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New in Bootzooka - Towards the Reactive

Recently we updated our seed project Bootzooka with new version of dependencies. The most important one is a transition from Slick 2.1 to 3.0. Slick is a library for accessing relational databases with typesafe queries built with a 'functional-relational mapping' paradigm. Its fresh release brings a greatly rebuilt API which is now asynchronous and monadic. Here's a great introduction to these concepts by Slick tech lead Stefan Zeiger. The consequences of such change are very significant - we can return Future instead of blocking not only in our data acess layer, but across all application layers. Let's take a look at a sample use case - new user registration (squeezed a bit here and there for brevity).

The web layer

Scalatra offers a simple way of handling aynchronous result. We can return an AsyncResult which wraps a Future returned by inner service.

post("/register", operation(register)) {
  if (!userService.isUserDataValid(loginOpt, emailOpt, passwordOpt)) {
    haltWithBadRequest("Wrong user data!")
  } else {
    val paramLogin = login
    val paramPass = password
    val paramEmail = email
    new AsyncResult {
      val is = userService.checkUserExistenceFor(paramLogin, paramEmail).flatMap {
        case Left(error) => Future { haltWithConflict(error) }
        case _ =>
          userService
            .registerNewUser(escapeHtml4(paramLogin), paramEmail, paramPass).map(
              _ => Created(StringJsonWrapper("success")))
      }
    }
  }
}

Notice that we extract paramLogin, paramPass and paramEmail before proceeding. That's because login, password and email are methods and we can't call them inside a Future. That's a tricky thing that one must remember when working with async Scala code - avoid closing over mutable state! Our AsyncResult has a field called is and this field should carry our result wrapped in a Future. To build it, we are sequentially calling two async services. First one, checkUserExistenceFor does a background check if a user with given name or email already exists. Since it returns a Future, we need to map or flatMap over it to define how we would like to proceed with the wrapped result. If requested user data does not yet exist, we are going to call another async service (registerNewUser) and return its result as the final one. Since we don't want to end up with a Future[Future[T]], we used flatMap. Let's now see how does the registration service look like:

The service layer

def registerNewUser(login: String, email: String, password: String): Future[Unit] = {
  val salt = Utils.randomString(128)
  val token = UUID.randomUUID().toString
  userDao.add(User(login, email.toLowerCase, password, salt, token))
    .flatMap(_ => {
    val confirmationEmail = emailTemplatingEngine.registrationConfirmation(login)
    emailScheduler.scheduleEmail(email, confirmationEmail)
  })
}

Here's a similar situation: our call to userDao is asynchronous and if it succeeds, we want to send an e-mail. Again we are using flatMap to compose these two values of type Future in a monadic way. Let's now peek into our DAO where we used new Slick 3.0 API:

The data access layer

def add(user: User): Future[Unit] = {
  val action = (for {
    userByLoginOpt <- findByLowerCasedLoginAction(user.login)
    userByEmailOpt <- findByEmailAction(user.email)
    _ <- addOrFailOnExistingAction(userByLoginOpt, userByEmailOpt, user)
  } yield ()).transactionally

  db.run(action)
}

private def addOrFailOnExistingAction(userByLoginOpt: Option[User], 
                             userByEmailOpt: Option[User], 
                             user: User) = {
  if (userByLoginOpt.isDefined || userByEmailOpt.isDefined)
    DBIO.failed(new IllegalArgumentException("User already exists"))
  else
    users += user
  }

Here we compose a transaction out of three Slick DBIOActions. First two actions are queries to find the user by login and email. The third action is dependent on their result. If a user is found, we terminate with an exception, otherwise we add a new user. This whole composite action is executed as a single transaction when we pass it to the db.run() call.

Why 'Reactive'?

We saw that our codebase is now crawling with Futures. Thanks to for comprehension and flatMap we can compose them like any other monad and define our logic based on eventually returned values. This is the main elegance of monads - being able to specify computations on some data wrapped in context without explicitly unwrapping this data. But what are real gains of the new approach? Wasn't it simpler before to just define our logic without all the extra code?

Elasic threads

In typical approach, a synchronous call creates a thread in the web layer and this thread becomes blocked until our database call completes. In case of slow responses, a part of our application can easily become a bottleneck producing hanging threads and affecting the availability and responsiveness of all the other features. With async DB calls threads get released quickly and they come back re-allocated after a reponse is ready to be returned.

Error isolation

Each async call requires an implicit ExecutionContext which defines what thread pool will be used. You can now have much better control over different parts of the application, assigning dedicated ExecutionContext to them. In case when all threads in some part of your system hang due to a bug, there's still a possibility that other parts have their own thread pools and will continue to work without resource starvation. This approach to isolation of failure zones is called the Bulkhead Pattern.

Conclusions

There are still parts in Bootzooka where we had to work synchronously. For example: Scalatra has an internal API for authentication which requires overriding a method that immediately returns current user, so we had to use Await.result() to handle this case. However, other parts of the code got transformed pretty seamlessly. With little effort, our system becomes much more elastic and resilient which, as we learned, are not only hot buzzwords from the Reactive Manifesto but actual important values.

Bootzooka is now also available as a Typesafe Activator Template

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