Quest for (persistable) Groovy immutability

This is my first post about how to implement DDD concepts with Groovy and deliver them with Grails. My goal is to have real domain logic, without any (or as few as possible) dependencies on surrounding frameworks.

Grails is, at first glance, very pervasive and induces you to mix your business logic with concerns that should be treated separately. On the other hand, the framework is more modular with every release – which means you can disable some parts of it. Under the hood uses some popular libraries (which can be used directly by circumventing the framework).

This article is about implementing immutable Value Objects in Groovy and plugging in the persistence in a Grails application. First part discusses strategies of achieving immutability with Groovy. Second part describes how to persist our Value Object with in a relational database with Hibernate and in document-oriented database (MongoDB).

Create an immutable Value Object class

Value Objects, a building block of DDD should be implemented as immutable. Which options do we have in Groovy?

  • annotate your VO with @Immutable annotation
    • very straightforward, however, no customization options. You get a tuple/map constructor for free, but it sets all your VO properties to null by default. No way to specify mandatory properties and validate them.
  • declare all fields as final (with the default Groovy visibility). Fields will have getters but no setters. Then:
    • use @TupleConstructor
      • no boilerplate code
      • map constructor provided by this annotation is useless with final fields. It calls first the no-arg constructor and then tries to set fields with provided values. As there no setters, you will get a nice groovy.lang.ReadOnlyPropertyException exception.
      • all parameters of the tuple constructor have a default null value. As for @Immutable, no way to enforce mandatoriness.
    • implement custom constructor(s)
      • some boilerplate code, reduced with AST transformations GContracts
      • fully customizable

Example of an immutable Value Object with custom constructor:

Persist the FullName Value Object inside an aggregate

Our sample FullName VO is a part of a larger Person aggregate:

To persist it, I won’t use GORM (at least not directly).

Relational DB, Hibernate

I’m going to use external Hibernate mapping. With this approach my Person aggregate won’t be coupled at the source code level to any persistence configuration.

Hibernate imposes some restrictions on what your objects should provide in order to be persistable. The ORM must be able to instantiate an empty object. It requires a default constructor, although it can be protected.

At this point I have to make a small concession to Hibernate and add a default constructor1 to FullName:

protected FullName() {}

For filling the object with values read from the database, it doesn’t require to have setters. Hibernate is able to ‘reach’ a field through Java reflection (even if it’s final).

I map the VO as Hibernate component (an object mapped to the same table as the aggregate). As there are no setters, I instruct Hibernate to access VO properties directly with access="field":

As the gateway to the persistence I use a repository, an object that can provide the illusion of an in-memory collection (Eric Evans, Domain-Driven Design). As a rule of thumb, there is one repository per aggregate.

I don’t define an explicit interface; I’m going to use a duck type thorough the domain. The implementation is a delivery detail and I put it in the infrastructure layer.

By mapping the Person aggregate as an external Grails entity, it gets all persistence and query methods provided by GORM. However, the rest of domain and infrastructure code doesn’t notice them. The repository (PersonGormRepository) is the only component allowed to use the GORM API on Person.

Result: GORM becomes a plugin to the domain, totally unaware of it. There is only one place where the coupling application – GORM happens – in the implementation of the repository.

The code is pretty straightforward:

MongoDB

Mongo is a document-oriented database perfectly suited for storing objects modelled with composition – one document per each aggregate instance, together with all contained entities and value objects.

There is a MongoDB GORM plugin for Grails. I won’t use it, since I had to pollute my aggregate with mapping configuration (and it should be oblivious to the persistence). I’m going to use GMongo library, a Groovy wrapper over the official MongoDB driver. MongoDB for GORM offers no such thing like external MongoDB mapping.

My repository implementation uses quite a low-level API (compared to GORM). I don’t have to tweak my FullName value object. I can use all-property constructor when filling the VO from the Mongo document.

findById method is quite simple:

save method is more complex comparing with the Hibernate repository. It has to deal with the insert and update semantics.

Additionally, it’s in charge of generating numerical ids (in case of a relational database, we relied on the DB engine). I copied the implementation from the GORM MongoDB plugin source code. It uses a collection with single document containing the recently used ID. When obtaining it I use the Mongo atomic operation findAndModify, increasing the id by one and getting the increased value.

Summary

It is relatively easy to decouple your domain from persistence and make the latter a pluggable detail. You are leaving a well-trodden path of GORM, but the journey outside of it is not scary. You enter other safe routes guarded with decade old patterns and implementation techniques. Sometimes there are few signposts. You have to dig inside blogs and scarce documentation in order to move forward. Sometimes you just wander around and need to make some experiments in order to find the right way.

As reward you will get a cohesive object-oriented domain code that focuses only on one thing: solve problems of your/your customer’s business. Code that shifts data back and forth is strictly separated. Two worlds meet together at the repository frontier. And that’s fine, why should your business deal with database quirks?

Full source code

You find the complete source code in grails-ddd GitHub repo:
* master branch does not implement any persistence (yet)
* MongoDB and Hibernate (with H2 embedded database) repositories are implemented in mongo and hibernate branches, respectively.


  1. before adding a default protected constructor, I tried implementing a CompositeUserType and a custom tuplizer. Unfortunately, both need to create an empty object before filling it from the database result set. 
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One thought on “Quest for (persistable) Groovy immutability

  1. Hi Marcin,
    nice post and something I was looking for earlier.

    1. About the use of @Immutable: I filed a jira request at the groovy guys (see http://jira.codehaus.org/browse/GROOVY-6555) that would let you validate the input parameters. I don’t know if they’re going to implement it, but if it receives enough votes, perhaps they will.

    I also created my own AST transformation which is based on the @Immutable one that allows you to specify you own constructor. In that case you would have the same functionality but it would be up to you to specify how you would create the VO. It’s not that complicated if you can base yourself on the existing ASt transformation for @TupleConstructor & @Immutable

    On the other hand, the solution you showed using final fields isn’t that bad either.

    2. About persistence in hibernate. Using components only will get you so far. It has severe limitations:
    a. you can’t compose components within other component. However in DDD I compose VO’s within other VO’s all the time.
    b. if you want to use a collection of components, hibernate requires you to have only non-null fields. Also you can’t have collections inside your VO when used as a collection inside an aggregate.

    For the 2 above reasons I came to the conclusion I should not use VO’s in my hibernate mapping. Currently I’m designing my aggregates by keeping the state in a separate class like the following:

    class Person{
    State state

    Person(FullName fullName){
    state.fullName = fullName
    }

    @Entity
    static class State{
    @Column
    String fullName_first
    @Column
    String fullName_last

    setFullName(FullName fullName){
    this.fullName_first = fullName.first
    this.fullName_last = fulName.last
    }
    }
    }

    This allows me to keep my aggregate clean, not caring about hibernate persistence. All the nasty hibernate details are in its State companion.

    Greetings
    Jan

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