Not able to return messages to Spray HttpService Actor

Question

I am learning Spray and Akka. Have built a simple Spray routing App with one of the route stubs as -

path("blog" / LongNumber) {
  blogId =>
    respondWithMediaType(MediaTypes.`application/json`) {
      get {
        request => BlogFetchActor ! Get(blogId)
      }
    }
 }

In the above code I dispatch a message in my spray route definition (in a class implementing HttpService) to another Actor with an ID (blogId) in the request. The BlogFetchActor is supposed to fetch the data from database and respond to the sender. This I have coded as below -

def receive: Receive = LoggingReceive {
  case Get(id: Long) => {
    log.debug("Retrieving blog with id %d".format(id))
      sender ! ReturnBlog(get(id))
  }
 }

The Actor message from the route is getting to my BlogFetchActor. My BlogFetchActor completes the job of getting the data from the database too. But when I try to send the response back to the sender HttpService, the ReturnBlog message, it does not work. The message ends up in DeadLetters and I see the below in my logs -

Message [in.bharathwrites.BlogFetchActor$ReturnBlog] from Actor[akka://on-spray-can/user/blog-service/blog#1301907662] to Actor[akka://on-spray-can/deadLetters] was not delivered.

Why does the response back to the sender not work? What am I doing wrong? How else can I achieve this? I tried reading the Spray docs and ScalaDoc but could not figure out the problem. Am not proficient enough to read the Spray code and fathom the reasons... Thanks in advance

Sergiy Prydatchenko Sergiy Prydatchenko · Accepted Answer · 2013-12-09T18:31:53

If you want to ask another actor before completing a request, you should use Akka's ask pattern (? in the example below).

You may also consider returning an Option[ReturnBlog] instead of ReturnBlog to allow Spray to generate 404 status response automatically if such a blog does not exists.

path("blog" / LongNumber) {
  blogId =>
    respondWithMediaType(MediaTypes.`application/json`) {
      get {
        complete { (BlogFetchActor ? Get(blogId)).mapTo[ReturnBlog] }
      }
    }
}

Not able to return messages to Spray HttpService Actor

1 Answers