[`InMemoryBM25Retriever`](../pipeline-components/retrievers/inmemorybm25retriever.mdx): A keyword-based Retriever that fetches documents matching a query from a temporary in-memory database.
[`InMemoryEmbeddingRetriever`](../pipeline-components/retrievers/inmemoryembeddingretriever.mdx): Compares the query and document embeddings and fetches the documents most relevant to the query.