Appendix A. Back end architecture

The back end code is organized in three layers, illustrated in the preceding graphic.

org.optaweb.vehiclerouting
├── domain
├── plugin          # Infrastructure layer
│   ├── persistence
│   ├── planner
│   ├── routing
│   └── websocket
└── service         # Application layer
    ├── demo
    ├── distance
    ├── location
    ├── region
    ├── reload
    ├── route
    └── vehicle

The service package contains the application layer that implements use cases. The plugin package contains the infrastructure layer.

Code in each layer is further organized by function. This means that each service or plug-in has its own package.

Compile-time dependencies are only allowed to point from outer layers towards the center. Following this rule helps to keep the domain model independent of underlying frameworks and other implementation details and models the behavior of business entities more precisely. With presentation and persistence being pushed out to the periphery, it is easier to test the behavior of business entities and use cases.

The domain has no dependencies.

Services only depend on the domain. If a service needs to send a result (for example to the database or to the client), it uses an output boundary interface. Its implementation is injected by the Inversion of Control (IoC) container.

Plug-ins depend on services in two ways. First, they invoke services based on events such as a user input or a route update coming from the optimization engine. Services are injected into plug-ins which moves the burden of their construction and dependency resolution to the IoC container. Second, plug-ins implement service output boundary interfaces to handle use case results, for example persisting changes to the database or sending a response to the web UI.

The domain package contains business objects that model the domain of this project, for example Location, Vehicle, Route. These objects are strictly business-oriented and must not be influenced by any tools and frameworks, for example object-relational mapping tools and web service frameworks.

The service package contains classes that implement use cases. A use case describes something that you want to do, for example adding a new location, changing vehicle capacity, or finding coordinates for an address. The business rules that govern use cases are expressed using the domain objects.

Services often need to interact with plug-ins in the outer layer, such as persistence, web, and optimization. To satisfy the dependency rules between layers, the interaction between services and plug-ins is expressed in terms of interfaces that define the dependencies of a service. A plug-in can satisfy a dependency of a service by providing a bean that implements the boundary interface of the service. The Spring IoC container creates an instance of the plug-in bean and injects it to the service at runtime. This is an example of the inversion of control principle.

The plugin package contains infrastructure functions such as optimization, persistence, routing, and network.