scala Doobie & Hikari CP transactor handling

Question

If you do this, HikariCP is init and shut down each time. Is there any way to avoid this and execute various queries?

// Resource yielding a transactor configured with a bounded connect EC and an unbounded
// transaction EC. Everything will be closed and shut down cleanly after use.
  val transactor: Resource[IO, HikariTransactor[IO]] =
  for {
    ce <- ExecutionContexts.fixedThreadPool[IO](32) // our connect EC
    be <- Blocker[IO] // our blocking EC
    xa <- HikariTransactor.newHikariTransactor[IO](
      "org.h2.Driver", // driver classname
      "jdbc:h2:mem:test;DB_CLOSE_DELAY=-1", // connect URL
      "sa", // username
      "", // password
      ce, // await connection here
      be // execute JDBC operations here
    )
  } yield xa

run

transactor.use(sql"select 42".query[Int].unique.transact[IO]).unsafeRunSync()

Answer 1

This is not how you use Resources in application.

You do do .use somewhere on main level, and then let your whole code requiring Transactor get that value passed, e.g:

val actorSystemResource: Resource[IO, ActorSystem]
val transactorResource: Resource[IO, Transactor[IO]]

// initialize controllers, services, etc and create routes for them
def routes(actorSystem: ActorSystem, transactor: Transactor[IO]): Route

val resources = for {
  transactor <- transactorResource
  actorSystem, <- actorSystemResource
  route = routes(actorSystem, transactor)
} yield (transactor, actorSystem, route)

resources.use { case (_, actorSystem, route) =>
  implicit system = actorSystem

  IO.fromFuture {
    Http().bindAndHandle(route, "localhost", 8080)
  }
}

Alternatively, you can use resource.allocated, but it almost certainly would be a bad idea, resulting in a code that never runs the release part of the Bracket because it is easy to mess up and e.g. not calling it if some exception is thrown.