Services

Services

Sometimes, it just isn't a thing.

In some cases, the clearest and most pragmatic design includes operations that do not conceptually belong to any object. Rather than force the issue, we can follow the natural contours of the problem space and include SERVICES explicitly in the model.

There are important domain operations that can't find a natural home in an ENTITY or VALUE OBJECT. Some of these are intrinsically activities or actions, not things, but since our modeling paradigm is objects, we try to fit them into objects anyway.

Now, the more common mistake is to give up too easily on fitting the behavior into an appropriate object, gradually slipping toward procedural programming. But when we force an operation into an object that doesn't fit the object's definition, the object loses its conceptual clarity and becomes hard to understand or refactor. Complex operations can easily swamp a simple object, obscuring its role. And because these operations often draw together many domain objects, coordinating them and putting them into action, the added responsibility will create dependencies on all those objects, tangling concepts that could be understood independently.

Sometimes services masquerade as model objects, appearing as objects with no meaning beyond doing some operation. These "doers" end up with names ending in "Manager" and the like. They have no state of their own nor any meaning in the domain beyond the operation they host. Still, at least this solution gives these distinct behaviors a home without messing up a real model object.

Some concepts from the domain aren't natural to model as objects. Forcing the required domain functionality to be the responsibility of an ENTITY or VALUE either distorts the definition of a model-based object or adds meaningless artificial objects.

A SERVICE is an operation offered as an interface that stands alone in the model, without encapsulating state, as ENTITIES and VALUE OBJECTS do. SERVICES are a common pattern in technical frameworks, but they can also apply in the domain layer.

The name service emphasizes the relationship with other objects. Unlike ENTITIES and VALUE OBJECTS, it is defined purely in terms of what it can do for a client. A SERVICE tends to be named for an activity, rather than an entity—a verb rather than a noun. A SERVICE can still have an abstract, intentional definition; it just has a different flavor than the definition of an object. A SERVICE should still have a defined responsibility, and that responsibility and the interface fulfilling it should be defined as part of the domain model. Operation names should come from the UBIQUITOUS LANGUAGE or be introduced into it. Parameters and results should be domain objects.

SERVICES should be used judiciously and not allowed to strip the ENTITIES and VALUE OBJECTS of all their behavior. But when an operation is actually an important domain concept, a SERVICE forms a natural part of a MODEL-DRIVEN DESIGN. Declared in the model as a SERVICE, rather than as a phony object that doesn't actually represent anything, the standalone operation will not mislead anyone.

A good SERVICE has three characteristics.

Statelessness here means that any client can use any instance of a particular SERVICE without regard to the instance's individual history. The execution of a SERVICE will use information that is accessible globally, and may even change that global information (that is, it may have side effects). But the SERVICE does not hold state of its own that affects its own behavior, as most domain objects do.

When a significant process or transformation in the domain is not a natural responsibility of an ENTITY or VALUE OBJECT, add an operation to the model as a standalone interface declared as a SERVICE. Define the interface in terms of the language of the model and make sure the operation name is part of the UBIQUITOUS LANGUAGE. Make the SERVICE stateless.

SERVICES and the Isolated Domain Layer

This pattern is focused on those SERVICES that have an important meaning in the domain in their own right, but of course SERVICES are not used only in the domain layer. It takes care to distinguish SERVICES that belong to the domain layer from those of other layers, and to factor responsibilities to keep that distinction sharp.

Most SERVICES discussed in the literature are purely technical and belong in the infrastructure layer. Domain and application SERVICES collaborate with these infrastructure SERVICES. For example, a bank might have an application that sends an e-mail to a customer when an account balance falls below a specific threshold. The interface that encapsulates the e-mail system, and perhaps alternate means of notification, is a SERVICE in the infrastructure layer.

It can be harder to distinguish application SERVICES from domain SERVICES. The application layer is responsible for ordering the notification. The domain layer is responsible for determining if a threshold was met—though this task probably does not call for a SERVICE, because it would fit the responsibility of an "account" object. That banking application could be responsible for funds transfers. If a SERVICE were devised to make appropriate debits and credits for a funds transfer, that capability would belong in the domain layer. Funds transfer has a meaning in the banking domain language, and it involves fundamental business logic. Technical SERVICES should lack any business meaning at all.

Many domain or application SERVICES are built on top of the populations of ENTITIES and VALUES, behaving like scripts that organize the potential of the domain to actually get something done. ENTITIES and VALUE OBJECTS are often too fine-grained to provide a convenient access to the capabilities of the domain layer. Here we encounter a very fine line between the domain layer and the application layer. For example, if the banking application can convert and export our transactions into a spreadsheet file for us to analyze, that export is an application SERVICE. There is no meaning of "file formats" in the domain of banking, and there are no business rules involved.

On the other hand, a feature that can transfer funds from one account to another is a domain SERVICE because it embeds significant business rules (crediting and debiting the appropriate accounts, for example) and because a "funds transfer" is a meaningful banking term. In this case, the SERVICE does not do much on its own; it would ask the two Account objects to do most of the work. But to put the "transfer" operation on the Account object would be awkward, because the operation involves two accounts and some global rules.

We might like to create a Funds Transfer object to represent the two entries plus the rules and history around the transfer. But we are still left with calls to SERVICES in the interbank networks. What's more, in most development systems, it is awkward to make a direct interface between a domain object and external resources. We can dress up such external SERVICES with a FACADE that takes inputs in terms of the model, perhaps returning a Funds Transfer object as its result. But whatever intermediaries we might have, and even though they don't belong to us, those SERVICES are carrying out the domain responsibility of funds transfer.

Granularity

Although this pattern discussion has emphasized the expressiveness of modeling a concept as a SERVICE, the pattern is also valuable as a means of controlling granularity in the interfaces of the domain layer, as well as decoupling clients from the ENTITIES and VALUE OBJECTS.

Medium-grained, stateless SERVICES can be easier to reuse in large systems because they encapsulate significant functionality behind a simple interface. Also, fine-grained objects can lead to inefficient messaging in a distributed system.

As previously discussed, fine-grained domain objects can contribute to knowledge leaks from the domain into the application layer, where the domain object's behavior is coordinated. The complexity of a highly detailed interaction ends up being handled in the application layer, allowing domain knowledge to creep into the application or user interface code, where it is lost from the domain layer. The judicious introduction of domain services can help maintain the bright line between layers.

This pattern favors interface simplicity over client control and versatility. It provides a medium grain of functionality very useful in packaging components of large or distributed systems. And sometimes a SERVICE is the most natural way to express a domain concept.

Access to SERVICES

Distributed system architectures, such as J2EE and CORBA, provide special publishing mechanisms for SERVICES, with conventions for their use, and they add distribution and access capabilities. But such frameworks are not always in use on a project, and even when they are, they are likely to be overkill when the motivation is just a logical separation of concerns.

The means of providing access to a SERVICE is not as important as the design decision to carve off specific responsibilities. A "doer" object may be satisfactory as an implementation of a SERVICE's interface. A simple SINGLETON (Gamma et al. 1995) can be written easily to provide access. Coding conventions can make it clear that these objects are just delivery mechanisms for SERVICE interfaces, and not meaningful domain objects. Elaborate architectures should be used only when there is a real need to distribute the system or otherwise draw on the framework's capabilities.