Support Kotlin's Flow [DATACMNS-1608]

[spring-projects-issues]

smallufo opened DATACMNS-1608 and commented

Hi , I hope Spring Data JPA support Kotlin's Flow like this :

 interface UserDao {
   @Query("select u from User u where ....")
   @Hints("batch=:batch") // something like that
   fun findUsers(@Param(name="batch") batchSize:Int): Flow<User>
}

There should be some hint in the @Query annotation

---

Issue Links:

• DATACMNS-1508 Add support for Coroutines repositories
  ("duplicates")

[spring-projects-issues]

Jens Schauder commented

What is your question/ proposal here?

Do you want support for Kotlin sFlow?
Support for specifying the batch size when reading into streamable result types?

[spring-projects-issues]

smallufo commented

Prior to JPA 2.2 , the only way to emit Kotlin's Flow is like this :

 override fun findSomeUsers(batch: Int): Flow<User> {
    return flow {
      (em.delegate as Session).sessionFactory.openSession().use { session ->
        val query = session.createQuery("select u from User u where ...")
        query.fetchSize = batch
        query.isReadOnly = true

        query.scroll(ScrollMode.FORWARD_ONLY).use { results ->
          while (results.next()) {
            val u = results.get(0) as User
            emit(u)
          }
        }
      }
    }
  } 

It has some drawbacks : 

It downcasts entityManager to Hibernate's session , and that's a little boilerplate , which should be able to code generated. 

 

Since JPA 2.2 supports streaming result , I hope Spring-Data-JPA also support Kotlin's flow as returning type (with some batchSize hint annotations).

 

Related SO link : https://stackoverflow.com/questions/58700235/spring-data-with-jpa-2-2-resultstream-to-kotlins-flow

 

 

[spring-projects-issues]

Mark Paluch commented

Kotlin's Flow is a Coroutine type. Allowing that sort of type lets users expect Coroutine-like behavior which is clearly not available from JPA as all I/O is blocking without a true push-data structure experience.

Please instead use Stream or Spring Data R2DBC which can return Flow as part of its Coroutines support

[spring-projects-issues]

Mark Paluch commented

Closing this ticket as per previous comment above. We have a general ticket handling Kotlin Coroutine repositories that is linked now