billing skeleton: bun + hono + sqlite, seed + happy path

.gitignore

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+node_modules/
+data.db
+data.db-wal
+data.db-shm

README.md

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+# monk-code-screen
+
+Billing/subscriptions/invoicing backend skeleton. Bun + Hono + `bun:sqlite` (zero network deps at runtime; TS runs natively, no build step).
+
+## ⚡ START COMMAND (sticky)
+
+```sh
+bun run dev          # http://localhost:3000, hot reload
+```
+
+Other commands:
+
+```sh
+bun run start        # no watch
+bun run db:reset     # wipe data.db and reseed
+bunx tsc --noEmit    # typecheck
+```
+
+## Layout
+
+```
+src/index.ts    Hono app + routes
+src/db.ts       opens data.db, applies schema, seeds if empty (runs on import)
+src/schema.sql  customers / plans / subscriptions / invoices (idempotent)
+```
+
+DB is `data.db` at repo root (WAL, FK on). Boot always re-applies schema, so adding a table = edit schema.sql + restart.
+
+## Routes
+
+```sh
+curl localhost:3000/health
+curl localhost:3000/customers
+curl localhost:3000/customers/cus_ada/invoices
+curl -X POST localhost:3000/subscriptions \
+  -H 'content-type: application/json' \
+  -d '{"customer_id":"cus_grace","plan_id":"plan_basic"}'
+```
+
+`POST /subscriptions` is the happy path: validates with zod, creates subscription + first invoice in one transaction, returns both (201).
+
+## Seed data
+
+- customers: `cus_ada`, `cus_grace`
+- plans: `plan_basic` ($10/mo), `plan_pro` ($50/mo)
+- `cus_ada` has an active `sub_1` on pro with open `inv_1`
+
+## Extension cheatsheet (interview)
+
+- New route: add to `src/index.ts`, `db.query(...).all()/get()`, `db.run(sql, [params])`
+- Multi-write: `db.transaction(() => { ... })()`
+- Validation: `z.object({...}).safeParse(await c.req.json())`
+- New table: append `CREATE TABLE IF NOT EXISTS` to schema.sql, restart (or `db:reset` if changing existing tables)

bun.lock

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+{
+  "lockfileVersion": 1,
+  "configVersion": 1,
+  "workspaces": {
+    "": {
+      "name": "monk-code-screen",
+      "dependencies": {
+        "hono": "^4.13.1",
+        "zod": "^4.4.3",
+      },
+      "devDependencies": {
+        "@types/bun": "^1.3.14",
+      },
+    },
+  },
+  "packages": {
+    "@types/bun": ["@types/bun@1.3.14", "", { "dependencies": { "bun-types": "1.3.14" } }, "sha512-h1hFqFVcvAvD9j9K7ZW7vd82aSA+rTdznZa+5bwvCwqSB1jmmfLcbIWhOLx1/+boy/xmjgCs/OMUL8hRJSmnPw=="],
+
+    "@types/node": ["@types/node@26.2.0", "", { "dependencies": { "undici-types": "~8.3.0" } }, "sha512-5IviulTZeRNp2vAJ514cc/HUlY5nZ9fCbq9DMyC52BrhFZACo3nI0R7qBxhQmo/d27NFe96ur/b7Wwxklda+kg=="],
+
+    "bun-types": ["bun-types@1.3.14", "", { "dependencies": { "@types/node": "*" } }, "sha512-4N0ig0fEomHt5R0KCFWjovxow98rIoRwKolrYdCcknNwMekCXRnWEUvgu5soYV8QXtVsrUD8B95MBOZGPvr6KQ=="],
+
+    "hono": ["hono@4.13.1", "", {}, "sha512-kdJoFVv2xmayw6cY09H7AbMJMt8Jn5jdlEdXsP7AGBdF2DIptVlKlOLKXP41yPip4/a3yQPv9gVcJYI8YY04dw=="],
+
+    "undici-types": ["undici-types@8.3.0", "", {}, "sha512-j375ScV60dom+YkPFIfTLcOiPxkN/buHz5GobjLhixFuANaNs3C9l4GmrWqejgXWJ7BbJcFYpTEUkS1Ge8bpZQ=="],
+
+    "zod": ["zod@4.4.3", "", {}, "sha512-ytENFjIJFl2UwYglde2jchW2Hwm4GJFLDiSXWdTrJQBIN9Fcyp7n4DhxJEiWNAJMV1/BqWfW/kkg71UDcHJyTQ=="],
+  }
+}

package.json

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+{
+  "name": "monk-code-screen",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "bun --watch src/index.ts",
+    "start": "bun src/index.ts",
+    "db:reset": "rm -f data.db data.db-wal data.db-shm && bun src/db.ts"
+  },
+  "dependencies": {
+    "hono": "^4.13.1",
+    "zod": "^4.4.3"
+  },
+  "devDependencies": {
+    "@types/bun": "^1.3.14"
+  }
+}

src/db.ts

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+import { Database } from "bun:sqlite";
+import { join } from "node:path";
+
+export const db = new Database(join(import.meta.dir, "..", "data.db"), { create: true });
+db.exec("PRAGMA journal_mode = WAL;");
+db.exec("PRAGMA foreign_keys = ON;");
+
+// Migrate: schema is idempotent (CREATE TABLE IF NOT EXISTS)
+db.exec(await Bun.file(join(import.meta.dir, "schema.sql")).text());
+
+// Seed once, only if empty
+const { n } = db.query<{ n: number }, []>("SELECT count(*) AS n FROM customers").get()!;
+if (n === 0) {
+  db.transaction(() => {
+    db.run(
+      `INSERT INTO customers (id, name, email) VALUES
+       ('cus_ada',   'Ada Lovelace',   'ada@example.com'),
+       ('cus_grace', 'Grace Hopper',   'grace@example.com')`,
+    );
+    db.run(
+      `INSERT INTO plans (id, name, amount_cents, currency, interval) VALUES
+       ('plan_basic', 'Basic',  1000, 'usd', 'month'),
+       ('plan_pro',   'Pro',    5000, 'usd', 'month')`,
+    );
+    db.run(
+      `INSERT INTO subscriptions (id, customer_id, plan_id, status, current_period_start, current_period_end) VALUES
+       ('sub_1', 'cus_ada', 'plan_pro', 'active', datetime('now'), datetime('now', '+1 month'))`,
+    );
+    db.run(
+      `INSERT INTO invoices (id, subscription_id, customer_id, amount_cents, currency, status, due_at) VALUES
+       ('inv_1', 'sub_1', 'cus_ada', 5000, 'usd', 'open', datetime('now', '+14 days'))`,
+    );
+  })();
+  console.log("seeded database");
+}
+
+// `bun run db:reset` executes this file directly after deleting data.db
+if (import.meta.main) {
+  console.log("db ready:", db.filename);
+}

src/index.ts

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+import { Hono } from "hono";
+import { logger } from "hono/logger";
+import { z } from "zod";
+import { db } from "./db";
+
+const app = new Hono();
+app.use(logger());
+
+app.get("/health", (c) => c.json({ ok: true }));
+
+// --- Customers -------------------------------------------------------------
+
+app.get("/customers", (c) => {
+  return c.json(db.query("SELECT * FROM customers ORDER BY created_at").all());
+});
+
+app.get("/customers/:id/invoices", (c) => {
+  const rows = db
+    .query("SELECT * FROM invoices WHERE customer_id = ? ORDER BY created_at DESC")
+    .all(c.req.param("id"));
+  return c.json(rows);
+});
+
+// --- Subscriptions (happy path: subscribe -> first invoice) -----------------
+
+const CreateSubscription = z.object({
+  customer_id: z.string(),
+  plan_id: z.string(),
+});
+
+app.post("/subscriptions", async (c) => {
+  const parsed = CreateSubscription.safeParse(await c.req.json().catch(() => null));
+  if (!parsed.success) return c.json({ error: z.treeifyError(parsed.error) }, 400);
+  const { customer_id, plan_id } = parsed.data;
+
+  const customer = db.query("SELECT id FROM customers WHERE id = ?").get(customer_id);
+  if (!customer) return c.json({ error: "customer not found" }, 404);
+
+  const plan = db
+    .query<{ id: string; amount_cents: number; currency: string; interval: string }, [string]>(
+      "SELECT * FROM plans WHERE id = ?",
+    )
+    .get(plan_id);
+  if (!plan) return c.json({ error: "plan not found" }, 404);
+
+  const subId = `sub_${crypto.randomUUID().slice(0, 8)}`;
+  const invId = `inv_${crypto.randomUUID().slice(0, 8)}`;
+
+  db.transaction(() => {
+    db.run(
+      `INSERT INTO subscriptions (id, customer_id, plan_id, status, current_period_start, current_period_end)
+       VALUES (?, ?, ?, 'active', datetime('now'), datetime('now', ?))`,
+      [subId, customer_id, plan_id, `+1 ${plan.interval}`],
+    );
+    db.run(
+      `INSERT INTO invoices (id, subscription_id, customer_id, amount_cents, currency, status, due_at)
+       VALUES (?, ?, ?, ?, ?, 'open', datetime('now', '+14 days'))`,
+      [invId, subId, customer_id, plan.amount_cents, plan.currency],
+    );
+  })();
+
+  const subscription = db.query("SELECT * FROM subscriptions WHERE id = ?").get(subId);
+  const invoice = db.query("SELECT * FROM invoices WHERE id = ?").get(invId);
+  return c.json({ subscription, invoice }, 201);
+});
+
+export default {
+  port: Number(process.env.PORT ?? 3000),
+  fetch: app.fetch,
+};
+
+console.log(`listening on http://localhost:${process.env.PORT ?? 3000}`);

src/schema.sql

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+CREATE TABLE IF NOT EXISTS customers (
+  id         TEXT PRIMARY KEY,
+  name       TEXT NOT NULL,
+  email      TEXT NOT NULL UNIQUE,
+  created_at TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE TABLE IF NOT EXISTS plans (
+  id           TEXT PRIMARY KEY,
+  name         TEXT NOT NULL,
+  amount_cents INTEGER NOT NULL,
+  currency     TEXT NOT NULL DEFAULT 'usd',
+  interval     TEXT NOT NULL DEFAULT 'month' CHECK (interval IN ('month', 'year'))
+);
+
+CREATE TABLE IF NOT EXISTS subscriptions (
+  id                   TEXT PRIMARY KEY,
+  customer_id          TEXT NOT NULL REFERENCES customers(id),
+  plan_id              TEXT NOT NULL REFERENCES plans(id),
+  status               TEXT NOT NULL DEFAULT 'active' CHECK (status IN ('active', 'past_due', 'canceled')),
+  current_period_start TEXT NOT NULL,
+  current_period_end   TEXT NOT NULL,
+  created_at           TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE TABLE IF NOT EXISTS invoices (
+  id              TEXT PRIMARY KEY,
+  subscription_id TEXT NOT NULL REFERENCES subscriptions(id),
+  customer_id     TEXT NOT NULL REFERENCES customers(id),
+  amount_cents    INTEGER NOT NULL,
+  currency        TEXT NOT NULL DEFAULT 'usd',
+  status          TEXT NOT NULL DEFAULT 'open' CHECK (status IN ('open', 'paid', 'void')),
+  due_at          TEXT NOT NULL,
+  created_at      TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_subscriptions_customer ON subscriptions(customer_id);
+CREATE INDEX IF NOT EXISTS idx_invoices_customer ON invoices(customer_id);

tsconfig.json

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+{
+  "compilerOptions": {
+    "target": "ESNext",
+    "module": "Preserve",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "noEmit": true,
+    "skipLibCheck": true,
+    "types": ["bun"]
+  },
+  "include": ["src"]
+}