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, the Solr search indexes are created automatically and no additional external software is required.
For Elasticsearch, a separate installation is required since Opencast version 9.0.
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.
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 powers the external API as well as the administrative user interface of Opencast.