Domain Driven Architecture

One problem with early versions of opentaps is that the ofbiz framework which we used is not an object-oriented framework. Instead, it is based on a data model which is fundamentally relational, and that data model is accessed via a map-like Java object called GenericValue. Most of the services in the business tier used a GenericDelegator to retrieve GenericValues from the database, performed operations on them, and then stored them back into the database again using the same GenericDelegator.

While this lightweight architecture could do a lot of things, as opentaps grew it became apparent that some of the application could significantly benefit from an object-oriented architecture. A few months ago, we started down this path and thought about how to write more object-oriented code with the ofbiz framework. More recently, after reading about Domain Driven Design and Domain Driven Design Quickly, we realized that what we really needed was not just object-oriented code, but rather a more formal classification of our business logic into domains. This document explains what domain driven architecture is, how we have implemented it, and how it could help you structure your code.

What is Domain Driven Design?

The basic idea behind a domain is to group together all the "domain expertise," or business knowledge, of an application and separate it from the application and its infrastructure. It is a different way of thinking about how to organize large software applications and complements the popular Model View Controller (MVC) architecture, which we also use in opentaps. With the Model View Controller architecture, the application's user interface (View) is separated from its business logic (Model), and a Controller directs requests from the view layer to the relevant business logic found in the model layer. The advantage of doing this is that the same business logic could then be reused elsewhere, either in another page in the view layer or as part of other business logic in the model layer.

MVC, however, doesn't really say how your model should be structured. Should it be object-oriented, or should it all be written in procedural languages or just SQL? Should they reside in separate components and packages, or could you just have one big file, which has all of your business logic? The domain driven design answers this question by separating the model layer ("M") of MVC into an application tier, a domain tier, and an infrastructure tier. The infrastructure tier is used to retrieve and store data. The domain tier is where the business knowledge or expertise is. The application tier is responsible for coordinating the infrastructure and domain tiers to make a useful application. Typically, it would use the infrastructure tier to obtain the data, consult the domain tier to see what should be done, and then use the infrastructure tier again to achieve the results.

For example, let's say that you wanted to assess late charges on all of your customers' outstanding invoices. MVC would tell you that your application should have a screen which shows you a list of outstanding invoices, and when the user says "Assess Late Charges", the controller would pass the users' input parameters to business logic in the model tier to do the dirty work of assessing those late charges.

With a domain driven design, we would look more deeply at what that dirty work actually involved. The application tier would call upon the infrastructure tier to retrieve all the invoices which may get assessed charges. Then, it would present that list of invoices to the domain tier, which has the business expertise to say "Should this invoice get charged?" and if so "How much should this invoice get charged?" The domain tier would then return the late charges for each invoice to the application tier. The application tier would then call on the infrastructure tier again to store the late charges into the database.

Why Domain Driven Design?

Why do we want to do all this?

You will be able to work with opentaps more easily

The first and most obvious benefit of domain driven design is that it helps us organize our application into natural domains, so you don't have to come in contact with all the 800+ tables in opentaps and the over 1,200 services that support them. For example, a domain driven design would allow us to break an application down into a few large domains, such as Customer, Order, and Invoice, and hide all the details within each of those domains from developers who don't need to work with them. Thus, if you are working on the Order domain, you may need to know a little bit about a Customer, such as his home address, shipping addresses, payment methods, but you don't really need to know all the tables used to track the relationship of customer information and their histories.

A related advantage is that it allows us to separate business tier expertise from infrastructure expertise. Thus, if you are working primarily with implementing business processes, you can write code which basically work with the different domains. You'll be happy to leave the database to somebody whose job is working on the infrastructure tier, and who's probably glad not to have to worry about your business processes.

You will be able to extend opentaps more easily

Imagine that you worked in an industry or a company that had customers, but they did some special things for their customers that most other companies don't. With an object-oriented domain driven design, you will be able to extend the existing Customer domain objects from opentaps with new methods specific to your industry or company, while still using everything from the opentaps Customer.

You will be able to use opentaps in novel ways

A potentially more valuable advantage is that domain driven design gets us closer to a plug-and-play application. Imagine again that your application is broken down into the Customer and Order domains, so that the Order domain interacts with customer information only through the Customer domain. What if you wanted to use the opentaps order entry and order management tool with another CRM application, like SugarCRM or SalesForce.com? With good domain separation, it would be a matter of just implementing the Customer domain objects used by the Order domain to call the new CRM application. Alternatively, if you wanted to use opentaps CRM with a legacy order management system, you could implement the Order domain objects used by the Customer domain in opentaps.

Finally, by separating out the domain tier of business knowledge from the infrastructure tier, it also allows us to deploy opentaps on a different infrastructure tier later as well. For example, instead of using the entity engine, you could use Hibernate or even the Google storage API instead. This frees your application from lock-in to a particular framework.

If these advantages sound familiar, they should be. They are in fact the advantages of encapsulation, polymorphism, and inheritance of object oriented programming. Domain driven design is essentially a practice for realizing those advantages in a large-scale application.

Terminology

Now let's look at some of the terminology used by Domain Driven Design, which will serve as our starting point:

• Domain is a body of business expertise. For example, you might have a domain of all business expertise about customers -- who is responsible for them, what prices they should get, how to contact them, etc.
• Entity is an object which has a distinct identity. For example, a Customer entity has a distinct identity with an ID.
• Value Object is an object which has no distinct identity. For example, the color of a product does not have a distinct identity if you think the "blue" of two blue shirts are the same thing.
• Aggregate is a higher level entity which could be viewed from the outside and in turn links you to other entities and value objects. For example, Customer might be an aggregate, so you can view Customer from Orders, Invoices, etc., but a Customer's addresses and phone numbers should only be retrieved by going through Customer first.
• Infrastructure is where the lower level infrastructure of your application is available. For example, it would provide you with the ability to access databases, remote web services, etc.
• Factory is used to create Entities. For example, a Factory might create an Invoice entity (and its related entities and value objects) from an Order entity.
• Repository is used to retrieve, store, and delete Entities from the database. For example, a Repository might help you store the Invoice (and related entities) your Factory created and then bring them back from the database.
• Service is business logic that involves several domain Entities or Aggregates. For example, creating Invoices from Orders is a service.

How Domain Driven Design is Implemented

When we started to implement the domain driven design, we faced a common issue for many developers: How could we need true to the spirit of a domain driven design, but at the same time live with our existing framework and code base?

What we did is first implement a set of foundation classes in org.opentaps.foundation.* to support the Entity, Repository, Inrastructure, Factory, and Service concepts under the ofbiz framework. For each of these, we implemented an interface, and then we implemented a specific version for the ofbiz framework. Thus, we have the following interfaces:

* org.opentaps.foundation.entity.EntityInterface
* org.opentaps.foundation.repository.RepositoryInterface
* org.opentaps.foundation.factory.FactoryInterface
* org.opentaps.foundation.infrastructure.Infrastructure
* org.opentaps.foundation.service.ServiceInterface

Then, for each of these we implemented a version for the ofbiz framework:

* org.opentaps.foundation.entity.ofbiz.Entity
* org.opentaps.foundation.repository.ofbiz.Repository
* org.opentaps.foundation.factory.ofbiz.Factory
* org.opentaps.foundation.infrastructure.ofbiz.Infrastructure
* org.opentaps.foundation.service.Service

Each of these is designed to map legacy code from ofbiz and ofbiz-based portions of opentaps into the concepts of the domain driven design:

Entity

The Entity object is a Java class equivalent for ofbiz GenericValues. There is a package of entity classes which are automatically generated for each entity defined by the entitymodel XMLs, including all the original ofbiz applications, the opentaps applications, and any of your own custom applications which extend existing entities or add new ones.

You can use than base entities as they are, converting back and forth between the GenericValue or Java Maps, or you can extend the base entities with additional methods that encapsulate business logic. For example, there is an Invoice base entity class with fields such as invoiceId, invoiceDate, referenceNumber and accessor method such as getInvoiceId(), setInvoiceId(String invoiceId), etc. Then, there is an Invoice class in the billing domain which extends the base Invoice class and has methods such as getInvoiceTotal(), getAppliedAmount(), etc.

Infrastructure

The Infrastructure class is a global directory of infrastructure resources, such as database access connections, located across all the frameworks and platforms of opentaps. Initially, it can be used to obtain the delegator and dispatcher of the ofbiz framework, but as more applications are added to opentaps, it will also return the infrastructure their frameworks require, including JDBC connections or hibernate session factories. These infrastructure resources are passed to the Repository and the Factory classes so that they can interact with the database and external Web services.

Part of the infrastructure package is the User class, which constructs a cross-framework user object that can be used by all the different applications and their frameworks. For example, you can create a User from the ofbiz UserLogin GenericValue, or you will also be able to create Users from legacy or external applications, kerberos tokens, or LDAP and returned their ofbizUserLogin. Note that this User class is not an extension of the UserLogin base entity class. The UserLogin class is designed to model user logins as data, whereas the User classes designed to pass user authentication between applications.

Factory

The Factory class is designed to create Entity objects based on other parameters. For example, you might want to create an Invoice Entity based on customer and invoice terms, or you might want to create an Invoice Entity based on an existing Order, taking its customer and list of items as a starting point. The Factory class is meant to be extended to create Factories for the different domain aggregates such as Invoice, Customer, Order, etc.

Under the ofbiz framework, the Factory often needs to use legacy services. Note that this is an interesting issue: in a classic domain driven design, the Factory would create the Entity as pure objects, and then the Repository would be responsible for storing them to the database. Such separation of roles is not present in the ofbiz framework, however, where virtually every service would access the database, create new data, and then store it back into the database. Thus, to reuse these existing services, our Factories sometimes end up storing the objects to the database first by calling an ofbiz service, then retrieve them again and return them as Entity objects.

Repository

The Repository is designed to help retrieve and store Entities and is meant to be extended for the major Entities, so the foundation Repository should be extended to CustomerRepository, InvoiceRepository, and OrderRepository to support Customer, Invoice, and Order Entities.

For the ofbiz framework, the preferred way to retrieve and store data could either use the service dispatcher or the delegator. Therefore, the Repository could be instantiated either with the delegator alone or with the delegator, dispatcher, and user login. The Repository should offer a set of methods for retrieving or persisting its related Entity and then either use the delegator or call the service to do it.

As a good design pattern, a repository should fetch objects directly and not associations. The modeling details of a particular implementation should be hidden from the domain. As an example, the repository should expose methods such as getPostalAddress() and getPhoneNumber() while the association tables PartyContactMech and InvoiceContactMech are dealt with within the implementation of these methods.

Repositories or Factories?

Since almost all legacy ofbiz services store values into the database, it may not be clear at first which one you should use. Remember that Factories are intended to create new Entity objects, while Repositories are intended to retrieve and store them. Therefore, we would follow the following rules for Factories and Repositories:

1. Use Factories for create and Repositories for get and store
2. Always return the domain's Entity object from your Factory, so it looks like a real object Factory
3. Factories will almost always use the service dispatcher, whereas Repositories will usually use the dispatcher but may sometimes use the delegator

Services

Services are designed to encapsulate business logic that span multiple Entities, such creating Invoice from an Order. With the opentaps Service foundation, you can create your services as plain Java objects (POJOs), similar to the Spring framework or JBoss Seam. When your services object is instantiated, it will be created with Infrastructure, a User object, and the locale. From these objects, you can obtain the ofbiz framework's delegator, dispatcher, and UserLogin GenericValue. The Service foundation class also can load the domains directory (see below) for you, and this is done automatically if you use the POJO Service Engine. Parameters for your service are passed into your service via set methods, the execution of your services via a void method, errors are propagated by exceptions, and the results of your service are passed back via get methods. This is in contrast to the ofbiz framework, where services are defined as static Java methods (don't ever write one in minilang!), the parameters are passed in a map, the results and any error messages are returned in a map.

Because these services are Java objects, we follow the convention to group services with similar parameters together into one class. For example, all services which create Invoices from Order should be in one class, and all services which create Invoices from Shipment should be in another. This allows them to share set and get methods without having one service class which is too long.

What domain should a service be a part of? By convention, we recommend that the service is part of the domain of its output, so all services which create Invoices should be part of the Invoice domain, whether the invoices are created from orders, shipments, or recurring agreements.

Exceptions

All exceptions should be implemented as subclasses of org.opentaps.foundation.exceptions.FoundationException, which is a base class that in its turn extends the ofbiz GeneralException class. EntityException, ServiceException, RepositoryException, InfrastructureException, and FactoryException all extend the base FoundationException class. You can in turn implement specific exceptions which subclass these general exceptions.

The FoundationException class can be instantiated with an exception message or another exception. It allows you to set a locale, a message UI label, and a Map context for the message UI label:

  throw new ServiceException("Service failed");
  throw new ServiceException("OpentapsErorLabel", UtilMisc.toMap("orderId", orderId), locale);

The FoundationException class's getMessage() method has been overridden to expand the UI label with the context map and the locale.

It also allows you to set your error message in one level in your code and then localize it at a higher level once the locale is known. For example, in your repository, you can throw a not found exception without setting the locale, and then the POJO Service Engine will catch that exception and automatically localize it:

  // in repository:
  throw new EntityNotFoundException("OpentapsErrorLabel", UtilMisc.toMap("orderId", orderId));

  // POJOJavaEngine 
  ... catch (Throwable t) {
      if (t instanceof FoundationException) {
          (FoundationException) t.setLocale(locale);
          return ServiceUtil.returnError(t.getMessage());
  }

Finally, the FoundationException class allows you to set whether your exception requires a global rollback or not. By default, exceptions do require rollback, but you can turn it off with setRequiresRollback:

   ServiceException ex = new ServiceException(...);
   ex.setRequiresRollback(false);

Setting requires rollback to false will cause the POJO Service Engine to use ServiceUtil.returnFailure instead of ServiceUtil.returnError. This will cause the service to return an error message, but it will not cause the other services in a chain to abort. You can also use the requires rollback flag for your own exception management.

Specifications

Most developers know that they should not use literal strings in their code. For example, we all feel that it would be bad to write code like this:

  if (order.getStatusId().equals("ORDER_APPROVED")) {
     // .... }

Our first reaction is always to define a file of string literals, and then reuse the predefined literals:

  public static final String ORDER_APPROVED = "ORDER_APPROVED";
  //...
  if (order.getStatusId().equals(ORDER_APPROVED)) {
     // ... }

This is nice: now, the compiler will be able to check if our status IDs are correct, and if somebody decides to change the ID code, all we have to do is change in one place.

But inevitably, we run into other problems with this kind of code. Somebody might decide that instead of having one state called approved, we want to have several states, like: approved, in production, pending shipment, etc. that have the same meaning as being approved. Later, somebody else might want to have more complicated logic: an order might considered approved if it is either in the approved state or does not contain certain hazardous materials and is in the in production or pending shipment state, for example.

Now we'll have to change all our code again. A developer's job is never done!

The main problem we all face is one of logic: most ERP software, like opentaps, operate on a set of data in one logical state (i.e., orders that are approved) and transform them into other data and other logical states (i.e., invoices that are created.) The problem is that these logical states are usually denoted as strings in a database field, but they are sometimes much more subtle and complex in real life. Thus, we developers are lulled into thinking that all logical states can be modeled as literal strings. This usually works, but in those 10% of the cases when it's not the case, our code is usually not well structured enough to deal with it.

The solution is to avoid using string literals. Instead, separate the logical checking code of a domain into a separate class, so that it can be modified as needed. This is the role for Specifications: defining literal values and logical states.

In practice, we recommend having one Specification class for each domain. For example, for the order domain, there should be an OrderSpecification class and a corresponding interface. Because in practice specifications are usually closely related to the way data is modeled in your database, we have kept it linked through the Repository class of each domain. Thus, to get the OrderSpecification, use its repository:

 OrderRepositoryInterface orderRepository = orderDomain.getOrderRepository();
 OrderSpecificationInterface orderSpecification = orderRepository.getOrderSpecification();

We have also found the following best practices to be helpful when implementing specifications:

1. If you need to check whether a condition is true or not, implement Boolean methods in the specifications, which your domain objects can use, rather than using literal strings directly in the domain objects. For example, instead of:

 if (orderSpecification.ORDER_APPROVED.equals(order.getOrderStatusId())) 

use:

 if (orderSpecification.isApproved(order))

1. If you need to get certain values from your specification for other purposes, get lists of values instead of string literals. For example, if you need to get OrderRole objects related to an order in the role of customer, instead of implementing a literal like orderSpecification.BILL_TO_CUSTOMER_ROLE, implement a method which returns a list of potential roles: orderSpecification.billToCustomerRoleTypeIds. You can then use the SQL IN operator to retrieve parties in all possible bill to customer roles and thus not be constrained to use only one potential role.

In both cases, by abstracting logical states and by making type codes more general-purpose, your code will be able to handle changing requirements much more easily.

An Example Using Domains

Now let's consider an example. Suppose we want to create an invoice for all the order items which are not physical products and which have been marked as performed (See Fulfilling Orders for Services.) Using the ofbiz framework, we would first define a service:

    <service name="opentaps.invoiceNonPhysicalOrderItems" engine="java"
        location="com.opensourcestrategies.financials.invoice.InvoiceServices" invoke="invoiceNonPhysicalOrderItems">
        <description>Creates an invoice from the non-physical items on the order.  
        It will invoice from the status in the orderItemStatusId,
        or if it is not supplied, default to ITEM_PERFORMED.  
        After the invoice is created, it will attempt to change the items' status
        to ITEM_COMPLETE.</description>
        <attribute name="orderId" type="String" mode="IN" optional="false"/>
        <attribute name="orderItemStatusId" type="String" mode="IN" optional="true"/>
        <attribute name="invoiceId" type="String" mode="OUT" optional="false"/>
    </service>

Then, we would create a static Java method for the service:

    public static Map invoiceNonPhysicalOrderItems(DispatchContext dctx, Map context) {
        LocalDispatcher dispatcher = dctx.getDispatcher();
        GenericValue userLogin = (GenericValue) context.get("userLogin");
        Locale locale = (Locale) context.get("locale");

        String orderId = (String) context.get("orderId");
        String orderItemStatusId = (String) context.get("orderItemStatusId");

        try {
            // validate that the order actually exists and get list of non-physical
            GenericValue order = delegator.findByPrimaryKey("OrderHeader", UtilMisc.toMap("orderId", orderId));
            if (UtilValidate.isEmpty(order)) {
                return ServiceUtil.returnError("Order [" + orderId + "] not found");
            }

            // set default item status
            if (UtilValidate.isEmpty(orderItemStatusId)) {
                Debug.logInfo("No status specified when invoicing non-physical items on order [" 
                              + orderId + "], using ITEM_PERFORMED", module);
                orderItemStatusId = "ITEM_PERFORMED";
            }

            // get the non-physical items which have been performed
            List<GenericValue> orderItems = order.getRelatedByAnd("OrderItem", 
                                                  UtilMisc.toMap("statusId", orderItemStatusId));
            List<GenericValue> itemsToInvoice = new ArrayList();
            for (GenericValue orderItem:orderItems) {
                if (!UtilOrder.isItemPhysical(orderItem)) {
                    itemsToInvoice.add(orderItem);
                }
            }

            // check if there are items to invoice
            if (UtilValidate.isEmpty(itemsToInvoice)) {
                return UtilMessage.createAndLogServiceError(
                       "OpentapsError_PerformedItemsToInvoiceNotFound", locale, module );
            }

            // create a new invoice for the order items
            Map tmpResult = dispatcher.runSync("createInvoiceForOrder", 
                            UtilMisc.toMap("orderId", orderId, "billItems", 
                                           itemsToInvoice, "userLogin", userLogin), 
                            7200, false);  // no new transaction
            if (ServiceUtil.isError(tmpResult)) {
                return tmpResult;
            }

            // change the status of the order items to COMPLETED
            for (GenericValue orderItem:itemsToInvoice) {
                tmpResult = dispatcher.runSync("changeOrderItemStatus", 
                             UtilMisc.toMap("orderId", orderItem.getString("orderId"), 
                             "orderItemSeqId", orderItem.getString("orderItemSeqId"), 
                             "statusId", "ITEM_COMPLETED", "userLogin", userLogin));
            
            // return invoiceId of new invoice created
            String invoiceId = (String) tmpResult.get("invoiceId");

            tmpResult = ServiceUtil.returnSuccess();
            tmpResult.put("invoiceId", invoiceId);
            return tmpResult;
        } catch (GeneralException e) {
            return UtilMessage.createAndLogServiceError(e, module);
        }
    }

So what's there not to love about this code?

1. It is closely tied to the database. Even though there's not a single line of SQL here, you have to know that orders are stored in "OrderHeader", and that it is related to "OrderItem", and that there are fields like statusId. You also have to use the string literals for status, like ITEM_COMPLETED, ITEM_PERFORMED, etc.
2. This method depends on things spread out in different parts of the application, like the UtilOrder class and the createInvoiceForOrder and changeOrderItemStatus services.
3. This code is completely dependent on the ofbiz framework's GenericValue, entity engine delegator, and local dispatcher.
4. Static Java methods like this, while easier to work with than minilang, do not enjoy the benefits of real object-oriented programming.

In other words, for somebody to write this code, they have to know a lot about the framework, the data model, and the application tier.

Here's a re-write of the everything inside the try ... catch block using the domain driven design:

       // validate that the order actually exists and get list of non-physical
       OrderRepository orderRepository = new OrderRepository(new Infrastructure(dispatcher), userLogin));
       Order order = orderRepository.getOrderById(orderId);
       if (UtilValidate.isEmpty(orderItemStatusId)) {
           Debug.logInfo("No status specified when invoicing non-physical items on order [" 
                         + orderId + "], using [" + 
                         OrderSpecification.ITEM_STATUS_PERFORMED + "]", module);
           orderItemStatusId = OrderSpecification.ITEM_STATUS_PERFORMED;
       }
       List<GenericValue> itemsToInvoice = order.getNonPhysicalItemsForStatus(orderItemStatusId);

       // check if there are items to invoice
       if (UtilValidate.isEmpty(itemsToInvoice)) {
           return UtilMessage.createAndLogServiceError(
                              "OpentapsError_PerformedItemsToInvoiceNotFound", locale, module );
       }

       // create a new invoice for the order items
       Map tmpResult = dispatcher.runSync("createInvoiceForOrder", 
                                          UtilMisc.toMap("orderId", orderId, "billItems", itemsToInvoice, 
                                                         "userLogin", userLogin), 
                                          7200, false);  // no new transaction
       if (ServiceUtil.isError(tmpResult)) {
           return tmpResult;
       }

       // change the status of the order items to COMPLETED
       order.setItemsStatus(itemsToInvoice, OrderSpecification.ITEM_STATUS_COMPLETED);
            
       // return invoiceId of new invoice created
       String invoiceId = (String) tmpResult.get("invoiceId");

       tmpResult = ServiceUtil.returnSuccess();
       tmpResult.put("invoiceId", invoiceId);
       return tmpResult;

This code is the programming equivalent of the missing link: it has many features of the old code, but a few important differences as well. What we have done is push everything related to orders to the Order Entity object, its OrderRepository, and OrderSpecification. We don't care where the order came from, how we can get the items of an order, or even how the status codes of an order are defined any more, because those are all responsibilities of the Order domain objects. (Even the validation that an order was obtained is handled by the OrderRepository, which will throw a RepositoryException if nothing is found from orderId.) We are also no longer tied to the delegator, although the Order domain may itself require the delegator. (The casting of itemsToInvoice to GenericValue is vestigal -- remember that our Entity object extends GenericValue, and a specific Java object may in turn extend Entity.)

We are, however, still tied to the createInvoiceForOrder service and the ofbiz service engine. That will have to wait until the next evolutionary step (which happened the next day). Using the Service class from above, we can implement a POJO version of this service:

public class OrderInvoicingService extends Service {

    private static final String module = OrderInvoicingService.class.getName();

    protected String orderId = null;
    protected String invoiceId = null;
    // by default, non-physical order items in this state will be invoiced
    protected String statusIdForNonPhysicalItemsToInvoice = OrderSpecification.ITEM_STATUS_PERFORMED;

    public OrderInvoicingService(Infrastructure infrastructure, User user, Locale locale) 
           throws ServiceException {
        super(infrastructure, user, locale);
    }

    public void setOrderId(String orderId) {
        this.orderId = orderId;
    }

    public String getInvoiceId() {
        return this.invoiceId;
    }

    /**
     * Set the status id of non-physical order items to be invoiced by invoiceNonPhysicalOrderItems, or
     * OrderSpecification.ITEM_STATUS_PERFORMED will be used
     * @param statusId
     */
    public void setStatusIdForNonPhysicalItemsToInvoice(String statusId) {
        if (statusId != null) {
            statusIdForNonPhysicalItemsToInvoice = statusId;
        }
    }

    public void invoiceNonPhysicalOrderItems() throws ServiceException {
        try {
            // validate that the order actually exists and get list of non-physical
            OrderRepository orderRepository = new OrderRepository(new Infrastructure(dispatcher), user);
            Order order = orderRepository.getOrderById(orderId);
            List<GenericValue> itemsToInvoice = order.getNonPhysicalItemsForStatus(statusIdForNonPhysicalItemsToInvoice);

            // check if there are items to invoice
            if (UtilValidate.isEmpty(itemsToInvoice)) {
                // TODO: Fix localization of errors
                throw new ServiceException("OpentapsError_PerformedItemsToInvoiceNotFound");
            }

            // create a new invoice for the order items
            // because of the way createInvoiceForOrder is written (665 lines of code!) 
            // we'd have to do some re-factoring before we can add the items to an existing invoice
            Map tmpResult = getDispatcher().runSync("createInvoiceForOrder", 
                                 UtilMisc.toMap("orderId", orderId, "billItems", itemsToInvoice, 
                                                 "userLogin", user), 
                                 7200, false);  // no new transaction
            if (ServiceUtil.isError(tmpResult)) {
                throw new ServiceException(ServiceUtil.getErrorMessage(tmpResult));
            }

            // change the status of the order items to COMPLETED
            order.setItemsStatus(itemsToInvoice, OrderSpecification.ITEM_STATUS_COMPLETED);

            // set the invoiceId of new invoice created
            this.invoiceId = (String) tmpResult.get("invoiceId");
        } catch (GeneralException ex) {
            throw new ServiceException(ex) ;
        }
    }

}

Then, the original Java static method simply has to pass the parameters to it, execute the method in the OrderInvoicingService, get its result, and pass it back. Here's the content of that try ... catch block again:

            OrderInvoicingService invoicingService = new OrderInvoicingService(
                                                          new Infrastructure(dispatcher), 
                                                          new User(userLogin), 
                                                          locale);
            invoicingService.setOrderId(orderId);
            invoicingService.setStatusIdForNonPhysicalItemsToInvoice(orderItemStatusId);
            invoicingService.invoiceNonPhysicalOrderItems();

            Map tmpResult = ServiceUtil.returnSuccess();
            tmpResult.put("invoiceId", invoicingService.getInvoiceId());

Congratulations! Now your business logic is a POJO. You can add annotations, use dependency injection with it, and use it with other Java frameworks now. (Is this how that missing link felt, seeing all those primordial forests for the first time?)

Your service is using a legacy ofbiz service "createInvoiceForOrder" still through its getDispatcher() method, but that's not so bad. If you want to use an ofbiz service, you should use its dispatcher. In this example, however, you still had to write a static Java method for your service, because you are using the ofbiz static Java method service engine. With the POJO Service Engine, however, that is no longer necessary, and you can remove the code in InvoiceServices.java and call OrderInvoicingServices.invoiceNonPhysicalOrderItems() directly.

A final round of enhancements used the base entities instead of GenericValues and the domains directory to load the order domain and the order repository, so this order invoicing service could function independent of the order management system. See POJO Service Engine for the code sample.

Putting It All Together

Now, let's see how we could put all this together to create applications around the domain driven architecture. As we discussed before, related data Entities could be grouped together as an Aggregate, which will have related Factories, Repositories, and Services. For example, an aggregate of concepts related to invoicing might include the Invoice, InvoiceItem, InvoiceStatus, InvoiceContactMech, InvoiceAttribute entities as well as invoice factories, invoice repositories, and several invoicing services:

Several of these Aggregates may then form a Domain of related business knowledge. For example, the Billing domain may consist of Invoice and Payment aggregates and their related factories, repositories, and services. This Domain would interact with other domains, such as Organization, Ledger, Party, and Product:

An application, such as opentaps Financials application, could be built from several relatively independent domains:

To keep them relatively independent of each other, an interface should be declared for each domain, and they should return interfaces to the repositories, factories, and services. Interfaces are not necessary for the entities, however, since entities represent a data model, which must be implemented in the same way for all opentaps applications. For example, Invoice will always have to have an invoice ID field, and the getInvoiceId() method should always return the value of that field. If different underlying invoicing systems use different types of invoice IDs, it is the responsibility of the invoice repository to parse that and store it in the invoice ID field of Invoice. The Invoice entity does not need to be changed. Here is an example of the interface for the billing domain, defined in org.opentaps.domain.billing.BillingDomainInterface:

import org.opentaps.domain.billing.invoice.InvoiceRepositoryInterface;

public interface BillingDomainInterface {

    public InvoiceRepositoryInterface getInvoiceRepository();
}

To make life a little easier, an abstract Domain class is available to encapsulate Infrastructure and User, so you don't have to set the Infrastructure and User after getting each repository, factory, or service. Instead, you can associate the Infrastructure and User with an actual implementation of a domain, and it will automatically populated Infrastructure and User for you. Here is its corresponding implementation in opentaps financials, org.opentaps.financials.domain.billing:

public class BillingDomain extends Domain implements BillingDomainInterface {

    public InvoiceRepository getInvoiceRepository() {
        InvoiceRepository repository = new InvoiceRepository();
        repository.setInfrastructure(getInfrastructure());
        repository.setUser(getUser());
        return repository;
    }

}

There should only be one directory of domains at any one time, so that all the opentaps applications use the same domains. In opentaps, this domain directory is defined in the DomainsDirectory class, and the actual domains are defined in hot-deploy/opentaps-common/config/domains-directory.xml:

<beans>

    <bean id="opentapsBillingDomain" class="org.opentaps.financials.domain.billing.BillingDomain"/>

    <bean id="domainsDirectory" class="org.opentaps.domain.DomainsDirectory">
        <property name="billingDomain"><ref bean="opentapsBillingDomain"/></property>
    </bean>
    
</beans>

Note that domains are declared explicitly in the DomainsDirectory, rather than as a Map. To add a new domain, you must modify the DomainsDirectory class to add a new member plus accessor (set/get) methods. To change your domains, you can just modify this xml file. For example:

<beans>

    <bean id="myBillingDomain" class="com.mycompany.domain.billing.BillingDomain"/>
    
    <bean id="domainsDirectory" class="org.opentaps.domain.DomainsDirectory">
        <property name="billingDomain"><ref bean="myBillingDomain"/></property>
    </bean>
    
</beans>

When you restart opentaps, the new domains will be loaded.

To load your domains, use DomainsLoader, which can be instantiated with Infrastructure and User:

// get the domain
DomainsLoader dl = new DomainsLoader(new Infrastructure(dispatcher), new User(admin));
DomainsDirectory domains = dl.loadDomainsDirectory();
BillingDomainInterface billingDomain = domains.getBillingDomain();

// now we can use it
InvoiceRepositoryInterface invoiceRepository = billingDomain.getInvoiceRepository();
Invoice invoice = invoiceRepository.getInvoiceById("10000");

Domain Driven Architecture and opentaps

To see how you can use the domain driven architecture in opentaps, see Working with the Domain Driven Architecture