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Samvera defaults to oai_dc as its metadata format in the OAI-PMH feed. However, many repositories have custom formats. SoftServ has implemented code in Hyku for easier customization of OAI-PMH data (https://playbook-staging.notch8.com/en/samvera/oai-feeds ). For example, Hyku for Consortia uses the custom metadata prefix oai_hyku in its Hyku Commons repository. This custom prefix has been added to hyku. See the pull request: https://github.com/samvera/hyku/pull/1934/files
The standard OAI XSLT transformation does not fully support displaying the custom metadata prefixes. See example below. However, one can right click to view the View Page Source to see the raw XML. One can get all the XML data with the custom prefixes relatively easily or a developer can configure that. This includes the custom header specs and custom encodings. It can generate the information from a SOLR service query.
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In Hyku, Hyku for Consortia/Hyku Commons provides the thumbnail link in its custom oai_hyku format in the thumbnail_url field (this is custom).
Example: https://au-archives.hykucommons.org/downloads/5d20d75f-0744-4f81-bcb0-998b1ff08e70?file=thumbnail
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