How to properly use spray.io LruCache

Question

I am quite an unexperienced spray/scala developer, I am trying to properly use spray.io LruCache. I am trying to achieve something very simple. I have a kafka consumer, when it reads something from its topic I want it to put the value it reads to cache. Then in one of the routings I want to read this value, the value is of type string, what I have at the moment looks as follows:

object MyCache {

  val cache: Cache[String] = LruCache(
    maxCapacity      = 10000,
    initialCapacity  = 100,
    timeToLive       = Duration.Inf,
    timeToIdle       = Duration(24, TimeUnit.HOURS)
   )
}

to put something into cache i use following code:

def message() = Future { new String(singleMessage.message()) }
MyCache.cache(key, message)

Then in one of the routings I am trying to get something from the cache:

val res = MyCache.cache.get(keyHash)

The problem is the type of res is Option[Future[String]], it is quite hard and ugly to access the real value in this case. Could someone please tell me how I can simplify my code to make it better and more readable ? Thanks in advance.

AmigoNico AmigoNico · Accepted Answer · 2015-02-26T07:14:45

Don't try to get the value out of the Future. Instead call map on the Future to arrange for work to be done on the value when the Future is completed, and then complete the request with that result (which is itself a Future). It should look something like this:

path("foo") {
  complete(MyCache.cache.get(keyHash) map (optMsg => ...))
}

Also, if singleMessage.message does not do I/O or otherwise block, then rather than creating the Future like you are

Future { new String(singleMessage.message) }

it would be more efficient to do it like so:

Future.successful(new String(singleMessage.message))

The latter just creates an already completed Future, bypassing the use of an ExecutionContext to evaluate the function.

If singleMessage.message does do I/O, then ideally you would do that I/O with some library (like Spray client, if it's an HTTP request) that returns a Future (rather than using Future { ... } to create another thread which will block).

How to properly use spray.io LruCache

1 Answers