业务层模式:Business Delegate—业务委托模式

Context
A multi-tiered, distributed system requires remote method invocations to send and
receive data across tiers. Clients are exposed to the complexity of dealing with distributed
components.
Problem
Presentation-tier components interact directly with business services. This direct
interaction exposes the underlying implementation details of the business service
application program interface (API) to the presentation tier. As a result, the presentation-tier
components are vulnerable to changes in the implementation of the business services: When
the implementation of the business services change, the exposed implementation code in the
presentation tier must change too.
Additionally, there may be a detrimental impact on network performance because
presentation-tier components that use the business service API make too many invocations
over the network. This happens when presentation-tier components use the underlying API
directly, with no client-side caching mechanism or aggregating service.
Lastly, exposing the service APIs directly to the client forces the client to deal with
the networking issues associated with the distributed nature of Enterprise JavaBeans (EJB)
technology.
Forces
Presentation-tier clients need access to business services.
Different clients, such as devices, Web clients, and thick clients, need access to
business service.
Business services APIs may change as business requirements evolve.
It is desirable to minimize coupling between presentation-tier clients and the business
service, thus hiding the underlying implementation details of the service, such as lookup
and access.
Clients may need to implement caching mechanisms for business service information.
It is desirable to reduce network traffic between client and business services.
Solution
Use a Business Delegate to reduce coupling between presentation-tier clients and
business services. The Business Delegate hides the underlying implementation details of the
business service, such as lookup and access details of the EJB architecture.
The Business Delegate acts as a client-side business abstraction; it provides an
abstraction for, and thus hides, the implementation of the business services. Using a
Business Delegate reduces the coupling between presentation-tier clients and the system's
business services. Depending on the implementation strategy, the Business Delegate may
shield clients from possible volatility in the implementation of the business service API.
Potentially, this reduces the number of changes that must be made to the presentation-tier
client code when the business service API or its underlying implementation changes.
However, interface methods in the Business Delegate may still require modification if
the underlying business service API changes. Admittedly, though, it is more likely that
changes will be made to the business service rather than to the Business Delegate.
Often, developers are skeptical when a design goal such as abstracting the business
layer causes additional upfront work in return for future gains. However, using this pattern
or its strategies results in only a small amount of additional upfront work and provides
considerable benefits. The main benefit is hiding the details of the underlying service. For
example, the client can become transparent to naming and lookup services. The Business
Delegate also handles the exceptions from the business services, such as java.rmi.Remote
exceptions, Java Messages Service (JMS) exceptions and so on. The Business Delegate may
intercept such service level exceptions and generate application level exceptions instead.
Application level exceptions are easier to handle by the clients, and may be user friendly.
The Business Delegate may also transparently perform any retry or recovery operations
necessary in the event of a service failure without exposing the client to the problem until it
is determined that the problem is not resolvable. These gains present a compelling reason to
use the pattern.
Another benefit is that the delegate may cache results and references to remote
business services. Caching can significantly improve performance, because it limits
unnecessary and potentially costly round trips over the network.
A Business Delegate uses a component called the Lookup Service. The Lookup
Service is responsible for hiding the underlying implementation details of the business
service lookup code. The Lookup Service may be written as part of the Delegate, but we
recommend that it be implemented as a separate component, as outlined in the Service
Locator pattern (See "Service Locator" on page 368.)
When the Business Delegate is used with a Session Facade, typically there is a
one-to-one relationship between the two. This one-to-one relationship exists because logic
that might have been encapsulated in a Business Delegate relating to its interaction with
multiple business services (creating a one-to-many relationship) will often be factored back
into a Session Facade.
Finally, it should be noted that this pattern could be used to reduce coupling between
other tiers, not simply the presentation and the business tiers.