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Vector store — Lodestar.Extensions.VectorData

An in-process Microsoft.Extensions.VectorData provider. A consumer written against that abstraction — a Semantic Kernel pipeline, a retrieval-augmented chain, anything that takes a VectorStore — gets hybrid keyword and vector search with no database and no service running anywhere. Every other provider that offers hybrid search talks to a server, and the providers that run in process offer none; this one holds the records in the calling process and fuses both rankings there.

It adds no retrieval arithmetic of its own. The vector half is EmbeddingIndex, the keyword half is Bm25Index over a CountVectorizer vocabulary, and the fusion is RankFusion.Rrf. What this package owes is the interface contract: keys, upserts, deletes, filters, and the one place the two rankings meet.

The shape: records are the truth, indexes are caches

Neither index can honour an upsert as it stands — EmbeddingIndex only appends, and Bm25Index is built whole from a matrix and never changes. So each collection keeps its records in a dictionary keyed by the record's key, and both indexes are rebuilt from it on the first search after a write. A batch of writes costs one rebuild, and an updated record's old vector is gone rather than masked. decisions/0123 records the choice and the two alternatives it beat.

That is also the cost to know about: the search that follows a write pays for a rebuild over every record. A workload that alternates one write with one search rebuilds on every search; reading a record by key never rebuilds anything.

What it refuses, and why each refusal names its reason

• LodestarVectorStore.GetDynamicCollection throws. A dynamic record is a dictionary, and a filter here is an expression compiled against a record's properties.
• A string search value throws. Nothing in this package turns text into a vector; embed it first — Lodestar.Extensions.AI is one way.
• A filtered LodestarVectorStoreCollection.GetAsync with OrderBy throws, because a dictionary has no order of its own to sort from.
• LodestarVectorStoreCollection.HybridSearchAsync throws on a record type that marks no IsFullTextIndexed property: there is no keyword half to fuse. It also throws when ScoreThreshold is set, because a fused score is not a similarity.
• A vector property declaring a DistanceFunction other than CosineSimilarity is refused when the LodestarVectorStoreCollection is constructed: the index scores cosine only, and a declared distance would also turn ScoreThreshold around.

Not trimming- or AOT-safe on the filter path

A filter is an Expression<Func<TRecord, bool>>, run in process by Expression.Compile, which needs dynamic code. A trimmed or ahead-of-time compiled application can use every member here without a filter; with one, it cannot.

Types

Type	What it is
LodestarVectorStore	An in-process vector store: named collections, held for the store's lifetime.
LodestarVectorStoreCollection	One collection: records in a dictionary, a vector index and a BM25 index derived from them.
LodestarVectorStoreOptions	How a collection builds its keyword half, and the offset reciprocal-rank fusion uses.

See also

• Semantic search with embeddings — where the vectors come from.
• Keyword search — the BM25 half on its own.
• Python → C# equivalence.