# `arts/connection` — Design & Implementation Document

> **Historical note 2026-05-14.** This document predates the directory rename
> sweep. The canonical artifact name is now `connection`; `$connection` points
> to `src/arts/connection`. Mentions of `conn`, `sess`, `timr`, `logr`,
> `cach` and `aapp` below are historical design context unless they refer to
> literal wire/event keys.
>
> **Purpose.** Implement a zero-dependency runtime artifact for managing multiple realtime connections at App level.
>
> **Audience.** Claude Code / implementer familiar with TypeScript, Svelte 5 runes, SvelteKit, and the existing `src/arts/*` artifact architecture.
>
> **Status.** Design target for first implementation.
>
> **Core decision.** The artifact is called `conn` / `connections`, not `sock` / `sockets`, because the public model must support WebSocket, SSE, polling, WebTransport, mocks, and future transports through adapters.
>
> **Naming correction applied.** `EngineConnections` / `ActiveConnections` are the root registry names. An individual runtime unit is `Connection`. Any older references below to `EngineConnection` / `ActiveConnection` describe that unit conceptually, but are not implementation names.

---

## 1. Problem statement

The application needs first-class support for realtime connections at App level.

The system must support:

- Multiple independent connections.
- Each connection with its own lifecycle, transport, auth, reconnect policy, heartbeat, buffer policy, serializer, channels and event listeners.
- Different transport types: WebSocket now, SSE/polling/mock soon, WebTransport later.
- Integration with `ActiveApp`, `Session`, `Http`, `Logger`, and browser lifecycle when configured.
- Strict TypeScript types.
- Zero dependencies.
- No domain state inside `arts/conn`.

The correct mental model is:

```txt
App
  └── Connections
        ├── connection('main')
        │     ├── channel('orders')
        │     └── channel('cart')
        │
        ├── connection('chat')
        │     ├── channel('room:123')
        │     └── channel('presence:room:123')
        │
        └── connection('market')
              └── channel('prices')
```

A connection is not necessarily a WebSocket. A connection is an abstract realtime transport endpoint.

---

## 1.1 Mandatory v1 scope

The first implementation of `arts/conn` MUST include the following capabilities.

This list is normative. If any item is missing, the implementation is incomplete.

### 1.1.1 Multi-connection registry

`arts/conn` must provide an App-level registry of named connections.

Required API:

```ts
const Connections = App.createActiveConnections<AppConnections>();

const Main = Connections.createConnection('main', options);
const Chat = Connections.createConnection('chat', options);
const Market = Connections.createConnection('market', options);

Connections.connection('main');
Connections.hasConnection('main');
Connections.connectionNames();
```

Required behavior:

- More than one connection may exist at the same time.
- Each connection has independent lifecycle/state/config.
- Connections are addressed by stable string names.
- Duplicate names are rejected.
- Missing names throw a typed error.
- The registry can open, close and reconnect connections by name.

Required hub methods:

```ts
openConnection(name): Promise<ConnectionConnectResult>;
closeConnection(name, reason?: string): void;
reconnectConnection(name, reason?: string): Promise<ConnectionConnectResult>;

openAll(): Promise<readonly ConnectionConnectResult[]>;
closeAll(reason?: string): void;
reconnectAll(reason?: string): Promise<readonly ConnectionConnectResult[]>;
```

### 1.1.2 Transport adapter

The connection engine MUST be transport-agnostic.

The engine may not hardcode WebSocket behavior directly.

Every physical transport must implement:

```ts
export interface ConnectionTransport {
	readonly kind: string;
	readonly state: ConnectionTransportState;
	readonly bufferedAmount: number;
	readonly canSend: boolean;

	open(): Promise<void>;
	send(data: string | ArrayBuffer): Promise<void> | void;
	close(code?: number, reason?: string): void;

	onOpen(listener: () => void): () => void;
	onMessage(listener: (message: string | ArrayBuffer) => void): () => void;
	onClose(listener: (event: ConnectionCloseEvent) => void): () => void;
	onError(listener: (error: unknown) => void): () => void;
}
```

Required behavior:

- `EngineConnection` only talks to `ConnectionTransport`.
- Reconnect, heartbeat, ack, channels and buffering work above the transport.
- Future transports must be addable without changing the engine core.

### 1.1.3 WebSocket transport

v1 MUST include:

```ts
createWebSocketTransport(options);
```

Required behavior:

- Opens a native `WebSocket`.
- Sends string or binary payloads.
- Receives messages and emits them through the adapter interface.
- Exposes `bufferedAmount`.
- Maps browser `open`, `message`, `close`, `error` events.
- Supports dynamic URL:

```ts
url: string | (() => string);
```

Required usage:

```ts
const Main = Connections.createConnection('main', {
	transport: createWebSocketTransport({
		url: () => '/realtime'
	})
});
```

### 1.1.4 Mock transport

v1 MUST include:

```ts
createMockTransport();
```

Required behavior:

- Implements `ConnectionTransport`.
- Allows deterministic tests without browser/network.
- Can manually emit open/message/close/error events.
- Captures sent messages for assertions.

Required testing helpers:

```ts
transport.emitOpen();
transport.emitMessage(raw);
transport.emitClose(event);
transport.emitError(error);
transport.sentMessages();
```

The engine test suite should primarily use the mock transport.

### 1.1.5 State per connection

Each connection MUST expose its own state.

Required state:

```ts
export type ConnectionState =
	| 'idle'
	| 'connecting'
	| 'open'
	| 'reconnecting'
	| 'closing'
	| 'closed'
	| 'failed';
```

Required `EngineConnection`/`ActiveConnection` properties:

```ts
readonly state: ConnectionState;
readonly connected: boolean;
readonly generation: number;
readonly error: unknown | null;
readonly openedAt: number | null;
readonly closedAt: number | null;
readonly lastMessageAt: number | null;
readonly reconnectAttempt: number;
```

Required event:

```ts
onState(listener: (change: ConnectionStateChange) => void): () => void;
```

Required change shape:

```ts
export interface ConnectionStateChange {
	readonly connection: string;
	readonly from: ConnectionState;
	readonly to: ConnectionState;
	readonly generation: number;
	readonly error?: unknown;
	readonly at: number;
}
```

### 1.1.6 Aggregated reactive state

`ActiveConnections` MUST expose an aggregated reactive view of all registered connections.

Required properties:

```ts
readonly size: number;
readonly activeNames: readonly string[];

readonly states: Readonly<Record<string, ConnectionState>>;

readonly connectedNames: readonly string[];
readonly connectingNames: readonly string[];
readonly reconnectingNames: readonly string[];
readonly failedNames: readonly string[];
readonly closedNames: readonly string[];

readonly allConnected: boolean;
readonly anyConnected: boolean;
readonly anyConnecting: boolean;
readonly anyReconnecting: boolean;
readonly anyFailed: boolean;
```

Required behavior:

- `Connection` owns the real state.
- `Connections` observes registered connections and derives aggregate state.
- There must not be a single ambiguous `Connections.state`.

### 1.1.7 Send / request / ack

v1 MUST support fire-and-forget sends and request/reply acks.

Required APIs:

```ts
send<TPayload>(
    type: string,
    payload: TPayload,
    options?: ConnectionSendOptions
): Promise<ConnectionSendResult>;

request<TPayload, TResult = unknown>(
    type: string,
    payload: TPayload,
    options?: ConnectionRequestOptions
): Promise<ConnectionAckResult<TResult>>;
```

Required frame fields:

```ts
id?: string;
type: string;
payload: unknown;
ack?: boolean;
replyTo?: string;
error?: ConnectionFrameError;
```

Required behavior:

- `request()` generates a message id.
- `request()` sends a frame with `ack: true`.
- Incoming frames with `replyTo` resolve the matching pending request.
- Requests time out using `timeoutMs`.
- Disconnect resolves all pending requests as closed.
- Normal runtime failures return tagged results, not uncaught exceptions.

### 1.1.8 Reconnect

v1 MUST support configurable reconnect.

Required options:

```ts
export interface ConnectionReconnectOptions {
	readonly enabled?: boolean;
	readonly minDelayMs?: number;
	readonly maxDelayMs?: number;
	readonly factor?: number;
	readonly jitterMs?: number;
	readonly maxAttempts?: number;
	readonly reconnectOnVisible?: boolean;
	readonly reconnectOnOnline?: boolean;
}
```

Required behavior:

- Unexpected close triggers reconnect when enabled.
- Intentional `disconnect()` does not reconnect.
- Backoff uses factor + jitter.
- Successful reconnect resets attempts.
- Reconnect increments connection generation.
- After reconnect, configured channels rejoin.
- After reconnect, auth re-runs if configured.
- Reconnect delay must be computed through the shared timer/backoff primitives
  (`$libs/timers.computeBackoffDelay`) and scheduled through injected
  `TimerScheduler`/`App.timers`, not raw `setTimeout` inside the connection
  engine. A native fallback is allowed only in the standalone engine path.

### 1.1.9 Heartbeat

v1 MUST support heartbeat.

Required options:

```ts
export interface ConnectionHeartbeatOptions {
	readonly enabled?: boolean;
	readonly intervalMs?: number;
	readonly timeoutMs?: number;
	readonly pingType?: string;
	readonly pongType?: string;
}
```

Required behavior:

- When connection is open, heartbeat sends ping frames if transport can send.
- Receiving pong or any message updates liveness.
- Timeout closes/reconnects according to reconnect policy.
- Heartbeat can be disabled per connection.

### 1.1.10 Channels

v1 MUST support logical channels/topics inside each connection.

Required API:

```ts
const Orders = Main.channel('orders');
const Cart = Main.channel('cart');
```

Required behavior:

- `connection.channel(name)` caches by name.
- Same channel name returns same channel instance.
- Frames with `topic` are routed to the matching channel.
- Frames without `topic` go to connection-level listeners.
- Channels expose `join`, `leave`, `send`, `request`, `on`, `onAny`.
- Channels may auto-join and rejoin after reconnect.

Required channel methods:

```ts
join(params?: unknown): Promise<ConnectionChannelJoinResult>;
leave(): Promise<void>;

send(type, payload, options?): Promise<ConnectionSendResult>;
request(type, payload, options?): Promise<ConnectionAckResult>;

on(type, listener): () => void;
onAny(listener): () => void;
```

### 1.1.11 Session integration opt-in

Session integration MUST be per connection and opt-in.

Required option:

```ts
session:
    | false
    | {
          readonly enabled?: boolean;
          readonly reauthOnRefresh?: boolean;
          readonly disconnectOnExpire?: boolean;
      };
```

Required behavior:

- Connections with `session: false` never react to session events.
- Connections with session integration may reauth on `REFRESHED`.
- Connections with session integration may disconnect on `EXPIRED`/`REVOKED`.
- No connection should assume JWT/Bearer.
- Auth payload is provided by user code.

Required auth shape:

```ts
auth?: {
    getAuth?: () =>
        | Record<string, unknown>
        | null
        | Promise<Record<string, unknown> | null>;
}
```

or shorthand:

```ts
auth: () => Record<string, unknown> | null | Promise<Record<string, unknown> | null>;
```

### 1.1.12 Minimum v1 deliverables

The first implementation is not accepted unless these files exist and are tested:

```txt
engine-connections.ts
active-connections.svelte.ts
engine-connection.ts
active-connection.svelte.ts
transport.ts
transports/websocket.ts
transports/mock.ts
serializer.ts
serializers/json.ts
channel.ts
reconnect.ts
heartbeat.ts
ack.ts
backpressure.ts
types.ts
errors.ts
consts.ts
```

Presence, SSE and polling may be deferred, but WebSocket + mock are mandatory.

---

## 2. Explicit non-goals

`arts/conn` must NOT implement:

- Chat UI.
- Notification UI.
- Business/domain event models.
- Authorization policies.
- User/session storage.
- Webhook receiving/signing.
- Durable queues.
- CRDTs/collaboration protocols.
- Server-side room management.
- Provider-specific SDK logic for Ably, Pusher, Supabase, etc.
- Socket.IO protocol compatibility.

Webhooks are a separate concern and should become a separate artifact, for example:

```txt
arts/webh
```

Do not put webhooks inside `arts/conn`.

---

## 3. Relationship with existing artifacts

### 3.1 With `aapp`

`ActiveApp` is the composition root.

`connections` should live on App, not on Session:

```ts
const Connections = App.createActiveConnections<AppConnections>();
```

or, once integrated:

```ts
App.connections.connection('main');
```

Do **not** put this under:

```ts
Sess.connections;
Sess.socket;
Sess.realtime;
```

Session is only a source of auth/lifecycle events.

### 3.2 With `sess`

`arts/sess` owns session lifecycle:

```txt
ADOPTED
REFRESHED
EXPIRED
REVOKED
EXTERNAL_CHANGED
```

`arts/conn` may subscribe to those events through App auto-wiring, but only per connection and only if configured.

Example:

```ts
connections: {
    main: {
        session: {
            enabled: true,
            reauthOnRefresh: true,
            disconnectOnExpire: true
        }
    },

    market: {
        session: false
    }
}
```

Public connections may ignore session entirely.

### 3.3 With `http`

`arts/conn` should not depend on `arts/http`, but transport adapters may use an HTTP client if injected.

Example SSE adapter:

```ts
createSseTransport({
	url: () => '/events',
	send: async (frame) => {
		await App.http.post('/events/send', { body: frame });
	}
});
```

### 3.4 With `logr`

Logger injection is allowed.

All logs should use a stable scope, for example:

```txt
conn
conn:main
conn:main:orders
```

### 3.5 With `stor`

No persistence by default.

Connection state is runtime state. Do not persist connections in storage.

### 3.6 With `timr`

`arts/conn` should use `App.timers` when built from `aapp`. Reconnect,
heartbeat and pending-ack timeouts must be keyed timers so app-level debug
panels and `App.dispose()` can see and cancel them uniformly.

Recommended key shape:

```txt
conn:<connection>:reconnect
conn:<connection>:heartbeat
conn:<connection>:ack:<id>
```

The engine accepts a structural scheduler option compatible with
`TimerScheduler` instead of importing Svelte-specific active wrappers.

### 3.7 With `libs/http` and `libs/timers`

Protocol vocabulary that is not connection-specific belongs in `libs/*`.

- HTTP method/header/status names used by polling/SSE transports must come
  from `$libs/http`.
- Reconnect backoff must reuse `$libs/timers`.
- `arts/conn` owns only connection-specific constants: states, events,
  transport kinds, frame keys, close reasons, logger messages and error names.

---

## 4. Naming

Artifact:

```txt
src/arts/connection/
```

Public names:

```ts
EngineConnections;
ActiveConnections;

EngineConnection;
ActiveConnection;

ConnectionChannel;
ConnectionPresence;

ConnectionTransport;
ConnectionSerializer;
```

Factory names:

```ts
createEngineConnections;
createActiveConnections;

createWebSocketTransport;
createSseTransport;
createPollingTransport;
createMockTransport;

jsonConnectionSerializer;
```

Avoid names such as:

```ts
SocketService;
RealtimeManager;
ConnectionManager;
```

---

## 5. Directory structure

Target structure:

```txt
src/arts/connection/
  index.ts

  consts.ts
  types.ts
  errors.ts

  engine-connections.ts
  active-connections.svelte.ts

  engine-connection.ts
  active-connection.svelte.ts

  channel.ts
  presence.ts

  transport.ts
  transports/
    websocket.ts
    sse.ts
    polling.ts
    mock.ts

  serializer.ts
  serializers/
    json.ts

  reconnect.ts
  heartbeat.ts
  backpressure.ts
  ack.ts

  app-integration.ts

  engine-connections.test.ts
  engine-connection.test.ts
  channel.test.ts
  transports.test.ts
```

Rules:

- `.ts` files must be runes-free.
- `.svelte.ts` files may use `$state`.
- `index.ts` must not accidentally force server-side imports of runes files where inappropriate.
- Prefer explicit file imports in App integration if necessary.

---

## 6. Core abstractions

### 6.1 Connection hub

The hub owns a registry of named connections.

```ts
export interface EngineConnections<TConnections extends ConnectionMap = ConnectionMap> {
	createConnection<K extends keyof TConnections & string>(
		name: K,
		options: ConnectionOptions<TConnections[K]>
	): EngineConnection<TConnections[K]>;

	createConnection<TChannels extends ConnectionChannelMap = ConnectionChannelMap>(
		name: string,
		options: ConnectionOptions<TChannels>
	): EngineConnection<TChannels>;

	connection<K extends keyof TConnections & string>(name: K): EngineConnection<TConnections[K]>;

	connection<TChannels extends ConnectionChannelMap = ConnectionChannelMap>(
		name: string
	): EngineConnection<TChannels>;

	has(name: string): boolean;

	names(): readonly string[];

	close(name: string, reason?: string): void;

	closeAll(reason?: string): void;

	dispose(): void;
}
```

Behavior:

- `createConnection(name, options)` creates and registers a connection.
- Creating the same name twice should throw `ConnectionAlreadyExistsError`.
- `connection(name)` returns an existing connection or throws `ConnectionNotFoundError`.
- `closeAll()` disconnects all registered connections.
- `dispose()` disconnects everything and clears listeners.

### 6.1.1 Hub lifecycle methods

In addition to create/lookup methods, the hub MUST expose lifecycle methods by connection name:

```ts
openConnection(name: string): Promise<ConnectionConnectResult>;
closeConnection(name: string, reason?: string): void;
reconnectConnection(name: string, reason?: string): Promise<ConnectionConnectResult>;

openAll(): Promise<readonly ConnectionConnectResult[]>;
closeAll(reason?: string): void;
reconnectAll(reason?: string): Promise<readonly ConnectionConnectResult[]>;
```

These are convenience methods over:

```ts
Connections.connection(name).connect();
Connections.connection(name).disconnect();
Connections.connection(name).reconnect();
```

Both levels are intentionally supported.

### 6.2 Active hub

```ts
export interface ActiveConnections<
	TConnections extends ConnectionMap = ConnectionMap
> extends EngineConnections<TConnections> {
	readonly size: number;
	readonly activeNames: readonly string[];
}
```

`ActiveConnections` wraps `EngineConnections` but exposes reactive derived state.

Important:

- `createEngineConnections()` must never create active/runes objects.
- `createActiveConnections()` may internally create `EngineConnections`.

Correct:

```ts
const Connections = createActiveConnections();
const Main = Connections.createConnection('main', options);
```

Avoid:

```ts
createEngineConnections().createActiveConnection(...)
```

That mixes layers.

---

## 7. Connection abstraction

Each connection has its own:

- name
- transport
- serializer
- state
- reconnect policy
- heartbeat policy
- buffer policy
- auth provider
- channel registry
- event listeners
- pending acks/requests

```ts
export interface EngineConnection<TChannels extends ConnectionChannelMap = ConnectionChannelMap> {
	readonly name: string;
	readonly state: ConnectionState;
	readonly connected: boolean;
	readonly generation: number;

	connect(): Promise<ConnectionConnectResult>;

	disconnect(reason?: string): void;

	reconnect(reason?: string): Promise<ConnectionConnectResult>;

	reauthenticate(): Promise<ConnectionAuthResult>;

	send<TPayload>(
		type: string,
		payload: TPayload,
		options?: ConnectionSendOptions
	): Promise<ConnectionSendResult>;

	request<TPayload, TResult = unknown>(
		type: string,
		payload: TPayload,
		options?: ConnectionRequestOptions
	): Promise<ConnectionAckResult<TResult>>;

	channel<K extends keyof TChannels & string>(
		name: K,
		options?: ConnectionChannelOptions
	): ConnectionChannel<TChannels[K]>;

	channel<TEvents extends ConnectionEventMap = ConnectionEventMap>(
		name: string,
		options?: ConnectionChannelOptions
	): ConnectionChannel<TEvents>;

	channels(): readonly ConnectionChannel[];

	hasChannel(name: string): boolean;

	leaveChannel(name: string): Promise<void>;

	onState(listener: (change: ConnectionStateChange) => void): () => void;

	onAny(listener: (frame: ConnectionFrame, meta: ConnectionMessageMeta) => void): () => void;

	dispose(): void;
}
```

`ActiveConnection` extends `EngineConnection` and exposes reactive properties:

```ts
export interface ActiveConnection<
	TChannels extends ConnectionChannelMap = ConnectionChannelMap
> extends EngineConnection<TChannels> {
	readonly state: ConnectionState;
	readonly connected: boolean;
	readonly generation: number;
	readonly channelNames: readonly string[];
}
```

---

## 8. State model

```ts
export type ConnectionState =
	| 'idle'
	| 'connecting'
	| 'open'
	| 'reconnecting'
	| 'closing'
	| 'closed'
	| 'failed';
```

State transitions:

```txt
idle -> connecting -> open
idle -> connecting -> failed

open -> reconnecting -> open
open -> closing -> closed
open -> closed

reconnecting -> open
reconnecting -> failed
reconnecting -> closed

failed -> connecting
closed -> connecting
```

Rules:

- `connect()` when already open should return `{ ok: true, state: 'open', reused: true }`.
- `connect()` while connecting should share the existing promise.
- `reconnect()` increments generation.
- `disconnect()` must cancel reconnect timers, heartbeat timers and pending acks.
- `dispose()` must call `disconnect()` and remove all listeners.

---

## 9. Transport adapter

The connection engine must not know whether the transport is WebSocket, SSE or polling.

```ts
export type ConnectionTransportState =
	| 'idle'
	| 'opening'
	| 'open'
	| 'closing'
	| 'closed'
	| 'failed';

export interface ConnectionCloseEvent {
	readonly code?: number;
	readonly reason?: string;
	readonly clean?: boolean;
}

export interface ConnectionTransport {
	readonly kind: string;
	readonly state: ConnectionTransportState;
	readonly bufferedAmount: number;
	readonly canSend: boolean;

	open(): Promise<void>;

	send(data: string | ArrayBuffer): Promise<void> | void;

	close(code?: number, reason?: string): void;

	onOpen(listener: () => void): () => void;

	onMessage(listener: (message: string | ArrayBuffer) => void): () => void;

	onClose(listener: (event: ConnectionCloseEvent) => void): () => void;

	onError(listener: (error: unknown) => void): () => void;
}
```

### 9.1 WebSocket transport

```ts
createWebSocketTransport({
	url: () => '/realtime',
	protocols: ['app-v1']
});
```

Must support:

- `open`
- `send`
- `close`
- `bufferedAmount`
- `onOpen`
- `onMessage`
- `onClose`
- `onError`

### 9.2 SSE transport

SSE is receive-first. Sending is optional.

```ts
createSseTransport({
	url: () => '/events',

	send: async (frame) => {
		await App.http.post('/events/send', { body: frame });
	}
});
```

If no `send` function is provided:

```ts
canSend === false;
```

Calling `send()` should return:

```ts
{ ok: false, reason: 'send_not_supported' }
```

at the connection layer.

### 9.3 Polling transport

```ts
createPollingTransport({
	poll: async (cursor) => {
		return App.http.get('/events', { query: { cursor } });
	},

	send: async (frame) => {
		await App.http.post('/events', { body: frame });
	},

	intervalMs: 5_000
});
```

### 9.4 Mock transport

Required for tests.

```ts
const transport = createMockTransport();

transport.emitMessage(raw);
transport.emitClose({ code: 1006, clean: false });
transport.emitError(new Error('boom'));
```

The mock transport must expose deterministic test helpers but should still implement the same `ConnectionTransport` interface.

---

## 10. Serializer adapter

Do not hardcode JSON deeply into the engine.

```ts
export interface ConnectionSerializer<TFrame = ConnectionFrame> {
	encode(frame: TFrame): string | ArrayBuffer;
	decode(raw: string | ArrayBuffer): TFrame;
}
```

Default:

```ts
jsonConnectionSerializer();
```

`jsonConnectionSerializer` must:

- encode frames with `JSON.stringify`
- decode string frames with `JSON.parse`
- reject binary frames unless explicitly configured
- return tagged errors or throw only internally and have the engine convert errors to `ConnectionError`

---

## 11. Frame model

Base frame:

```ts
export interface ConnectionFrame<TType extends string = string, TPayload = unknown> {
	readonly id?: string;
	readonly topic?: string;
	readonly type: TType;
	readonly payload: TPayload;
	readonly ts?: number;
	readonly ack?: boolean;
	readonly replyTo?: string;
	readonly error?: ConnectionFrameError;
}
```

Meanings:

- `id`: unique message id generated by the client when needed.
- `topic`: optional channel name.
- `type`: event type.
- `payload`: event payload.
- `ts`: epoch ms.
- `ack`: when true, sender expects reply.
- `replyTo`: id of original request when this is an ack/reply.
- `error`: reply error payload.

Global events have no `topic`.

Channel events include `topic`.

---

## 12. Event maps and typing

```ts
export type ConnectionEventMap = Record<string, unknown>;

export type ConnectionChannelMap = Record<string, ConnectionEventMap>;

export type ConnectionMap = Record<string, ConnectionChannelMap>;
```

Example:

```ts
type AppConnections = {
	main: {
		orders: {
			'order.created': OrderCreated;
			'order.updated': OrderUpdated;
		};

		cart: {
			'cart.changed': CartChanged;
		};
	};

	chat: {
		rooms: {
			'message.created': MessageCreated;
			'typing.started': TypingStarted;
		};
	};

	market: {
		prices: {
			'price.tick': PriceTick;
		};
	};
};
```

Usage:

```ts
const Connections = App.createActiveConnections<AppConnections>();

const Main = Connections.createConnection('main', { ... });
const Orders = Main.channel('orders');

Orders.on('order.updated', (payload) => {
    payload.orderId;
});
```

The generic typing should help at call-sites, but runtime validation is optional and schema-driven.

---

## 13. Channels

A channel is a logical topic within one connection.

```ts
export type ConnectionChannelState = 'idle' | 'joining' | 'joined' | 'leaving' | 'left' | 'failed';

export interface ConnectionChannel<TEvents extends ConnectionEventMap = ConnectionEventMap> {
	readonly name: string;
	readonly state: ConnectionChannelState;

	join(params?: unknown): Promise<ConnectionChannelJoinResult>;

	leave(): Promise<void>;

	send<K extends keyof TEvents & string>(
		type: K,
		payload: TEvents[K],
		options?: ConnectionSendOptions
	): Promise<ConnectionSendResult>;

	request<K extends keyof TEvents & string, TResult = unknown>(
		type: K,
		payload: TEvents[K],
		options?: ConnectionRequestOptions
	): Promise<ConnectionAckResult<TResult>>;

	on<K extends keyof TEvents & string>(
		type: K,
		listener: (payload: TEvents[K], meta: ConnectionMessageMeta) => void
	): () => void;

	onAny(listener: (frame: ConnectionFrame, meta: ConnectionMessageMeta) => void): () => void;

	dispose(): void;
}
```

Rules:

- `connection.channel(name)` should cache and return the same instance for the same name.
- Channels do not create physical connections.
- Channels route frames by `frame.topic === channel.name`.
- `leave()` sends a leave frame if connected, then marks local state as left.
- Rejoin after reconnect is configurable.

Channel options:

```ts
export interface ConnectionChannelOptions {
	readonly autoJoin?: boolean;
	readonly rejoinOnReconnect?: boolean;
	readonly params?: () => unknown | Promise<unknown>;
}
```

---

## 14. Presence

Presence is optional and channel-scoped.

Do not implement a complex distributed presence model in v1. Provide a minimal abstraction that can be driven by frames.

```ts
export interface ConnectionPresence<TPresence = unknown> {
	readonly members: readonly ConnectionPresenceMember<TPresence>[];

	onJoin(listener: (member: ConnectionPresenceMember<TPresence>) => void): () => void;

	onLeave(listener: (member: ConnectionPresenceMember<TPresence>) => void): () => void;

	onSync(listener: (members: readonly ConnectionPresenceMember<TPresence>[]) => void): () => void;
}

export interface ConnectionPresenceMember<TPresence = unknown> {
	readonly id: string;
	readonly payload: TPresence;
	readonly joinedAt?: number;
}
```

Presence may be implemented later if v1 scope is too large. Do not block the base connection engine on presence.

---

## 15. Send/request/ack semantics

### 15.1 Send

```ts
export type ConnectionSendResult =
	| { readonly ok: true; readonly id?: string }
	| {
			readonly ok: false;
			readonly reason:
				| 'closed'
				| 'send_not_supported'
				| 'buffer_full'
				| 'invalid_frame'
				| 'serialize_failed'
				| 'transport_error';
			readonly error?: unknown;
	  };
```

`send()` should not throw for normal runtime failures.

### 15.2 Request/ack

```ts
export type ConnectionAckResult<T = unknown> =
	| { readonly ok: true; readonly payload: T }
	| {
			readonly ok: false;
			readonly reason: 'timeout' | 'closed' | 'rejected' | 'transport_error' | 'invalid_reply';
			readonly error?: unknown;
	  };
```

Request behavior:

- Generate id.
- Send frame with `ack: true`.
- Store pending resolver by id.
- When frame with `replyTo` arrives, resolve matching promise.
- Timeout rejects as tagged result.
- Disconnect resolves all pending acks with `{ ok: false, reason: 'closed' }`.

Options:

```ts
export interface ConnectionRequestOptions extends ConnectionSendOptions {
	readonly timeoutMs?: number;
}
```

---

## 16. Reconnect

```ts
export interface ConnectionReconnectOptions {
	readonly enabled?: boolean;
	readonly minDelayMs?: number;
	readonly maxDelayMs?: number;
	readonly factor?: number;
	readonly jitterMs?: number;
	readonly maxAttempts?: number;
	readonly reconnectOnVisible?: boolean;
	readonly reconnectOnOnline?: boolean;
}
```

Defaults:

```ts
{
    enabled: true,
    minDelayMs: 500,
    maxDelayMs: 15_000,
    factor: 1.8,
    jitterMs: 500,
    maxAttempts: undefined,
    reconnectOnVisible: true,
    reconnectOnOnline: true
}
```

Rules:

- Use exponential backoff with jitter.
- Reset attempt count after successful open.
- Do not reconnect after intentional `disconnect()` unless `reconnect()` is called.
- Reconnect increments generation.
- After reconnect:
  1. reauthenticate if configured
  2. rejoin channels that had `rejoinOnReconnect !== false`
  3. resume message flow

---

## 17. Heartbeat

```ts
export interface ConnectionHeartbeatOptions {
	readonly enabled?: boolean;
	readonly intervalMs?: number;
	readonly timeoutMs?: number;
	readonly pingType?: string;
	readonly pongType?: string;
}
```

Defaults:

```ts
{
    enabled: true,
    intervalMs: 25_000,
    timeoutMs: 10_000,
    pingType: 'connection.ping',
    pongType: 'connection.pong'
}
```

Behavior:

- When open, periodically send ping if transport can send.
- If no pong/any message is received within timeout, close/reconnect.
- For SSE receive-only transports, heartbeat may be receive-based only.

---

## 18. Backpressure and buffering

```ts
export interface ConnectionBufferOptions {
	readonly policy?: 'buffer' | 'drop' | 'fail';
	readonly maxMessages?: number;
	readonly maxBytes?: number;
}
```

Rules:

- If disconnected and policy is `buffer`, enqueue messages up to `maxMessages`.
- If disconnected and policy is `drop`, return ok false or drop according to options.
- If disconnected and policy is `fail`, return `{ ok: false, reason: 'closed' }`.
- If `transport.bufferedAmount > maxBytes`, return `{ ok: false, reason: 'buffer_full' }`.
- Buffered messages flush after reconnect, before normal sends.
- Request/ack messages should generally not be buffered unless explicitly allowed.

Default:

```ts
{
    policy: 'fail',
    maxMessages: 100,
    maxBytes: 1_000_000
}
```

---

## 19. Auth and reauthentication

Auth is connection-specific and opaque.

```ts
export type ConnectionAuthPayload = Record<string, unknown>;

export interface ConnectionAuthOptions {
	readonly getAuth?: () => ConnectionAuthPayload | null | Promise<ConnectionAuthPayload | null>;

	readonly authType?: string;
}
```

Reauth behavior:

```ts
reauthenticate(): Promise<ConnectionAuthResult>
```

Implementation may send a frame:

```ts
{
    type: 'connection.auth',
    payload: await getAuth(),
    ack: true
}
```

Result:

```ts
export type ConnectionAuthResult =
	| { readonly ok: true }
	| {
			readonly ok: false;
			readonly reason: 'no_auth_provider' | 'closed' | 'rejected' | 'timeout' | 'transport_error';
			readonly error?: unknown;
	  };
```

Do not hardcode Bearer/JWT.

---

## 20. App integration

### 20.1 App options

Suggested App-level options:

```ts
export interface ActiveAppConnectionsOptions {
	readonly defaults?: Partial<ConnectionOptions>;

	readonly connections?: Record<string, Partial<ConnectionOptions>>;

	readonly autoCreate?: boolean;
}
```

But do not force all connections to be declared in App config. Support both patterns:

Declarative:

```ts
const App = createActiveApp({
	connections: {
		connections: {
			main: {
				reconnect: { enabled: true }
			},
			market: {
				buffer: { policy: 'drop' }
			}
		}
	}
});
```

Typed creation:

```ts
const Connections = App.createActiveConnections<AppConnections>();

const Main = Connections.createConnection('main', {
	transport: createWebSocketTransport({
		url: () => '/realtime'
	})
});
```

### 20.2 Why factory on App

Use an App factory, not a generic `App.connections` baked into `ActiveApp<T>` by default.

Recommended:

```ts
const Connections = App.createActiveConnections<AppConnections>();
```

Reasons:

- Type flows at call site.
- App itself avoids becoming generic on every connection map.
- Mirrors the already accepted factory pattern for typed runtime helpers.
- Avoids forcing connections in apps that do not need realtime.

### 20.3 Auto-wiring

Auto-wire only obvious infrastructure:

- `Logger` injected into connections.
- App defaults merged into connection options.
- Session events wired only when connection opts into session integration.
- Frontend visibility/online events used only when reconnect options enable them.

Do not auto-wire domain behavior.

---

## 21. Options

```ts
export interface ConnectionOptions<TChannels extends ConnectionChannelMap = ConnectionChannelMap> {
	readonly transport: ConnectionTransport | (() => ConnectionTransport);

	readonly serializer?: ConnectionSerializer;

	readonly reconnect?: ConnectionReconnectOptions | false;

	readonly heartbeat?: ConnectionHeartbeatOptions | false;

	readonly buffer?: ConnectionBufferOptions;

	readonly auth?:
		| ConnectionAuthOptions
		| (() => ConnectionAuthPayload | null | Promise<ConnectionAuthPayload | null>);

	readonly session?:
		| false
		| {
				readonly enabled?: boolean;
				readonly reauthOnRefresh?: boolean;
				readonly disconnectOnExpire?: boolean;
		  };

	readonly channels?: {
		readonly [K in keyof TChannels & string]?: ConnectionChannelOptions;
	};

	readonly loggerScope?: string;
}
```

Note:

- If `transport` is a function, call it on each fresh physical connect/reconnect if the previous transport cannot be reused.
- If a transport instance can reopen safely, reuse is allowed, but keep behavior deterministic.

---

## 22. Errors

Use custom errors for programmer errors, not normal runtime failures.

Examples:

```ts
ConnectionAlreadyExistsError;
ConnectionNotFoundError;
ConnectionInvalidNameError;
ConnectionInvalidFrameError;
ConnectionDisposedError;
ConnectionChannelAlreadyExistsError;
ConnectionChannelNotFoundError;
```

Runtime network failures should generally return tagged results or emit state/error events, not throw unexpectedly.

---

## 23. Public constants

```ts
export const CONNECTION_STATE_IDLE = 'idle';
export const CONNECTION_STATE_CONNECTING = 'connecting';
export const CONNECTION_STATE_OPEN = 'open';
export const CONNECTION_STATE_RECONNECTING = 'reconnecting';
export const CONNECTION_STATE_CLOSING = 'closing';
export const CONNECTION_STATE_CLOSED = 'closed';
export const CONNECTION_STATE_FAILED = 'failed';

export const DEFAULT_RECONNECT_MIN_DELAY_MS = 500;
export const DEFAULT_RECONNECT_MAX_DELAY_MS = 15_000;
export const DEFAULT_RECONNECT_FACTOR = 1.8;
export const DEFAULT_RECONNECT_JITTER_MS = 500;

export const DEFAULT_HEARTBEAT_INTERVAL_MS = 25_000;
export const DEFAULT_HEARTBEAT_TIMEOUT_MS = 10_000;

export const DEFAULT_ACK_TIMEOUT_MS = 10_000;

export const TRANSPORT_KIND_WEBSOCKET = 'websocket';
export const TRANSPORT_KIND_MOCK = 'mock';

export const FRAME_KEY_ID = 'id';
export const FRAME_KEY_TYPE = 'type';
export const FRAME_KEY_TOPIC = 'topic';
export const FRAME_KEY_PAYLOAD = 'payload';
export const FRAME_KEY_REPLY_TO = 'replyTo';

export const LOGGER_CATEGORY = 'conn';
export const LOG_MSG_LISTENER_THREW_PREFIX = 'Listener threw on ';

export const ERROR_NAME_DISPOSED = 'ConnectionDisposedError';
export const ERROR_NAME_ALREADY_EXISTS = 'ConnectionAlreadyExistsError';
export const ERROR_NAME_NOT_FOUND = 'ConnectionNotFoundError';
export const ERROR_NAME_INVALID_NAME = 'ConnectionInvalidNameError';
export const ERROR_NAME_INVALID_FRAME = 'ConnectionInvalidFrameError';
```

Prefer exported constants over magic strings/numbers. Engine bodies should not
inline connection states, event names, transport kinds, close reasons, frame
field names, logger categories or error names.

---

## 24. Implementation algorithm

### 24.1 createEngineConnections

- Keep `Map<string, EngineConnection>`.
- Validate connection names.
- Merge hub defaults with per-connection options.
- Create `EngineConnection`.
- Register it.
- Return it.
- Dispose closes all.

### 24.2 createEngineConnection

Internal state:

```ts
let state: ConnectionState = 'idle';
let generation = 0;
let disposed = false;
let connectPromise: Promise<ConnectionConnectResult> | null = null;

const channels = new Map<string, ConnectionChannel>();
const listeners = new Set<...>();
const pendingAcks = new Map<string, PendingAck>();
const outboundBuffer: ConnectionFrame[] = [];
```

On `connect()`:

1. If disposed, throw `ConnectionDisposedError`.
2. If open, return reused success.
3. If connecting, return existing promise.
4. Create/open transport.
5. Attach transport listeners.
6. Transition to `connecting`.
7. Await transport open.
8. Transition to `open`.
9. Authenticate if configured.
10. Start heartbeat.
11. Flush buffer.
12. Auto-join configured channels.
13. Return success.

On transport message:

1. Decode with serializer.
2. Validate frame shape.
3. If `replyTo`, resolve pending ack.
4. If heartbeat pong, mark heartbeat alive.
5. If `topic`, route to channel.
6. Emit global listeners.

On transport close:

1. Stop heartbeat.
2. Resolve pending acks as closed.
3. If intentional, transition closed.
4. If not intentional and reconnect enabled, transition reconnecting and schedule reconnect.
5. Else transition failed/closed.

On `disconnect()`:

1. Mark intentional close.
2. Stop reconnect timer.
3. Stop heartbeat.
4. Close transport.
5. Resolve pending acks.
6. Transition closed.

---

## 25. Validation

Runtime frame validation should be minimal by default:

```ts
function isConnectionFrame(value: unknown): value is ConnectionFrame {
	return (
		typeof value === 'object' &&
		value !== null &&
		typeof (value as { type?: unknown }).type === 'string' &&
		'payload' in value
	);
}
```

Optional Standard Schema support may be added for event payloads, but do not make it required for v1.

If added later:

```ts
schemas: {
    orders: {
        'order.updated': OrderUpdatedSchema
    }
}
```

No vendor-specific validation library.

---

## 26. Testing checklist

### Engine hub

- creates connection
- rejects duplicate connection
- retrieves connection
- throws on missing connection
- closes one connection
- closes all connections
- disposes cleanly

### Connection lifecycle

- connects successfully
- connect dedups in-flight promise
- disconnect closes transport
- reconnect increments generation
- no reconnect after intentional disconnect
- reconnect after unexpected close
- failed state after max attempts

### Transport

- WebSocket transport maps events correctly
- mock transport works deterministically
- SSE transport reports `canSend = false` when no send provided
- polling transport receives messages repeatedly

### Channels

- channel is cached by name
- routes messages by topic
- global messages do not hit channel listeners
- channel leave updates state
- rejoin after reconnect when configured

### Send/request

- send serializes frame
- send fails when closed and buffer policy fail
- send buffers when policy buffer
- send drops/fails on buffer full
- request resolves on replyTo
- request times out
- disconnect resolves pending requests as closed

### Heartbeat

- sends ping
- receives pong
- reconnects/closes on heartbeat timeout
- disabled heartbeat does nothing

### Session integration

- session refresh triggers reauth only for configured connections
- session revoke disconnects only configured connections
- public connections remain open

### Active wrappers

- `state` reactive
- `connected` reactive
- `generation` reactive
- channel names reactive if exposed

---

## 27. Implementation order for Claude Code

Implement in this order:

1. `consts.ts`
2. `types.ts`
3. `errors.ts`
4. `serializer.ts` + `serializers/json.ts`
5. `transport.ts`
6. `transports/mock.ts`
7. `engine-connection.ts`
8. `channel.ts`
9. `engine-connections.ts`
10. Tests for engine with mock transport
11. `transports/websocket.ts`
12. `reconnect.ts`
13. `heartbeat.ts`
14. `backpressure.ts`
15. `ack.ts`
16. `active-connection.svelte.ts`
17. `active-connections.svelte.ts`
18. `app-integration.ts`
19. Optional SSE/polling transports
20. Documentation examples

Do not implement presence in the first pass unless everything above is stable.

---

## 28. Example usage

```ts
type AppConnections = {
	main: {
		orders: {
			'order.updated': {
				orderId: string;
				status: string;
			};
		};

		cart: {
			'cart.changed': {
				cartId: string;
				total: number;
			};
		};
	};

	market: {
		prices: {
			'price.tick': {
				symbol: string;
				price: number;
			};
		};
	};
};

const Connections = App.createActiveConnections<AppConnections>();

const Main = Connections.createConnection('main', {
	transport: createWebSocketTransport({
		url: () => '/realtime'
	}),

	auth: () => {
		const session = Sess.current;

		return session?.credential ? { token: session.credential.accessToken } : null;
	},

	session: {
		enabled: true,
		reauthOnRefresh: true,
		disconnectOnExpire: true
	}
});

const Orders = Main.channel('orders');

Orders.on('order.updated', (payload) => {
	console.log(payload.orderId, payload.status);
});

await Main.connect();
```

Second connection:

```ts
const Market = Connections.createConnection('market', {
	transport: createWebSocketTransport({
		url: () => 'wss://prices.example.com/stream'
	}),

	reconnect: {
		minDelayMs: 250,
		maxDelayMs: 3_000
	},

	buffer: {
		policy: 'drop',
		maxMessages: 50
	},

	session: false
});

const Prices = Market.channel('prices');

Prices.on('price.tick', (tick) => {
	console.log(tick.symbol, tick.price);
});

await Market.connect();
```

---

## 29. Final architectural rule

`arts/conn` owns realtime connection infrastructure.

It does not own auth, session, permissions, user state, cache, or webhooks.

The final layering should remain:

```txt
arts/sess
    session lifecycle

arts/http
    request/response

arts/conn
    realtime connections

arts/webh
    signed webhook receive/send infrastructure

arts/cache
    future derived state/query cache

ActiveApp
    composition root that wires them when configured
```