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may also provide specific methods that retrieve metadata related to quality of ser 

vices, parameter constraints, and so forth. Publishing a schema for a document

that a service accepts is another way to provide metadata.

When deciding what information to store in a directory, try to store data that is

useful to more than one application. Storing metadata in a directory also helps if

you need to analyze the impact of any changes to the application. 

Also, consider implementing directory based data sharing, since this enables

applications to operate collectively. Such data sharing reduces management costs

and improves an enterprise's overall responsiveness to business change. If you use

this approach, be sure to set up authorization policies to control access to the data. 

It is also important when using a directory service to locate authoritative or

reliable sources of the data. A major reason for not using a directory service is

outdated or suspect data. Users quickly learn that they cannot rely on the service

and hence stop using it. Keep in mind, however, that a single application or data 

base is rarely authoritative for all required data, and this may cause a data integra 

tion problem of its own. 

6.5.4 Using Registries for Integration 

You may want to consider using registries to integrate an application with an exist 

ing EISs, especially when you want a loose binding between the application and the

systems with which it is integrated. For example, in the Web services approach, you

can store the URL and the WSDL for the Web service in a UDDI registry. However,

using a registry comes with the additional overhead of running and maintaining a

registry server and the added programming complexity to have your application use

the registry. The registry server may also be a single point of failure for your system. 

Generally, it is not worthwhile to use registries for a small enterprise whose

applications integrate with just a few EISs. Registries make more sense for

medium and large integration architectures. You want to maximize the use of the

registry among as many applications as possible. 

6.5.5 Versioning and Evolution

Applications integrated with EISs change over time, as do the EISs. As these

systems and applications evolve over their lifetimes, new versions of them emerge.

Integration of the applications and EISs must be able to handle this evolution and

versioning process.