writeonce/docs/stories/language-runtime-database/20-cross-program-tables.md

iteration	status
20	hold

Iteration 20 — cross-program tables: attach to a running program's database

Format: product/story-iteration-template. Part of Story — one language, one runtime, one database, one binary.

Inserted 2026-08-15, hence 20. It follows 9b because a program attaching to another's tables wants the same typed statements and queries the owner has — a surface that must exist before it can be shared — and precedes iteration 25 because HTTP is the external face of a program; this iteration is the writeonce-native face, program to program on the same machine.

No spec exists yet. This iteration frames the outcome and records the open forks; the design must be brainstormed before a plan is written. The forks in Info are genuine decisions, not details.

Goals

• A running program A with a persistent database (WO_DATA, iteration 9) can be attached by a second writeonce program B: B names A's IPC connection string in its own wo.toml, and from then on reads and writes A.Table rows with the same typed statements it uses on its own tables — checked by B's compiler against A's declared table shapes.
• A stays the single writer. B never opens A's WAL, never maps A's slabs: every statement B issues travels the IPC channel and executes inside A's engine, through the same choke-point row API A's own statements use. The ownership doctrine survives contact with a second process because the second process never touches the memory.
• Access is granted, never assumed. A's manifest registers B by name with explicit rights (read, or read+write); an unregistered client is refused at connect, an under-privileged statement is refused at execute with a trap B can catch. No registration, no access — including on the same uid.

Acceptance Criteria

• What to achieve?
  • Given A running with [share] registering client "b" as read+write, and B's wo.toml carrying [connect.a] with A's IPC string,
  • when B executes insert a.AuditLog { … } and a query over a.AuditLog,
  • then the row exists in A (visible to A's own queries, WAL-logged before B's insert acknowledges), and B's query returns it — with B's compiler having checked every field name against A's declared shape.
• What to achieve?
  • Given A registers client "c" as read-only,
  • when C executes a query it succeeds, and when C attempts an insert,
  • then the insert traps with the access-denied code inside C (catchable), and A's log records the refusal; nothing was applied, nothing was WAL-logged.
• What to achieve?
  • Given a program with no registration in A's manifest,
  • when it presents A's IPC string and attempts to attach,
  • then the connect itself is refused — rights are checked at the door, not per statement only.
• What to achieve?
  • Given B attached and mid-statement,
  • when A shuts down cleanly (SIGTERM) or crashes,
  • then B's in-flight statement traps with a connection error B can catch (never a hang), and B can re-attach after A reboots and replays — with every previously acknowledged write still present.
• What to achieve?
  • Given the employee sample running as A with its departments and employees tables,
  • when a second sample program (a thin reporting client) attaches read-only and runs the GroupBy report over a.Employee,
  • then it prints the same report the owner prints — the demonstration that attach + query compose.

Out Of Scope

• Remote machines. The IPC string names a local channel; cross-host access is the HTTP/service layer's job (iteration 25) or a much later network protocol. Same-machine is what "attach" means here.
• B caching A's rows. Every read crosses the channel; a client-side cache (and its invalidation) is a later performance iteration, if ever.
• Cross-program transactions. A statement is atomic inside A exactly as A's own statements are; B cannot open a transaction spanning its own tables and A's. That is 2PC territory, recorded with the database track's deferred items.
• LIVE subscriptions over the channel — composes with the subscription registry later (the client-api phase doc already sketches the wire shape).
• Schema migration while attached — a blue-green swap in A while B holds an attachment is iteration 26's compatibility problem; this iteration may simply drop attachments on swap.

Info

Prior art in the tree: docs/runtime/database/04-client-api.md already designs a native binary wire protocol for external clients (length-prefixed, typed, subscription-ready) — this iteration's channel should be its same-machine profile, not a new invention. The WAL's typed value encoding (database/src/wal.c, iteration 9 Task 2) is a working engine-value wire format today: statements and rows can ride the same encoding the log already uses. The wo.toml manifest exists and is compiler-read (woc <dir>), so both ends' declarations have a natural home.

Forks the spec must settle:

1. What carries the channel — and what does the IPC string name? A unix domain socket is the obvious carrier (peer credentials for free, net-stdlib adjacency); the string would be unix:/path/a.sock in B's [connect.a] and A would listen beside its WO_DATA directory. The alternatives — a FIFO pair, shared memory + doorbell — buy latency at the cost of the credential story and the crash-detection story (a dead socket peer is unambiguous; a dead shm peer is a protocol). Leaning: unix socket, one connection per attached client, A serving requests on its event loop (iteration 8's shard-actor loop when it lands; a dedicated accept loop until then — which is also the fork's dependency question: how much of iteration 8 does this need?).

2. How does B's compiler know A's table shapes? B typechecks a.Employee { … } against A's declarations, so B needs them at compile time. Options: B's [connect.a] names A's project directory and woc reads A's types straight from A's source (simple, but couples B's build to A's checkout); A exports a schema file (a .wob-adjacent digest of its class table) that B's manifest points at (decoupled, but a new artifact with a staleness story); or shared type definitions in a common module both import (cleanest language story, needs the module system to span projects). A runtime schema handshake must exist regardless — B's compiled expectation of a.Employee's shape is verified against A's live class table at attach, and a mismatch refuses the attachment with both sides' shapes named. Leaning: project-directory reference for the milestone plus the mandatory handshake; the export artifact when the staleness story matters.

3. What exactly does A's registration grant? The request's shape is per-client rights: [share] clients = [{ name = "b", rights = "rw" }] or per-table refinement (tables = ["AuditLog"]). Identity: the client NAME must be bound to something a peer cannot fake — unix peer credentials (uid), a token A mints, or both. Leaning: name + uid via SO_PEERCRED for the milestone (same-machine, same-trust-domain), rights whole-database read or read+write (per-table refinement deferred until a workload needs it), and the registration is A's manifest so a grant is a config change + restart, not an API. Superseded as the end state (2026-08-15): identity is a keypair and grants name public keys — iteration 21 owns that; the uid check is only this iteration's bootstrap and must be flagged pre-21 wherever it ships.

4. What does B's statement actually block on? B's insert crosses the channel, executes in A (RAM + WAL + fsync), and acknowledges back — a blocking round-trip on B's thread, exactly like B's own WO_DATA inserts block on their own fsync. Queries stream results back whole (materialized; no cursors over the wire this iteration). The alternative — async statements with completion callbacks — has no language surface to stand on (no function values) and waits for fibers (iteration 11). Leaning: blocking, with the stop-flag rule from the log-watcher work applying (a SIGTERM'd B parked on a channel read exits cleanly).

Proposed Solution

• Brainstorm the spec first, settling the four forks; then a plan. Expected shape: A-side — a listener beside the engine, a request dispatcher that executes through the same statement executors iteration 9 built (database/src/db.c), the registration check at accept and per statement; B-side — [connect.<name>] manifest surface, compiler namespace <name>.Table binding table statements/queries to channel stubs instead of local engine builtins; both — the client-api phase doc's wire protocol, profiled for unix sockets, values in the WAL's encoding.
• The acceptance workload extends the employee sample: A = the employee program with [share]; B = docs/examples/employee-list (pre-authored 2026-08-15, sample-first — both manifests designed as a pair), attaching read-only for the list/report/staff modes and proving the rights matrix with its probe-write mode. The sample stays the test.
• Depends on iterations 9 (engine, WAL — done through Task 3 as of 2026-08-15) and 9b (typed statements and queries worth sharing); wants iteration 8's event loop for A's serving side but can prototype on a dedicated accept loop the way the MCP sample serves today.