Chord is an application-composition runtime for systems assembled from plugins/extensions. It provides facets, services, replicated state, and a pluggable remote-service boundary. It is developed as a standalone package in the Pi monorepo, but it is not a Pi package: it does not depend on any other Pi workspace package and can be used by unrelated applications.

What Chord is for

A single application feature may need to run in several environments: for example, an agent worker, a terminal UI, and a remote WebUI. Chord provides the generic machinery to write such extensions in a way that is both delightful for humans as well as agents.

The design has a few connected pieces:

Plugins are synchronous setup units that declare the services they provide and require. After every plugin has declared its shape, a host validates the complete dependency graph, binds services, activates providers before consumers, and disposes resources in reverse dependency order. These units are called facets.
Facets are parts of a plugin. Each facet is bundled up separately and runs in the process or environment where it's supposed to run. You can use facets to split a plugin into separate pieces that need to be loaded into different processes and environments (think backend, browser, TUI etc.)
Services are typed, stable tokens with either one provider (singleton) or dynamic keyed instances (keyed). A service can be process-local, with an unrestricted JavaScript contract, or remotely exposable. Consumers retain a stable facade while a provider disconnects or is replaced.
Replicated state exposes authoritative state to local and remote connected consumers. Producers publish atomic copy-on-write transactions with change(context, callback); consumers receive complete immutable values. Draft proxies exist only during the callback and are revoked afterward. Chord compares the previous and next immutable revisions to produce one decoded operation batch, while each remote client/state stream owns independent path-codec state. Replicas become unready on disconnect or replacement until they are rehydrated.
Delta tracking records and coalesces operations over tracked plain JSON. It preserves common string and array operations, supports durable base batches, and validates untrusted operations as they are applied. Batches guarantee convergence but are not canonical or necessarily minimal.
Remote service sources advertise services available outside a facet host and open bindings for the services its facets require. Bindings carry logical calls and subscriptions through an application-supplied adapter. Chord requires strict-JSON arguments, results, snapshots, updates, and catalogues, but does not prescribe framing, routing, transport, or an application wire envelope. JsonRepresentation<T> derives a wire-safe type for application data with unknown payloads, while isJsonValue() validates received values at an adapter boundary. Symmetric RPC peers are planned as one optional implementation of this boundary.
Context Chord provides a Go-like context system for cancellation and invocation-scoped application values. Applications can carry permissions or telemetry through those values without Chord depending on either.

The current runtime exports service tokens, singleton and keyed providers, remote bindings, replicated state, facet hosts, and facet loaders from @earendil-works/chord. Import public types and general runtime APIs from the package root. Context constants and functions live in @earendil-works/chord/context because their generic names should not pollute the root API. Chord-owned identifiers use the chord.* namespace and its reserved service prefix is $chord.*.

Remote service adapters

Chord owns its transport-independent service wire grammar. Consumer adapters use createServiceCatalogueCall(), createServiceSubscribeCall(), and createServiceUnsubscribeCall() for $chord.service control calls. createRemoteServiceEndpoint() handles those calls for one provider consumer, including subscription activation and cleanup. parseServiceCall(), parseServiceCatalogue(), and the decoded/wire snapshot and update parsers validate Chord semantics after an adapter has established a strict-JSON boundary. RemoteServiceErrorCode and REMOTE_SERVICE_ERROR_CODES define the service errors that may cross that boundary.

Replicated state operations use one createServiceStateEncoder() at the provider side and one createServiceStateDecoder() at the consumer side for each subscription. Those registries create an independent Delta path dictionary for every instance/member state and reset it on replacement, unavailability, close, or fresh hydration. Applications may place these values inside any routing, request, response, or event envelope; Chord does not prescribe that outer protocol.

Tracking JSON deltas

Import the standalone delta primitive from @earendil-works/chord/delta:

import { apply, track } from "@earendil-works/chord/delta";

const changes = track({ output: "", count: 0 });
changes.flush(); // opening base batch
changes.state.output += "done\n";
changes.state.count += 1;

const ops = changes.flush();
const replica = apply({ output: "", count: 0 }, ops);

The first flush is always a complete base batch. Later flushes contain path-based changes. applyImmutable() applies those batches while preserving prior replica revisions. Replicated state instead uses transaction-scoped copy-on-write drafts:

const status = env.replicatedState({ output: "", count: 0 });
status.change(context, (draft) => {
	draft.output += "done\n";
	draft.count += 1;
});

A successful change() publishes exactly one atomic revision. If its callback throws, the original value and sequence remain unchanged. Draft handles are revoked when the callback returns and assigned containers are copied by value. Unchanged subtrees are shared between immutable revisions. Chord derives string append and front-truncate operations, array splices and permutations, sets, and deletes from the two revisions; a large delta falls back to a complete snapshot. Remote connection plumbing encodes each batch independently for every client/state pairing. replace(context, value) publishes a detached complete value directly.

The standalone Delta guide documents the lower-level mutable tracker, which remains available separately from replicated state.

Bundling and loading facets

@earendil-works/chord/bundler uses esbuild to turn ESM or TypeScript application entries into independent, content-addressed CommonJS files. The package-level API reads plugin identity and build configuration from package.json, then applies facet path conventions supplied by the host application:

{
  "name": "@example/my-plugin",
  "version": "1.0.0",
  "type": "module",
  "peerDependencies": {
    "@earendil-works/chord": "^0.84.4"
  },
  "chord": {
    "facets": {
      "worker": "./src/custom-worker.ts",
      "presentation": false
    }
  }
}

import { bundleFacetPackage } from "@earendil-works/chord/bundler";

await bundleFacetPackage({
	packagePath: "/path/to/my-plugin",
	outdir: "/application-owned/plugin-builds/my-plugin",
	defaultFacets: {
		worker: "src/worker.ts",
		presentation: "src/presentation.ts",
	},
});

Existing conventional files become entries unless chord.facets overrides or disables them. Peer dependencies are externalized and resolved against the host when loading. Chord never installs dependencies or runs package lifecycle scripts. bundleFacets() remains available as the lower-level API for callers that already have explicit plugin identity and entry mappings.

The output directory contains one .cjs file per entry plus chord-facets.json. Load one application-selected entry through the Node-only loader:

import { createFacetBundleLoader } from "@earendil-works/chord/node";

const loader = createFacetBundleLoader({
	manifestPath: "/application-owned/plugin-builds/my-plugin/chord-facets.json",
	entry: "worker",
	resolveExternal: (specifier) => import.meta.resolve(specifier),
});
const loaded = await loader.load();

Each load() verifies SHA-256 integrity and compiles the CommonJS body directly with node:vm instead of putting the plugin into Node's CommonJS or ESM module cache. Externals are resolved by the host and loaded through a restricted require; esbuild lowers dynamic imports so they use the same path. Disposing a retired generation releases the loader's facet references, making its compiled code eligible for garbage collection once plugin-owned resources are also gone.

For transport to another Node host, readFacetBundleArtifact() packages one verified manifest entry with its source, and createFacetBundleArtifactLoader() materializes fresh temporary generations while resolving externals against the receiving host.

To reload, load a candidate, pass its facets to FacetHost.reload(), dispose the candidate on failure, and dispose the retired LoadedFacets only after a successful cutover. The host activates and validates the candidate while the currently active providers remain routed, then replaces each singleton directly without an unavailable interval. Stable service handles therefore do not become disconnected during an ordinary reload. Keyed instances remain incarnation-specific and replacements receive fresh generations. The bundler writes a complete temporary directory before replacing the previous output, so loaders do not observe partially built generations.

See PLANNING.md for the broader RPC and generation-loading architecture.