active-svelte/src/arts/http/README.md

# http

Professional HTTP client. **Zero external dependencies.** Tagged-result error
model. Per-call schema validation against any Standard Schema (Sium, Zod,
Valibot, ArkType). Idempotent-by-default retry with `Retry-After` parsing.
Per-attempt and total-budget timeouts. SvelteKit-native via injectable
`fetch`. Hooks for auth refresh, logging, transformation.

```ts
import { createEngineHttp } from '$http';

const http = createEngineHttp({ baseUrl: 'https://api.example.com' });

const r = await http.get('/users/me', { schema: UserSchema });

if (r.ok) {
    console.log(r.value.email);
} else if (r.kind === 'http') {
    console.error(`HTTP ${r.status}: ${r.statusText}`);
} else if (r.kind === 'validation') {
    console.error('Server returned an unexpected shape:', r.issues);
} else {
    console.error('Network failure:', r.error);
}
```

---

## Why another one

`fetch` exists. `axios`, `ky`, `ofetch`, `wretch` exist. None ship with the
combination this artifact targets:

- **Zero dependencies.** `axios` carries IE polyfills, `wretch` ships
  middlewares as separate addons, `ofetch` depends on `unjs/ufo`.
- **Tagged-result error model.** Every other library throws on non-2xx.
  `arts/http` returns a discriminated union — errors are data, not exceptions
  (matches `$sium`'s `Result<O>` for uniform pattern matching).
- **Standard Schema first-class.** Per-call `schema` parameter validates the
  response body. Returns `Out<S>` typed. Works with Sium / Zod / Valibot /
  ArkType identically. No runtime dep on Sium — the artifact only imports
  the `StandardSchemaV1` interface from `$libs/standard-schema`.
- **`bodySchema` for write methods.** Validate the request payload before
  it leaves the engine. Catches "I sent the wrong shape" — a class of bugs
  no other generic HTTP client catches.
- **SvelteKit-aware.** `engine.with({ fetch: event.fetch })` returns a scoped
  client that inherits cookies, resolves relative URLs and skips network
  roundtrips when the route is local.
- **Retryer extracted.** The retry primitive is a separable export — a
  future `arts/cache` can reuse it without a duplicate implementation.

---

## Architecture

```
http/
├── index.ts             Barrel exports
├── types.ts             EngineHttp, HttpResult, HttpInit, HttpHooks, RetryConfig
├── consts.ts            DEFAULT_RETRY, DEFAULT_TIMEOUT, NULL_BODY_STATUSES, ...
├── engine-http.ts       Factory createEngineHttp()
├── errors.ts            HttpNetworkError / HttpTimeoutError / HttpAbortError /
│                        HttpBodyValidationError + type guards
├── retry.ts             parseRetryAfter, shouldRetryRequest, computeRetryDelay,
│                        delayWithSignal (reusable by arts/cache)
├── timeout.ts           composeSignals, attemptTimeoutSignal, totalTimeoutSignal,
│                        classifyAbort
├── body.ts              isJSONSerializable, serializeBody, parseBody, mergeHeaders
├── search.ts            normalizeSearch, appendSearch, resolveUrl
└── test/
    ├── engine-http.test.ts   end-to-end with stub fetch
    ├── retry.test.ts          Retry-After parsing, policy, delay
    ├── timeout.test.ts        signal composition + classification
    ├── body.test.ts           JSON serialization + content-type sniffing
    ├── search.test.ts         search + URL helpers
    └── errors.test.ts         hierarchy + type guards
```

## Alias

Configured in `svelte.config.js`:

```js
alias: { $http: 'src/arts/http' }
```

---

## HttpResult — the discriminated union

Every method returns `Promise<HttpResult<T>>`:

```ts
type HttpResult<T> =
    | { ok: true;  value: T;            response: Response }
    | { ok: false; kind: 'http';        status; statusText; body; response: Response }
    | { ok: false; kind: 'validation';  issues;             response: Response }
    | { ok: false; kind: 'network';     error };
```

The `ok` discriminator matches Sium's. Pattern-match identically:

```ts
if (r.ok) {
    use(r.value);
} else switch (r.kind) {
    case 'http':       toast(`Server returned ${r.status}`); break;
    case 'validation': console.error(r.issues); break;
    case 'network':    toast('Offline?'); break;
}
```

The `response: Response` is preserved on the three branches that have one —
auth flows, rate-limit headers and ETags all live there. The `network`
branch deliberately omits `response` because no HTTP exchange happened.

---

## Validation — opt-in via `schema`

No schema → `value` is `unknown`, body is not parsed:

```ts
const r = await http.get('/api/health'); // r.value: unknown (undefined)
```

With a schema → body is parsed (JSON or text) and validated:

```ts
const UserSchema = sium.object({ id: sium.string(), email: sium.email() });

const r = await http.get('/api/users/me', { schema: UserSchema });
if (r.ok) r.value.email; // string, validated
```

Works with any Standard Schema vendor:

```ts
import { z } from 'zod';
const r = await http.get('/api/posts', {
    schema: z.array(z.object({ id: z.string() }))
});
```

For `POST` / `PUT` / `PATCH`, validate the request body too:

```ts
await http.post('/api/users', {
    body:       payload,
    bodySchema: CreateUserSchema, // pre-flight check; throws if invalid
    schema:     UserSchema         // post-flight check on response
});
```

When `bodySchema` rejects, the engine throws `HttpBodyValidationError`
synchronously. This is a programmer error (you sent the wrong shape), not
a runtime data condition — different from `kind: 'validation'` which is
about the *server's* response.

---

## API

### `createEngineHttp(options?)`

```ts
const http = createEngineHttp({
    baseUrl:      'https://api.example.com',
    headers:      { Accept: 'application/json' },
    fetch:        globalThis.fetch,    // override per-engine
    timeout:      10_000,              // per-attempt (ms)
    totalTimeout: 30_000,              // total budget covering all attempts (0 = off)
    retry: {
        limit:        2,
        statusCodes:  [408, 425, 429, 500, 502, 503, 504],
        methods:      ['GET', 'HEAD', 'PUT', 'DELETE', 'OPTIONS'], // idempotent
        delay:        (n) => 1000 * 2 ** (n - 1),                  // exponential
        backoffLimit: 30_000,
        jitter:       false
    },
    hooks: {
        beforeRequest:  [],
        beforeRetry:    [],
        afterResponse:  [],
        beforeError:    []
    },
    logger: App.Logger // wired automatically when used via App.Http
});
```

### `engine.with(overrides)`

Returns a new engine with `overrides` merged on top of the current defaults.
Hook arrays concatenate (parent first); scalar fields are replaced. **Never
mutates** the parent engine.

```ts
const scoped = http.with({ fetch: event.fetch }); // SvelteKit pattern
const apiV2  = http.with({ baseUrl: 'https://api.example.com/v2' });
```

### Method shortcuts

```ts
http.get   <S>(url, init?)   → Promise<HttpResult<Out<S>>>
http.head  <S>(url, init?)   → Promise<HttpResult<Out<S>>>
http.delete<S>(url, init?)   → Promise<HttpResult<Out<S>>>
http.options<S>(url, init?)  → Promise<HttpResult<Out<S>>>
http.post  <S, B>(url, init?) → Promise<HttpResult<Out<S>>>
http.put   <S, B>(url, init?) → Promise<HttpResult<Out<S>>>
http.patch <S, B>(url, init?) → Promise<HttpResult<Out<S>>>
```

`Out<S>` is `unknown` when `S` is undefined, otherwise `InferOutput<S>`.

### Per-call init

```ts
http.get('/users', {
    schema:    UserSchema,                     // validate response body
    bodySchema: NewUserSchema,                 // (write methods only) validate body
    body:      { name: 'Ada' },                // any HttpBodyInit
    headers:   { 'X-Trace-Id': '...' },        // merged on top of engine defaults
    search:    { page: 2, q: 'ada' },          // appended to URL
    fetch:     event.fetch,                    // per-call fetch override
    timeout:   5_000,                          // per-attempt (ms)
    signal:    abortCtrl.signal,               // user signal — aborts cancel retry
    retry:     { limit: 0 },                   // override retry policy (or `false`)
    hooks:     { beforeRequest: [trace] }      // per-call hooks (engine first)
});
```

---

## Retry policy

Defaults — *idempotent-by-default*: `GET`, `HEAD`, `PUT`, `DELETE`,
`OPTIONS` retry; `POST` and `PATCH` do not (unless `methods` is overridden).
Status codes: `408 425 429 500 502 503 504`.

### Retry-After

Five header variants are parsed in priority order:

```
Retry-After, RateLimit-Reset, X-RateLimit-Reset,
X-Rate-Limit-Reset, X-RateLimit-Retry-After
```

Numeric values below `1.7e9` are treated as delta-seconds; above as
epoch-seconds (the threshold is "no realistic retry delay is that large; no
realistic past timestamp is that small"). HTTP-dates (`Sun, 06 Nov 1994
08:49:37 GMT`) are also accepted. When the server signals a delay, it
overrides the policy's `delay(attempt)`.

### Signal hierarchy (no surprises)

| Signal source             | Reaction                                       |
| ------------------------- | ---------------------------------------------- |
| User-provided `signal`    | Aborts the current attempt; **no retry**       |
| `totalTimeout` exceeded   | Aborts the current attempt; **no retry**       |
| Per-attempt `timeout`     | Aborts the attempt; **retries** if budget left |

Internal classification uses `signal.reason` (an `HttpTimeoutError` with
`scope: 'attempt' | 'total'`, or an `HttpAbortError` for user signals) so
the retryer makes the right call without ambiguity.

---

## Hooks lifecycle

Per attempt, in order:

```
beforeRequest[0..k] → fetch() → afterResponse[0..k] (success)
                              ↘ beforeRetry[0..k] → wait(delay) → next attempt

Final failure: → beforeError[0..k] (may rescue with a Response)
```

Mutate `ctx.request.headers` in place — the change persists into the
`fetch` call. To rewrite the URL, mutate `ctx.url`. To short-circuit with a
cached response, return it from `beforeRequest`. To rescue a failed attempt,
return a `Response` from `beforeError` and the engine resolves as if it had
been received cleanly.

```ts
const http = createEngineHttp({
    hooks: {
        beforeRequest: [
            (ctx) => {
                ctx.request.headers.set('X-Trace-Id', crypto.randomUUID());
            }
        ],
        beforeRetry: [
            ({ attempt, retryDelay, error }) => {
                console.warn(`retry ${attempt}, waiting ${retryDelay}ms`, error);
            }
        ],
        afterResponse: [
            ({ response }) => {
                metrics.observe('http.duration', response.headers.get('x-time'));
            }
        ],
        beforeError: [
            ({ response }) => {
                if (response?.status === 401) {
                    return refreshAndRetry(); // returns a new Response or undefined
                }
            }
        ]
    }
});
```

### Auth refresh pattern

Use a header **hook** so the token is read per attempt — `beforeRetry` can
update the underlying token between attempts and the next attempt's
headers reflect the new value:

```ts
let token = await getToken();

const http = createEngineHttp({
    headers: () => ({ Authorization: `Bearer ${token}` }),
    hooks: {
        beforeRetry: [
            async ({ error, response }) => {
                if (response?.status === 401) {
                    token = await refreshToken();
                }
            }
        ]
    }
});
```

---

## SvelteKit integration

In a `+page.ts` / `+page.server.ts` / `+layout.ts` `load`, scope the engine
to the request via `event.fetch`:

```ts
// +page.server.ts
import type { PageServerLoad } from './$types';

export const load: PageServerLoad = async ({ fetch }) => {
    const api = App.Http.with({ fetch }); // inherits cookies + relative URLs
    const user = await api.get('/api/users/me', { schema: UserSchema });
    if (!user.ok) throw error(401, 'Not authenticated');
    return { user: user.value };
};
```

Or per-call:

```ts
const r = await App.Http.get('/api/users/me', {
    fetch: event.fetch,
    schema: UserSchema
});
```

Without `event.fetch`, server-side requests use `globalThis.fetch` and lose
SvelteKit's automatic cookie forwarding and relative-URL resolution.

---

## Errors

Five subclasses, all with stable literal `name` and exported type guards:

| Class                       | When                                       | Surfaces as                |
| --------------------------- | ------------------------------------------ | -------------------------- |
| `HttpNetworkError`          | Wrapper for `fetch` rejection              | `kind: 'network'`          |
| `HttpTimeoutError`          | Per-attempt or total timeout fired         | `kind: 'network'` (final)  |
| `HttpAbortError`            | User signal aborted                        | `kind: 'network'` (final)  |
| `HttpBodyValidationError`   | `bodySchema` rejected the payload          | **Thrown** (programmer err)|

Type guards: `isHttpNetworkError`, `isHttpTimeoutError`, `isHttpAbortError`,
`isHttpBodyValidationError`. Each carries stable `name` so they survive
serialization across worker boundaries.

---

## Composition with App

When used through `createActiveApp(...)`, `App.Http` is built with the
shared `Logger` injected automatically:

```ts
const App = createActiveApp({
    http: {
        baseUrl: 'https://api.example.com',
        timeout: 10_000
    }
});

await App.Http.get('/users/me', { schema: UserSchema });
```

Per-request `event.fetch` for SSR:

```ts
const api = App.Http.with({ fetch: event.fetch });
```

Validation messages with the App's locale:

```ts
const r = await App.Http.get('/api/users/me', { schema: UserSchema });
if (!r.ok && r.kind === 'validation') {
    const sium = App.createSiumEngine();
    console.error(sium.resolveIssues(r.issues));
}
```

---

## Testing

The engine takes any `fetch`-compatible function. Stub it:

```ts
import { createEngineHttp } from '$http';

const http = createEngineHttp({
    retry: { limit: 0 },
    timeout: 0,
    fetch: ((_url, _init) =>
        Promise.resolve(new Response('{"ok":true}', {
            status: 200,
            headers: { 'content-type': 'application/json' }
        }))) as typeof fetch
});
```

Or queue several responses for retry scenarios — see
`src/arts/http/test/engine-http.test.ts` for the test helpers.

---

## Bundle profile

| Layer                           | Approx. size (min) |
| ------------------------------- | ------------------ |
| Engine + Retryer + helpers      | ~5 KB              |
| Errors + type guards            | ~1 KB              |
| Total (everything reached)      | ~6 KB              |

Zero runtime dependencies. The `StandardSchemaV1` import is type-only.