wanderbooks/docs/receipt-store.md

Receipt & document store + persistence spec

Three pieces: the journal (the source of truth), the blob store (the bytes), and SQLite (the queryable projection). All immutable-first, matching the ledger's philosophy: the books never forget, and neither does the filing cabinet.

The journal: the source of truth

SQLite is the query engine, not the source of truth. The source of truth is an append-only journal of domain events: one JSON object per line (JSONL), hash-chained. This is the deliberate answer to "insanely durable, easily backupable, journable":

• Durable. Every state change is an immutable fact appended to journal.jsonl, fsync'd per append. Each line carries the SHA-256 of the previous line's canonical bytes (prev), forming a tamper-evident chain: bit rot, torn writes, and casual editing are detected on read/verify, not discovered at tax time. Honest scope: this detects corruption and casual tampering. It does not stop a determined attacker with full write access from rewriting the chain — nothing local does.
• Backupable. The backup is cp -r $WANDERBOOKS_DATA /backup: the journal file plus the blob directory. Plain files, no WAL-file timing windows, no special backup API. A torn tail from a mid-copy is detectable (the last line fails hash/parse) and recoverable (truncate to the last good seq — the projection never saw it). Better: the journal is git-friendly, so an optional wanderbooks backup that commits it to a private git repo gives versioned, distributed, deduplicated backup with full history.
• Journable. It literally is a journal: {seq, ts, type, payload, prev}. Greppable, diffable, replayable. The audit trail isn't a feature bolted on — it's the storage format.

Write path

There is exactly one write path: append to the journal, fsync, then apply to SQLite. No dual writes, no background applier thread (the volume doesn't justify it): the MCP server appends, fsyncs, applies the new lines to the projection synchronously, and returns. A crash between append and apply is safe — on startup the applier replays every journal line with seq > last_applied_seq.

Line format

{"seq": 42, "ts": "2026-10-10T18:20:00Z", "type": "entry.posted",
 "payload": { "...": "..." }, "prev": "abc123..."}

• seq: 1-based, gapless, assigned by the writer.
• ts: UTC, RFC 3339. Recorded, never trusted for ordering (seq orders).
• type: account.added, entry.posted, entry.reversed, receipt.attached, receipt.linked, receipt.unlinked, document.filed, attachment.withdrawn (plus the A/R, A/P, asset, period, payroll, and class events listed on JournalLine).
• payload: the full domain facts (account fields, entry with lines, blob sha, ...). Everything the applier needs; nothing it must look up elsewhere.
• prev: hex SHA-256 of the previous line's canonical JSON (keys sorted, no whitespace). The genesis line uses "prev": "000...0" (64 zeros).

Drafts are journaled (durable)

Draft entries are durable: entry.created and entry.line_added journal lines record the draft as the agent builds it, so drafts (and receipts attached to them) survive server restarts. Drafts never touch the SQLite projection until posted — reports only see posted entries. A draft that never posts leaves its created/line_added lines in the journal, which is correct: the journal records what happened, including abandoned work.

Verification

wanderbooks verify (a ledger-store API, later a CLI subcommand): re-read the journal, recompute the hash chain, replay every line into a fresh in-memory Ledger, and compare its reports against the SQLite projection's. Any mismatch is a loud failure. Run it on startup (cheap at this scale) and before backups.

Blob store

• Content-addressed. The key is SHA-256 of the exact bytes. Path: $DATA/blobs/<first2>/<next2>/<full-hex>. Same bytes, same path, forever.
• Immutable. Blobs are never modified or deleted by the application. A retention/deletion policy is an explicit future human decision, not an accident of implementation.
• Deduplicated. Attaching the same receipt twice stores one blob and two attachment rows. attach_receipt / file_document return deduplicated: true when the blob already existed.
• Limits. 25 MB per file by default, configurable. MIME type is accepted from the client and stored; server-side sniffing is future work.
• Metadata lives in SQLite (blobs table), not in the filesystem: sha256 PK, size_bytes, mime_type, original_name, created_at.

Attachments & documents

• attachments: id PK, blob_sha FK, entry_id FK NULL, category, description, created_at.
  • entry_id is nullable: a receipt can be filed before its journal entry exists, then linked with books.link_receipt.
  • category mirrors today's Drive layout: formation, legal, tax, accounting (today's 01 Formation / 02 Legal / 03 Tax / 04 Accounting), plus receipts.
• Documents filed via books.file_document and receipts via books.attach_receipt share the blob store; they differ only in metadata.
• Receipts change by events, never by edits. receipt.unlinked (attachment_id, entry_id, reason) detaches a receipt from its entry; attachment.withdrawn (attachment_id, reason) retires an unlinked receipt or a document (withdrawn_at, withdrawn_reason columns, schema v5). Linking requires an unlinked, non-withdrawn attachment, so evidence only leaves an entry through a recorded unlink. Blobs are untouched: a withdrawn file is still hash-verified and readable, and the journal shows when it was filed, linked, unlinked, and withdrawn, and why.
• The store is policy-free: it records attachments faithfully. Whether a posting requires a receipt is decided by the MCP layer's receipt policy (docs/mcp-interface.md), which reads the attachments table.

ledger-store: the SQLite projection

SQLite is the read-optimized projection of the journal. Queries, reports, the MCP server's reads, and the future GUI all hit SQLite — and SQLite is disposable by design: rebuild drops it and replays the journal. Suspected corruption is fixed by rebuild, never by forensics.

• Location. $WANDERBOOKS_DATA/wanderbooks.db, falling back to ~/.local/share/wanderbooks/wanderbooks.db. The journal lives next to it at $WANDERBOOKS_DATA/journal.jsonl; blobs under $WANDERBOOKS_DATA/blobs/. WAL mode on, synchronous=FULL, integrity_check on startup.
• Schema. meta(key, value) — schema_version, base_currency, last_applied_seq; accounts, entries, lines, blobs, attachments as above; events(seq, type, payload, created_at) — the applier's output, a queryable mirror of the journal stream and the feed the future GUI tails. (Poll WHERE seq > ?; later SSE over the HTTP transport. This is the correct answer to "watch its files to update live" — watch the event stream, not the files.)
• Amounts as TEXT, never REAL. Unchanged and still load-bearing: a REAL column would silently reintroduce binary floats into the money path.
• Append-only, three layers. (1) The engine's API never mutates posted state. (2) The journal is append-only by file discipline plus the hash chain. (3) The applier only ever INSERTs into the projection. The SQLite triggers from the earlier draft are deliberately dropped: they protected the projection, which is disposable — the journal is the trust anchor, and piling mechanisms on the disposable layer adds complexity without adding safety. Simplicity is a durability feature too.
• Migrations. Numbered SQL files, applied in order, bumping meta.schema_version. Every migration is tested by rebuilding a projection from the journal and comparing reports before/after. A newer schema version than the binary understands is a hard error, not a guess.

Migration from Google Drive

Backfill walks the Drive Wanderland Labs LLC folders and files each document via the equivalent of books.file_document with its category. The Drive folders stay exactly as they are until the capability-parity cutover rule (see docs/capability-parity.md) is satisfied. Migration is an implementation task, not part of this spec — but the spec is written so the migration is a straightforward script, not a redesign.