Followers of this blog must have known by now that I am a big fan of a clean domain model. And domain driven design, espoused by Eric Evans is the way to go when you are modeling a complex domain and would like to have your model survive for quite some time in the future. Recently I have been experimenting a bit with domain driven design using some amount of asynchronous message passing techniques particularly in the services and the storage layer.

The Repository, as Eric says, is the domain centric abstraction on top of your data storage layer. It gives your model back the feeling that you are dealing with domain concepts instead of marshalling data across your storage layers. Typically you have contracts for repositories at the aggregate root level. The underlying implementation commits to a platform (like JPA) and ensures that your object graph of the aggregate root rests in peace within the relational database. It need not be a relational database - it can be file system, it can be a NoSQL database. That's the power of abstraction that Repositories add to your model.

Ever since I started playing around with Erlang, I have been toying with thoughts of making repositories asynchronous. I blogged some of my thoughts in this post and even implemented a prototype using Scala actors.

Enter Akka and its lightweight actor model that offers transaction support over an STM. Akka offers seamless integration with a variety of persistence engines like Cassandra, MongoDB and Redis. It has plans of adding to this list many of the relational stores as well. The richness of the Akka stack makes for a strong case in designing a beautiful asynchronous repository abstraction.

Consider a very simple domain model for a Bank Account ..

case class Account(no: String, 
  name: String, 
  dateOfOpening: Date, 
  dateOfClose: Option[Date],
  balance: Float)

We can model typical operations on a bank account like Opening a New Account, Querying for the Balance of an Account, Posting an amount in an Account through message dispatch. Typical messages will look like the following in Scala ..

sealed trait AccountEvent
case class Open(from: String, no: String, name: String) extends AccountEvent
case class New(account: Account) extends AccountEvent
case class Balance(from: String, no: String) extends AccountEvent
case class Post(from: String, no: String, amount: Float) extends AccountEvent

Note all messages are immutable Scala objects, which will be dispatched by the client, intercepted by a domain service, which can optionally do some processing and validation, and then finally forwarded to the Repository.

In this post we will look at the final stage in the lifecycle of a message, which is how it gets processed by the Repository. In the next post we will integrate the whole along with an abstraction for a domain service. Along the way we will see many of the goodness that Akka transactors offer including support for fault tolerant processing in the event of system crashes.

trait AccountRepository extends Actor

class RedisAccountRepository extends AccountRepository {
  lifeCycle = Some(LifeCycle(Permanent))    
  val STORAGE_ID = "account.storage"

  // actual persistent store
  private var accounts = atomic { RedisStorage.getMap(STORAGE_ID) }

  def receive = {
    case New(a) => 
      atomic {
        accounts.+=(a.no.getBytes, toByteArray[Account](a))
      }

    case Balance(from, no) =>
      val b = atomic { accounts.get(no.getBytes) }
      b match {
        case None => reply(None)
        case Some(a) => 
          val acc = fromByteArray[Account](a).asInstanceOf[Account]
          reply(Some(acc.balance))
      }

      //.. other message handlers
  }

  override def postRestart(reason: Throwable) = {
    accounts = RedisStorage.getMap(STORAGE_ID)  
  }
}

The above snippet implements a message based Repository abstraction with an underlying implementation in Redis. Redis is an advanced key/value store that offers persistence for a suite of data structures like Lists, Sets, Hashes and more. Akka offers transparent persistence to a Redis storage through a common set of abstractions. In the above code you can change RedisStorage.getMap(STORAGE_ID) to CassandraStorage.getMap(..) and switch your underlying storage to Cassandra.

The above Repository works through asynchronous message passing modeled with Akka actors. Here are some of the salient points in the implementation ..

1. Akka is based on the let-it-crash philosophy. You can design supervisor hierarchies that will be responsible for controlling the lifecycles of your actors. In the Actor abstraction you can configure how you would like to handle a crash. LifeCycle(Permanent) means that the actor will always be restarted by the supervisor in the event of a crash. It can also be Lifecycle(Temporary), which means that it will not be restarted and will be shut down using the shutdown hook that you provide. In our case we make the Repository resilient to crashes.


2. accounts is the handle to a Map that gets persisted in Redis. Here we store all accounts that the clients open hashed by the account number. Have a look at the New message handler in the implementation.


3. With Akka you can also provide a restart hook when you repository crashes and gets restarted automatically by the supervisor. postRestart is the hook where we re-initialize the Map structure.


4. Akka uses multiverse, a Java based STM implementation for transaction handling. In the code mark your transactions using atomic {} and the underlying STM will take care of the rest. Instead of atomic, you can also use monadic for-comprehensions for annotating your transaction blocks. Have a look at Akka documentation for details.

Asynchronous message based implementations decouple the end points, do not block and offer more manageability in distribution of your system. Typical implementations of actor based models are very lightweight. Akka actors take around 600 bytes which means that you can have millions of actors even in a commodity machine. Akka supports various types of message passing semantics which you can use to organize interactions between your collaborating objects.

In the next post we will see how the Repository interacts with Domain Services to implement client request handling. And yeah, you got it right - we will use more of Akka actors.