The Open Archives Initiative Protocol for Metadata Harvesting is a simple protocol that allows services to expose metadata for harvesting. This means that other services can regularly gather metadata.
OAI-PMH is a metadata harvester that locates and aggregates metadata from different data sets. A metadata harvester can be used to integrate data from a number of sources into one catalogue; users can then search the OPAC, locate the metadata within the catalogue and be linked to the content at the original repository’s site. OAI allows multiple forms of metadata to be exposed but mandates DC as a minimum. Many data providers do not provide richer metadata formats. Only the metadata needs to be stored locally and not the files themselves.
OAI-PMH is based on common standards: HTTP and XML. It is intended as a low-barrier solution, but a level of manual intervention in harvesting metadata is required and incremental harvesting may be problematic. There is a high level of flexibility in the protocol and in Dublin Core, which can create obstacles for service providers.
Elastic Search (general information)