parththakkar106andClaude Opus 5
03c6707ca7
Two plan documents from designing the branching story tree. The tree work turned up that memory retrieval fetches the whole bank's embeddings every turn -- 96% of a turn's database traffic -- and that is the one read a tree cannot window, so it lands first. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_015CYEJKobJ2Re4Dv7qUoSA7
128 lines
6.5 KiB
Markdown
128 lines
6.5 KiB
Markdown
# 13 — Memory-bank embedding cost (round three of the egress work)
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**Goal:** stop every turn fetching the entire memory bank's embeddings. Measured at
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**3,024 KB per turn** on adventure 25 against **129 KB** for everything else a turn
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reads — the memory bank is ~96% of a turn's database traffic, and it is fetched fresh
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every single turn to pick `memory_top_k = 5` memories.
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Found 2026-08-16 while designing the story tree (see `14-phase-story-tree.md`), because
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memory retrieval is the one read a tree **cannot** window — it is long-range recall by
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design, so it always spans the full path. That makes this the cost floor of a turn under
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the tree, which is why it lands first.
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## The measurement (production, Neon SQL editor)
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```sql
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SELECT count(*), avg(json_array_length(embedding))::int,
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avg(length(embedding::text))::int,
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pg_size_pretty(sum(length(embedding::text))::bigint)
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FROM memories WHERE embedding IS NOT NULL;
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-- 134 memories | 1536 dims | 30,971 bytes each | 4,053 kB total
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```
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| adventure | active | fetched | per turn |
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|---|---|---|---|
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| 25 | 100 | 100 | **3,024 kB** |
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| 21 | 18 | 18 | 545 kB |
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| 12 | 12 | 12 | 363 kB |
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| 20 | 4 | 4 | 121 kB |
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Break-even against the rest of a turn is **4.2 memories**, i.e. about action 25. Every
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adventure past that is dominated by this.
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`active == fetched` everywhere: nothing has been evicted yet, so the Python-side
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`forgotten` filter currently costs nothing. It becomes a real leak the moment eviction
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starts.
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## Why round two missed it
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`retrieve_memories` needs `settings.embedding_model`, and embedding providers are
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**BYOK-only by construction** — they never touch the demo key. So the public demo never
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embeds anything, and the round-two stress harness (which had no embedding model
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configured) measured the turn loop with its heaviest read switched off. The 23.0 MB
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figure for a 200-action playthrough is the memory-bank-**off** number; with it on,
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adventure 25 is closer to 300 MB.
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**Rule going forward: any egress measurement must run with an embedding model set.**
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## Root cause
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`memorybank.py:208`
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```python
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candidates = [m for m in adventure.memories if not m.forgotten and m.embedding]
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```
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Walks the relationship, so every memory row for the adventure loads with its embedding.
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`embedding` is `Mapped[list]` on a `JSON` column — 1536 floats serialised as text is
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~31 KB. Cosine ranking happens in Python (a deliberate choice, documented at
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`models.py:144`), so all of it must cross the wire. The comment sized it by **count**
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("fine at a few hundred") rather than by **bytes**.
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Not the same bug as migration 36/37 — the column is not a repeating group and there is
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no denormalisation to fix. It is a *format* problem plus a *fetch-frequency* problem.
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## Decisions (settled 2026-08-16)
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- **Packed float32, keep 1536 dimensions.** 31 KB → 6 KB, a straight 5x, with **zero
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retrieval-quality risk**. Explicitly rejected dropping to 512/768 dims: the size fix
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plus the cache makes the extra 3x unnecessary, and it would have meant re-embedding.
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- **No re-embedding.** Dimensions are unchanged, so the migration is a pure format
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conversion of the 134 existing rows — read the JSON, write packed bytes, no API calls.
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One-time 4 MB read.
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- **In-process cache alongside the size fix**, not sequenced after it. Turns for one
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adventure arrive back-to-back, so a dict keyed by adventure id takes steady-state cost
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to ~0. 100 vectors as float32 is 600 KB of RAM — negligible.
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- **`memory_bank_capacity` 200 → ~80.** Taken on *quality* grounds as much as cost:
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ranking 200 memories to pick 5 dilutes retrieval. Note this will start evicting on
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adventure 25 immediately (it sits at 100).
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- **Not pgvector.** It is the structural answer and would keep vectors in the database
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entirely, but it breaks SQLite dev parity — which the codebase protects deliberately
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(`context/history.py:42`, the `replace()`/`trim()` dialect dance). Revisit only if the
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bank grows past what Python cosine can handle.
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## Work, in order
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1. **Rebuild the byte-meter harness — in the repo this time.** `backend/tools/dbmeter.py`
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+ `stress_session.py`. The originals lived outside the repo and are gone. **Default it
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to running with an embedding model configured**, since that omission is precisely what
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hid this finding. Do this first so every item below is measured, not assumed.
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2. **Migration 38 — `memories.embedding_blob` (`LargeBinary`).** Backfill in Python
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(`struct.pack(f"<{n}f", *vec)`); the conversion cannot be expressed in portable SQL, so
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unlike migration 36/37 this one does pay a one-time 4 MB read. Drop the old JSON column
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in a follow-up migration once verified, not in the same one.
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3. **Read path.** `retrieve_memories` queries `memories` directly with
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`forgotten = false AND embedding_blob IS NOT NULL` in **SQL**, not Python. Unpack with
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`struct`/`numpy`.
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4. **Vector cache.** Keyed by `adventure_id`, invalidated on memory create, evict and
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delete. Must survive the retry/undo paths that prune memories.
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5. **Capacity default 200 → 80.** Existing adventures inherit it, so adventure 25 evicts
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on its next turn — check that the eviction path is sane at scale before shipping.
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6. **Infinite scroll upward in `Play.jsx`** for the remaining 423 KB page load of a
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finished adventure — the last open item from round two. Load the newest turns, fetch
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older ones as the reader scrolls up.
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## Guardrails to add with this work
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- **Query-count / byte assertions per endpoint**, extending the `test_egress.py` idea:
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assert an endpoint issues at most N queries and fetches under X KB against
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production-sized fixtures. This class of bug is invisible at ten rows.
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- **Explicit column projections on read paths.** List endpoints name the fields they
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need rather than loading whole entities, so the next heavy column is opt-**in**. This
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is the structural version of what `deferred=True` does by hand.
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- **Row-width review rule.** Any new large column justifies itself or goes out-of-line.
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`actions` now carries five JSON columns.
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Deliberately **not** taken: moving `context_snapshot` out of the database. It costs
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nothing on reads now that it is deferred, and storage is ~$0.02/mo. Revisit only if
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backups or storage start to hurt.
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## Verification
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- Harness: one turn on adventure 25, memory bank **on**, before and after. Target is
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3,024 kB → ~600 kB cold, ~0 warm.
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- Re-run the round-two shapes with an embedding model configured, so the 200-action
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playthrough number is finally honest.
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- The existing `test_egress.py` guard must still pass — nothing here should touch the
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deferred action columns.
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