It is important to devote time to validating assumptions about services, including: where they exist, the purpose of the service, the information bound to the service, dependencies (what other services a service may call upon), and security issues.
A services directory is the best place to aggregate all that painstakingly collected information about services and data. As with other directories, this is a repository for gathered information about available services, along with the documentation (or pointers to documentation) for each service -- what it does, information passed to a service, information coming from a service, and so on. This directory is used (along with application semantics) to define the points of integration for all systems in the domain.
You could certainly do this using Excel or a small database, but today’s SOA registries or repositories do a much better job. As noted previously, an SOA registry is a resource that enterprises use to publish, discover, and consume Web services. A repository, on the other hand, holds additional content such as XML Schemas, DTDs (Document Type Definitions), and WSDL documents. Think of a repository as a persistence mechanism that stores information published to the registry. Today, pretty much all the products in this area have both capabilities, whether they call themselves registries or repositories.
One of the better-known offerings is Infravio’s X-Registry Platform, which calls itself an SOA governance platform and allows designers to maintain both semantic and service information. (Governance involves the development, management, and enforcement of policies that set parameters for how services should be built and accessed.) Then there’s Systinet Registry, recently acquired by Mercury Interactive, which provides a simple, standards-based means for publishing and discovering reusable business services and SOA artifacts. Systinet 2 is a governance and lifecycle platform built around the registry.
Other registry/repository products include Flashline SOA, RainingData’s TigerLogic SOA Repository, and various functionality built into the “stacks” offered by BEA, IBM, Sun, and other big vendors -- BEA actually licenses the Systinet Registry. All of these are evolving rapidly and most integrate with governance solutions. They make it easier to manage data and service information you’ve collected, but unfortunately they can’t do much to relieve the hard labor of gathering that information in the first place.