cc-os/docs/memory-system/03-architecture-decisions.md

Architecture Decision Records

A running log of decisions and why. Format per entry: Context · Decision · Rationale · Alternatives rejected · Status. Newest decisions extend the log; supersede rather than delete.

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ADR-001 — Two memory types, kept as separate systems

• Context: Earlier attempts to make one tool serve both "what happened" and "how do we do X" felt forced (e.g. trying to make memsearch filter knowledge by tags).
• Decision: Model episodic memory and semantic/knowledge memory as two separate systems with different tools.
• Rationale: They have different lifecycles (episodic accretes and decays; knowledge is deliberately maintained), different write paths (auto-captured vs curated with guardrails), and different query patterns ("when did we…" vs "how do we…"). Separation dissolves the earlier integration tension entirely.
• Alternatives rejected: One unified store (memsearch-for-everything, or OpenBrain's single thoughts table) — conflates the two and forces awkward filtering.
• Status: Accepted.

ADR-002 — memsearch for the episodic layer

• Context: Need timeline/"what happened" memory (Goal 3) that's NL-queryable and lazy.
• Decision: Adopt memsearch (Zilliz) off-the-shelf for episodic memory.
• Rationale: It already implements the OpenClaw daily-notes + "dreaming" pattern and the markdown-as-truth / disposable-shadow-index philosophy we'd otherwise build. Embedded Milvus Lite (single file), hybrid BM25+vector+RRF search, local ONNX embeddings (no API key/cost), a FileWatcher that handles deletions — no Docker, no server. Two-line install.
• Alternatives rejected: claude-mem (MCP-based — Claude must actively call search; opaque blobs vs readable markdown; overkill features). Hand-building daily notes + dreaming ourselves (reinventing a solved tool).
• Status: Accepted.

ADR-003 — Flat vault with namespaced tags, not folders

• Context: Connelly/Huryn organize by folders (tools/, domain/). User wants a flat Obsidian vault with tags as virtual indexes, and cross-cutting filters (client × tool × convention).
• Decision: One flat markdown vault; organize via namespaced, nested tags (tool/, client/, domain/, convention/, scope/). Slashes are valid Obsidian nested tags, so #tool matches all children.
• Rationale: A note can carry several namespaces at once (tool/semrush + client/sesame3g + convention/react-ts) — folders can't express that. Enables "filter by client+tool to narrow the index." Enumerable virtual indexes ("what clients/tools exist").
• Alternatives rejected: Folder hierarchy (single-axis; can't do cross-cutting filters). Pure-prefix path filtering via memsearch source_prefix (would force directories back in).
• Trade-off accepted: Tags give the human/Obsidian free filtering, but the AI gets nothing for free from tags — we must materialize them into a queryable index (see ADR-004).
• Status: Accepted.

ADR-004 — SQLite + Sequel (Ruby) tag index as the knowledge-layer cache

• Context: The AI can't use Obsidian tags directly; tag filtering needs a machine-queryable index. A previous ~/Documents/SecondBrain/ tag database was lost track of.
• Decision: A small Ruby program using the Sequel ORM over SQLite, exposed as a CLI. Schema: files(path, mtime, summary, scope), tags(name), files_tags join (many_to_many). The summary is a column on files (an attribute), not a join.
• Rationale: Normalized tags table makes enumerating the vocabulary a first-class cheap query (the "virtual index" goal). The summary column is what turns the index from a finder into a router — the AI sees enough to pick a file without opening it (progressive disclosure, low tokens). Ruby + Sequel + CLI keeps the contract clean and the DB swappable; the AI never touches SQLite directly.
• Failure-mode guard (the lost-SecondBrain lesson): markdown is always authoritative; the SQLite file is a disposable cache that is never synced and can be rebuilt from frontmatter anytime (index update --rebuild).
• Alternatives rejected: Plain-markdown generated INDEX.md (must regenerate; grep-at-scale is token-heavy). Frontmatter grep on demand (scales badly). Milvus/Postgres for knowledge (overkill; QMD/memsearch prove SQLite is enough — see ADR-006/008).
• Query output: returns path + summary + matched tags (option C) — tags are cheap and show why a result matched, useful for cross-client queries.
• Status: Accepted.

ADR-005 — Structured-first; semantic search over the vault deferred

• Context: Tag filtering ("client/sesame3g + tool/semrush") may miss notes whose wording doesn't match the query ("how do we use semrush" vs a note titled "search analytics integration").
• Decision: Ship the knowledge layer structured-only (tags + summaries). Defer meaning-based search over the vault until it demonstrably bites.
• Rationale: Structured tagging is the lightweight/fast thing the user wants, and the summary+tag design is built to make it work. Follow the video's "only level up when it bites."
• Status: Accepted (semantic deferred).

ADR-006 — QMD as the (deferred) semantic-over-knowledge layer

• Context: When ADR-005's structured-only proves insufficient, we want a set-and-forget semantic layer over the vault, local and Docker-free.
• Decision: Earmark QMD (github.com/tobi/qmd) for that role; do not install yet.
• Rationale: Local markdown search using SQLite + FTS5/BM25 + local vector embeddings (EmbeddingGemma-300M GGUF) + LLM rerank; CLI + optional MCP server; no Docker, no API keys. Validates that SQLite + a local vector model suffices (no Milvus/Postgres for knowledge). Complements the tag index (QMD filters by path/collection context, not first-class frontmatter tags), so it adds semantic recall without replacing structured filtering.
• Alternatives rejected: Pointing memsearch at the vault (mixes episodic and knowledge corpora; its filtering is path-prefix not tags). A bespoke embedding index (reinvents QMD).
• Status: Deferred / earmarked.

ADR-007 — Lazy freshness: write-hook + session-start reconcile, no daemon/cron

• Context: The cache must reflect new/edited/deleted/renamed notes without becoming a resource hog or going stale on renames.
• Decision: Option A (lazy). A PostToolUse hook updates the index on AI writes (single-file, prunes on delete). Manual edits are caught by a session-start reconcile (index update --since + prune of vanished paths). No daemon, no cron.
• Rationale: The AI is the primary writer, so write-time hooks give event-driven freshness with no polling. The user rarely edits the vault by hand, so a session-start reconcile is enough; a continuous inotify daemon (the listen gem) would add an always-on process to manage/sync for negligible benefit. Matches the user's "lazy sync is fine" stance.
• Alternatives rejected: inotify/listen daemon (live freshness, but always-on process to manage — unnecessary). Cron reconcile ("seems silly" per user; session-start covers it).
• Status: Accepted.

ADR-008 — Markdown-as-truth; sync the vault, not the indexes

• Context: Must be accessible on a VPS / multiple machines but run local-fast (Goal 4).
• Decision: Sync the markdown vault to the VPS via git or Syncthing (choice deferred to build time). Indexes (Milvus Lite, future QMD) are rebuilt per machine and never synced.
• Rationale: Markdown is plain text — git/Syncthing sync it trivially; lazy (hourly or continuous-async) is enough. Indexes are disposable caches; syncing binary DBs invites conflicts for no gain. Local reads stay fast; ownership and portability stay with the user.
• Alternatives rejected: OpenBrain / Mem0 hosted DBs — always-remote, adds per-query latency and monthly cost, conflicts with local-fast; ownership weaker (Mem0 especially). Only worth it for real-time cross-tool memory, which the user called overkill.
• Status: Accepted.

ADR-009 — Package as a global Claude Code plugin with skills

• Context: Every project, on every machine, should know how to use the vault — write conventions, query patterns, the hooks, and the CLI — without per-project setup.
• Decision: Ship hooks + scripts + CRUD know-how as a global Claude Code plugin with skills, installed at the user level.
• Rationale: Skills carry the "when to write / what conventions / how & when to query" guidance to the model; the plugin registers the session-start / session-end / PostToolUse hooks and bundles the Ruby CLI. Global install = consistent behavior everywhere; single source of truth for the conventions themselves.
• Status: Accepted (to be built — see 04-build-plan.md).

Rejected tools (summary)

Tool	Why rejected for our use
MemPalace (L4)	Storage not readable markdown; isolated drawers (knowledge not interconnected); fights self-managing + cross-linking goals
Recall / LightRAG (L5)	Content knowledge bases / deep research, not operational memory; Recall = hosted, you don't own data; LightRAG = enterprise overkill
OpenBrain / Mem0 (L6)	Always-remote DB → latency + cost; conflicts with local-fast lazy-sync; only pays off for real-time cross-tool memory (user: overkill)
Postgres / Milvus server	Unnecessary — SQLite (tag index) + Milvus Lite (memsearch) + QMD's SQLite cover everything locally with no Docker
claude-mem	MCP-based (Claude must call search); opaque blobs vs readable markdown; feature overkill