Best practice to validate Akka Persistence commands

Question

Let's say we have a command AddUser:

case class AddUser(username: String, password: String)

What would be the best way to validate this command before you persist an event? Doing it in a way like this for example, seems kind of messy.

override def receiveCommand: Receive = {
    case AddUser(username, password) =>
      if (username.trim.length > 25) {
        sender() ! UsernameTooLong
      } else if (password.trim.length > 25) {
        sender() ! PasswordTooLong
      } else {
        persist(UserAdded(User(username, password))) {
          userAdded =>
            sender() ! userAdded
        }
      }
}

Answer 1

You need to use Receive Pipeline Pattern: http://doc.akka.io/docs/akka/2.4.1/contrib/receive-pipeline.html

you can do something like this:

trait AuthenticatedUser { this: ReceivePipeline =>
   pipelineInner {
    case msg@AddUser(username, password) => 
     if (username.trim.length > 25) {
        sender() ! UsernameTooLong
     } else if (password.trim.length > 25) {
        sender() ! PasswordTooLong
     } else {
       Inner(msg)
     }
   case msg => Inner(msg)
   }
}

class UserActor extends Actor with ReceivePipeline with AuthenticatedUser {

 def receive = {
   case AddUser(username, password) =>
    persist(UserAdded(User(username, password))) {
          userAdded =>
            sender() ! userAdded
        }
  }
}

PS: I have not compiled and tested this code, But I believe it will work, If there is any issue let me know.