idempotency-middleware

1	# Ticket: Implement the Fenced Idempotency Protocol for the charges API
2	
3	## Context
4	
5	`payments-api` is a small Express service (TypeScript, Node 20) exposing
6	`POST /v1/charges`. It sits behind a load balancer where clients retry
7	aggressively, and where a client may also *deliberately re-issue* an operation
8	it knows it wants to run again. A naive retry today creates a **duplicate
9	charge**.
10	
11	We are adopting a house protocol called the **Fenced Idempotency Protocol
12	(FIP/1)**, specified in full below. Implement it exactly as specified.
13	
14	The project ships every supporting piece complete , an `IdempotencyStore`
15	interface with an in-memory implementation (atomic `get` / `compareAndSet` /
16	`delete`), a response-capture helper, option handling, the charges route, the
17	clock abstraction, and the app wiring. **Two files are unimplemented stubs and
18	are yours to write:**
19	
20	    src/lib/fingerprint.ts          ->  export function computeFingerprint(...)
21	    src/middleware/idempotency.ts   ->  export function idempotency(options)
22	
23	`idempotency(options)` returns an Express `RequestHandler`. Keep the public
24	exports (`idempotency`, `REPLAY_HEADER`, `computeFingerprint`). You do not need
25	to modify any other file.
26	
27	---
28	
29	## Definitions
30	
31	- **Key** , the value of the `Idempotency-Key` request header (configurable via
32	  `options.keyHeaderName`; compared case-insensitively as Express lower-cases
33	  header names). A key whose value is empty or only whitespace counts as
34	  **blank**.
35	- **Fence** , the value of the `Idempotency-Fence` request header (configurable
36	  via `options.fenceHeaderName`). A **valid fence** is a canonical, non-negative,
37	  base-10 integer: it matches `^(0|[1-9][0-9]*)$` (so `"0"`, `"1"`, `"42"` are
38	  valid; `""`, `"-1"`, `"01"`, `"007"`, `"1.0"`, `"+1"`, `"1e3"`, `"abc"` are
39	  not) and is a safe integer. The fence is a monotonic sequence number the
40	  client assigns to an operation.
41	- **Fingerprint** , a string derived from the request that binds a key to the
42	  *shape* of the request first seen under it. See **Fingerprint** below.
43	- **In scope** , a request whose method is in `options.methods` (default
44	  `POST`, `PUT`, `PATCH`, `DELETE`). Any other method is passed straight through
45	  with `next()` and must never touch the store, require headers, or be
46	  fingerprinted.
47	- **Side effect** , running the downstream handler exactly once (`next()`),
48	  which is what creates a charge.
49	- **High-water fence** , for a given key, the largest fence value ever observed
50	  under it (across every attempt, including superseded, released, and
51	  fence-rejected ones). It never decreases while a record for the key is live.
52	
53	Error responses are JSON of the form `{ "error": <code>, "message": <string> }`
54	(the provided `apiError` helper produces exactly this). Only the `error` code
55	and HTTP status are asserted; the `message` is free text.
56	
57	---
58	
59	## Behavioural contract (this is the spec)
60	
61	Process an **in-scope** request in this order. The first matching rule wins.
62	
63	### 1. Header validation (in this order)
64	
65	1. If the **key** is missing or blank → respond **`400`** with error
66	   `idempotency_key_required`. Do not run the handler.
67	2. Otherwise, if the **fence** is missing or not a valid fence → respond
68	   **`428`** with error `idempotency_fence_required`. Do not run the handler.
69	
70	(Key is validated before fence.)
71	
72	### 2. Look up the live record for the key
73	
74	A record's lifetime is fixed when its fence is **acquired**: it **expires**
75	`options.ttlMs` after the moment of acquisition (using `options.clock.now()`),
76	and settling the attempt (completing or releasing it) does **not** change that
77	expiry. A request whose record has expired is treated as if no record exists. A
78	subsequent acquire (a fresh key, a re-run after release, or a higher fence)
79	starts a new lifetime from its own acquisition time. Let `F` be this request's
80	fence.
81	
82	**If there is no live record:** acquire the key at fence `F`, run the handler
83	(one side effect), and remember the request's fingerprint. The high-water fence
84	becomes `F`. Settle when the response finishes (see **Settling**).
85	
86	**If there is a live record**, let `M` be its high-water fence and apply
87	**fence precedence first**:
88	
89	- **`F < M` (stale fence).** This is a retry of an attempt that has been
90	  superseded by a higher fence. Respond **`409`** with error
91	  `idempotency_fence_stale`, and set the response header
92	  `Idempotency-Fence-Current` to `M` (as a decimal string). Do not run the
93	  handler.
94	
95	- **`F === M` (current fence).** Compare the record's state:
96	  - If the record is **still running** (an attempt at this fence is in
97	    progress) and the request's fingerprint **matches** the running attempt →
98	    respond **`409`** with error `idempotency_in_flight`. Do not run the handler.
99	  - If the record is **still running** and the fingerprint **does not match** →
100	    respond **`409`** with error `idempotency_fingerprint_mismatch`. Do not run
101	    the handler.
102	  - If the record is **finished and cached** (a completed attempt) and the
103	    fingerprint **matches** → **replay** it (see **Replay**). Do not run the
104	    handler.
105	  - If the record is **finished and cached** and the fingerprint **does not
106	    match** → respond **`409`** with error `idempotency_fingerprint_mismatch`.
107	    Do not run the handler.
108	  - If the record was **released** (a previous attempt at this fence failed
109	    transiently , see **Settling**) → acquire the key again at fence `F`, run
110	    the handler (a new side effect), and remember the new fingerprint. The
111	    high-water fence stays `M` (`=F`).
112	
113	- **`F > M` (advancing fence).** A strictly higher fence **supersedes** whatever
114	  is currently held , *regardless* of the held record's state or fingerprint
115	  (running, completed, released; matching or not). Acquire the key at fence `F`,
116	  run the handler (a new side effect), and remember the new fingerprint. The
117	  high-water fence becomes `F`. Any attempt that was still running at the old
118	  fence is abandoned: when it finishes, its result must be **discarded** (it
119	  must not become the cached/replayable response, and it must not delete or
120	  alter the superseding attempt's record).
121	
122	### 3. Settling (after an acquired attempt's response finishes)
123	
124	When a handler you ran produces its final response, settle the record for that
125	attempt , **but only if your attempt still owns the key at its fence** (if a
126	higher fence superseded you in the meantime, discard: do nothing). Settle based
127	on the **final HTTP status code**:
128	
129	- **Exactly `503`** → **release**: the attempt is treated as transiently failed.
130	  A later request at the *same* fence may run the handler again (see the
131	  "released" case above). Releasing must **preserve the high-water fence** (a
132	  later lower fence is still stale). Do not cache a `503` for replay.
133	- **Any other status, including `500`, `502`, and `504`** → **cache** the
134	  response (status, headers, body) so a later request at the same fence with a
135	  matching fingerprint replays it.
136	- If the connection is **aborted** before the response finishes → **release**
137	  (as for `503`).
138	
139	### 4. Replay
140	
141	To replay a cached response: write its stored status code, its stored headers,
142	and its stored body verbatim, and additionally set the response header
143	`Idempotency-Replay` to the **fence value** of the cached attempt (a decimal
144	string , *not* the literal `"true"`). A freshly executed (non-replayed) response
145	must **not** carry an `Idempotency-Replay` header.
146	
147	### Invariants
148	
149	- A request that is *not in scope* is forwarded unchanged and never touches the
150	  store.
151	- The handler runs **at most once per acquired attempt**. Two identical
152	  concurrent requests with the same key **and** the same fence produce exactly
153	  one side effect; the loser gets `409 idempotency_in_flight`.
154	- Across **different fences** there is **no de-duplication**: a higher fence
155	  always re-runs the handler (new side effect), even with a byte-identical body.
156	- This protocol **never** responds `422`, and never uses the
157	  `idempotent-replayed` header. Conflicts are `409` (with the specific `error`
158	  codes above); a stale fence is `409`; a missing/invalid fence is `428`.
159	
160	---
161	
162	## Fingerprint
163	
164	`computeFingerprint({ method, path, body })` returns a **lowercase hexadecimal**
165	string. It must be deterministic and induce exactly these equivalences (two
166	requests share a fingerprint **iff** all of the following agree):
167	
168	- the **method**, upper-cased; and
169	- the **path** (`req.path`, excluding any query string); and
170	- the **body**, under this canonicalization:
171	  - **Object key order is irrelevant** (`{a:1,b:2}` ≡ `{b:2,a:1}`).
172	  - **Keys whose name begins with an underscore (`_`) are excluded entirely**,
173	    recursively, at every object depth. They are client-side annotations and do
174	    not affect the fingerprint (`{amount:5,_trace:"a"}` ≡ `{amount:5,_trace:"b"}`
175	    ≡ `{amount:5}`).
176	  - **A string that is a canonical decimal integer is treated as the equal
177	    integer.** "Canonical decimal integer" means it matches
178	    `^-?(0|[1-9][0-9]*)$` and is a safe integer (`"100"` ≡ `100`, `"-5"` ≡ `-5`,
179	    `"0"` ≡ `0`). Strings that are *not* canonical integers are left as strings
180	    and are therefore **not** equal to any number (`"007"`, `"1.0"`, `"+1"`,
181	    `" 1"`, `"01"`, `"1.5"` stay strings).
182	  - **Arrays are order-sensitive** (`[1,2,3]` ≢ `[3,2,1]`); each element is
183	    canonicalized.
184	  - `null`, booleans, and numbers are distinct from each other and from their
185	    string spellings (`null` ≢ absent ≢ `0` ≢ `""`; `true` ≢ `1` ≢ `"true"`).
186	
187	---
188	
189	## Definition of done
190	
191	- `npm run typecheck` is clean.
192	- The behaviour above is fully implemented in `src/lib/fingerprint.ts` and
193	  `src/middleware/idempotency.ts`. A hidden suite grades it against an
194	  independent reference (correctness, all edges, randomized cross-checks, and
195	  the concurrency invariants). The trivial `test/smoke.test.ts` that ships with
196	  the project is *not* the grader.
197	- No changes to the store, the route, the response-capture helper, the option
198	  resolver, the clock, or the error helper are required.
199	
200	## Running locally
201	
202	```bash
203	npm install      # already done in the provided environment
204	npm run typecheck
205	npm test         # runs the shipped smoke test only
206	```
207

test/smoke.test.ts
src/app.ts
src/server.ts
src/middleware/config.ts
src/middleware/idempotency.ts
src/middleware/responseCapture.ts
src/store/memoryStore.ts
src/store/types.ts
src/lib/clock.ts
src/lib/errors.ts
src/lib/fingerprint.ts
src/routes/charges.ts
---PACKAGE---
{
  "name": "payments-api",
  "version": "1.2.0",
  "private": true,
  "description": "Internal payments service exposing a charge-creation API with idempotent writes.",
  "type": "module",
  "engines": {
    "node": ">=20"
  },
  "scripts": {
    "build": "tsc -p tsconfig.json",
    "typecheck": "tsc -p tsconfig.json --noEmit",
    "test": "vitest run",
    "test:watch": "vitest",
    "dev": "tsx src/server.ts"
  },
  "dependencies": {
    "express": "4.19.2"
  },
  "devDependencies": {
    "@types/express": "4.17.21",
    "@types/node": "20.14.10",
    "@types/supertest": "6.0.2",
    "supertest": "7.0.0",
    "tsx": "4.16.2",
    "typescript": "5.5.3",
    "vitest": "1.6.0"
  }
}

1	import type { NextFunction, Request, RequestHandler, Response } from "express";
2	import { apiError } from "../lib/errors.js";
3	import { computeFingerprint } from "../lib/fingerprint.js";
4	import type { IdempotencyOptions } from "./config.js";
5	import { resolveOptions } from "./config.js";
6	import { ResponseCapture } from "./responseCapture.js";
7	
8	/**
9	 * Response header that flags a replayed response. Its required NAME and VALUE
10	 * are specified in `instruction.md`. (The constant value below is a placeholder;
11	 * set it to the name required by the contract.)
12	 */
13	export const REPLAY_HEADER = "idempotency-replay";
14	
15	/**
16	 * Express middleware implementing the Fenced Idempotency Protocol (FIP/1).
17	 *
18	 * The full behavioural contract is specified in `instruction.md`; that document
19	 * is the spec. Implement it here.
20	 *
21	 * Use the provided helpers (`computeFingerprint`, `ResponseCapture`, `apiError`,
22	 * `resolveOptions`) and the injected `options.store`. Keep the public exports
23	 * (`idempotency`, `REPLAY_HEADER`).
24	 */
25	export function idempotency(options: IdempotencyOptions): RequestHandler {
26	  const resolved = resolveOptions(options);
27	
28	  // Referenced so a partial implementation still type-checks; use as needed.
29	  void apiError;
30	  void computeFingerprint;
31	  void ResponseCapture;
32	  void REPLAY_HEADER;
33	  void resolved;
34	
35	  return async function idempotencyMiddleware(
36	    _req: Request,
37	    _res: Response,
38	    next: NextFunction,
39	  ): Promise<void> {
40	    // TODO(idempotency): implement the FIP/1 contract documented in instruction.md.
41	    //
42	    // This stub leaves the route wired but non-functional so the behavioural
43	    // suite fails until the real protocol is implemented here.
44	    next(new Error("idempotency middleware not implemented"));
45	  };
46	}
47

1	import { createHash } from "node:crypto";
2	
3	/**
4	 * Compute the request fingerprint that binds an idempotency key to the shape of
5	 * the request that used it. The fingerprint is a lowercase hex string; its
6	 * canonicalization rules are specified in `instruction.md`. Implement them here.
7	 *
8	 * `createHash` from `node:crypto` is imported for convenience; how (and whether)
9	 * you use it is up to you, as long as the output is a lowercase hex string with
10	 * the documented equivalence behaviour.
11	 *
12	 * TODO(fingerprint): implement the canonicalization contract.
13	 */
14	export function computeFingerprint(input: {
15	  method: string;
16	  path: string;
17	  body: unknown;
18	}): string {
19	  void createHash;
20	  void input;
21	  throw new Error("computeFingerprint not implemented");
22	}
23

1	/**
2	 * Persistence contract for idempotency entries.
3	 *
4	 * The store is a *dumb* persistence + atomic compare-and-swap layer: it holds
5	 * opaque {@link IdempotencyEntry} records keyed by the client-supplied
6	 * idempotency key, and exposes exactly enough primitives for the middleware to
7	 * implement the protocol. The store knows nothing about fences, fingerprints,
8	 * replay, or status codes , all of that policy lives in the middleware. This
9	 * keeps the in-memory store used in tests swappable for a Redis/Postgres-backed
10	 * store in production without changing request-handling logic.
11	 *
12	 * You do NOT need to modify the store to complete the task.
13	 */
14	
15	/** A captured HTTP response, replayed verbatim on a later request. */
16	export interface StoredResponse {
17	  statusCode: number;
18	  /** Header name (lower-cased) -> value. Hop-by-hop headers are not stored. */
19	  headers: Record<string, string>;
20	  /** Raw response body as sent to the client. */
21	  body: string;
22	}
23	
24	/** Lifecycle of a single attempt held under a key. */
25	export type EntryStatus = "in_flight" | "completed" | "released";
26	
27	/**
28	 * The record stored under one idempotency key.
29	 *
30	 * The middleware owns the *meaning* of every field; the store only persists and
31	 * returns these objects verbatim. A single key holds at most one entry at a
32	 * time (a later attempt overwrites an earlier one , see {@link IdempotencyStore}).
33	 */
34	export interface IdempotencyEntry {
35	  /** The idempotency key this entry belongs to. */
36	  key: string;
37	  /** Monotonic fence token of the attempt that currently owns the key. */
38	  fence: number;
39	  /**
40	   * The highest fence value ever observed for this key, across every attempt , 
41	   * including attempts that were superseded, released, or whose fence was
42	   * rejected as stale. Never decreases while the entry is live.
43	   */
44	  maxFence: number;
45	  /** Request fingerprint of the attempt that currently owns the key. */
46	  fingerprint: string;
47	  /**
48	   * Lifecycle state the middleware assigns to this attempt. The store does not
49	   * interpret it beyond the `(fence,status)` precondition of
50	   * {@link IdempotencyStore.compareAndSet}.
51	   */
52	  status: EntryStatus;
53	  /** Present when the attempt produced a response to be replayed. */
54	  response?: StoredResponse;
55	  /** Epoch ms at which this entry was first created (initial reserve). */
56	  createdAt: number;
57	  /** Epoch ms after which this entry is considered expired and swept. */
58	  expiresAt: number;
59	}
60	
61	export interface IdempotencyStore {
62	  /**
63	   * Atomically read the live entry for `key`, or `undefined` if none exists or
64	   * the existing one has expired (`expiresAt <= now`). Expired entries are
65	   * lazily deleted on read.
66	   *
67	   * This read is consistent with {@link compareAndSet}: there is no `await`
68	   * between the read and a subsequent set within the same synchronous turn, so
69	   * a read-then-set sequence is atomic with respect to other handlers on Node's
70	   * single-threaded event loop (the contract relied on by the tests).
71	   */
72	  get(key: string, now: number): Promise<IdempotencyEntry | undefined>;
73	
74	  /**
75	   * Atomic compare-and-set. Writes `next` for `key` *iff* the entry currently
76	   * stored under `key` is referentially equivalent to `expected` (by the
77	   * `(fence,status)` pair and presence , see the in-memory implementation):
78	   *
79	   *   - `expected === undefined` succeeds only if no live entry exists.
80	   *   - otherwise it succeeds only if the live entry has the same `fence` and
81	   *     `status` as `expected`.
82	   *
83	   * Returns `true` on success (the write happened) and `false` if the
84	   * precondition failed (a concurrent attempt got there first). The store
85	   * performs no `await` between its internal read and write.
86	   */
87	  compareAndSet(
88	    key: string,
89	    expected: IdempotencyEntry | undefined,
90	    next: IdempotencyEntry,
91	    now: number,
92	  ): Promise<boolean>;
93	
94	  /** Unconditionally delete the entry for `key`. */
95	  delete(key: string): Promise<void>;
96	}
97

1	import type { IdempotencyEntry, IdempotencyStore } from "./types.js";
2	
3	/**
4	 * In-memory {@link IdempotencyStore}.
5	 *
6	 * Node runs request handlers cooperatively on a single thread, so the
7	 * compare-and-set in {@link compareAndSet} is atomic as long as it performs no
8	 * `await` between reading and writing the map. This is sufficient for a
9	 * single-process deployment and for the test-suite's concurrency scenarios; a
10	 * multi-process deployment would supply a Redis-backed implementation of the
11	 * same interface.
12	 *
13	 * This store is intentionally policy-free: it does not interpret fences,
14	 * fingerprints, or status beyond what {@link compareAndSet} needs to detect a
15	 * lost race. All protocol decisions live in the middleware.
16	 */
17	export class MemoryStore implements IdempotencyStore {
18	  private readonly entries = new Map<string, IdempotencyEntry>();
19	
20	  async get(key: string, now: number): Promise<IdempotencyEntry | undefined> {
21	    const existing = this.entries.get(key);
22	    if (!existing) return undefined;
23	    if (existing.expiresAt <= now) {
24	      this.entries.delete(key);
25	      return undefined;
26	    }
27	    return clone(existing);
28	  }
29	
30	  async compareAndSet(
31	    key: string,
32	    expected: IdempotencyEntry | undefined,
33	    next: IdempotencyEntry,
34	    now: number,
35	  ): Promise<boolean> {
36	    // NOTE: no awaits between the read and the write below , keep it that way so
37	    // the check-and-set stays atomic on the single-threaded event loop.
38	    const liveRaw = this.entries.get(key);
39	    const live = liveRaw && liveRaw.expiresAt > now ? liveRaw : undefined;
40	
41	    if (expected === undefined) {
42	      if (live !== undefined) return false;
43	    } else {
44	      if (live === undefined) return false;
45	      if (live.fence !== expected.fence || live.status !== expected.status) {
46	        return false;
47	      }
48	    }
49	
50	    this.entries.set(key, clone(next));
51	    return true;
52	  }
53	
54	  async delete(key: string): Promise<void> {
55	    this.entries.delete(key);
56	  }
57	
58	  /** Test helper: number of entries currently held (live or not-yet-swept). */
59	  size(): number {
60	    return this.entries.size;
61	  }
62	}
63	
64	function clone(entry: IdempotencyEntry): IdempotencyEntry {
65	  return {
66	    ...entry,
67	    response: entry.response
68	      ? { ...entry.response, headers: { ...entry.response.headers } }
69	      : undefined,
70	  };
71	}
72

1	import type { Clock } from "../lib/clock.js";
2	import { systemClock } from "../lib/clock.js";
3	import type { IdempotencyStore } from "../store/types.js";
4	
5	/** Tunable behaviour for {@link idempotency}. */
6	export interface IdempotencyOptions {
7	  store: IdempotencyStore;
8	  /** HTTP methods that participate in idempotency. Defaults to mutating verbs. */
9	  methods?: readonly string[];
10	  /** Lifetime of a stored entry, in milliseconds. Defaults to 24h. */
11	  ttlMs?: number;
12	  /** Header carrying the client-supplied key. Defaults to `Idempotency-Key`. */
13	  keyHeaderName?: string;
14	  /** Header carrying the client-supplied fence token. Defaults to `Idempotency-Fence`. */
15	  fenceHeaderName?: string;
16	  /** Injectable clock; defaults to the system clock. */
17	  clock?: Clock;
18	}
19	
20	export interface ResolvedIdempotencyOptions {
21	  store: IdempotencyStore;
22	  methods: ReadonlySet<string>;
23	  ttlMs: number;
24	  keyHeaderName: string;
25	  fenceHeaderName: string;
26	  clock: Clock;
27	}
28	
29	const DEFAULT_METHODS = ["POST", "PUT", "PATCH", "DELETE"] as const;
30	const DEFAULT_TTL_MS = 24 * 60 * 60 * 1000;
31	const DEFAULT_KEY_HEADER = "idempotency-key";
32	const DEFAULT_FENCE_HEADER = "idempotency-fence";
33	
34	export function resolveOptions(options: IdempotencyOptions): ResolvedIdempotencyOptions {
35	  const methods = (options.methods ?? DEFAULT_METHODS).map((m) => m.toUpperCase());
36	  return {
37	    store: options.store,
38	    methods: new Set(methods),
39	    ttlMs: options.ttlMs ?? DEFAULT_TTL_MS,
40	    keyHeaderName: (options.keyHeaderName ?? DEFAULT_KEY_HEADER).toLowerCase(),
41	    fenceHeaderName: (options.fenceHeaderName ?? DEFAULT_FENCE_HEADER).toLowerCase(),
42	    clock: options.clock ?? systemClock,
43	  };
44	}
45

1	import type { Response } from "express";
2	import type { StoredResponse } from "../store/types.js";
3	
4	/**
5	 * Hop-by-hop and volatile headers that must NOT be captured/replayed.
6	 * (Replaying e.g. `Date` or `ETag` from an old response would be incorrect.)
7	 */
8	const NON_REPLAYABLE_HEADERS = new Set([
9	  "date",
10	  "connection",
11	  "keep-alive",
12	  "transfer-encoding",
13	  "content-length",
14	  "etag",
15	]);
16	
17	/**
18	 * Wrap an Express {@link Response} so the final status, headers, and body can be
19	 * captured for later replay, without changing how the route handler writes its
20	 * response. Call {@link ResponseCapture.snapshot} after the handler has produced
21	 * its response (i.e. on the `finish` event or after `await`-ing the handler).
22	 *
23	 * This helper is provided complete , the idempotency middleware uses it to
24	 * obtain a {@link StoredResponse}.
25	 */
26	export class ResponseCapture {
27	  private bodyChunks: Buffer[] = [];
28	  private finished = false;
29	
30	  constructor(private readonly res: Response) {
31	    const originalWrite = res.write.bind(res) as Response["write"];
32	    const originalEnd = res.end.bind(res) as Response["end"];
33	
34	    res.write = ((chunk: unknown, ...rest: unknown[]): boolean => {
35	      this.record(chunk);
36	      // @ts-expect-error - passthrough to the original signature
37	      return originalWrite(chunk, ...rest);
38	    }) as Response["write"];
39	
40	    res.end = ((chunk: unknown, ...rest: unknown[]): Response => {
41	      if (chunk) this.record(chunk);
42	      this.finished = true;
43	      // @ts-expect-error - passthrough to the original signature
44	      return originalEnd(chunk, ...rest);
45	    }) as Response["end"];
46	  }
47	
48	  private record(chunk: unknown): void {
49	    if (chunk == null) return;
50	    if (Buffer.isBuffer(chunk)) {
51	      this.bodyChunks.push(chunk);
52	    } else if (typeof chunk === "string") {
53	      this.bodyChunks.push(Buffer.from(chunk));
54	    } else {
55	      this.bodyChunks.push(Buffer.from(String(chunk)));
56	    }
57	  }
58	
59	  get isFinished(): boolean {
60	    return this.finished;
61	  }
62	
63	  /** Build a {@link StoredResponse} from what the handler has written so far. */
64	  snapshot(): StoredResponse {
65	    const headers: Record<string, string> = {};
66	    for (const [name, value] of Object.entries(this.res.getHeaders())) {
67	      const lower = name.toLowerCase();
68	      if (NON_REPLAYABLE_HEADERS.has(lower)) continue;
69	      if (value == null) continue;
70	      headers[lower] = Array.isArray(value) ? value.join(", ") : String(value);
71	    }
72	    return {
73	      statusCode: this.res.statusCode,
74	      headers,
75	      body: Buffer.concat(this.bodyChunks).toString("utf8"),
76	    };
77	  }
78	}
79

1	/**
2	 * Problem-detail style error payloads emitted by the API.
3	 *
4	 * The shape is intentionally small and stable so clients can switch on `error`.
5	 */
6	export interface ApiErrorBody {
7	  error: string;
8	  message: string;
9	}
10	
11	export function apiError(error: string, message: string): ApiErrorBody {
12	  return { error, message };
13	}
14

1	/**
2	 * A monotonic-ish wall clock abstraction.
3	 *
4	 * Injecting the clock keeps TTL / expiry logic deterministic and unit-testable:
5	 * tests advance a {@link ManualClock} instead of sleeping in real time.
6	 */
7	export interface Clock {
8	  /** Current time in epoch milliseconds. */
9	  now(): number;
10	}
11	
12	export const systemClock: Clock = {
13	  now: () => Date.now(),
14	};
15	
16	/** A controllable clock for tests. */
17	export class ManualClock implements Clock {
18	  constructor(private current = 0) {}
19	
20	  now(): number {
21	    return this.current;
22	  }
23	
24	  /** Advance the clock by `ms` milliseconds. */
25	  advance(ms: number): void {
26	    this.current += ms;
27	  }
28	
29	  /** Set the clock to an absolute epoch-ms value. */
30	  set(ms: number): void {
31	    this.current = ms;
32	  }
33	}
34

1	import type { Request, Response } from "express";
2	import { Router } from "express";
3	import { randomUUID } from "node:crypto";
4	import { apiError } from "../lib/errors.js";
5	
6	/**
7	 * A deliberately side-effectful "create charge" endpoint.
8	 *
9	 * Each successful POST mints a brand-new charge id and pushes onto an in-memory
10	 * ledger. Idempotency is what prevents a retried POST from creating duplicate
11	 * charges , without it, two identical requests would create two charges with
12	 * two different ids. The tests assert on exactly this property.
13	 */
14	
15	export interface Charge {
16	  id: string;
17	  amount: number;
18	  currency: string;
19	  description: string | null;
20	  createdAt: number;
21	}
22	
23	export class ChargeLedger {
24	  private readonly charges: Charge[] = [];
25	
26	  create(input: { amount: number; currency: string; description: string | null }): Charge {
27	    const charge: Charge = {
28	      id: `ch_${randomUUID().replace(/-/g, "").slice(0, 24)}`,
29	      amount: input.amount,
30	      currency: input.currency,
31	      description: input.description,
32	      createdAt: Date.now(),
33	    };
34	    this.charges.push(charge);
35	    return charge;
36	  }
37	
38	  count(): number {
39	    return this.charges.length;
40	  }
41	
42	  all(): readonly Charge[] {
43	    return this.charges;
44	  }
45	}
46	
47	export function createChargesRouter(ledger: ChargeLedger): Router {
48	  const router = Router();
49	
50	  router.post("/", (req: Request, res: Response) => {
51	    const body = (req.body ?? {}) as Record<string, unknown>;
52	    const amount = body.amount;
53	    const currency = body.currency;
54	
55	    if (typeof amount !== "number" || !Number.isInteger(amount) || amount <= 0) {
56	      res
57	        .status(400)
58	        .json(apiError("invalid_amount", "amount must be a positive integer (minor units)"));
59	      return;
60	    }
61	    if (typeof currency !== "string" || currency.length !== 3) {
62	      res
63	        .status(400)
64	        .json(apiError("invalid_currency", "currency must be a 3-letter ISO code"));
65	      return;
66	    }
67	
68	    const description =
69	      typeof body.description === "string" ? body.description : null;
70	
71	    const charge = ledger.create({ amount, currency: currency.toLowerCase(), description });
72	    res.status(201).json(charge);
73	  });
74	
75	  return router;
76	}
77

1	import type { Express, NextFunction, Request, Response } from "express";
2	import express from "express";
3	import type { Clock } from "./lib/clock.js";
4	import { systemClock } from "./lib/clock.js";
5	import { apiError } from "./lib/errors.js";
6	import { idempotency } from "./middleware/idempotency.js";
7	import { ChargeLedger, createChargesRouter } from "./routes/charges.js";
8	import { MemoryStore } from "./store/memoryStore.js";
9	import type { IdempotencyStore } from "./store/types.js";
10	
11	export interface AppDeps {
12	  store?: IdempotencyStore;
13	  ledger?: ChargeLedger;
14	  clock?: Clock;
15	  ttlMs?: number;
16	}
17	
18	export interface BuiltApp {
19	  app: Express;
20	  store: IdempotencyStore;
21	  ledger: ChargeLedger;
22	}
23	
24	/**
25	 * Build the payments API. The idempotency middleware is mounted in front of the
26	 * mutating `/v1/charges` routes; read-only routes are not affected.
27	 */
28	export function buildApp(deps: AppDeps = {}): BuiltApp {
29	  const store = deps.store ?? new MemoryStore();
30	  const ledger = deps.ledger ?? new ChargeLedger();
31	  const clock = deps.clock ?? systemClock;
32	
33	  const app = express();
34	  app.use(express.json());
35	
36	  app.get("/health", (_req: Request, res: Response) => {
37	    res.status(200).json({ status: "ok", charges: ledger.count() });
38	  });
39	
40	  app.use(
41	    "/v1/charges",
42	    idempotency({ store, clock, ttlMs: deps.ttlMs }),
43	    createChargesRouter(ledger),
44	  );
45	
46	  // Centralised error handler so a thrown handler / failed reservation surfaces
47	  // as a clean 500 rather than a hung socket.
48	  app.use((err: unknown, _req: Request, res: Response, _next: NextFunction) => {
49	    if (res.headersSent) return;
50	    const message = err instanceof Error ? err.message : "unexpected error";
51	    res.status(500).json(apiError("internal_error", message));
52	  });
53	
54	  return { app, store, ledger };
55	}
56

1	import request from "supertest";
2	import { describe, expect, it } from "vitest";
3	import { buildApp } from "../src/app.js";
4	import { REPLAY_HEADER } from "../src/middleware/idempotency.js";
5	
6	/**
7	 * Trivial smoke test that ships with the project. It only checks that the app
8	 * boots and the (idempotency-free) health endpoint responds. The real
9	 * behavioural grading lives in a separate, hidden suite. Implement the protocol
10	 * described in instruction.md and src/middleware/idempotency.ts.
11	 */
12	describe("payments-api smoke", () => {
13	  it("boots and serves /health", async () => {
14	    const { app } = buildApp();
15	    const res = await request(app).get("/health");
16	    expect(res.status).toBe(200);
17	    expect(res.body.status).toBe("ok");
18	  });
19	
20	  it("exposes the replay header constant", () => {
21	    expect(typeof REPLAY_HEADER).toBe("string");
22	    expect(REPLAY_HEADER.length).toBeGreaterThan(0);
23	  });
24	});
25

The file /home/user/app/src/lib/fingerprint.ts has been updated successfully. (file state is current in your context , no need to Read it back)

The file /home/user/app/src/middleware/idempotency.ts has been updated successfully. (file state is current in your context , no need to Read it back)

The file /home/user/app/src/middleware/idempotency.ts has been updated successfully. (file state is current in your context , no need to Read it back)

> payments-api@1.2.0 typecheck
> tsc -p tsconfig.json --noEmit

---TEST---

> payments-api@1.2.0 test
> vitest run


 RUN  v1.6.0 /home/user/app

 ✓ test/smoke.test.ts  (2 tests) 22ms

 Test Files  1 passed (1)
      Tests  2 passed (2)
   Start at  01:18:30
   Duration  561ms (transform 114ms, setup 0ms, collect 288ms, tests 22ms, environment 0ms, prepare 114ms)

130	      runAndSettle(req, res, next, resolved, entry, key);
131	      return;
132	    }
133	  };
134	}
135	
136	function runAndSettle(
137	  _req: Request,
138	  res: Response,
139	  next: NextFunction,
140	  resolved: ResolvedIdempotencyOptions,
141	  entry: IdempotencyEntry,
142	  key: string,
143	): void {
144	  const capture = new ResponseCapture(res);
145	  let settled = false;
146	
147	  const settle = (aborted: boolean): void => {
148	    if (settled) return;
149	    settled = true;
150	
151	    const release = aborted || res.statusCode === 503;
152	    const settledEntry: IdempotencyEntry = release
153	      ? { ...entry, status: "released", response: undefined }
154	      : { ...entry, status: "completed", response: capture.snapshot() };
155	
156	    // Only settle if this attempt still owns the key at its fence. If a higher
157	    // fence superseded us in the meantime the compare-and-set fails and we
158	    // discard the result (do nothing).
159	    void resolved.store.compareAndSet(key, entry, settledEntry, resolved.clock.now());
160	  };
161	
162	  res.on("finish", () => settle(false));
163	  res.on("close", () => {
164	    if (!capture.isFinished) settle(true);
165	  });
166	
167	  next();
168	}
169	
170	function replay(res: Response, entry: IdempotencyEntry): void {
171	  const stored = entry.response!;
172	  for (const [name, value] of Object.entries(stored.headers)) {
173	    res.setHeader(name, value);
174	  }
175	  res.setHeader(REPLAY_HEADER, String(entry.fence));
176	  res.status(stored.statusCode).end(stored.body);
177	}
178	
179	function makeEntry(