pi-mono/packages/chord/docs/guide-plugins-hosts/book.md

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title: Hosts and Plugins: a tutorial for Chord v2 cover: Hosts / and Plugins kicker: A guide · Chord v2 · 2026-11-02 subtitle: Build a small notes app step by step, from one host to plugins on a server and on workers, edited while it runs stats:

• 3 | kinds of host in an app: server, worker, client
• 1 | file per host kind in a plugin
• 0 | restarts to apply a plugin edit
• 1 | rule: plugins talk only through object types and registries note: | A tutorial for people building apps on Chord v2, written by the redesign session from design.md, slice-7c.md, slice-10.md, slice-12.md, slice-13.md, the decision log and the code in packages/chord/src. API names are the current ones. The early chapters wire hosts by hand to show the parts; the last chapter shows the app kit (chord/app), which does that wiring for an app. colophon: Hosts and Plugins · third edition · 2026-11-03 · Chord v2 slices 1-10, 12 and 13, messages and languages

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Before we begin

You want to build a notes app. You type a note in a browser tab, and every other tab sees it at once. Each project gets its own notes in its own process, so a project that misbehaves cannot take the others down. And you, or an agent you work with, add features while the app runs: a search, an export button, a word count, without restarting anything.

Chord gives you three things for that. Hosts: the processes of your app (the server, a worker per project, the browser tabs), connected to each other. Objects: the state and methods your app is made of, living on one host and used from any other. Plugins: directories of code that declare objects for each kind of host, loaded and reloaded while the app runs.

This book builds the app in that order. Each chapter adds one concept and shows complete code.

Seven things to know

:::findings

1. A host is one participant. The server, each worker, each browser tab and each terminal UI is a host; hosts connect over links. Tabs and terminal UIs are clients: no address, they connect to the server and ask.
2. An object has a type, and a type is a contract. The contract names the object's state and methods; any host can ask for an object by type and params, and Chord finds where it lives.
3. A facet declares, an object acts. A facet is a plugin's code for one host kind: it says which object types this host serves. The objects hold the state and do the work.
4. A plugin is a directory (or a single file). A package.json with the plugin's name, and one file per host kind: server.ts, worker.ts, web.tsx. It imports its host only through the app's versioned API (@app/api/v1).
5. The server builds, every host evaluates. The plugins component builds each facet into a small script; the server, workers and tabs load the scripts for their kind.
6. Plugins meet through types and registries. One plugin uses another's objects through its contract, or contributes to a registry another defines. Nothing else crosses between plugins.
7. An edit is a rollout. Change a file, build, roll out: the server, then every worker, swap the plugin's objects in place; a broken edit keeps the old code running and reports why. :::

:::flow wide animate | How it all fits: plugins on disk are built once on the server; each host evaluates the facets of its kind; tabs and the TUI are clients that reach workers' objects through the server. row "On disk" muted box disk "plugins/notes/" "server.ts, worker.ts, web.tsx, tui.ts, contract.ts" muted box core "src/ (base code)" "@app/api/v1: Chord's surface, host kinds, principals" muted row "Server process" accent box comp "plugins component" "discover, build every facet, watch, roll out" accent box server "server host" "server facets; routes Project → workers" row "Clients and workers" box tab "client: tab or TUI" "web or tui facets, local UI objects; holds Notes { p1 }" blue box w1 "worker p1" "worker facets; serves Notes { p1 }" arrow disk -> comp "read, build" arrow core -> comp "host packages" arrow comp -> server "server scripts" arrow comp -> tab "web or tui scripts" arrow tab -> server "hold Notes { p1 }" arrow server -> w1 "start (worker scripts), relay" note "Each host loads only its own kind's facets. Plugins meet only through object types and registries." :::

Words used throughout

A host is one participant of the app, made with createHost. A host kind (Server, Session) describes addressable hosts; an address (Session({ id: "p1" })) names one. An object type is a contract: local (used on its own host only) or remote (used from any host). An object is one instance of a type, per params. A facet is a plugin's declarations for one host kind. A plugin is a directory with a package.json. A reference is what you get when you ask for an object (host.get, self.get, ctx.get). A document is data an object stores, changed in transactions; a table holds many records with queries, in the host's store, and a live query keeps some of its rows current; a derive is a value computed from others; a native is an outside resource an object opens and closes. A registry is an extension point: one plugin defines it, others contribute. A rollout applies a build to the hosts in order.

One Host

Start without plugins. One host, one object that keeps notes:

import { createHost, defineFacet, defineState, local, method, obj, arr, str } from "@earendil-works/chord";

const NoteList = obj({ items: arr(str()) });          // one schema, used by the contract and the document

// the contract: what a Notes object offers
const Notes = local("notes.store", {
	version: 1,
	params: {},                                        // one object per host: no params
	members: {
		list: NoteList,                               // state anyone can read and watch
		add: method(obj({ text: str() })),            // a method: arguments checked by Chord
	},
});

// the document the object stores (its own version: storage can change without changing the contract)
const NotesDoc = defineState("notes", { schema: NoteList, version: 1, initial: () => ({ items: [] }) });

// the facet: which types this host serves, and how to make each object
const facet = defineFacet((host) => {
	host.serve(Notes, (_params, self) => {
		const doc = self.state(NotesDoc);
		return {
			list: doc,
			add: async ({ text }, ctx) => self.tx(async (tx) => { (await tx.draft(doc)).items.push(text); }, ctx),
		};
	});
});

const server = createHost({ onError: console.error });
await server.load([{ name: "@app/notes", facet }], ctx);
const notes = await server.get(Notes, {}, ctx);        // a reference: live, with its state
await notes.add({ text: "buy milk" }, ctx);
console.log(notes.list.value);                          // { items: ["buy milk"] }

Four things happened. The contract Notes says what the object offers; the document says what it stores. Here they have the same shape, so one schema serves both; they are separate because they change separately (an object may offer a computed value or a shortened view instead of what it stores, and storage can change its shape without users noticing). The facet says this host serves Notes, with a factory that declares the object's parts: a document, and a method writing it in a transaction. load publishes the facet. get returns a reference once the object is live; its members are the contract's.

The factory only declares: it is synchronous and opens nothing. Chord activates the object (loads its documents, opens what it asked to open) and then serves calls. That is why the same factory can run again when the code changes.

load comes first. A get, hold or registry on a host whose load was not called throws not_ready at once ("call host.load first"): it would wait for types nobody published. A client with nothing to load calls load([]). Mistakes that would otherwise look like slowness are reported through onError instead: a request waiting for a type or registry nobody serves (unbound_long, after 5 s; it keeps waiting until its readyTimeout), a registry with contributions and no provider (registry_undeclared, when load publishes), and a plugin name that is no npm name makes load throw.

An Object's Parts

A factory builds an object from a handful of parts, each declared with one call on self. The notes object grows to show all of them.

Documents hold the object's data: self.state(NotesDoc) gives a handle whose .value is the current data. Data changes only in a transaction: self.tx(fn, ctx) runs fn with tx, and await tx.draft(handle) gives a mutable copy of the document's data; whatever you change on it is committed at once when fn returns, or not at all if it throws.

add: async ({ text }, ctx) => self.tx(async (tx) => {
	const draft = await tx.draft(doc);          // the document's data, writable inside this transaction only
	draft.items.push(text);                      // recorded as a change, committed when the function returns
}, ctx),

Documents live in memory unless they are persisted: defineState(..., { persist: true, checkpointWhen }) keeps one in the host's store (a SQLite file the app opens and gives to createHost({ store })), loaded when the object activates. For many records with queries, an object declares a table:

const NoteRows = defineTable("notes", {
	version: 1,
	columns: { id: str(), text: str(), at: int() },
	key: ["id"],
	indexes: { by_time: ["at"] },
});

host.serve(NoteStore, (_params, self) => {               // a local type with empty params: it may persist
	const rows = self.table(NoteRows);                    // created (or migrated) when the object activates
	const recent = query(NoteRows).orderBy((n) => n.at, "desc");
	return {
		add: async ({ text }, ctx) => self.tx(async (tx) => tx.put(rows, { id: uuid(), text, at: Date.now() }), ctx),
		page: async ({ after }, ctx) => rows.page(recent, 50, ctx, { after }),   // 50 at a time, with a cursor
	};
});

Persisted data lives only in local objects with empty params (one per host and type), so its address never depends on who asked or with which params; an object with params (a view per project) reads from such a store. Writes go through the same transactions as documents: a note row and a document can change in one commit.

A table has no watch of its own. To show rows live (the newest notes in a tab), the object declares a live query over its own table, with a limit:

const newest = self.query(rows, recent, 50);             // the 50 newest rows, a value like a document's
return { newest, add, page };                            // a member: tabs watch it and get only the changes

Chord runs the query before the object goes live, and again in every commit of the object that writes rows, and sends what changed. Only the object's own tables: it is their only writer, so its commits are exactly when the rows can change, and a commit that writes no such table costs nothing. Each writing commit runs the query once more, at most the limit's rows: right for data people edit, wrong for a table written hundreds of times a second (there may be dragons; keep what the tab shows in a document the object writes with each change instead). The limit is required.

A derive is a value computed from other values, recomputed whenever they change. It is how an object offers something it does not store:

const count = self.derive([doc], ([notes]) => notes.items.length);   // 0, 1, 2, … as notes are added
return { list: doc, count, add };                                      // `count` can be a member like `list`

Watching is how anyone follows a value as it changes: a document handle, a derive, a reference's member. watch gives the current value, then one call per change:

const watch = await notes.count.watch(ctx);
watch.start(async (call) => {
	if (call.kind === "value") console.log("notes:", call.value);   // "failed" calls say a value cannot be computed now
});

A routine is the object watching on its own behalf: self.on(source, fn) runs fn with each new value, for as long as the object lives (a reconciler: "whenever the notes change, rebuild the export file").

A native is something outside Chord the object opens and must close: a file watcher, a database connection, a child process. self.open(open, close) opens it while the object activates and closes it when the object ends:

const inbox = self.open(async (ctx) => watchDirectory(inboxPath, ctx.abortSignal), (watcher) => watcher.close());
self.onLive(async (ctx) => {                                  // a routine started once the object is live
	for await (const file of inbox.value.files(ctx.abortSignal)) await importNote(file, ctx);
});

Natives are why some objects restart when their plugin changes: an open connection cannot be moved to new code, so Chord closes it and opens it again. Objects without natives are swapped in place. A first read of a table is no native: declare it as a live query, not with self.open, or the object restarts at every change of its plugin.

Await what you start in a root or a handler. What is started with a context ends with it. Code outside objects gets its ctx from host.root(fn, ctx), and that scope seals when fn returns; a handler's context ends when its caller cancels or its object ends. A change still waiting when its context ends is cancelled before it commits:

await server.root(async (ctx) => {
	void notes.add({ text: "lost" }, ctx).catch(() => {});   // fn returns, the scope seals: the call's change is cancelled
}, ctx);
await server.root(async (ctx) => notes.add({ text: "kept" }, ctx), ctx);   // awaited: it commits

The cancelled change rejects, and since an empty catch would hide that it never happened, Chord also reports it through onError as transaction_cancelled, naming the object and, in its error, the scope that ended.

Several Hosts

Now give each project its own process. Declare the kinds of addressable host, and make notes a remote type that lives on its project's worker:

export const Server = hostKind("server");
export const Project = hostKind("project", { id: str() });

export const Notes = remote("notes", {
	version: 1,
	params: { project: str() },
	hostAddress: ({ project }) => Project({ id: project }),   // where an object of these params lives
	variants: { editor: { state: { list: NoteList }, methods: { add: method(obj({ text: str() })) } } },
});

Two things are new. A remote type can be used from any host, and it has variants: different sets of members for different callers, chosen by the factory (an editor variant with add, a reader variant for guests without it). And hostAddress says where its objects live; it is the whole routing story.

A tab asks for Notes { project: "p1" }. Its host computes the address, Project(p1), and sends the request to its parent, the server. The server has a route for Project: it starts (or finds) the worker for p1 and passes the request on. The worker serves Notes. The tab gets a reference whose state follows the worker's document, one frame per change.

:::flow wide animate | A tab holds a project's notes: the server routes by address to the project's worker and relays, without reading the data. row "Clients" blue box tab1 "tab A" "holds Notes { project: p1 }" box tab2 "tab B" "holds Notes { project: p1 }" row "Server" accent box server "server" "route Project → worker" row "Workers" muted box w1 "worker p1" "serves Notes { p1 }" box w2 "worker p2" "serves Notes { p2 }" arrow tab1 -> server "hold" arrow tab2 -> server "hold" arrow server -> w1 "relay" :::

Each host is created the same way, with what it is:

// server.ts
const server = createHost<P>({ address: Server(), principal: { kind: "server" }, onError });

// worker.ts (started by the server for one project)
const worker = createHost<P>({ address: Project({ id }), principal: { kind: "worker", project: id }, onError });
worker.connect(parentLink, { principal: { kind: "server" }, trusted: true }, ctx);

// a browser tab: no address, it is a client
const tab = createHost<P>({ onError });
tab.connect(webSocketLink(new WebSocket("/chord")), { principal: { kind: "server" } }, ctx);

P is your app's type of principal, who is calling: an owner, a guest, the server, a worker. Links stamp it; a remote object is made per principal, so a guest can get a read-only variant of the same notes.

Who a reference acts as

A reference acts as whoever made it. host.get and self.get in a local object act as the host; self.get in a remote object acts as that object's principal; ctx.get in a method acts as the method's caller. That is what lets a guest's call reach a worker as the guest and get the guest's variant, never more than the guest may see.

It surprises one case: a server object that should act as itself towards a worker. The owner calls Search.rebuildIndex, which asks each project's worker to scan its files:

// server.ts: wrong, the worker sees the owner
rebuildIndex: async ({ project }, ctx) => {
	const scan = await ctx.get(IndexScan, { project });   // acts as ctx.principal: the owner, the method's caller
	return scan.run(ctx);
},

// worker.ts: the factory serves only the server
host.serve(IndexScan, (params, principal, self) => {
	if (principal.kind !== "server") throw new Error("IndexScan serves the server only");   // refuses the owner
	…
});

The worker's factory gets { kind: "owner" } and refuses. To act as the server, take the reference where the server is the maker: self.get(IndexScan, { project }) in a local server object of its own (a dependency of that object, which acts as the host), or, in base code that has the host, a root of the server's host:

// base code: a root acts as the host (the server), not as the method's caller
rebuildIndex: async ({ project }, ctx) =>
	server.root(async (own) => (await own.get(IndexScan, { project })).run(own), ctx),

Calls in progress

There is no limit on calls in progress per link. A tab sending 200 calls at once over its socket gets all 200 run: at most N (256 by default, announced in the handshake) are in flight per origin, the rest wait in order in the link's call queue, none is refused for being too many. The bounds are that call window and the part limits on what arrives (sizes, not counts). An object that must limit work per caller does so in its methods, or runs the work as a job.

:::note | The app kit does this wiring Starting workers, routing a host kind to them, the WebSocket entry and authentication are slice 10's components (createWorkers, chord.http). An app built with the app kit (chord/app, the last chapter) does not wire them: startServer(...) on the server and startWorker(...) in a worker's entry do. :::

Client Hosts

A browser tab and a terminal UI are hosts like the server, with one difference: they have no address. Nobody can send a request to a tab; a tab only connects to the server and asks for things. That is a deliberate rule: a tab runs on a device the server does not control, so nothing on the server ever waits for, or trusts, a tab's state.

// a browser tab (slice 11's page host will do this wiring for you)
const tab = createHost<P>({ onError });                               // no address, no principal: a client
tab.connect(webSocketLink(new WebSocket("/chord")), { principal: { kind: "server" } }, ctx);
await tab.load([{ name: "@app/web", facet: webCore }, ...webFacets], ctx);

const notes = tab.hold(Notes, { project: "p1" }, ctx);               // a reference at once, live soon
(await notes.list.watch(ctx)).start(async (call) => {
	if (call.kind === "value") render(call.value.items);              // every change, as it commits on the worker
});
await notes.add({ text: "from the tab" }, ctx);                        // a call: tab → server → worker

Who the tab is is not the tab's to say. The server authenticates the WebSocket when it connects (a cookie, the owner on loopback, a guest's share link) and stamps that principal on everything the tab asks for. The worker's Notes factory receives it and may give a guest a read-only variant. A tab cannot claim to be the owner.

What a tab serves is only local types: state of this tab's UI, never shared. A draft that survives switching panels, a selection, a tab-local cache:

// plugins/notes/web.tsx: the web facet serves a local type, and contributes a panel
export default defineFacet((host, self) => {
	host.serve(NoteDraft, (_params, self) => ({ text: self.state(DraftDoc) }));   // lives in this tab only
	self.contribute(Panels, { title: "Notes", Component: NotesPanel });
});

When the connection drops (a phone in a tunnel, a server restart), the tab keeps its references. Their values stay readable, their state says loading, calls made meanwhile wait; on reconnect every reference is held again and gets the current value in one frame. The UI shows a banner from ref.state, nothing more.

A terminal UI is the same in Node: a client host connected to the server over a WebSocket, loading the plugins' tui facets, rendering with whatever terminal library the app chose. What a client never does: serve a remote type, be routed to, or keep data others rely on. Data that must outlive a tab belongs in an object on the server or a worker.

Text in the viewer's language is a client's job too, because two people looking at the same object may read different languages. A plugin writes its texts once, in a shared messages.ts; a client renders them in its host's language (the user's pick, else the device's), and objects store messages as data, never sentences (design §23, "Messages and languages"):

// plugins/notes/messages.ts: the first catalog types the others
export const messages = defineMessages("notes", { en: { empty: "No notes yet" }, de: { empty: "Noch keine Notizen" } });
// plugins/notes/web.tsx: the facet contributes its catalogs; a component renders in the page's language
self.contribute(Translations, messages.translations);
const { t } = useMessages(messages);                       // re-renders when the user picks another language
// plugins/notes/worker.ts: a notice as data; every client renders it with t.render(notice)
draft.notice = messages.message("empty");

Plugins

So far the facets were part of the app's own code. A plugin is the same code in its own directory:

plugins/notes/
  package.json        { "name": "@app/notes" }
  contract.ts         the Notes contract (other plugins import it)
  worker.ts           the facet for workers: serves Notes
  server.ts           the facet for the server: an export route
  web.tsx             the facet for browser tabs: a notes panel

The plugin's name is its package.json name. Each file named after a host kind is that kind's facet, its default export a defineFacet(...). A plugin has only the files it needs: a plugin that only adds a route has a server.ts and nothing else.

// plugins/notes/worker.ts
import { defineFacet } from "@app/api/v1";
import { Notes } from "./contract.ts";

export default defineFacet((host) => {
	host.serve(Notes, ({ project }, principal, self) => {
		const doc = self.state(NotesDoc);
		return { variant: "editor", members: { list: doc, add: async ({ text }, ctx) => /* … */ } };
	});
});

@app/api/v1 is your app's API, version 1: the part of its base code plugins import, as a host package (the plugin's build leaves it out; every host passes its own copy in). It holds what plugins share and never reloads: Chord's builders re-exported (local, remote, obj, str, ...), host kinds, the principal type, a defineFacet bound to it. A plugin never imports Chord itself; the version in the path says what it was written against.

Loading Plugins

A plugin is source code on the server's disk: TypeScript files that import each other, import other plugins' contracts, import npm packages, and import things the app itself provides. None of the processes that run the plugin can use those files as they are. A browser tab has no file system and no TypeScript; a worker could read the files, but then every worker would compile every plugin again, and two workers started a minute apart could see two different versions. So loading a plugin is two separate steps:

1. Build: turn each plugin facet (notes/worker.ts, notes/web.tsx) into one self-contained script. This happens once per edit, on the server, for every host kind.
2. Evaluate: run a script inside one process, which gives back the facet, ready for host.load. This happens in every process that hosts that kind: the server for its own facets, each worker for the worker facets, each tab for the web facets.

The two steps need different things, and that is why the API has two calls.

Host packages: what a script leaves out

Look at what notes/worker.ts imports:

import { defineFacet, obj, remote, Session, str } from "@app/api/v1";   // the app's API: its own part and Chord's
import { Notes } from "./contract.ts";                                  // the plugin's own file
import slugify from "slugify";                                          // an npm package in its dependencies

The last two go into the script: the plugin's own files and its npm dependencies are copied in. The first must not. If the script carried its own copy of the API, and with it of Chord, the worker would run two Chords: the plugin's ChordError would not be the worker's ChordError, and the plugin's defineFacet would make a facet the worker's host does not recognize. The plugin must see the very Session host kind and the very Panels registry the process already has, not a copy.

Packages the process already has, and that every plugin must share, are its host packages. A script leaves them out and asks for them by name when it runs. Building therefore needs only their names: "do not copy @app/api/v1 into this script".

Building: createPlugins, on the server

const plugins = createPlugins({
	roots: [{ path: "./plugins", required: true }],      // where the plugin directories are
	api: { name: "@app/api", versions: ["v1"] },          // the app's API and the versions it provides
	kinds: {
		server: { platform: "node", hostPackages: ["@app/api/v1", "@app/api/v1/server"] },
		worker: { platform: "node", hostPackages: ["@app/api/v1", "@app/api/v1/worker"] },
		web: { platform: "browser", hostPackages: ["@app/api/v1", "@app/api/v1/web", "react", "react/jsx-runtime"] },
	},
	onError,
}, ctx);

createPlugins is the builder, and it builds for every kind, because only the server has the sources. For each kind it needs two facts. The platform says where the scripts run: a node script may import node:fs, a browser script may import a stylesheet but not node:fs. The host package names say what to leave out; they differ per kind, because a tab also shares React with its plugins and a worker does not, and the API's per-kind modules (/v1/web) belong to their kind only. With api set, the build checks every import of the API: a version the app does not list, another kind's module, two versions in one facet, or Chord imported directly is a build error naming the file and line. Nothing here runs a plugin.

Evaluating: plugins.load, in the process that hosts

Evaluating is where the names become things. When the script runs and asks for @app/api/v1, someone has to hand it the actual module, the one this process imported when it started. Only the process itself has it:

// server.ts
import * as api from "./api/v1/index.ts";                 // the app's API, the module plugins import as "@app/api/v1"
import * as apiServer from "./api/v1/server.ts";
import { serverFacet } from "./core.ts";

const report = await plugins.load(server, {
	kind: "server",                                          // evaluate the server kind's scripts
	modules: { "@app/api/v1": api, "@app/api/v1/server": apiServer },   // this process's own instances
	core: [{ name: "@app/core", facet: serverFacet }],       // base-code facets, loaded beside the plugins
}, ctx);

modules is this process's answer to every host package name. The server passes its own; a worker passes its own in workerMain; a page passes its own in startPage. Each process has different instances, which is exactly the point: inside each process, plugins share that process's Chord and base code.

core is the app's base code in facet form. It is a facet like a plugin's, but it is not a plugin: it is made in the process's own code, often from values only this process has (core.workerFacet(start) closes over a worker's start data and store), and it never reloads, because it defines what all plugins share. Passing it here loads it together with the plugins, so they are published in one step.

Why plugins.load(host), not host.load

A host has its own load, and plugins.load calls it. The host's load takes facets that already exist: functions in memory. It knows nothing about directories, builds, scripts or modules, on purpose: the same host runs in a test with facets imported directly, or in a Cloudflare Durable Object where plugins are bundled at deploy time and nothing is built or evaluated at runtime.

The plugins component is the part that knows about directories. It finds the plugins, builds them, evaluates them with the process's modules, and then calls host.load with the result. It also remembers, per host, which facet each plugin runs, so that a later reload can hand the host its complete new plugin set, with unchanged plugins as the very same objects. That memory is why the host is passed to it: plugins.load(server, …) on the server, plugins.load(worker, …) in a worker (where workerMain gives it a plugins that only evaluates), the same call in every process.

:::flow wide | Building on the server needs names; evaluating in each process needs that process's modules. row "Server: build (once per edit)" accent box src "plugin sources" "notes/worker.ts, imports @app/api/v1" muted box build "createPlugins" "copies own files and npm deps; leaves host packages out" accent box script "worker script" "require(\"@app/api/v1\") left as a name" row "Each worker: evaluate" blue box mods "the worker's modules" "its own @app/api/v1, its own Chord" muted box load "plugins.load(worker, …)" "runs the script, answers each name" blue box facet "the facet" "uses the worker's Session, the worker's defineFacet" arrow src -> build arrow build -> script arrow script -> load "shipped" arrow mods -> load arrow load -> facet :::

What happens, step by step

:::steps | plugins.load(server, { kind: "server", … }) +++ Discover The component lists ./plugins and reads each package.json. Its name is the plugin's name; the files named after a kind are its facets. A directory without a package.json, or with two server files, is reported, not loaded. Files at the root's top named <name>.<kind>.ts (hello.server.ts) are single-file plugins, grouped by name. +++ Build For each plugin with a server facet it builds one script, if no current one exists. A syntax error or an import the rules do not allow (a path into base code, a package not in the plugin's dependencies) is a failed build with the file and line, never a crash. +++ Evaluate The script runs. Each host package name it asks for is answered from modules; anything else it may not have. The script's default export must be a facet. +++ Load host.load runs the setups of core and of every plugin facet and publishes them together. A plugin whose build, evaluation or setup failed is left out and listed in report.failed with the reason; the others load. Objects that need it wait until a later edit loads it. :::

Only the server builds. Workers and clients (browser tabs, terminal UIs) only evaluate: the server builds their kinds too, a worker gets its scripts in its start message, a tab or a TUI gets them from the server over HTTP, and each evaluates them with its own modules (a worker's from workerMain, a page's from its base bundle).

Plugins Using Plugins

A search plugin wants to search the notes. It imports the contract and asks for the object:

// plugins/search/worker.ts
import { Notes } from "../notes/contract.ts";   // a plugin of the same root, by relative path: copied into search's script

host.serve(Search, ({ project }, principal, self) => {
	const notes = self.get(Notes, { project });   // a dependency: Search goes live once Notes is
	const index = self.derive([notes.list], ([list]) => buildIndex(list.items));
	return { variant: "main", members: { find: async ({ q }) => lookup(index.value, q) } };
});

Which file a plugin imports decides which version of the contract it claims, so the rule is: a plugin carries the contract it was written against. Two cases:

• Plugins that change together (the app's own, in its repository; a user's, in one root) import each other by relative path, ../notes/contract.ts: the same commit holds both sides. The file is copied into the importer's script, so only contracts and other side-effect-free files belong there (a copied file may import only the API and files of its own plugin). A relative path leaving the root (into base code, into another root) is a build error.
• Plugins from another repository take contracts as a dependency at a fixed version, by package name: @bob/notes (its contract.ts, through the package's exports), @bob/notes/web (its contract.web.tsx), pinned in package.json by an npm version or a git ref ("@bob/notes": "github:bob/notes#<commit>"). The build inlines the installed version: when Bob's notes move to contract version 2, a consumer built against version 1 keeps claiming version 1, and the provider serves it through accepts or the consumer waits "contract mismatch", visible, never silently wrong. A plugin inside a repository holding several (which lists them in its own package.json, "chord": { "plugins": ["./notes", "./search"] }) is a dependency on the whole repository, imported by path into it: @bob/plugins/notes/contract.ts.

A contract file may be for one kind: contract.web.tsx (React components in a type, a panel's registry token) is a web file. Importing it, directly or through other files, from a facet of another kind is a build error naming the chain (server.ts → view.ts → ../notes/contract.web.tsx). The shared contract.ts mentions native things only through import type.

self.get in a factory declares a dependency: Chord makes Search live only once Notes is, and if the notes plugin is missing, Search waits with a clear reason ("waiting for notes: not served here") instead of failing.

The other way is a registry: an extension point one plugin defines and others contribute to. The app's API defines Panels in its web module, and every plugin with a UI contributes one:

// plugins/notes/web.tsx
import { Panels } from "@app/api/v1/web";
export default defineFacet((host, self) => self.contribute(Panels, { title: "Notes", Component: NotesPanel }));

What a plugin may not do is reach into another's data. Search never reads notes' documents; it uses the Notes object's state. That rule is what lets the notes plugin change its storage without breaking search.

Changing a Plugin

You change the notes plugin: notes now have a timestamp. The watcher sees the saved file and marks the plugin dirty. You (or an agent) build and roll out:

const built = await plugins.build(plugins.state.value.dirty, ctx);
const result = await plugins.rollout([plugins.step("server", server), workers.step("workers")], built, ctx);

The rollout goes host by host, server first, then every running worker. On each host Chord compares the new plugin set with the loaded one, runs the new factories against a recording self to see what each object would declare, and then gives every object of the changed plugin its fate:

The object	What happens
holds no natives (open files, sockets), the new code neither	swapped in place: new methods, same documents; running calls finish on the old code
holds natives	restarted: calls wait, running ones finish, natives close and reopen with the new code
its type was removed	ended; its users wait until something serves it again

The tab's reference follows the new object and gets one frame with the difference. Nothing reconnects. If the edit is broken (a syntax error, a factory that throws), that plugin keeps its old code on that host and the report says why; every other plugin of the rollout applies.

A tab applies a new web script the same way, without reloading the page, and the UI keeps what lives outside the changed plugin. The changed plugin's own components are new functions, so React mounts them anew and their useState starts over. What must survive the update (a half-typed note, a scroll position) goes into page state, kept by the tab's host under a key: usePageState(key, initial) from chord/react instead of useState, and scope.pageState(key, initial) on a terminal's TuiScope (design §21, "UI on client hosts").

:::note | Versions A change to a contract (a new field in Notes's state) gets a new version. A tab built with the old contract keeps working if the provider maps the old version (accepts); otherwise it waits with "version mismatch" until it reloads. :::

Base Code and Plugins

Every app has two kinds of code, and the line between them decides what can change while the app runs.

Base code is the app's own program: src/, started by node server.ts. It shrinks to three things: the API (the versioned modules plugins import), the bootstrap (creating hosts, opening the store, components such as createHttp and createWorkers) and one small start-data facet serving process values (start data, the opened store) as core objects. Base code changes only with a restart: if it reloaded, the process would hold two Panels registries, and old and new plugins would no longer meet.

Plugins are everything else: notes, search, export, a terminal, the harness, the providers of the app's core registries. They reload on an edit, a broken one is left out alone, and an agent can write one while the app runs.

The API, by version and by kind

Plugins import their host only through the API, a version in the path:

// src/api/v1/index.ts: "@app/api/v1", for every kind
export { defineRegistry, local, obj, remote, str } from "@earendil-works/chord";   // the Chord surface the app supports
export { defineFacet, Project, Server } from "../../core.ts";                           // bound to the app's principal
// src/api/v1/web.ts: "@app/api/v1/web", for pages only
export const Panels = defineRegistry<Panel, readonly Panel[]>("app.panels", { version: 1 });

The app lists the versions it provides, newest first (api: { name: "@app/api", versions: ["v2", "v1"] }), and each kind's modules in kinds.<kind>.hostPackages. When the app changes its API in a way that breaks plugins, it adds /v2 and keeps /v1 as a thin layer over it for a while; once /v1 goes, a plugin importing it fails to build with "@app/api/v1 is not available: this app provides v2", the plugin alone, the message saying what to change. One facet uses one version. Contracts the app promises others are re-exported in the API (export { Notes } from "../../plugins/notes/contract.ts"), so a plugin built against /v1 keeps v1's contract files.

Plugins the app ships

An app comes with plugins of its own. They follow the same rules as anyone's, in a root inside the app, next to the root where users and agents add theirs:

const plugins = createPlugins({
	roots: [{ path: "./plugins", required: true }, join(dataDir, "plugins")],   // the app's own, then the user's
	api: { name: "@app/api", versions: ["v1"] },
	kinds,
	onError,
}, ctx);

They are built at startup like any plugin (a quarter of a second for twenty), reload like any plugin, and are left out alone when broken at a reload. The app's root is required: a plugin of it failing at startup makes plugins.load reject with a PluginError naming it (after the load settled), so the app stops instead of running without its core. A failure in the user's root is reported and the app runs on. If two roots have a plugin with the same name, the first root wins, and the other is reported.

Small plugins, installed plugins, tested plugins

• A single file at a root's top is a plugin: plugins/hello.server.ts and plugins/hello.web.tsx are the plugin hello with two facets. It has no package.json and imports only the API; when it needs a second file or a package, it becomes the directory hello/, which a reload treats as the same plugin.
• Installing from git or npm: plugins.installPlugin({ git: url, ref }, root, ctx) clones the ref (a branch, tag or commit) into the root, { npm: name, version } extracts the package; then its dependencies are installed. Removal is deleting the directory.
• Testing a plugin alone: testPlugin("./plugins/stats", { kind: "worker", api, modules, stubs }, ctx) builds it as production does, loads it on an in-memory host beside stub facets serving what it uses, collects what it contributes to registries nobody serves, and lists the contracts it waits for (docs/plugin-authoring.md).

The App Kit

The chapters so far wired an app by hand: a store, a host, createHttp, createWorkers, createPlugins, an authenticator, a worker entry connecting to its parent. Every app needs the same wiring, and most of it has one right answer. The app kit (chord/app) is that answer, opinionated on purpose: one data directory layout, one way to sign in, one place for files.

A whole app

The notes app of example 53 is four files of base code and two plugins:

// start.ts
const app = await startServer<NotesPrincipal>({
	name: "notes",                                  // the data directory: ~/.notes unless dataDir says otherwise
	root: "./plugins",                              // the app's own plugins: a required root
	api: { name: "@notes/app", versions: ["v1"], modules: { "@notes/app/v1": v1, "@notes/app/v1/server": v1server } },
	kinds: { server: { platform: "node" }, worker: { platform: "node" } },
	principal: { server: { kind: "server" }, worker: (address) => ({ kind: "worker", project: address.key }) },
	workers: [{
		kind: Project,                              // one worker per project
		entry: "./worker.ts",
		start: (address) => ({ project: address.key }),
		authorize: (principal) => principal.kind === "owner" || principal.kind === "server",
	}],
	onError: (error) => log(error),
}, ctx);
console.log(`Open ${app.ownerLink()}`);

// worker.ts
startWorker({ api: API, kinds: KINDS, onError: (error) => log(error) });

startServer creates the data directory (server/store.sqlite, workers/<kind>/<address>/, plugins/, logs/, owner-token), opens the server's store, creates the server host, HTTP, a workers component per worker kind and the plugins component over two roots: the app's plugins/ and the user's <dataDir>/plugins/. Then it loads the server's plugins with the kit's own facets beside them and resolves. startWorker does the worker's half: the project's directory and store, the host at the project's address, the link to the server first, the worker's plugins.

What plugins get from the kit

Plugins still import only the app's API; the API re-exports the kit's objects it offers:

• AppInfo on the server (data directory, app name, API version) and WorkerInfo on workers (the address and the start data start(address) made): the start-data objects. Process values reach plugins only through them.
• Files, one per host: put for small files, begin, append and finish for large ones, ranged read, remove and a watchable list, stored in the host's directory with caps per host.
• Sessions on the server: issue, revoke, list.

This finishes what the previous chapter started. Base code is the API, the start script and the worker's entry. The providers of the app's own registries live in its required root: example 53's API declares a Sections registry, and its home plugin serves it and reads AppInfo, so it reloads like any other plugin.

Who may come in

HTTP denies by default: a request without a principal gets 401 before any plugin code runs, unless the route says public: true. Someone gets a principal through a session: a random token, kept in the store only as a hash, sent as a signed cookie by a browser or as Authorization: Bearer by a command-line client.

The first session is the owner's. At each start the kit makes a one-time owner link, http://127.0.0.1:8788/_chord/owner#<token>, valid ten minutes; the start script prints it. The token is in the fragment, which browsers never send, so it never reaches a log: the page posts it, the kit consumes it and sets the owner's cookie. A command-line client on the same machine reads <dataDir>/owner-token (mode 0600). One on another machine pairs: it asks for a code, the owner approves the code in a signed-in browser, and the client's next poll returns its token (example 54).

Users other than the owner come from optional plugins the app adds: defineUsers(profile) for accounts, inviteFacet() for invite links, ephemeralLoginFacet() for "pick a name and you're in".

Applying changes, moving projects

The kit never reloads by itself. An edit or an install marks plugins dirty; app.plugins.state drives the app's "apply changes" button, and app.applyChanges(ctx) builds and rolls out to the server, then the workers.

A project's data is what its worker created: its store and its files. app.exportData(Project({ id }), path, ctx) writes them as one archive (a manifest, the store's export, the files) and stops nothing: a running worker writes its own store at one consistent point. app.importData(path, Project({ id: "copy" }), ctx) makes a new project from it and refuses one that already has data (example 55).

Reference

Call	What it does
createHost<P>({ address?, principal?, store?, onError })	a host: addressable with an address, else a client
host.load([{ name, facet }], ctx) / host.reload(...)	publish facets / apply a new plugin set
host.get(Type, params, ctx) / host.hold(...)	a live reference / a reference at once that follows its object
local(name, { version, params, members }) / remote(name, { version, params, hostAddress, variants })	contracts
defineFacet((host, self) => …), host.serve(Type, factory), self.contribute(R, value)	a facet
factory self: state, derive, get, registry, open, on, onLive, job, tx	an object's parts
createPlugins({ roots, api, kinds, onError }, ctx)	the plugins component
plugins.load(host, { kind, core, modules }, ctx), build(names), rollout(steps, built)	load, build, roll out
plugins.installPlugin(source, root, ctx), testPlugin(dir, options, ctx)	install a plugin, test one alone
hostKind(name, params?), Kind(params)	host kinds and addresses
startServer<P>({ name, dataDir?, root?, listen?, api, kinds, principal, workers?, files?, onError }, ctx)	an app's server, with the kit
startWorker({ api, kinds, busy?, onError })	a worker's entry, with the kit
app.ownerLink(), app.applyChanges(ctx), app.plugins.install(source, ctx)	sign in, apply changes, install
app.exportData(address, path, ctx), app.importData(path, address, ctx)	move a worker's data
AppInfo, WorkerInfo, Files, Sessions	the kit's objects plugins get