Actions as Controllers

Actions can be used to implement MVC Controllers by acting as the external interface of a service, receiving requests, operating over the requests values and forwarding the call to other components in the same service.

To illustrate how you can use an Action as a Controller, we will build on top of the Value Entity Shopping Cart example, adding a new Action to the existing shopping cart service.

Forwarding Commands

The forward effect allows us to transform or further validate an incoming request before passing it on to another component and have the response message directly passed back to the client making the request. The response from the forwarded operation must have the same response type as the original command.

In this example we accept the same command as the entity, AddLineItem, but add some additional verification of the request and only conditionally forward the request to the entity if the verification is successful:

Java
src/main/java/com/example/shoppingcart/ShoppingCartActionImpl.java
  @Override
  public Effect<Empty> verifiedAddItem(ShoppingCartApi.AddLineItem addLineItem) {
    if (addLineItem.getName().equalsIgnoreCase("carrot")) { (1)
      return effects().error("Carrots no longer for sale"); (2)
    } else {
      DeferredCall<ShoppingCartApi.AddLineItem, Empty> call =
          components().shoppingCart().addItem(addLineItem); (3)
      return effects().forward(call); (4)
    }
  }
1 check if the added item is carrots
2 if it is "carrots" immediately return an error, disallowing adding the item
3 for allowed requests, use components().shoppingCart().addItem() to get a DeferredCall
4 The DeferredCall is used with effects().forward() to forward the request to the entity
Scala
src/main/java/com/example/shoppingcart/ShoppingCartActionImpl.scala
  override def verifiedAddItem(addLineItem: AddLineItem): Action.Effect[Empty]=
    if (addLineItem.name.equalsIgnoreCase("carrot")) (1)
      effects.error("Carrots no longer for sale") (2)
    else {
      val call = components.shoppingCart.addItem(addLineItem) (3)
      effects.forward(call) (4)
    }
1 check if the added item is carrots
2 if it is "carrots" immediately return an error, disallowing adding the item
3 for allowed requests, use components.shoppingCart.addItem() to get a DeferredCall
4 The DeferredCall is used with effects.forward() to forward the request to the entity

Transform Request and Response to Another Component

The asyncReply and asyncEffect effects allow us to process and transform a request before calling another component and then also transform the response.

As an example, let us look at the problem of creating a new entity with an id generated by the service itself.

In this example we implement an Initialize command for the controller Action which returns the message NewCartCreated with the entity id that can subsequently be used to interact with the cart.

Java
src/main/java/com/example/shoppingcart/ShoppingCartActionImpl.java
  @Override
  public Effect<ShoppingCartController.NewCartCreated> initializeCart(ShoppingCartController.NewCart newCart) {
    final String cartId = UUID.randomUUID().toString(); (1)
    CompletionStage<Empty> shoppingCartCreated =
        components().shoppingCart().create(ShoppingCartApi.CreateCart.newBuilder().setCartId(cartId).build()) (2)
        .execute(); (3)

    // transform response
    CompletionStage<Effect<ShoppingCartController.NewCartCreated>> effect =
        shoppingCartCreated.handle((empty, error) -> { (4)
          if (error == null) {
            return effects().reply(ShoppingCartController.NewCartCreated.newBuilder().setCartId(cartId).build()); (5)
          } else {
            return effects().error("Failed to create cart, please retry"); (6)
          }
        });

    return effects().asyncEffect(effect); (7)
  }
1 We generate a new UUID
2 We use components().shoppingCart().create(…​) to create a DeferredCall for create on the shopping cart
3 execute() on the DeferredCall immediately triggers a call and returns a CompletionStage for the response
4 Once the call succeeds or fails the CompletionStage is completed or failed, we can transform the result from CompletionStage<Empty> to CompletionStage<Effect<NewCartCreated>> using handle.
5 On a successful response, we create a reply effect with a NewCartCreated
6 If the call leads to an error, we create an error effect asking the client to retry
7 effects().asyncEffect() allows us to reply with a CompletionStage<Effect<NewCartCreated>>
Scala
src/main/java/com/example/shoppingcart/ShoppingCartActionImpl.scala
  override def initializeCart(newCart: NewCart): Action.Effect[NewCartCreated] = {
    val cartId = UUID.randomUUID().toString (1)

    val created: Future[Empty] =
      components.shoppingCart.create(CreateCart(cartId)).execute() (2)

    val effect: Future[Action.Effect[NewCartCreated]] = (3)
      created.map(_ => effects.reply(NewCartCreated(cartId))) (4)
        .recover(_ => effects.error("Failed to create cart, please retry")) (5)

    effects.asyncEffect(effect) (6)
  }
1 We generate a new UUID
2 We use components.shoppingCart.create(…​) to create a DeferredCall for create on the shopping cart
3 execute() on the DeferredCall immediately triggers a call and returns a Future for the response
4 On a successful response, we map the Empty reply to a reply effect with the reply NewCartCreated
5 If the call leads to an error, we recover and return an error effect asking the client to retry
6 effects.asyncEffect() allows us to reply with a Future[Effect[NewCartCreated]] rather than a reply we already have created

The action generates a UUID to use as entity id for the shopping cart. UUIDs are extremely unlikely to lead to the same id being generated, but to completely guarantee two calls can never be assigned the same shopping cart we make use of the "boundary of consistency" provided by the entity - the entity will only process a single command at a time and can safely make decisions based on its state - for example to only allow creation once by storing something in its state signifying that it has been created.

In this case we mark that the entity has been created using a creation timestamp in the shopping cart state stored on first create call (when the timestamp has the default value of 0). If the cart has already been stored with a timestamp we return an error effect:

Java
src/main/java/com/example/shoppingcart/domain/ShoppingCart.java
  @Override
  public Effect<Empty> create(ShoppingCartDomain.Cart currentState, ShoppingCartApi.CreateCart createCart) {
    if (currentState.getCreationTimestamp() > 0L) {
      return effects().error("Cart was already created");
    } else {
      return effects().updateState(currentState.toBuilder().setCreationTimestamp(Instant.now().toEpochMilli()).build())
          .thenReply(Empty.getDefaultInstance());
    }
  }
Scala
src/main/java/com/example/shoppingcart/domain/ShoppingCart.scala
  override def create(currentState: Cart, createCart: shoppingcart.CreateCart): ValueEntity.Effect[Empty] =
    if (currentState.creationTimestamp > 0)
      effects.error("Cart was already created")
    else
      effects.updateState(currentState.copy(creationTimestamp = Instant.now().toEpochMilli))
        .thenReply(Empty.defaultInstance)

Composing calls

The async call shown in the previous section, can also be used to chain or compose multiple calls to a single action response.

In this example we build on the previous cart creation by adding an initial item in the cart once it has been created, but before we return the new id to the client:

Java
src/main/java/com/example/shoppingcart/ShoppingCartActionImpl.java
  @Override
  public Effect<ShoppingCartController.NewCartCreated> createPrePopulated(ShoppingCartController.NewCart newCart) {
    final String cartId = UUID.randomUUID().toString();
    CompletionStage<Empty> shoppingCartCreated =
        components().shoppingCart().create(ShoppingCartApi.CreateCart.newBuilder().setCartId(cartId).build())
            .execute();

    CompletionStage<Empty> cartPopulated = shoppingCartCreated.thenCompose(empty -> { (1)
      ShoppingCartApi.AddLineItem initialItem = (2)
          ShoppingCartApi.AddLineItem.newBuilder()
              .setCartId(cartId)
              .setProductId("e")
              .setName("eggplant")
              .setQuantity(1)
              .build();

      return components().shoppingCart().addItem(initialItem).execute(); (3)
    });

    CompletionStage<ShoppingCartController.NewCartCreated> reply =
        cartPopulated.thenApply(empty -> (4)
          ShoppingCartController.NewCartCreated.newBuilder().setCartId(cartId).build()
        );

    return effects().asyncReply(reply); (5)
  }
1 CompletionStage#thenCompose allow us to perform an additional async operation, returning a CompletionStage once the current one completes successfully
2 Create a request to add an initial item to the cart
3 Executing the addItem call returns a CompletionStage<Empty> once it succeeds
4 handle allows us to transform the successful completion of addItem with Empty to the response type of this method - NewCartCreated
5 effects().asyncReply() lets us reply once the CompletionStage<NewCartCreated> completes
Scala
src/main/java/com/example/shoppingcart/ShoppingCartActionImpl.scala
  def createPrePopulated(newCart: NewCart): Action.Effect[NewCartCreated] = {
    val cartId = UUID.randomUUID().toString

    val reply: Future[NewCartCreated] =
      for { (1)
        created <- components.shoppingCart.create(CreateCart(cartId)).execute()
        populated <- components.shoppingCart.addItem(AddLineItem(cartId, "e", "eggplant", 1)).execute()
      } yield NewCartCreated(cartId) (2)

    effects.asyncReply(reply) (3)
  }
1 For comprehensions (or directly using flatMap) allow us to compose the individual async steps returning Future
2 Once both steps have completed, create a NewCartCreated leading to a Future[NewCartCreated] coming out of the for-comprehension
3 effect.asyncReply lets us reply once the Future[NewCartCreated] completes

In this sample it is safe to base a subsequent call to the entity on the reply of the previous one, no client will know of the cart id until createPrePopulated replies.

For many other use cases it is important to understand that there is no transaction or consistency boundary outside of the entity, so for a sequence of calls from an action to an entity, the state of the entity could be updated by other calls it receives inbetween.

For example, imagine an action that for a cart id retrieves the state using getState to verify if too many items are already in the cart, and once that has been verified, it adds the item to the cart.

Java
src/main/java/com/example/shoppingcart/ShoppingCartActionImpl.java
  @Override
  public Effect<Empty> unsafeValidation(ShoppingCartApi.AddLineItem addLineItem) {
    // NOTE: This is an example of an anti-pattern, do not copy this
    CompletionStage<ShoppingCartApi.Cart> cartReply = components().shoppingCart().getCart(
            ShoppingCartApi.GetShoppingCart.newBuilder()
                .setCartId(addLineItem.getCartId())
                .build())
        .execute(); (1)

    CompletionStage<Effect<Empty>> effect = cartReply.thenApply(cart -> {
      int totalCount = cart.getItemsList().stream()
          .mapToInt(ShoppingCartApi.LineItem::getQuantity)
          .sum();

      if (totalCount < 10) {
        return effects().error("Max 10 items in a cart");
      } else {
        return effects().forward(components().shoppingCart().addItem(addLineItem)); (2)
      }
    });

    return effects().asyncEffect(effect);
  }
1 Between this call returning
2 And this next call to the same entity, the entity could accept other commands that change the total count of items in the cart
Scala
src/main/java/com/example/shoppingcart/ShoppingCartActionImpl.scala
  override def unsafeValidation(addLineItem: AddLineItem): Action.Effect[Empty] = {
    // NOTE: This is an example of an anti-pattern, do not copy this
    val cartReply = components.shoppingCart.getCart(GetShoppingCart(addLineItem.cartId))
      .execute() (1)

    val effect =
      cartReply.map { cart =>
        val totalCount = cart.items.map(_.quantity).sum

        if (totalCount < 10)
          effects.error("Max 10 items in a cart")
        else
          effects.forward(components.shoppingCart.addItem(addLineItem)) (2)
      }

    effects.asyncEffect(effect)
  }
1 Between this call returning
2 And this next call to the same entity, the entity could accept other commands that changes the total count of items in the cart

The problem with this is that an addItem call directly to the entity happening between the getState action returning and the addItem call from the action would lead to more items in the cart than the allowed limit.

Such validation depending on state can only safely be done handling the command inside of the entity.