Qdrant is an open-source vector similarity search engine written in Rust, storing vectors alongside JSON payloads and allowing results to be filtered by those payloads during the search rather than after it. It solves the problem that retrieval usually needs a condition as well as a similarity, such as restricting to one tenant or one document type. It exists because filtering after a vector search silently loses recall: the top matches may all be filtered away. Qdrant can be self-hosted or used as a managed service.
Milvus is an open-source vector database built as a distributed system, separating storage and compute so a collection can grow beyond one machine. It solves the problem that a single-node vector index eventually stops fitting in memory or stops keeping up with query load. It exists for the upper end of the scale range, where the operational complexity of a distributed deployment is repaid by horizontal growth. It supports several index types so recall and latency can be traded against each other.
