WorkCache.scala

import cats.effect.{Deferred, IO}

import java.util.concurrent.ConcurrentHashMap

trait WorkCache[Key, Result] {
  private val map = new ConcurrentHashMap[Key, Deferred[IO, Result]]

  protected def persisted(key: Key): IO[Option[Result]]

  protected def work(key: Key): IO[Result]

  def apply(key: Key): IO[Result] = IO(Option(map.get(key))).flatMap {
    case Some(d) => d.get
    case None => persisted(key).flatMap {
      case Some(result) => IO.pure(result)
      case None => Deferred[IO, Result].flatMap { d =>
        map.put(key, d)
        work(key).flatMap { result =>
          d.complete(result).map { _ =>
            map.remove(key)
            result
          }
        }
      }
    }
  }
}