Epsilla Vector Database vs OpenSearch

Epsilla Vector Database

GPL-3.0 server you can run from one Docker image. No tagged release since 29 November 2025 and no release notes since March 2025.

OpenSearch

OpenSearch 3.9.0 runs vector, text and hybrid search under Apache-2.0, with an OpenAPI 3.1 description of its REST API and API keys limited by permission and index pattern. The owner runs the cluster, no status page or SLA comes with the software, and no tool in the MCP server sets a read-only or destructive annotation.

Epsilla Vector Database

1. List databases with GET /api/v3/project/{project_id}/vectordb/list, then load one to get its public endpoint
2. Hybrid search is assembled client-side with as_search_engine().add_retriever(...).set_reranker(type="rrf")
3. If you use pyepsilla against the cloud, patch or wrap it to verify TLS
4. Ask a person to create the database in the console first. The API can't

OpenSearch

1. Create the index with index.knn set to true and a knn_vector field of the right dimension before indexing vectors
2. Put the filter clause inside the knn query for filtering during the search. A filter outside it runs afterwards and can return fewer than k hits
3. Ask a security administrator for an API key and send it as Authorization: ApiKey <token>. The token is shown once and lasts 90 days at most
4. Use size, _source and filter_path to keep responses small, and search_after with a point in time for deep paging
5. Set OPENSEARCH_SETTINGS_ALLOW_WRITE=false on the MCP server unless writes are needed. GenericOpenSearchApiTool can call any endpoint