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If you want to provide a simple isolation layer, you should consider writing a

data access object, or DAO class, to hide the internal details of the EIS. A data

access object encapsulates access to persistent data, such as that stored in a data

management system. It decouples the user of the object from the programming

mechanism for accessing the underlying data. It can also expose an easier to use

API. Typically, you develop a data access class for a specific business function or

set of functions, then client applications use these classes. Another common use is

to layer data access objects on top of connectors, or resource adapters, thus isolat 

ing applications from the details of the connector interactions. Often, each

resource adapter interaction has a corresponding data access object, whose

methods pass input records to the adapter as parameters and return output records

to the client application as return values. 

Using a connector reduces the complexity of the integration layer. Note that

when using the connector approach, the EAI application is tightly coupled with

the EIS since the application directly uses the data model and functionality of the

EIS. Since there is minimal layering, it also increases performance. However, it is

an approach that works best when the integration problem is small in scope. Since

it does not put into place a true integration architecture, this approach may limit

the ability to scale the integration layer as the enterprise grows. Given these

advantages and disadvantages, consider using this approach as a basic building

block for other strategies. 

6.3.4 Combining Approaches into an Integration Architecture

In many cases, architects combine these various integration layers into a single inte 

gration architecture. The end result the mix of integration layer types is in large

measure driven by the requirements of each enterprise's situation. Architects need to

consider the realities of the current state of the technologies and weigh that against

future promises.

For example, Web services, as they exist today, have some shortcomings:

They do not deliver the heavy duty process integration, data transformation, and

security capabilities required by many EAI scenarios. Similarly, services for trans 

actional integrity and reliable messaging are not yet in place. Since security and

transactional context propagation are critical business requirements, these are

important factors to consider when using a Web services approach.

Data binding is another issue to consider. Web services are a good solution

when data binding requirements are straightforward, such as mapping simple data

types to Java data types. However, when it is necessary to manipulate complex