---
date: 2026-03-22
topic: "Embedding-Based Motion Similarity Cache"
status: validated
---

## Problem Statement

We have text embeddings and fused (SVD + text) embeddings stored for motions, but no usable similarity search. The current `database.search_similar()` is a full Python scan — it SELECTs all embeddings, parses JSON one by one, and computes cosine similarity with `zip` in pure Python. This is O(N) per query with no vectorized math, no indexing, and no caching. The similarity cache migration (`2026-03-22-add-similarity-cache.sql`) is a commented-out placeholder with no executable SQL.

Additionally, several infrastructure gaps block a working similarity system:
- The `embeddings` table is not created by `_init_database()` (only exists via migration file)
- The fusion pipeline has an N+1 query pattern (per SVD row queries embeddings separately)
- `ai_provider._post_with_retries` does not retry on 429 (rate limit) responses

## Constraints

- DuckDB only — no pgvector, no external vector store
- Vectors stored as JSON text columns (existing format, not changing)
- DuckDB connections are short-lived (open/close per method)
- Do not modify `app.py` or `scheduler.py`
- Tests must be offline (monkeypatch network calls)
- Functional style, Python, uv
- Logging via `getLogger`, no `print()`

## Approach

**Precomputed similarity cache** — batch-compute top-K nearest neighbors per motion and store results in a cache table. The UI reads the cache with a simple indexed lookup.

Rationale: the motion corpus changes slowly (new motions trickle in from parliament). Computing nearest neighbors at query time is wasteful. One offline O(N^2) pass via numpy matrix multiplication gives us O(1) lookups forever until the next recompute.

Alternatives rejected:
- **DuckDB vss extension (HNSW)**: experimental, requires vector format migration away from JSON text, overkill for ~thousands of motions
- **Real-time numpy search**: better than pure-Python zip, but still O(N) per query; caching eliminates repeated work
- **FAISS/Annoy ANN index**: designed for millions of vectors, unnecessary complexity at our scale

## Architecture

```
New files:
  similarity/
    __init__.py
    compute.py      -- batch pairwise cosine, extract top-K, write cache
    lookup.py       -- read cached results for a motion

Modified files:
  database.py       -- add similarity_cache + embeddings to _init_database,
                       add store/read/clear helpers, deprecate old search_similar
  migrations/2026-03-22-add-similarity-cache.sql  -- uncomment and finalize
  ai_provider.py    -- add 429 to retry branch
  pipeline/fusion.py -- fix N+1 with bulk JOIN
```

## Components

### similarity_cache table

```
similarity_cache (
    id              INTEGER DEFAULT nextval('similarity_cache_id_seq'),
    source_motion_id INTEGER NOT NULL,
    target_motion_id INTEGER NOT NULL,
    score           REAL NOT NULL,
    vector_type     TEXT NOT NULL,      -- 'text', 'fused', 'svd'
    window_id       TEXT,               -- NULL for text-only, set for fused/SVD
    created_at      TIMESTAMP DEFAULT CURRENT_TIMESTAMP
)
```

Composite index on `(source_motion_id, vector_type, window_id)` for fast lookups.

### similarity/compute.py

- Load all vectors of a given type into a numpy matrix in one query (parse JSON, stack into ndarray)
- Normalize rows to unit length
- Compute full cosine similarity matrix via `normalized @ normalized.T`
- Extract top-K per row (excluding self-similarity)
- Bulk-insert results into `similarity_cache`
- Idempotent: `clear_similarity_cache(vector_type, window_id)` then insert within same connection scope

Public function: `compute_similarities(vector_type='fused', window_id=None, top_k=10, db_path=None)`

### similarity/lookup.py

- `get_similar_motions(motion_id, vector_type='fused', window_id=None, top_k=10, db_path=None)` — SELECT from cache ordered by score DESC
- Returns list of dicts: `{motion_id, score}`
- Optionally join motion metadata (title, layman_explanation) for richer results
- Graceful degradation: empty cache returns empty list

### database.py changes

1. Add `embeddings` table creation to `_init_database()` — matches migration schema
2. Add `similarity_cache` table + sequence creation to `_init_database()`
3. New helpers:
   - `store_similarity_batch(rows: list[dict])` — bulk INSERT
   - `get_cached_similarities(source_motion_id, vector_type, window_id=None, top_k=10)` — read
   - `clear_similarity_cache(vector_type, window_id=None)` — DELETE for idempotent recompute
4. Deprecate `search_similar()` — mark with a log warning pointing to `similarity.lookup`

### ai_provider.py fix

- Add HTTP 429 to the retry branch in `_post_with_retries`
- If `Retry-After` header is present, use it as the backoff delay; otherwise fall back to existing exponential backoff
- This is a single-line condition change plus header parsing

### pipeline/fusion.py fix

- Replace the per-row SELECT from `embeddings` with a single bulk query:
  JOIN `svd_vectors` with latest `embeddings` per motion_id in one SQL statement
- Loop over joined results and concatenate in Python
- Eliminates N+1 query pattern

## Data Flow

1. Existing pipeline runs: extract MP votes → SVD → text embeddings → fusion
2. After fusion completes, `similarity/compute.py` loads all fused vectors for the window into a numpy matrix
3. Computes pairwise cosine similarity matrix, extracts top-K per motion
4. Bulk-inserts results into `similarity_cache` (clearing previous cache for that batch first)
5. Separately, text-only similarity can be computed across all motions (no window dependency)
6. UI calls `similarity/lookup.py` for a direct indexed read — instant response

## Error Handling

- **Missing vectors**: motions without embeddings are excluded from the similarity matrix; not an error
- **Empty matrix**: if no vectors exist for a vector_type/window, log warning and skip (don't write empty cache)
- **DB write failures**: wrap cache writes in try/except, log error, don't crash the pipeline; similarity is non-critical
- **Stale cache**: cache is fully replaced on each recompute (delete + insert in same connection scope); if recompute fails partway, old cache remains valid
- **Dimension mismatch**: vectors with inconsistent dimensions are padded or excluded with a warning (following existing clustering.py pattern)

## Testing Strategy

- **Unit: compute.py** — create known vectors with predictable cosine similarities (e.g., identical vectors → score 1.0, orthogonal → 0.0), verify matrix math produces correct top-K ordering
- **Unit: lookup.py** — seed cache table in temp DB, verify queries return correct ordered results, verify empty cache returns empty list
- **Unit: database helpers** — test store_similarity_batch / get_cached_similarities / clear_similarity_cache round-trip
- **Unit: ai_provider 429 retry** — monkeypatch requests.post to return 429, verify retry with backoff
- **Unit: fusion bulk join** — verify N+1 elimination produces same results as original
- **Migration test** — apply updated similarity_cache migration on temp DuckDB, verify schema matches expected columns
- **Integration test** — insert fake embeddings → run compute → verify cache populated → lookup returns expected results
- **All tests offline**: in-memory DuckDB, monkeypatched network calls

## Open Questions

None blocking. Future enhancements (not in scope):
- MP-to-MP similarity from SVD vectors (explorer UI is motion-focused for now)
- Real-time similarity for newly ingested motions before next batch recompute
