Gatling: Consuming all values from a Collection (Feeder) in parallel

I have a list of user ids, they can be stored in a Collection or Feeder. In the test I create users with provided ids (around 2000 users) in one POST request and then send GET requests to verify that the users were created. It may take 5-10 seconds to create all the users, but first users start to appear after 0.5-1 second. I am creating and verifying users simultaneously in two different scenarios. The problem is that verifyUsers(ids) will send GET requests for each id one by one, but I need them to be sent in parallel because each GET request will take some time and I don't want to create a queue.

def createUsers(ids: List[String]): ScenarioBuilder = scenario("Create users")
    .exec(createUsersForIds(ids))

def verifyUsers(ids: List[String]) = scenario("Verify users")
    .repeat(ids.size, "iterator") {
        exec(session => {
            val index: Int = session("iterator").as[Int]
            session.set("id", ids.take(index))
        })
            .exec(verifyUserWithId) // verifyUserWithId will then take a userId from the session and send a GET request with that id
    }

setUp(
    createUsers(ids).inject(atOnceUsers(1)),
    verifyUsers(ids).inject(atOnceUsers(1))
)

In short, you want multiple producers (virtual users in the createUsers scenario) and multiple consumers (virtual users in the verifyUsers scenario), with a queue in between.

Some hints:

• use a concurrent queue, such as java.util.concurrent.ConcurrentLinkedQueue
• use some counters such as AtomicLong to keep track of the number of records produced and consumed so you can detect when you've consumed all the records you've ultimately produced.
• implement the consumer side so that when there's no available records (possibly because you're consuming faster than you're producing), you do a loop (asLongAs) with a pause.

Edit: If your use case is actually that the producer produces all records at once, the solution is way easier:

• make the producer and consumer scenarios sequential, see https://docs.gatling.io/reference/script/core/injection/#sequential-scenarios
• have all your consumers poll the concurrent queue in a loop (AslongAs or doWhile depending on how you implement your logic) as long as the polled record is not empty

Hope it helps!