wanderbooks/docs/architecture.md

Architecture

The one big idea: engine-first, everything is a command

ledger-core owns all accounting knowledge. Every state change is a method on Ledger:

• add_account — extend the chart of accounts
• create_entry / add_line — build a draft journal entry
• post_entry — validate and apply (one-way)
• reverse_entry — correct via reversal (one-way)

Each command validates before it applies, and each failure is a typed LedgerError variant. There is no other mutation path: struct fields are private, and constructors are pub(crate) where it matters. This is what makes the ledger auditable and what will make the future oracle harness trustworthy — the harness only needs to drive the command surface.

Reports (trial_balance, balance_sheet, unclosed_earnings, snapshot) are free functions: pure, total, and deterministic. Same ledger in, same report out, every time, on every platform. That determinism is the entire basis of docs/oracle.md.

The crates (2026-10-28)

Two Cargo workspaces: the engine and its application in crates/, and the specification in bookspec/ (which never depends on the engine).

Crate	Role	Depends on (ours)
ledger-core	the semantics engine: money, chart, journal, Ledger commands, reports, periods, assets, schedules, parties, A/R and A/P, payroll, projects, settings and conventions, the kernel and its Kani harnesses	nothing
ledger-vault	the TIN vault: XChaCha20-Poly1305 records under the book's key	nothing
ledger-config	the wanderbooks.toml codec (adoption in, books.export_settings out) and the receipt policy until P4 moves it into the core	ledger-core
ledger-store	the source of truth: the hash-chained JSONL journal, its format gate, the SQLite projection (schema 16; a file per schema), the blob store, backups, the access log, Snapshot for readers	ledger-core
ledger-import	formats: QuickBooks CSV in; IIF, XLSX, Typst PDFs (statements, the invoice, labels) out	ledger-core
ledger-package	the shared read-only view over Store or Snapshot (PackageBooks): the CPA package, the 1099 worksheet, the invoice document, Needs attention	ledger-core, ledger-store, ledger-config, ledger-import, ledger-vault
ledger-mcp	the books server: the MCP tools (122), resources, the handbook, scopes, the door and the access log, backups' scheduling	all of the above
ledger-host	the host and its leases, the agent bridge (wanderbooks mcp), tokens in the OS keyring, the owner's commands, uploads and downloads	ledger-store, ledger-mcp, ledger-config
ledger-gui	the viewer: read-only, live, egui; the Books menu hands work to the server	ledger-core, ledger-store, ledger-config, ledger-import, ledger-package, ledger-vault, ledger-host
ledger-cli	wanderbooks, the one executable: every mode (serve, server, host, mcp, gui, setup, books, token, connect, doctor, …) and the over-the-wire test suites	everything
bookspec/checker, bookspec/runner	the example schema and parser, the rule catalogue, the Engine trait; the per-rule runner	nothing of ours (the adapter in ledger-core's tests depends on them)

Dependency direction is always inward toward ledger-core; nothing in the core depends on another crate of ours. UI concerns (formatting, locale, widgets) never leak into the core; the core speaks Decimal and ISO codes. A crate that was planned and never created is not missing: ledger-export became part of ledger-import; ledger-oracle is the test crates/ledger-core/tests/oracle_gnucash.rs with oracle/gnucash/ (just oracle); ledger-view and ledger-extract were closed by D-53 (PackageBooks is the shared view; extraction is in-process in ledger-import, P6).

Pending (the P6 branch, 2026-10-27, unmerged): P6 made ledger-store depend on ledger-import so the writer can extract a file's text at attach time and on the backfill command. That keeps the direction inward but pulls Typst, the fonts and hayro into the store's dependency closure; whether it stands, or extraction moves to the server before the store call, is decided at P6's review.

ledger-core itself (crates/ledger-core/src/): money, account, journal, ledger (the command surface and replay), kernel (every money arithmetic, generic over Amount) and proofs (its Kani harnesses), reports, cashflow, analysis, periods, assets, schedules, recurring, payouts, payroll, form1099, tax, classes, parties, arap, settings, conventions, calendar, shares, tax_capital, rules, error; its tests in tests/ (invariants.rs, properties.rs, spec_*.rs, kernel_bridge.rs, model.rs, bookspec.rs, golden/).

Dependency rules

ledger-core may depend only on:

• rust_decimal — exact decimal arithmetic
• thiserror — the error type
• chrono — dates (NaiveDate; no wall-clock reads in the core)
• serde / serde_json — the oracle interchange format
• jiff — the bundled time-zone database behind company.timezone ("today" is computed by the caller and passed in; the core reads no clock)

New dependencies need justification in the commit message. In particular: no I/O, no networking, no async runtime, no logging framework in the core. The core is a pure domain library and stays that way.

Why Rust

Exact decimal semantics, exhaustive pattern matching on the five account types, and a type system that makes invalid states (mutable posted entries, float money) unrepresentable. The storytold pipeline this project is modeled on is Rust + egui for the same reasons; when the UI crate arrives it will be one codebase compiling to native and WASM.

The GUI (ledger-gui)

Read-only by construction: it opens a ledger_store::Snapshot (journal verified and replayed into its own Ledger, projected into a private in-memory SQLite, blobs read through the verifying store) and never writes to the data directory. A loader thread watches the directory and applies appended journal lines; the UI renders an immutable view model rebuilt per change. Statements and series come from ledger-core (reports, analysis); the GUI only arranges them. See plan/gui.md.

Shared read-only views (ledger-package)

ledger-package is the shared read-only view: packages, the invoice PDF, and Needs attention, over Store or Snapshot. Its PackageBooks trait is what those builders read of the books (ledger, settings, journal, attachments and their links and bytes, statements, reconciliations), so the server and the viewer make the same package, the same invoice, and the same attention list from one piece of code; nothing in it writes. The viewer's attention module only maps each item's Target to its own route, and books.start turns the items into its next steps, so the human's list and the agent's cannot drift apart. The crate depends inward (ledger-core, ledger-store, ledger-config, ledger-import, ledger-vault), never on ledger-mcp or ledger-gui.

The journal format gate (ledger-store, P0)

Every journal line passes a gate after its parse, seq and chain checks (ledger_store::format): a type this version knows, or a journal.format_set this version reads. A line it can't read stops the read whole (the writer never opens, the viewer applies none of the new lines): never skipped, never half-read. A line with a key beside the envelope's (seq, ts, type, payload, prev, frozen across formats) is refused the same way, never read with the key dropped. The refusal is NewerJournal, not JournalCorrupt: verified ("line N is intact; this version can't check the lines after it") when the next line's prev, read leniently whatever format frames that line, matches the refused line's hash (the SHA-256 of its canonical JSON, every key included); corruption when a prev it can read doesn't match; "newer or damaged" when there is no next line or no prev it can read. The writer (Store::open) and the viewer (Snapshot) read the journal the same way (bytes split on newlines) and share the same gate and settling (format::gate, format::settle), so they always give the same answer for the same journal (fuzzed by fuzz_store_replay). The writer checks every complete line before it changes anything: a torn tail is set aside only when the gate passes, so an older version never alters newer books. Rule: docs/spec/rules.md B-27.

How the books are kept, in the books (P1)

Settings, conventions, decisions, open items and approvals are journaled (journal format 2, declared with journal.format_set before the first such line, so a P0 reader stops there with "update wanderbooks"). The engine keeps them beside the ledger (ledger_core::settings::Settings, ledger_core::conventions::Conventions, fields of Ledger), so the store, its replay and the viewer's Snapshot share one reading; Ledger:: differs_from and verify compare them, and the projection has a table for each (schema 16 on 2026-10-28; a file per schema). ledger-config is the codec between them and a wanderbooks.toml (codec::import, codec::export): adoption (wanderbooks books adopt) moves an older books dir's file in through it, once, and books.export_settings writes the books' settings out as one. The books are their settings' only source (D-44): the server (settings::Live) takes them whenever their last settings.set changes (on every lock of the store), the viewer reads them from the journal it shows, and neither reads a file; "today" is the company's time zone's, set per request. Rule: docs/spec/rules.md B-29.

The MCP server's layout (ledger-mcp, P0; U0a)

crates/ledger-mcp/src/server.rs is the server itself: BooksServer, the shared store and its lock, call_tool and get_info (the ServerHandler), tool_router, the request machinery and the helpers that cross groups. Tool handlers live one file per group in crates/ledger-mcp/src/tools/, each declared with #[path], opening with use super::*, and holding a #[tool_router(router = <group>_router, vis = "pub(super)")] impl BooksServer; server.rs's tool_router adds every group's router. The groups are:

• documents — invoices, bills, payments, voids, books.documents, books.document, the control check and the agings;
• sent — books.mark_sent, the record that an invoice went out (P4b S5);
• entries — books.draft_entry, books.add_line, books.post_entry, books.reverse_entry, books.get_entry, books.search_entries, the bank CSV import;
• reports — the statements, books.show, books.verify, books.digest, books.export, the CPA package, the P&L by class, and the export builders;
• assets, schedules (schedules and recurring), periods, reconciliation, payroll, payouts, tax;
• parties — parties, vendors, customers, roles, W-9s, the TIN vault, 1099s, entry parties;
• attachments — receipts, filed documents, and their links;
• settings — the handbook, books.settings and the recorded FX rates (the module is settings_tools, since crate::settings is the live settings);
• chart — the chart of accounts, the template and CSV import, renaming and renumbering, and the control roles;

plus the groups split earlier: kept (books.start, settings changes, conventions, decisions, open items, posting templates, approvals), projects, files (uploads), instruments, access and backup. A helper only one group uses lives in that group's file; the ones several groups share live in tools/helpers.rs and server.rs re-exports them, so no group file reaches into another group's. The tests that span the whole tool surface sit beside server.rs in server_tests.rs; a group's own tests live in its group file. The split changed no tool's name, description, annotations or behaviour.

The MCP server's store lock (ledger-mcp, P0)

One Store is shared by every session (SharedStore: a tokio::sync::Mutex and a stopped flag). Tool bodies run on tokio's blocking threads (run_blocking), never on the async workers, and take the lock with blocking_lock; the guard is !Send, so no tool can hold it across an .await. The store journals a command before it changes memory, so a panic between the two would leave memory behind the journal; and a tokio mutex never poisons. So the server fails closed: a panic in a request (its task, or while it holds the guard) sets stopped, and every later request, on every session, is refused until a restart, which re-verifies the journal.

bookspec (bookspec/, P13)

bookspec is the formal accounting specification (D-34): every rule stated normatively, sourced, with a symbolic form and concrete examples, and a conformance harness any engine can be run against. It lives in its own Cargo workspace inside this repository until the harness is stable enough to move to its own repository (P13 step 6, D-34, Z-17):

• bookspec/checker is engine-independent: the example schema and its parser (Example::from_toml, every structure deny_unknown_fields, the cross-field shape checked by Example::validate), the rule catalogue (rules(), RuleId BS-<area>-<n>, the refusal-to-rule table) and the Engine trait (load, apply, query, claims, and change for a [[changes]] entry, which posts no entry and so is neither a setup nor an event).
• bookspec/runner drives any Engine through the examples and reports per rule: pass, fail or not claimed. It compares the engine's answers with the examples' exact expected values; it never asks the engine whether its own answer is right.
• bookspec/spec/rules-index.md maps B-1 to B-45 to the bookspec ids; docs/spec/rules.md carries each stated rule's id under its heading.
• crates/ledger-core/tests/bookspec.rs is the wanderbooks adapter: it implements Engine over Store (a dev-dependency only, so no engine crate depends on the specification) and remains the loader that checks every figure. ledger-package answers the formats appendix's IIF queries through the same adapter, as the package test does.

Neither bookspec crate depends on ledger-core; the dependency points inward from the adapter. just check runs the bookspec workspace's format, clippy and tests beside the wanderbooks workspace's, so an example cannot rot (P13 step 1).

Self-host: the same server, another account (P9a, D-51)

The books can live on a machine the owner controls (plan 3.8). It is the same ledger-mcp server, the same store and the same tools; only the process boundary changes. wanderbooks serve runs as the dedicated account wanderbooks under deploy/wanderbooks-server.service, over a data root only that account can read (/srv/wanderbooks, 0700). Agents have no account on that machine, so the isolation is enforced by the operating system rather than advisory: the server is told --isolation enforced (the flag, never a guess) and books.start reports service_user (enforced) (server::Isolation).

The door is the same HTTP service on a second listener: --http 127.0.0.1:7411 binds the MCP routes beside the Unix sockets (3.9), and the address is loopback only (any other is refused before anything binds: the door is plaintext with a static bearer, so the TLS proxy or tunnel belongs in front of loopback); Caddy, Tailscale Funnel or cloudflared puts TLS and the public name in front of it. The bearer token guards every call exactly as on the socket; the tunnel only forwards. The socket's peer-credential check (OwnerOnly) is for local callers and does not apply to the loopback door, which is why the bearer and the loopback-only bind matter there. The flag travels serve → ledger-host's host → server (Opener::starting_with, start::start_host, host::Args), so a host already running with other flags keeps them; on a self-host machine serve is the unit's only holder.

The unit sets IPAddressDeny=any and IPAddressAllow=localhost: the server reaches nothing out. A book whose backup target is a remote git repository therefore needs that host allowed in a drop-in (D-31 backups still run on the server). The recipe, the head check for the move and the two-account CI job are in deploy/README.md, deploy/wanderbooks-server.service and .github/workflows/two-user.yml; the checks are crates/ledger-cli/tests/two_user.rs.