LEANN IVF Backend

FAISS IndexIVFFlat backend for LEANN with add and delete APIs for incremental updates (#231, #89, #141).

Install

uv sync --package leann-backend-ivf
# or
pip install leann-backend-ivf

Requires faiss-cpu. For query embedding during search, the IVF searcher uses the same embedding server as HNSW; install leann-backend-hnsw if you use recompute/query embedding.

Build

leann build my-index --docs ./src --backend-name ivf

Optional backend kwargs (e.g. via API): nlist (default 100), distance_metric ("l2" or "cosine"/"mips").

Add / Delete API

Follows the same idea as HNSW’s update path; use these for incremental workflows (e.g. after Merkle tree / file-change API):

• add_vectors(index_path, embeddings, passage_ids) Appends vectors to an existing IVF index. embeddings: (N, D) float32; passage_ids: list of N passage id strings (must not already exist).

• remove_ids(index_path, passage_ids) Removes vectors by passage id. Returns the number of vectors removed. Use after detecting changed chunk ids: delete old, then re-insert new.

Example:

from leann_backend_ivf import add_vectors, remove_ids

# After file-change API says chunk id "abc123" changed:
remove_ids("/path/to/index.leann", ["abc123"])
# Re-embed and add new chunk with same or new id
add_vectors("/path/to/index.leann", new_embeddings, ["abc123"])

Core integration (#89, #141)

LeannBuilder.update_index(index_path, remove_passage_ids=None) in leann-core supports IVF: it calls add_vectors for appends and, when remove_passage_ids is set, calls remove_ids first (e.g. from a file-change / Merkle API). Use for incremental build or reindex: detect changed chunk ids → update_index(path, remove_passage_ids=changed_ids) then add chunks for the new content.

Notes

• IVF stores vectors in flat lists per cluster; no compact/recompute mode like HNSW. Good for frequent add/delete.
• ID mapping is stored in ivf_id_map.json next to the index; passage ids are stable across add/delete.
• Search uses nprobe (default from complexity, capped by nlist). Tune nlist at build time for speed/recall.