How to save & return data from within Future callback - scala

Question

I've been facing an issue the past few days regarding saving & handling data from Futures in Scala. I'm new to the language and the concept of both. Lagom's documentation on Cassandra says to implement roughly 9 files of code and I want to ensure my database code works before spreading it out over that much code.
Specifically, I'm currently trying to implement a proof of concept to send data to/from the cassandra database that lagom implements for you. So far I'm able to send and retrieve data to/from the database, but I'm having trouble returning that data since this all runs asynchronously, and also returning that the data returned successfully.
I've been playing around for a while; The retrieval code looks like this:
override def getBucket(logicalBucket: String) = ServiceCall[NotUsed, String] {
request => {
val returnList = ListBuffer[String]()
println("Retrieving Bucket " + logicalBucket)
val readingFromTable = "SELECT * FROM data_access_service_impl.s3buckets;"
//DB query
var rowsFuture: Future[Seq[Row]] = cassandraSession.selectAll(readingFromTable)
println(rowsFuture)
Await.result(rowsFuture, 10 seconds)
rowsFuture onSuccess {
case rows => {
println(rows)
for (row <- rows) println(row.getString("name"))
for (row <- rows) returnList += row.getString("name")
println("ReturnList: " + returnList.mkString)
}
}
rowsFuture onFailure {
case e => println("An error has occured: " + e.getMessage)
Future {"An error has occured: " + e.getMessage}
}
Future.successful("ReturnList: " + returnList.mkString)
}
}
When this runs, I get the expected database values to 'println' in the onSuccess callback. However, that same variable, which I use in the return statement, outside of the callback prints as empty (and returns empty data as well). This also happens in the 'insertion' function I use, where it doesn't always return variables I set within callback functions.
If I try to put the statement within the callback function, I'm given an error of 'returns Unit, expects Future[String]'. So I'm stuck where I can't return from within the callback functions, so I can't guarantee I'm returning data).
The goal for me is to return a string to the API so that it shows a list of all the s3 bucket names within the DB. That would mean iterating through the Future[Seq[Row]] datatype, and saving the data into a concatenated string. If somebody could help with that, they'll solve 2 weeks of problems I've had reading through Lagom, Akka, Datastax, and Cassandra documentation. I'm flabbergasted at this point (information overload) and there's no clearcut guide I've found on this.
For reference, here's the cassandraSession documentation:
LagomTutorial/Documentation Style Information with their only cassandra-query example
CassandraSession.scala code

Answer 1

The key thing to understand about Future, (and Option, and Either, and Try) is that you do not (in general) get values out of them, you bring computations into them. The most common way to do that is with the map and flatMap methods.
In your case, you want to take a Seq[Row] and transform it into a String. However, your Seq[Row] is wrapped in this opaque data structure called Future, so you can't just rows.mkString as you would if you actually had a Seq[Row]. So, instead of getting the value and performing computation on it, bring your computation rows.mkString to the data:
//DB query
val rowsFuture: Future[Seq[Row]] = cassandraSession.selectAll(readingFromTable)
val rowToString = (row: Row) => row.getString("name")
val computation = (rows: Seq[Row]) => rows.map(rowToString).mkString
// Computation to the data, rather than the other way around
val resultFuture = rowsFuture.map(computation)
Now, when rowsFuture is completed, the new future that you created by calling rowsFuture.map will be fulfilled with the result of calling computation on the Seq[Row] that you actually care about.
At that point you can just return resultFuture and everything will work as anticipated, because the code that calls getBucket is expecting a Future and will handle it as is appropriate.
Why is Future opaque?
The simple reason is because it represents a value that may not currently exist. You can only get the value when the value is there, but when you start your call it isn't there. Rather than have you poll some isComplete field yourself, the code lets you register computations (callbacks, like onSuccess and onFailure) or create new derived future values using map and flatMap.
The deeper reason is because Future is a Monad and monads encompass computation, but do not have an operation to extract that computation out of them

Answer 2

Replace the select for your specific select and the field that you want to obtain for your specific field.The example is only for test, is not a architecture propose.
package ldg.com.dbmodule.model
/**
* Created by ldipotet on 05/11/17.
*/
import com.datastax.driver.core.{Cluster, ResultSet, ResultSetFuture}
import scala.util.{Failure, Success, Try}
import java.util.concurrent.TimeUnit
import scala.collection.JavaConversions._
//Use Implicit Global Context is strongly discouraged! we must create
//our OWN execution CONTEXT !
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.{Future, _}
import scala.concurrent.duration._
object CassandraDataStaxClient {
//We create here a CallBack in Scala with the DataStax api
implicit def resultSetFutureToScala(rf: ResultSetFuture):
Future[ResultSet] = {
val promiseResult = Promise[ResultSet]()
val producer = Future {
getResultSet(rf) match {
//we write a promise with an specific value
case Success(resultset) => promiseResult success resultset
case Failure(e) => promiseResult failure (new
IllegalStateException)
}
}
promiseResult.future
}
def getResultSet: ResultSetFuture => Try[ResultSet] = rsetFuture => {
Try(
// Other choice can be:
// getUninterruptibly(long timeout, TimeUnit unit) throws
TimeoutException
// for an specific time
//can deal an IOException
rsetFuture.getUninterruptibly
)
}
def main(args: Array[String]) {
def defaultFutureUnit() = TimeUnit.SECONDS
val time = 20 seconds
//Better use addContactPoints and adds more tha one contact point
val cluster = Cluster.builder().addContactPoint("127.0.0.1").build()
val session = cluster.connect("myOwnKeySpace")
//session.executeAsync es asyncronous so we'll have here a
//ResultSetFuture
//converted to a resulset due to Implicitconversion
val future: Future[ResultSet] = session.executeAsync("SELECT * FROM
myOwnTable")
//blocking on a future is strongly discouraged!! next is an specific
//case
//to make sure that all of the futures have been completed
val results = Await.result(future,time).all()
results.foreach(row=>println(row.getString("any_String_Field"))
session.close()
cluster.close()
}
}