The Open Archives Initiative Protocol for Metadata Harvesting Schema (OAI-PMH Schema) provides a formal structure for validating responses as part of the OAI-PMH Protocol. The OAI-PMH Protocol is a low-barrier mechanism for repository interoperability. Data Providers are repositories that expose structured metadata via OAI-PMH. Service Providers then make OAI-PMH service requests to harvest that metadata. OAI-PMH is a set of six verbs or services that are invoked within HTTP.