Friday, October 29, 2010

Using Scala implicits to limit client's view on the domain model

This is common problem in the Java Enterprise applications: how do we limit client's view on the domain model? I mean, we use JPA Entity Beans to model our domain, then we use it within a transaction in the Service layer to do some stuff and then we want to provide the results to the Presentation layer where is no access to the transaction anymore. The problem is that actually we do not give access to the full domain model, but only to a limited part of it. So, generally speaking, you cannot get all employees of a department if there was no special measures taken about it in the Service layer (or any other layer beneath).

Wouldn't it be nice if compiler would perform such checks? To me it sounds crazy impossible, while certainly desirable feature and I still don't know how to implement it in Java. Commonly used DTO approach quickly leads to several DTOs per entity (like shallow and deep copies + all variations per related entities and their entities and so forth). Code quickly becomes verbose, clumsy and filled with plumbing. In the Java world there is no escape from this looser choice: either DTO rabbit farm or careful programming of the (Web?) GUI part.

I think I've found a way to let compiler solve this problem in Scala.

Let's start with our domain model:

class Person(val id:Long, val email:String,
protected[domain] var dep:Department = null,
protected[domain] var address:Address = null)
class Department(val code:String, val name:String)
class Address(val street:String)

As you can see, entity relationships are hidden from the code outside the domain package. To provide such access let's define following trait:

trait PersonDepartment {
implicit def toPersonDepartment(p:Person) = new {
def getDepartment = p.dep
def setDepartment(d:Department) = p.dep = d

The code for PersonAddress trait is essentially the same, so I omit it here. And now is the tricky part: how do we import this implicit? Scala doesn't allow to import members of traits, only members of object (and packages, ok). Well, how about this code (like Service layer function):

def doThis = {
val p = new Person(1, "bb")
val dataView = new PersonDepartment {}
import dataView._
p.setDepartment(new Department("aa", "Whatever"))

It compiles! There is just one step left: provide the client with our dataView object:

(p, dataView)

We return our Person object together with an object who's implicits provide access to protected members of the domain object. The client code will look like this:

val (p, dataView) = doThis
import dataView._
val dep = p.getDepartment

And it compiles as well! But this one doesn't:

val a = p.getAddress

So, type checking works, compiler does here exactly what we want. Just for fun, let's mixin PersonAddress in the dataView in our function:

def doThis = {
val dataView = new PersonDepartment with PersonAddress {}

And our p.getAddress on the client side compiles now! Notice that we didn't changed the client code, only the function in the Service layer and still we can get compilation error if the client tries to access parts of the domain model we do not want him to. So, we found a way to manange the client's view of our domain model and we can do it from the Service layer. We can write another function and define there another subset of relationships and tell our client about limitations in type-safe way. All this will be enforced then by Scala compiler. Traits don't have to provide access to the same entity class by the way. Like we can define DepartmentPerson trait and mix it in the dataView together with PersonAddress, all in the same object, there are no obstacles for that from the type system.

This approach can also be used to provide deep-enough copies of our domain objects that will automatically be limited to the selected relationships. I think that relflection will be needed anyway and it rises another questions, but it's possible to write such code.

Hope you enjoyed it and

May the sources be with you! :)

Thursday, October 28, 2010

Transactional Monad for Scala

Scala is a nice programming language that allows both imperative and functional programming style. There is however no standard implementation of monads which would allow to work with outside world (like databases, i/o) in a pure functional way. Since there is no such thing, let's build it! Let's take a web shop as an example and step by step build our monad or whatever we might get along the way. Let's just stay practical and build some useful stuff.

So, how our database monad would look like? Monads are type constructors, so we start with a type constructor:

trait Transactional[A]

and this is how we would like to use it:

def findByPK(id:Long):Transactional[Product]

This type signature would indicate that we have a function that returns a Product object as a result of the database transaction. In JPA world it can be managed instance of Product entity. It's already useful on it's own, because now we can distinguish managed entities from not managed, which is a good thing. The code which requires managed entities will always use Transactional and the code which can live with detached ones will use objects directly.

How would we like to use our monad? I mean, encapsulating result of a transaction is good, but what can you do with it? I would say, anything you like. Literally:

trait Transactional[A] {
def map[B](f:A=>B):Transactional[B]

Look at List or Option, they all have this method implemented, even Google wouldn't be the same without such method as we know now. Our function thus could take a Product as a parameter and produce ... , whatever, say HTML:

def render(p:Product):String

We could write then:

def renderProduct(id:Long) = findByPK(id) map render

Neat and nice. Very simple, and we've got again something useful. We are still stuck with Transactional[String] but it's just because the end result of all our functions depends on the database transaction, so there is nothing to worry about yet, we are still on track.

It is good that we can combine pure functions with our Transactional thing, but how about other functions that also return Transactional? Like we've got the shopping basket rendered in the same way and want to combine it with the product HTML? In List and Option map() function has a brother which does exactly what we want:

trait Transactional[A] {
def map[B](f:A=>B):Transactional[B]
def flatMap[B](f:A=>Transactional[B]):Transactional[B]

So, now we can suck following function in our Transactional:

def renderShoppingBasket:Transactional[String]

and it will look like:

renderProduct(id) flatMap {_ => renderShoppingBasket}

Cool, but what happend to our product HTML? How do we plug functions which take more than 1 parameter? Say, we have this function to produce the final output:

def renderProductPage(productHTML:String, shoppingBasket:String):String

How do we plug such things in? This is where Scala for-comprehension come in place. The code looks like this:

for {
product <- findByPK(id)
productHTML <- renderProduct(product)
basketHTML <- renderShoppingBasket
} yield renderProductPage(productHTML, basketHTML)

Do not know about you, but I like this code. All this left arrows are actually translated to flatMap and map calls. To support 'if' we have to add also filter(p:A=>Boolean):Transactional[A]. Not big deal, but handy. We can read this arrows as extraction of object (managed entities?) out of Transactional. Keep in mind that the result will (and should!) always be another Transactional.

Our page is ready, but we still have Transactional[String], not the real String. How do we escape from it? We could simply add a get method which gives us this String, but then it will look more like a JavaBean, not a monad. Monad's are different. They abstract side-effects from pure functions. So, to get out from the Transactional, there must be a ... transaction which has to be commited to give us the outcome. Ok, here you are:

trait Transactional[A] {
def map[B](f:A=>B):Transactional[B]
def flatMap[B](f:A=>Transactional[B]):Transactional[B]
def commit:A // commit transaction and give us outcome

What about implementation? Let's start from the beginning, how would our findByPK function look like? How about this (JPA style) :

def findByPK(id:Long) = transactional {em:EntityManager => em.find(id, Product.class) }

I cannot think about simpler function. Let's make it compilable with this implementation:

object Transactional {
def transactional[A](body: EntityManager => A) = new Transactional {
def atomic = body

If you add import Transactional._ in your Scala code, the example above will compile. But wait, we just made very important design decision as to me. We do not contain the transaction outcome anymore, but a function that takes EntityManager as an argument and gives us the result of some operation on it. So, one more time, just because this is important: Transactional[A] contains an EntityManager => A function. It has following consequences:
  • We do not need an EntityManager instance when we construct Transactional object. Good, no dependency injection, no ThreadLocal anymore
  • Our Transactional becomes lazy. It doesn't do anything until we really need it. Cool, will see how it fly.
  • Whole transaction start/commit/rollback can be placed together, in 1 function. Great! no transaction/connection leaks!

Finally we can implement map and flatMap buddies to do our magic:

trait Transactional[A] {
def atomic:EntityManager => A
def map[B](f: A => B):Transactional[B] = transactional[B](f compose atomic)
def flatMap[B](f: A => Transactional[B]):Transactional[B] =
transactional {r => f(atomic(r)).atomic(r)}

Back to the commit function now. It doesn't really only commit our transaction, but executes it as a whole. Let's rename it to exec. It needs an instance of EntityManager to get the job done. Let's inject it as implicit parameter. Might get life easier. I omit implementation here, it is quite trivial and not in functional style. Yes, all side-effects happen there, so no need use functional style. At least, I cannot think of anything functionally-stylish and useful at the moment.

Well, it seems like we are done with our Transactional for now. If you ask me if we've build a real Monad the answer will be "not yet". We need to factor EntityManager out as a type parameter and add nonTransactional constructor that accepts 1 parameter of any type.

Notice that we didn't used any dependency injection, interceptors, ThreadLocals, there is no place for it anymore. In return we've got a way to combine our transactional code, distill pure functions from it and still be expressive and practical.