I have been thinking about the opportunities OAI-PMH has to offer for a while. At the beginning my acceptance of OAI-PMH was rather unreflective, it was just the right thing to do, something that helps to connect various digital resources. The literature usually did not provide too many guidelines as to how OAI-PMH can be employed as a collection development tool. There were a lot of articles about OAI-PMH, but many of those materials were rather technical and OAI-PMH was treated just as a tool to populate databases with records from other repositories. Interoperability was often mentioned, but there were few case studies that would show how these distributed resources can be aggregated in a meaningful way that would complement material offered by the institution. As a result there is not sufficient granularity in providing records for harvesting, few institutions offer meaningfully created sets. Those sets which exist are often indiscriminate aggregations of resources produced under different projects or by different agencies.
The OAI-PMH was developed for institutional repositories which often exist separately from special collections or archives within university libraries and they may be one of the reasons, why this technology has been underused in heritage repositories. Those huge pools of records with no clearly defined scope and audience had little to offer in terms of collection development and build-up, they could hardly supplement one's own material with complimentary resources from other repositories with similar area of interest. However, recently a number of OAI-PMH service providers appeared that harvest records from more narrowly specified sets, these services are mostly tied to a project, so the metadata seem more consistent than. This is the case of the Sheet Music collection that even posted its cataloging guidelines. Another project with a clearly defined scope, even if broader than the Sheet Music project is American Social History Online. Projects like this allow a high level of customization of their services. They can provide users with more precise and useful results, a metadata filter can be used that brings browsing users almost effortlessly several levels deep into collection hierarchy to resources they seek.
The use of Web 2.0 tools and intelligent use of client-side scripting can make useful browsing and searching even within more general aggregations. I was particularly impressed with the ELib service administered by University of Bremen, Germany that also presents harvested material in very intuitive and effective way. Taking advantage of subjects headings for creating browseable hierarchies, but also tag clouds for keywords and further refinement of query.
The DLF (Digital Library Federation) OAI Portal is a reminder of an earlier period when the OAI-PMH served merely for aggregating material from various resources without any further manipulation and repurposing of metadata. Users can use search or browse two browsable hierarchies, one based on subject headings that however are still very broad and then one based on data providers, but no other tools for narrowing the record sets are available. The records obviously originated in various formats and were based on different rules, and the effort to normalize them was rather limited.
In order to take advantage of the OAI-PMH and make it a useful tool for metadata sharing that can help to round out and complement virtual collections, the data providers need to make sure that the records are available in meaningful granular sets. Clearly defined metadata sets and cataloging that follows accepted standards and takes into account new contexts in which metadata can exist, make it possible for other repositories to integrate these records into their collections, and in such a way to provide additional exposure to those resources.
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