pi-mono/packages/chord/docs/spec.md

Chord specification

This document specifies the runtime in src. Where it and the code disagree, the disagreement is a bug. Known limitations and decisions lists what the runtime does not do yet and the choices the sections below leave open.

Hard constraints:

• Chord's runtime depends only on its own delta module (tracker, applyImmutable, encoder/decoder, diff) and Node/browser built-ins plus typebox for schemas. No other Pi packages.
• Built-in plugins are trusted, loaded from source in-process. The plugin loader uses dynamic import() with a cache-busting query (owner-approved exception, TODO(loader): replace with an isolating, evicting loader; old module copies leak; TUI plugins need isolation).
• Hot reload is required. Host restart only for Chord/runtime code changes or host-level crash recovery.

1. Concepts

Term	Meaning
type	Name + contract. remote (addressable from other hosts; schemas for input, variant state members, method args/results) or local (host-only; members may be anything, including functions and native objects, but must be listed in the contract: method, opaque or a state schema).
factory	(input, principal, self) => behavior. Synchronous. Registered with host.serve(type, factory, options) from a plugin facet.
record	Stable identity per (type, canonicalInput, principalKey). Owns cells, resources, holds, listeners, tasks, registrations. Survives reloads.
behavior	What a factory run returns (methods + state members; kind for remote variants). Immutable, per epoch.
handle	What get returns. Late-bound. Never the record or behavior.
hold	One handle's claim. A record lives while held.
principal	App-defined. Stamped by links. In-host code acts as the host principal.
root	One short Chord call tree, admitted at trusted ingress (link request without root, UI event, timer, host.root). Carries { rootId, epoch }.
epoch	One committed set of behaviors/registrations per host (server-issued id).
code holder	Long-lived execution that is NOT a root: Durable phases/tool runs, browsers, self.task loops. Every Chord call they make starts a fresh short root.

2. Contracts

Contracts are runtime values for every type (shipped bundles have no TypeScript; third-party plugins are built separately). TS types are inferred from them.

export const Chat = remote("pi.chat", {
	version: 1,                                   // contract version: a plain number, bumped only on breaking changes
	input: Type.Object({ session: Type.String(), conversation: Type.Number() }),
	variants: {
		viewer: { state: { view: ViewSchema } },
		participant: {
			state: { view: ViewSchema },
			methods: { send: method(SendSchema, Type.Object({ submissionId: Type.Number() })) },
		},
	},
});

export const Index = local("pi.index", {
	version: 1,
	input: Type.Object({ session: Type.String() }),
	members: {
		search: method(Type.Object({ query: Type.String() }), Type.Array(Hit)),
		onChange: method(Type.Function([Change], Type.Void()), Unsubscribe),   // non-JSON allowed for locals
		harness: opaque("pi.durable-harness@1"),                               // native object, by name+version
	},
});

• Input identity: normalize (apply defaults, strip unknown), validate, then canonical JSON with fields equal to their schema default omitted. Changing defaults or normalization requires a new type name.
• Remote calls validate args/results and state values at the provider boundary. Local calls are not validated per call.
• Local contracts list every member (members: { search: method(...), harness: opaque(...), status: Schema }). A handle that is still loading must know whether h.x is a method (a caller that waits until live), a state member (a readable) or an opaque value, before any behavior exists. Members missing from the contract are found only once the handle is live.
• Every bundle embeds the contract tokens it was built against (its own copy). Fingerprint = hash of the canonical description.

Compatibility rule

A consumer built against version M binds to a provider at version N iff N = M and the provider's description covers the consumer's; otherwise only through accepts[M] adapters declared by the provider (serve(type, factory, { accepts: { 1: (current) => v1View } })); else contract_mismatch.

Covers: every consumer member exists with the same kind; methods: provider accepts every argument the consumer can send (new fields optional, nothing newly required or retyped) and the provider's result fits the consumer's expected result (extra fields fine); state: provider value fits consumer schema (extra fields fine); opaque: same name@version.

Additive changes need no bump. Breaking changes (remove/rename/retype, newly required, or a change of meaning with the same shape) bump the number. Same version + non-covering description = forgotten bump, refused. Meaning changes without a bump are undetectable (author's responsibility, as with Durable document versions).

Checked at: get binding, reload prepare (new provider token vs every surviving consumer holding the type and every loaded plugin importing it), remote holds (consumer token in the hold frame).

The exact host-owned eager bootstrap hold is a lifecycle pin, not a surviving old-contract consumer. When its provider is replaced, commit replaces that hold with one bound to the new provider contract, acquiring it before releasing the old hold so the same record, state and unchanged resources survive. It needs no accepts adapter for its own previous contract. Prepare/abort do not change that hold. All other consumers retain their own tokens and the normal coverage/adapter checks; state-version and migration checks are unchanged.

3. Factory and declarations

Factories are synchronous; returning a promise throws. Declarations are identified across factory reruns by property names in object literals (not call order, not separate string keys). Names are unique per record per declaration kind; a declaration method may be called more than once if names don't repeat. Holds are identified by their address.

host.serve(StatusStore, ({ session }, _host, self) => {
	const state = self.state({
		status: cell(StatusSchema, { running: false, tasks: 0 }),   // schema required, initial value
		errors: cell(Type.Array(Type.String()), []),
	});
	const catalog = self.get(Catalog, {});                         // hold; identity = canonical address (+ at)
	const res = self.resources({
		watch: resource({
			rev: 1, deps: [catalog], exclusive: false,
			open: async ({ signal }) => openWatch(session, signal),
			close: async (w) => w.close(),
		}),
	});
	self.listen({
		changes: listener(res.watch, (w) => w.events, (event, ctx) => state.status.change(ctx, (d) => { d.tasks = event.tasks; })),
	});
	self.tasks({ poll: async (ctx) => { /* loop until ctx.signal aborts */ } });
	const derived = self.derive({ visible: derivation([state.status], (s) => project(s)) });  // synchronous
	self.register(Routes, { "GET /status": handler });              // registry keys are the registry's own keys; eager records only
	return { status: derived.visible, report: (args, ctx) => { state.status.replace(ctx, args); return null; } };
}, {
	state: { version: 2, migrate: { status: (old, from) => upgrade(old, from) } },  // migrate gets the old value and its version
	linger: 30_000,
	eager: false,
});

Declaration	Survives reload	Notes
state({ name: cell(schema, initial) })	same cell and value; migrated on state-version bump	Schema required (no TypeScript at runtime). change(ctx, draft => void), replace(ctx, value), value, subscribe. Writes through change touch only mutated paths.
get(type, input, { at? })	kept if declared again (same address)	returns a handle
resources({ name: resource({ rev, deps?, exclusive?, open, close }) })	kept unless rev/deps change	open runs at prepare (non-exclusive) or at the gate (exclusive); handle exposes .value
listen({ name: listener(source, select?, fn) })	same subscription while source (resource) and select (identity or source text) are unchanged; fn swapped	each event starts a fresh root in the current epoch; select is not re-run for a kept subscription
tasks({ name: fn })	restarted with new code at commit (old one aborted via ctx.signal)	a code holder; calls inside start fresh roots
derive({ name: derivation(sources, fn) })	kept; fn swapped	synchronous fn
register(registry, { key: value })	value swapped at commit	eager records only (v1)
expose(resource)	follows the resource	a local member (usually opaque) whose handle value is the resource's value once live

State members of a behavior may be cells, derivations, or state members of handles the record holds (they are defined once the record is live). cell.replace(ctx, value, { diff? }) publishes a structural diff when the new value shares containers with the old one (or diff: true), else r. Facet setup receives { serve, name, root, registry } of its host.

migrate per cell is (old, fromVersion) => new. A record may be several versions behind (author bumped more than once between reloads).

Removed vs guide: self.own, holdWhile, async factories, inferred dependencies.

Synchronous factory, asynchronous activation

self.get and self.resource only declare and return placeholders. Activation of a record:

1. run the factory synchronously (declarations);
2. await declared resources' open() (e.g. connect a DB);
3. await declared holds becoming live (dependencies activate first, coalesced);
4. record is live: handles live, ready resolves.

Inside the factory, placeholders must not be used (resource.value throws not_ready; handle calls throw). Methods/listeners/tasks run only on live records, so their dependencies are live. External consumers (UI, roots) holding a loading handle: calls wait until live (ctx cancellation, deadline); state members read undefined until hydrated. Activation failures propagate to dependents and all waiting gets; the next get retries.

Host eager readiness

• host.install(facet, { awaitEager? }) installs declarations synchronously and returns a promise for this facet's eager activations. host.load(facets, { awaitEager? }) installs the entire batch before awaiting its eager activations. awaitEager defaults to true. Unheld lazy objects are not activated.
• Factory, resource-open, dependency and activation-finalization failures reject the returned promise with the activation error (ChordError codes preserved; ordinary errors wrapped as unavailable). Eager failures also reach onError. Installation is not a rollback transaction; setup/registration errors from install throw synchronously. Abandoned startup should close the host.
• { awaitEager: false } starts the same eager objects but resolves after installation, without waiting for their activation. host.ready() awaits a snapshot of the host's eager holds at call time (including direct eager serve and deferred loads), rejects their activation failures, and does not acquire additional holds. It is not an ongoing health check and does not await task completion or remote state hydration.
• Local bootstrap readiness and upstream admission are separate. A worker connects its upstream link, loads with { awaitEager: false }, awaits its local bootstrap handle (e.g. Harness, with no upstream dependency), then announces control readiness so its parent can accept the link. Only after admission does it await host.ready(). Otherwise an eager reporter depending on server SessionStatus deadlocks with the parent waiting for control readiness. Deferred loads skip the whole barrier, not selected dependencies; callers explicitly choose local bootstrap handles, keep bootstrap busy until full readiness, and propagate later startup errors.

4. Principals, variants, handles

• createHost({ principalKey }) maps principal to a key that includes connection provenance and grant revision. Records are per principal key. host is a built-in principal for in-host calls.
• Remote factories return { kind, ...members }; kind must be a declared variant; members must match it. Local factories return any object.
• Handle: status (loading | live | released | gone), ready promise, kind, shape ({ kind, members, contract }), onStatus(listener) (every status change, with the error for gone), late-bound members, release(). A remote handle's state member holds a value only while subscribed (undefined after its last subscriber leaves). Shape changes (reload, variant) are pushed; members missing from the current variant reject not_in_variant; released handles reject released; forced disposal makes handles gone.
• Methods are (args, ctx) => Promise<result>. Factories still check arguments; variants are not complete authorization.
• Revocation: host.invalidatePrincipal(match) force-disposes matching records on this host and propagates over links; links re-stamp or close.

5. Lifetime

• Hold owners: record (declared get), call (ctx.get, released at settle), connection (remote holds; relay maps per origin connection), host (eager), UI.
• Activation coalesced per address; if all requesters leave before it completes, the result is disposed. Factory throw: unwind declarations, reject gets, next get retries.
• Disposal after last hold + linger: tasks aborted, listeners removed, resources closed (reverse order; all run; errors aggregated), holds released (cascade).
• Bulk forced disposal (host.close, invalidatePrincipal) goes in waves, holders before what they hold, so a resource's close can still use its record's holds; host.close closes links after the records.
• Forced disposal: handles gone immediately, new calls rejected, in-flight call contexts aborted, then cooperative cleanup.
• Hold graph acyclic in-host (checked at get); across hosts by convention.
• Links have ping/timeout; a dead link releases everything held through it.

6. Roots and epochs

• Ctx = { signal, rootId, epoch, get(...) , principal? }. Explicit; no AsyncLocalStorage dependency (may be used in dev to warn).
• A link creates a root for a request that carries none; nested remote calls carry {rootId, epoch}.
• Dispatch: a call with epoch E runs the record's behavior for E. A host retains at most current and previous epochs (two-version rule). A call for an epoch the host doesn't have (older than previous, or host started later) is rejected superseded (retryable by starting a new root). Never substitute another epoch.
• Browsers never carry epochs into the server: browser calls start roots at the server. Browsers learn of changes via shape/reset pushes and a server Manifest object; incompatible contract fingerprint -> upgrade_required.

7. Reload

Explicit trigger (reload_plugins tool, button, API). Serialized; no nesting.

1. Build/type-check gate (application tooling, outside Chord): dependency-closed set; refuse breaking remote contract changes without a contract version bump.
2. Load: on every affected host, import new facet modules (cache-busted).
3. Prepare: for each live record of each affected type, run the new factory in recording mode: existing cells readable, writes throw; listeners/tasks/registrations recorded, not started; holds recorded; non-exclusive resources whose key is new or rev/deps changed are opened as candidate-owned (closed on abort). Validate keys, variants, registry collisions. New records activated during prepare use the current epoch (old code). Any failure: abort, discard candidates. Vote.
4. Gate (only records whose state version was bumped or which replace an exclusive resource, plus records depending on those resources):
   • Admission cut: from now, an old-epoch root that has not touched the record gets upgrade_conflict (retryable), not queued. Roots that already hold an activation lease on it are grandfathered; their re-entry is admitted.
   • Wait for grandfathered activations to finish, deadline (default 5 s). Missed deadline: abort the reload, reopen, nothing changed (report blocking roots).
   • Exclusive resources: close old, open new; on failure reopen old and abort.
   • Run migrate into private copies; validate.
5. Commit (server commits itself, then sends commit to children; the decision is not persisted, see section 12; FIFO links guarantee no host receives a new-epoch root before committing): each host in one synchronous step installs migrated cell values and state versions, installs behaviors as epoch E', swaps registry tables, restarts tasks with new code, rebinds listeners, pushes shape/reset to subscribers, opens gates. E' current, E previous. The coordinator's result carries children: each participating child's prepared report (its vote).
6. Retire: when no roots of previous remain (or on the next reload, which first waits for retirement with a deadline), drop its behaviors and release declarations only it declared.

Compatible changes (no state bump, no exclusive resource change) skip the gate: no waiting.

State compatibility rule: reload prepare compares each cell's old and new schema. Unchanged: compatible (no gate). Changed with the state version bumped: gate + migrate, output validated against the new schema. Changed without a bump: reload refused. A change of meaning with an identical schema needs a bump too (undetectable; author's responsibility). Initial values and migrate outputs are validated; per-write validation in dev mode.

8. Registries

export const Routes = defineRegistry<RouteHandler>("http.routes", { conflict: "error" });
self.register(Routes, { "GET /download": handler });       // in an eager record; keys are the registry's keys
host.registry(Routes).get("GET /download", ctx);            // table of ctx.epoch (or current without ctx)
host.registry(Routes).subscribe((table, epoch) => ...);     // bridges for imperative consumers (Durable)

Per-epoch immutable tables; order = first registration order, kept on replace. An application can implement Durable's RegistryReader from the table (snapshot per epoch) so phases pin a complete snapshot; Chord does not ship that bridge.

9. Hosts and links

• createHost({ name, principalKey, upstream? }); host.accept(link) for children; host.route(prefix, { authorize(principal, hostName), resolve(hostName) => Promise<Link> }).
• Links: memoryLinkPair() (tests), Node IPC (child_process), WebSocket (server ws, browser WebSocket).
• Principal stamping: the accepting side stamps the principal of what connects (from its authenticator); forwarded principals accepted only from the parent link; payload principal fields ignored.
• A child that connects behind its parent's epoch is fenced (nothing routed to it) and reconciled by a reload of its modules to the parent's epoch. Calls its roots of the old epoch make upstream are admitted in the parent's current epoch (the parent never ran that epoch), not rejected superseded.
• Relay: server forwards frames for at hosts to the resolved child link, mapping holds per origin connection; authorizes before resolving/spawning and re-checks after.
• Wire frames (JSON): hello/bye, hold/release (a hold returns shape or gone; there is no separate get), shape, call/result/cancel, sub/unsub and state (snap/ops/reset with a per-subscription delta encoder), gone, root_end, retire, invalidate, ping/pong, reload control (prepare/vote/commit/abort).

10. Host manager (library)

createHostManager({ driver, keepAlive }): fork driver (entry, init, lock, env), serialized start/stop/replace, exit-before-replace, process-tree kill after grace, control channel (ready/fatal/shutdown), liveness callback (booting, activeCalls, viewers with heartbeats, durable tasks, idleFor).

11. Test coverage

The tests cover at least: lifetime/holds/linger/forced disposal/gone; activation coalescing and abandonment; principals and variants; remote calls over memory links incl. relay and principal forgery attempts; state snap/ops/reset over a link with reconnect; registries per epoch; reload: compatible (no wait, pinned roots), prepare isolation (writes throw, failing factory leaves everything unchanged), state bump with migrate and gate, upgrade_conflict for late old roots, deadline abort, exclusive resource replacement, cross-host commit ordering, two-version retirement and superseded.

12. Known limitations and decisions

Still open

• Retirement across hosts. A child retires an epoch only after its parent retired it (retire frame) and its own roots of that epoch ended; the server decides from its own roots. A root started on a worker that calls up to a server which has already retired the epoch both had still gets superseded (retryable).
• The commit decision is not persisted (section 7, step 5). A server crash between its own commit and a child's commit leaves that child on the old epoch until it reconnects and is reconciled.
• One level of reload participants. Only the coordinator's direct child hosts take part; a child does not forward prepare to its own children.
• Reconciliation needs the child's module list. It is remembered from cluster reloads (host.remoteModules), so a server restart forgets it, and a child behind a restarted server then reloads no modules: it gets the epoch id with its own code. The reconciliation vote deadline is fixed at 30 s; a failed reconciliation closes the link, so the host manager restarts the worker.
• Links do not resume. The provider releases everything held through a closed link at once, so re-held records are new ones. Keeping holds for a reconnect window would need session ids on links. Calls in flight when a link drops reject unavailable; whether they ran is unknown.
• Refused clients cannot tell. The WebSocket adapter closes unauthenticated sockets; the client sees a dropped upstream and its calls wait for a reconnect until their ctx is aborted.
• The publication queue is process-wide and synchronous. A slow observer delays every later notification in the process, including those of other hosts in the same process.
• No automatic restart. A terminal host failure is picked up by the host manager's keepAlive or fatal path, not inside Chord.
• The Node loader leaks. Every reload imports fresh module copies that are never evicted, and it does not isolate plugins (TODO(loader)).
• Stale socket locks. On Unix systems other than Linux, two processes that probe the same stale socket file at the same moment could both take the lock. Managers serialize starts per lock name.
• Listener select identity. A listener's subscription is kept across factory runs when its origin and select are unchanged; select counts as unchanged when identical or with the same source text, so a select whose behavior depends on captured values that change between runs is not detected.
• Shutdown and dead links. host.close() disposes records in waves (holders before what they hold) and closes links last. A close that awaits a remote call over a link that is already down waits for a reconnect; bound such calls.

Decisions

• Errors. The guide's codes; a reason field carries the detail, for example invalid/args, invalid/cycle, invalid/duplicate, invalid/read_only, invalid/not_eager, invalid/registry_conflict, or unavailable/unknown_type. A ChordError thrown by a factory or a dependency keeps its code through every dependent; other errors become unavailable, with the original as cause.
• Reload errors. There is no reload_aborted code. A failed reload throws the cause's code and reason with the message reload aborted: … (the cause is attached). Reasons of its own are aborted/gate_deadline (with the blocking roots in details), aborted/deadline, aborted/vote_deadline and unavailable/reload_running. Across hosts, the message names the host that failed. A child cannot reload on its own (unavailable/not_coordinator).
• New records during prepare. A reload pre-allocates the next epoch ("pending"). Types the same reload adds are served from it; their records install behavior only for the pending epoch and are force-disposed if the reload aborts.
• Gate admission. From the gate until commit or abort, creating a new record of a changed type fails with upgrade_conflict (retryable, reason gate); nothing is paused, so nothing can deadlock. Gated records get minEpoch = E': calls and writes carrying an older epoch are rejected upgrade_conflict permanently, and a gated record drops its previous-epoch behavior at commit. Only records whose state version or exclusive resources change are gated.
• Tasks and listeners during the gate. Tasks of gated records are aborted at the admission cut and restart with new code at commit, or with the old code on abort. Listener events for gated records are queued and replayed through the new listener at commit, or the old one at abort.
• Grandfathered roots are the roots that called into the record and are still running at the cut. Every host remembers on which links a root's calls went out and sends root_end on them when the root ends, so leases outlive calls across hosts.
• Removed types and missed retirement. Records of removed types are force-disposed at commit. When a retirement deadline is missed, retirement is forced, so those roots get superseded on their next call.
• Two-version rule. A record of a changed type created after a commit has only new code; a previous-epoch root calling it gets superseded.
• Validation. Remote calls are validated at handle dispatch, in-host too: a direct binding against the provider's schema, a binding through an adapter against the consumer's. A provider validates each snapshot and reset of a remote state member once per (re)subscription, against the consumer's contract through an accepts adapter and the provider's otherwise; ops are validated only in dev mode. At reload, the new provider's description must cover the old provider's when the version is unchanged, in addition to the check against live consumers and facets' uses.
• Contracts. Local contracts list their members, so a loading handle knows whether a member is a method, state or opaque. Canonical identity omits input fields equal to their default; with TypeBox, use Type.Optional(Type.X({ default })) to let callers omit them. Provider variants must cover consumer variants; members the consumer's contract does not list are hidden. Schema change detection compares canonical JSON, so reordering anyOf branches counts as a change. Fingerprints use a non-cryptographic 64-bit hash: synchronous and browser-safe; contracts are trusted.
• Principals. A root ctx at ingress acts as its principal (host.root(fn, { principal })); contexts inside objects act as host. The acceptor decides a link's kind and name (accept(link, { principal, kind, name })); only configured host links are addressable, carry roots and may vote, and control frames are accepted only from the parent. Clients never carry roots or epochs. Relaying needs a matching route; authorize(principal, hostName) runs before resolve and again after, and revoking or re-stamping a principal closes its relays at once. invalidatePrincipal(match) re-stamps or closes matching links and propagates pattern matches to child hosts.
• Cells and readables. A value returned from change replaces the value and is always validated; replace transfers ownership without copying; by default ({ diff } overrides) it publishes a diff when the new value reuses a container of the old one at the same key, else r. subscribe(fn) delivers values, starting with the current one; events(fn) delivers snap, then ops, or reset. Handle state members are stable readables, undefined while loading or outside the variant, and reset when their source moves. Remote state members hydrate only while subscribed and forget their value when the last listener leaves.
• Holds and calls. A hold's identity is its address; a second self.get of the same address in one run returns the same handle. A handle call without ctx, or with a task ctx, starts a fresh root in the current epoch; a ctx whose call or root already settled is refused (aborted/expired), one from another host invalid/foreign_ctx. Calling a handle inside a factory throws not_ready.
• Resources and disposal. New and replaced exclusive resources open at the gate, and so do non-exclusive resources of the same record that depend on them. A new record for an address that is still being disposed waits for that disposal. A resource open that completes after its signal aborted closes its value at once.
• Links and the host manager. Every received frame is validated against its schema; malformed frames, duplicate ids and control frames from the wrong side close the link. Each connection pings every pingInterval and closes after pingTimeout without a frame. The child holds a kernel lock (an abstract socket on Linux and Android, a named pipe on Windows, a probed socket file elsewhere), acquired by childControl().lock() before storage opens. The manager kills the child's process group on every exit path and derives booting and idleFor from the child's liveness reports. WebSocket support is structural, so Chord does not depend on ws.
• The state API stays. Objects keep named cells and derivations; giving each object exactly one state value was rejected. Do not reopen this without a new, concrete problem.