Opencast comes with multiple search indexes which act both as a cache and as a fast way to perform full text searches on metadata. By default, all search indexes are created automatically and no additional external software is required.
While this works well, all indexes can be deployed separately. This comes with the obvious drawback of a harder deployment but has also a few advantages like a smaller core system or being able to have some service redundancies which would not be possible otherwise.
Opencast currently uses two search index types: Solr and Elasticsearch.
Solr is mostly powering older services and replacing this index type is planned for the future. But for now it is still the back-end for the search service (LTI and engage tools), the workflow service and the series service.
Elasticsearch is the back-end for both the administrative user interface as well as the external API.