I am new to Scala and Play! programming but have a reasonable amount of experience with webapps in Django, and plenty of general programming experience.
I have been doing some exercises of my own to try to improve my understanding of Play! and this behaviour has me totally stumped. This is a follow-up question to the more general one here: Trying to understand Scala enumerator/iteratees
I am trying to query a database, using Enumerators, Iteratees and Futures.
When I code my controller thus:
def index = Action {
db.withConnection { conn=>
val stmt = conn.createStatement()
val result = stmt.executeQuery("select * from datatable")
val resultEnum:Enumerator[TestDataObject] = Enumerator.generateM {
logger.debug("called enumerator")
result.next() match {
case true =>
val obj = TestDataObject(result.getString("name"), result.getString("object_type"),
result.getString("quantity").toInt, result.getString("cost").toFloat)
logger.info(obj.toJsonString)
Future(Some(obj))
case false =>
logger.warn("reached end of iteration")
stmt.close()
Future(None)
}
}
val consume:Iteratee[TestDataObject,Seq[TestDataObject]] = {
Iteratee.fold[TestDataObject,Seq[TestDataObject]](Seq.empty[TestDataObject]) { (result,chunk) => result :+ chunk }
}
val newIteree = Iteratee.flatten(resultEnum(consume))
val eventuallyResult:Future[Seq[TestDataObject]] = newIteree.run
Ok(Await.result(eventuallyResult,60 seconds))
}
}
I get the expected result in the log:
10:50:27.765 [ForkJoinPool-3-worker-15] DEBUG TestDataObjectController - called enumerator
10:50:27.856 [ForkJoinPool-3-worker-15] INFO TestDataObjectController - {"name":"thingamajig","objtype":"widget","quantity":200,"cost":3.99}
10:50:27.860 [ForkJoinPool-3-worker-15] DEBUG TestDataObjectController - called enumerator
10:50:27.863 [ForkJoinPool-3-worker-15] INFO TestDataObjectController - {"name":"doofus","objtype":"widget","quantity":900,"cost":1.99}
10:50:27.863 [ForkJoinPool-3-worker-11] DEBUG TestDataObjectController - called enumerator
10:50:27.868 [ForkJoinPool-3-worker-11] INFO TestDataObjectController - {"name":"wotsit","objtype":"widget","quantity":30,"cost":0.49}
10:50:27.868 [ForkJoinPool-3-worker-13] DEBUG TestDataObjectController - called enumerator
10:50:27.871 [ForkJoinPool-3-worker-13] INFO TestDataObjectController - {"name":"foo","objtype":"thingy","quantity":490,"cost":1.49}
10:50:27.871 [ForkJoinPool-3-worker-11] DEBUG TestDataObjectController - called enumerator
10:50:27.871 [ForkJoinPool-3-worker-11] WARN TestDataObjectController - reached end of iteration
and I get the expected JSON object (with an implicit converter defined in the controller class, not shown here).
However, when I try to code it properly, using Action.async:
def index = Action.async {
db.withConnection { conn=>
val stmt = conn.createStatement()
val result = stmt.executeQuery("select * from datatable")
val resultEnum:Enumerator[TestDataObject] = Enumerator.generateM {
logger.debug("called enumerator")
result.next() match {
case true =>
val obj = TestDataObject(result.getString("name"), result.getString("object_type"),
result.getString("quantity").toInt, result.getString("cost").toFloat)
logger.info(obj.toJsonString)
Future(Some(obj))
case false =>
logger.warn("reached end of iteration")
stmt.close()
Future(None)
}
}
val consume:Iteratee[TestDataObject,Seq[TestDataObject]] = {
Iteratee.fold[TestDataObject,Seq[TestDataObject]](Seq.empty[TestDataObject]) { (result,chunk) => result :+ chunk }
}
val newIteree = Iteratee.flatten(resultEnum(consume))
val eventuallyResult:Future[Seq[TestDataObject]] = newIteree.run
eventuallyResult.map { data=> Ok(data)}
}
}
then the enumerator terminates on the first run!
[info] play.api.Play - Application started (Dev)
10:53:47.571 [ForkJoinPool-3-worker-13] DEBUG TestDataObjectController - called enumerator
10:53:47.572 [ForkJoinPool-3-worker-13] WARN TestDataObjectController - reached end of iteration
and I get a blank JSON array returned
It appears that result.next() is behaving differently in the two contexts, but I can't work out why. I was wondering whether different iterations are being called in parallel threads, but this is reproduced reliably every time I run the code so I would not expect it to be a thread concurrency issue. Thanks for your time!
