Richard Searle's Blog

Thoughts about software

Implementing Lookup

Posted by eggsearle on April 3, 2011

The previous blog entry was a broad brush description of the implementation concepts. This entry will cover the details of actually implementing the Lookup trait.

Lookup uses a curried apply to simplify the DSL, minimizing the visual clutter.

call is the actual implementation of the sub-service, launching the request-response interaction with arg and returning a CI instance that will appear in the corresponding response.

Pending provides a (Recursive)PartialFunction that matches the CI instance. It’s apply ignores the CI instance since its actual value has no impact on the processing. It returns a function that casts the request value to the expected type and evaluates the function originally provided in the call to Lookup.apply.

trait Lookup[A,R] {
  def apply(arg:A)(fn:R =>RPF):RPF = new Pending(call(arg),fn)
  protected def call(arg:A):CI
}

private class Pending[T](correlationId:CI,fn:T=>RPF) extends RPF{
    def isDefinedAt(ci:CI) = ci == correlationId
    def apply(ci:CI) =  {a:Any=>fn(a.asInstanceOf[T])}
}

case class CI(id:Int)

A trivial example Lookup that doubles the integer request

object DoubleLookup extends Lookup[Int,Int]{
   var responseQueue:List[(CI,Int)] = _
   protected def call(arg:Int) = {
      val ci = CorrelationAllocator()
      responseQueue = ci->arg*2 :: responseQueue
      ci
   }
}

Note responseQueue which provides a trivial implementation of the mechanism required to asynchronously deliver the response. It captures the CI (to match the Pending instance that triggered the request and the actual value that will be processed via that Pending.

CorrelationAllocator is an object that creates sufficiently unique CI values.

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