Reading Play-Slick DBAction code, I thought that this code might contain a race condition:
object DBAction{
// snip
def apply(r: (RequestWithDbSession) => Result)(implicit app:Application) = {
Action { implicit request =>
AsyncResult {
DB.withSession{ s:scala.slick.session.Session =>
Future(r( RequestWithDbSession(request,s) ))(executionContext)
}
}
}
}
The function r runs at a future time, after withSession has returned a Future[Result], and called session.close(). Is there a race condition in this code?
I am not sure if that is called a race condition. However to me it seems that you are correct that something is wrong here. The session might no longer be valid when the future executes the code.
It would be better to invert the execution and request a database session from within the future:
Async {
Future {
DB.withSession{ s:scala.slick.session.Session =>
r( RequestWithDbSession(request, s) )
}
}
}
I think your are right and fix suggested by EECOLOR looks correct. We are tracking this in a ticket: https://github.com/freekh/play-slick/issues/81
Thx
Related
I will start mentioning I am very new to Scala but I have now to maintain a legacy code where some new feature are being tried to be include.
I have the following code:
Where a list is coming as a parameter where a new output needs to be processed. However it seems like code is not waiting for the response to the external service when processing.
def historyBet(jackpotListUser : List[JackpotBetHistory])(implicit MC: AppMarkerContext) : List[LegacyJackpotHistoryResponse] =
for {
bet <- jackpotListUser
prize = jackpotIntegratorService.findJackpotByJackpotHumanId(bet.jackpotHumanId) match {
case Some(jackpot : JackpotResponse) =>
...
extra code extracting price from jackpot : JackpotResponse
...
extra code generating result with prize
} yield result
How can I do a call to jackpotIntegratorService.findJackpotByJackpotHumanId to execute at that time. instead of returning something that F[Option....?
def findJackpotByJackpotHumanId(
jackpotHumanId: JackpotHumanId
)(implicit MC: AppMarkerContext): F[Option[JackpotResponse]] =
jackpotIntegratorRepo.findJackpotByJackpotHumanId(jackpotHumanId)
where it is finally implemented as:
override def findJackpotByJackpotHumanId(
jackpotHumanId: JackpotHumanId
)(implicit mc: AppMarkerContext): IO[Option[JackpotResponse]] =
... code calling an API which return the IO.
Thanks!
I thought I could do IO.await somewhere... but not sure where or how...
because in the "historyBet" function I got a F[] when it was an IO... so what is the syntax to be able to wait for the response and the continue?
Extra Comment:
The real issue we notice is that the method call is starting (the logs shows part of it) but the caller with in the maps continues too.
prize = jackpotIntegratorService.findJackpotByJackpotHumanId
this part of the code continues even when prize, which we want the final object JackpotResponse, not the IO or F.
So, if your method needs to call an IO then it must return an IO unless you unsafeRunSync them... but, as the name suggest, you should not do that.
So the return type is now: IO[List[LegacyJackpotHistoryResponse]
And can be implemented like this:
def historyBet(jackpotListUser: List[JackpotBetHistory])(implicit MC: AppMarkerContext): IO[List[LegacyJackpotHistoryResponse]] =
jackpotListUser.traverse { bet =>
jackpotIntegratorService.findJackpotByJackpotHumanId(bet.jackpotHumanId).map {
case Some(jackpot) =>
// ...
case None =>
// ...
}
}
I write a custom spark sink. In my addBatch method I use ForEachPartitionAsync which if I'm not wrong only makes the driver work asynchronously, returning a future.
val work: FutureAction[Unit] = rdd.foreachPartitionAsync { rows =>
val sourceInfo: StreamSourceInfo = serializeRowsAsInputStream(schema, rows)
val ackIngestion = Future {
ingestRows(sourceInfo) } andThen {
case Success(ingestion) => ackIngestionDone(partitionId, ingestion)
}
Await.result(ackIngestion, timeOut) // I would like to remove this line..
}
work onSuccess {
case _ => // move data from temporary table, report success of all workers
}
work onFailure{
//delete tmp data
case t => throw t.getCause
}
I can't find a way to run the worker nodes without blocking on the Await call, as if I remove them a success is reported to the work future object although the future didn't really finish.
Is there a way to report to the driver that all the workers finished
their asynchronous jobs?
Note: I looked at the foreachPartitionAsync function and it has only one implementation that expects a function that returns a Unit (i would've expected it to have another one returning a future or maybe a CountDownLatch..)
I am building some custom directives with Spray and I find a problem:
When a route is rejected I thought that it was immediately completed with a rejection. However it is not and it tries to complete the inner route. For example, in this code:
path("users") {
post {
entity(as[User]){
user =>
directive_always_reject(){
println("Hello")
complete(200, "Hello")
}
}
}
}
The "hello" is always printed, even if the directive is rejected.
I always thought that when a route is rejected it does not continue to inner routes (or at least is how I understand it)
Did I misunderstand something with Spray or there is a bug somewhere in my code?
Thanks you!
[UPDATE 1]
An implementation of directive_always_reject() could be:
def directive_always_reject[T]() : Directive0 = {
reject(
new ValidationRejection("Some error")
)
}
[UPDATE 2]
I see that a solution could be put the println inside the complete function.
path("users") {
post {
entity(as[User]){
user =>
directive_always_reject(){
complete {
println("Hello")
"Hello"
}
}
}
}
}
However this solution does not apply to me because in my real code I pass the RequestContext to another actor who is the one that completes the request.
Is there any difference between these two ways of completing a failed Future? If so, which way is considered to be more "correct"?
Calling Promise.failure:
def functionThatFinishesLater: Future[String] = {
val myPromise = Promise[String]
Future {
// Do something that might fail
if (failed) {
myPromise.failure(new RuntimeException("message")) // complete with throwable
} else {
myPromise.success("yay!")
}
} (aDifferentExecutionContext)
myPromise.future
}
Or just throwing an exception
def functionThatFinishesLater: Future[String] = {
val myPromise = Promise[String]
Future {
// Do something that might fail
if (failed) {
throw new RuntimeException("message") // throw the exception
} else {
myPromise.success("yay!")
}
} (aDifferentExecutionContext)
myPromise.future
}
It looks to me like you're mixing paradigms. A Promise is an imperative way of completing a Future, but a Future can also be made completed by wrapping the computation in a Future constructor. You're doing both, which is probably not what you want. The second statement in both code fragments is of type Future[Promise[String]], and I'm almost certain you really want just Future[String].
If you're using using the Future.apply constructor, you should just treat the value produced as the Future, rather than using it to resolve a separate Promise value:
val myFuture = Future {
// Do some long operation that might fail
if (failed) {
throw new RuntimeException("message")
} else {
"yay!"
}
}
The way to use the Promise is to create the Promise, give its Future to some other piece of code that cares, and then use .success(...) or .failure(...) to complete it after some long running operation. So to recap, the big difference is that Future has to wrap the whole computation, but you can pass a Promise around and complete it elsewhere if you need to.
Alright so the title is a little obscure since I was unsure how to word it, but essentially I am trying to do a try-catch statement that will timeout... Here's some pseudo-code that may help describe what I'm trying to do:
try (10 seconds) {
*make some connection and do some things*
} catch {
case ex1: TimeoutException => *do something*
case ex2: Exception => *do something else*
}
Currently there is a bug in the hardware I'm working with where the request for a connection never gets a response back, so it just sits there and doesn't catch any exceptions. Since it's only a bug (that should be temporary), I don't want to manipulate the architecture of the application (specifically I do not want to create a new actor just to account for something small) and it would be very ideal if I could implement this pseudo-code within the scope of the class.
So essentially my question is how do I implement the pseudo-code above within the scope of the class it's in?
Let me know if anything is unclear! Thank you!
Try:
import scala.concurrent._
import ExecutionContext.Implicits.global
val f = future {
// make some connection and do some things
}
try {
Await.result(f, 10 seconds);
} catch {
case e: TimeoutException => // do something
case _ => // do something else
}
More info: Futures and Promises