diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000000..718a0636a9
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+node_modules/
+dist/
+.env
+data/*.tmp
diff --git a/README.md b/README.md
new file mode 100644
index 0000000000..c6fa2f4558
--- /dev/null
+++ b/README.md
@@ -0,0 +1,154 @@
+# 99Tech Backend Assessment
+
+This repository contains my backend assessment solutions for 99Tech.
+
+## Problem 4
+
+File: `src/problem4.ts`
+
+The task is to provide three unique TypeScript implementations of:
+
+```ts
+function sum_to_n(n: number): number
+```
+
+For example:
+
+```ts
+sum_to_n(5) === 1 + 2 + 3 + 4 + 5 === 15
+```
+
+### Assumption
+
+The prompt says `n` can be any integer, but only defines the positive case.
+For `n <= 0`, I interpret "sum to n" as walking from `1` down to `n`.
+
+Example:
+
+```ts
+sum_to_n(-3) === 1 + 0 + (-1) + (-2) + (-3) === -5
+```
+
+The prompt also states that the result will always be less than
+`Number.MAX_SAFE_INTEGER`.
+
+### Implementations
+
+- `sum_to_n_a`: arithmetic formula, `O(1)` time and `O(1)` space.
+- `sum_to_n_b`: iterative loop, `O(|n|)` time and `O(1)` space.
+- `sum_to_n_c`: recursion, `O(|n|)` time and `O(|n|)` space.
+
+### Run
+
+```bash
+npm install
+npm run build
+npm run problem4
+```
+
+## Problem 5
+
+File: `src/problem5/server.ts`
+
+The task is to develop an ExpressJS backend server with TypeScript, CRUD APIs,
+basic filters, and simple data persistence.
+
+This solution uses a `Book` resource and stores data in `data/books.json`.
+The JSON file acts as a simple file-based database, so data remains after the
+server restarts.
+
+### Book Resource
+
+```ts
+type Book = {
+  id: string;
+  title: string;
+  author: string;
+  status: "available" | "borrowed" | "archived";
+  description?: string;
+  createdAt: string;
+  updatedAt: string;
+};
+```
+
+### Run Server
+
+```bash
+npm install
+npm run dev
+```
+
+The server starts on:
+
+```txt
+http://localhost:3000
+```
+
+You can change the port with:
+
+```bash
+PORT=4000 npm run dev
+```
+
+### API Endpoints
+
+Health check:
+
+```bash
+curl http://localhost:3000/health
+```
+
+Create a book:
+
+```bash
+curl -X POST http://localhost:3000/books \
+  -H "Content-Type: application/json" \
+  -d '{"title":"Clean Code","author":"Robert C. Martin","status":"available","description":"A book about writing readable code."}'
+```
+
+List books:
+
+```bash
+curl "http://localhost:3000/books"
+```
+
+List books with filters:
+
+```bash
+curl "http://localhost:3000/books?status=available&author=robert&q=clean&limit=10&offset=0"
+```
+
+Get book details:
+
+```bash
+curl http://localhost:3000/books/<book-id>
+```
+
+Update a book:
+
+```bash
+curl -X PATCH http://localhost:3000/books/<book-id> \
+  -H "Content-Type: application/json" \
+  -d '{"status":"borrowed"}'
+```
+
+Delete a book:
+
+```bash
+curl -X DELETE http://localhost:3000/books/<book-id>
+```
+
+### Build And Run Compiled Code
+
+```bash
+npm run build
+npm start
+```
+
+## Problem 6
+
+File: `problem6/README.md`
+
+The task is to write an architecture specification for a live scoreboard module.
+The documentation includes API contracts, data model, security requirements,
+execution flow, realtime update strategy, and improvement notes.
diff --git a/data/books.json b/data/books.json
new file mode 100644
index 0000000000..825ace245a
--- /dev/null
+++ b/data/books.json
@@ -0,0 +1,13 @@
+{
+  "books": [
+    {
+      "id": "846c1e8d-dc2e-45ad-b444-274f03df7aa2",
+      "title": "Clean Code",
+      "author": "Robert C. Martin",
+      "status": "available",
+      "description": "A book about writing readable code.",
+      "createdAt": "2026-04-17T07:55:08.185Z",
+      "updatedAt": "2026-04-17T07:55:08.185Z"
+    }
+  ]
+}
\ No newline at end of file
diff --git a/package-lock.json b/package-lock.json
new file mode 100644
index 0000000000..0af1a56669
--- /dev/null
+++ b/package-lock.json
@@ -0,0 +1,1159 @@
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+      "dev": true,
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+}
diff --git a/package.json b/package.json
new file mode 100644
index 0000000000..a16e100279
--- /dev/null
+++ b/package.json
@@ -0,0 +1,22 @@
+{
+  "name": "99tech-backend-assessment",
+  "version": "1.0.0",
+  "private": true,
+  "description": "Backend assessment solutions for 99Tech.",
+  "scripts": {
+    "build": "tsc",
+    "start": "node dist/problem5/server.js",
+    "dev": "ts-node src/problem5/server.ts",
+    "problem4": "ts-node src/problem4/index.ts",
+    "start:problem4": "node dist/problem4/index.js"
+  },
+  "dependencies": {
+    "express": "^4.21.2"
+  },
+  "devDependencies": {
+    "@types/express": "^4.17.25",
+    "@types/node": "^22.10.2",
+    "ts-node": "^10.9.2",
+    "typescript": "^5.7.2"
+  }
+}
diff --git a/src/problem4/index.ts b/src/problem4/index.ts
new file mode 100644
index 0000000000..6705a3aea9
--- /dev/null
+++ b/src/problem4/index.ts
@@ -0,0 +1,54 @@
+/*
+ * Problem 4: Three ways to implement sum_to_n(n).
+ *
+ * Assumption:
+ * The prompt says n can be any integer, but defines the operation with
+ * sum_to_n(5) === 1 + 2 + 3 + 4 + 5.
+ *
+ * For n <= 0, I interpret "sum to n" as walking from 1 down to n.
+ * Example: sum_to_n(-3) === 1 + 0 + (-1) + (-2) + (-3) === -5.
+ *
+ * The prompt also guarantees the result is less than Number.MAX_SAFE_INTEGER.
+ */
+
+export function sum_to_n_a(n: number): number {
+  if (n >= 1) {
+    return (n * (n + 1)) / 2;
+  }
+  const absoluteN = Math.abs(n);
+  return 1 - (absoluteN * (absoluteN + 1)) / 2;
+}
+
+export function sum_to_n_b(n: number): number {
+  let sum = 0;
+  const step = n >= 1 ? 1 : -1;
+
+  for (
+    let current = 1;
+    step === 1 ? current <= n : current >= n;
+    current += step
+  ) {
+    sum += current;
+  }
+
+  return sum;
+}
+
+export function sum_to_n_c(n: number): number {
+  const sumRange = (start: number, end: number): number => {
+    if (start === end) {
+      return start;
+    }
+
+    const middle = Math.floor((start + end) / 2);
+    return sumRange(start, middle) + sumRange(middle + 1, end);
+  };
+
+  return n >= 1 ? sumRange(1, n) : sumRange(n, 1);
+}
+
+const sampleInput = 5;
+
+console.log("sum_to_n_a:", sum_to_n_a(sampleInput));
+console.log("sum_to_n_b:", sum_to_n_b(sampleInput));
+console.log("sum_to_n_c:", sum_to_n_c(sampleInput));
diff --git a/src/problem5/bookService.ts b/src/problem5/bookService.ts
new file mode 100644
index 0000000000..b42624bc4a
--- /dev/null
+++ b/src/problem5/bookService.ts
@@ -0,0 +1,200 @@
+import { randomUUID } from "node:crypto";
+import { readDatabase, writeDatabase } from "./database";
+import { Book, BookFilters, BookStatus, CreateBookInput, UpdateBookInput } from "./types";
+
+const validStatuses: BookStatus[] = ["available", "borrowed", "archived"];
+
+function isBookStatus(value: unknown): value is BookStatus {
+  return typeof value === "string" && validStatuses.includes(value as BookStatus);
+}
+
+function isNonEmptyString(value: unknown): value is string {
+  return typeof value === "string" && value.trim().length > 0;
+}
+
+function normalizeText(value: string): string {
+  return value.trim();
+}
+
+function parsePositiveInteger(value: unknown, fallback: number): number {
+  if (typeof value !== "string") {
+    return fallback;
+  }
+
+  const parsedValue = Number.parseInt(value, 10);
+  return Number.isInteger(parsedValue) && parsedValue >= 0 ? parsedValue : fallback;
+}
+
+export function parseBookFilters(query: Record<string, unknown>): BookFilters {
+  return {
+    status: isBookStatus(query.status) ? query.status : undefined,
+    author: isNonEmptyString(query.author) ? normalizeText(query.author) : undefined,
+    q: isNonEmptyString(query.q) ? normalizeText(query.q) : undefined,
+    limit: parsePositiveInteger(query.limit, 20),
+    offset: parsePositiveInteger(query.offset, 0),
+  };
+}
+
+export function parseCreateBookInput(body: unknown): CreateBookInput {
+  if (typeof body !== "object" || body === null) {
+    throw new Error("Request body must be an object.");
+  }
+
+  const input = body as Record<string, unknown>;
+
+  if (!isNonEmptyString(input.title)) {
+    throw new Error("title is required.");
+  }
+
+  if (!isNonEmptyString(input.author)) {
+    throw new Error("author is required.");
+  }
+
+  if (input.status !== undefined && !isBookStatus(input.status)) {
+    throw new Error("status must be one of: available, borrowed, archived.");
+  }
+
+  if (input.description !== undefined && typeof input.description !== "string") {
+    throw new Error("description must be a string.");
+  }
+
+  return {
+    title: normalizeText(input.title),
+    author: normalizeText(input.author),
+    status: input.status ?? "available",
+    description: typeof input.description === "string" ? normalizeText(input.description) : undefined,
+  };
+}
+
+export function parseUpdateBookInput(body: unknown): UpdateBookInput {
+  if (typeof body !== "object" || body === null) {
+    throw new Error("Request body must be an object.");
+  }
+
+  const input = body as Record<string, unknown>;
+  const update: UpdateBookInput = {};
+
+  if (input.title !== undefined) {
+    if (!isNonEmptyString(input.title)) {
+      throw new Error("title must be a non-empty string.");
+    }
+
+    update.title = normalizeText(input.title);
+  }
+
+  if (input.author !== undefined) {
+    if (!isNonEmptyString(input.author)) {
+      throw new Error("author must be a non-empty string.");
+    }
+
+    update.author = normalizeText(input.author);
+  }
+
+  if (input.status !== undefined) {
+    if (!isBookStatus(input.status)) {
+      throw new Error("status must be one of: available, borrowed, archived.");
+    }
+
+    update.status = input.status;
+  }
+
+  if (input.description !== undefined) {
+    if (typeof input.description !== "string") {
+      throw new Error("description must be a string.");
+    }
+
+    update.description = normalizeText(input.description);
+  }
+
+  if (Object.keys(update).length === 0) {
+    throw new Error("At least one field is required to update.");
+  }
+
+  return update;
+}
+
+export async function createBook(input: CreateBookInput): Promise<Book> {
+  const database = await readDatabase();
+  const now = new Date().toISOString();
+
+  const book: Book = {
+    id: randomUUID(),
+    title: input.title,
+    author: input.author,
+    status: input.status ?? "available",
+    description: input.description,
+    createdAt: now,
+    updatedAt: now,
+  };
+
+  database.books.push(book);
+  await writeDatabase(database);
+
+  return book;
+}
+
+export async function listBooks(filters: BookFilters): Promise<{ items: Book[]; total: number }> {
+  const database = await readDatabase();
+  const searchText = filters.q?.toLowerCase();
+  const author = filters.author?.toLowerCase();
+
+  const filteredBooks = database.books.filter((book) => {
+    const matchesStatus = filters.status === undefined || book.status === filters.status;
+    const matchesAuthor = author === undefined || book.author.toLowerCase().includes(author);
+    const matchesSearch =
+      searchText === undefined ||
+      book.title.toLowerCase().includes(searchText) ||
+      book.author.toLowerCase().includes(searchText) ||
+      book.description?.toLowerCase().includes(searchText);
+
+    return matchesStatus && matchesAuthor && matchesSearch;
+  });
+
+  const offset = filters.offset ?? 0;
+  const limit = filters.limit ?? 20;
+
+  return {
+    items: filteredBooks.slice(offset, offset + limit),
+    total: filteredBooks.length,
+  };
+}
+
+export async function getBookById(id: string): Promise<Book | undefined> {
+  const database = await readDatabase();
+  return database.books.find((book) => book.id === id);
+}
+
+export async function updateBook(id: string, input: UpdateBookInput): Promise<Book | undefined> {
+  const database = await readDatabase();
+  const bookIndex = database.books.findIndex((book) => book.id === id);
+
+  if (bookIndex === -1) {
+    return undefined;
+  }
+
+  const currentBook = database.books[bookIndex];
+  const updatedBook: Book = {
+    ...currentBook,
+    ...input,
+    updatedAt: new Date().toISOString(),
+  };
+
+  database.books[bookIndex] = updatedBook;
+  await writeDatabase(database);
+
+  return updatedBook;
+}
+
+export async function deleteBook(id: string): Promise<boolean> {
+  const database = await readDatabase();
+  const bookIndex = database.books.findIndex((book) => book.id === id);
+
+  if (bookIndex === -1) {
+    return false;
+  }
+
+  database.books.splice(bookIndex, 1);
+  await writeDatabase(database);
+
+  return true;
+}
diff --git a/src/problem5/database.ts b/src/problem5/database.ts
new file mode 100644
index 0000000000..8f9f64d749
--- /dev/null
+++ b/src/problem5/database.ts
@@ -0,0 +1,39 @@
+import { mkdir, readFile, rename, writeFile } from "node:fs/promises";
+import path from "node:path";
+import { Book } from "./types";
+
+type DatabaseContent = {
+  books: Book[];
+};
+
+const databasePath = path.join(process.cwd(), "data", "books.json");
+const temporaryDatabasePath = path.join(process.cwd(), "data", "books.json.tmp");
+
+const emptyDatabase: DatabaseContent = {
+  books: [],
+};
+
+async function ensureDatabaseFileExists(): Promise<void> {
+  await mkdir(path.dirname(databasePath), { recursive: true });
+
+  try {
+    await readFile(databasePath, "utf8");
+  } catch {
+    await writeDatabase(emptyDatabase);
+  }
+}
+
+export async function readDatabase(): Promise<DatabaseContent> {
+  await ensureDatabaseFileExists();
+
+  const fileContent = await readFile(databasePath, "utf8");
+  return JSON.parse(fileContent) as DatabaseContent;
+}
+
+export async function writeDatabase(content: DatabaseContent): Promise<void> {
+  await mkdir(path.dirname(databasePath), { recursive: true });
+
+  const json = JSON.stringify(content, null, 2);
+  await writeFile(temporaryDatabasePath, json);
+  await rename(temporaryDatabasePath, databasePath);
+}
diff --git a/src/problem5/postman_collection/postman_collection.json b/src/problem5/postman_collection/postman_collection.json
new file mode 100644
index 0000000000..6716703874
--- /dev/null
+++ b/src/problem5/postman_collection/postman_collection.json
@@ -0,0 +1,189 @@
+{
+  "info": {
+    "_postman_id": "3c5bc72b-22b0-45c0-b241-b4bf6e722b2f",
+    "name": "99Tech Problem 5 - Book API",
+    "description": "Postman collection for the Problem 5 Express book management API.",
+    "schema": "https://schema.getpostman.com/json/collection/v2.1.0/collection.json"
+  },
+  "variable": [
+    {
+      "key": "baseUrl",
+      "value": "http://localhost:3000"
+    },
+    {
+      "key": "bookId",
+      "value": "846c1e8d-dc2e-45ad-b444-274f03df7aa2"
+    }
+  ],
+  "item": [
+    {
+      "name": "Health Check",
+      "request": {
+        "method": "GET",
+        "header": [],
+        "url": {
+          "raw": "{{baseUrl}}/health",
+          "host": [
+            "{{baseUrl}}"
+          ],
+          "path": [
+            "health"
+          ]
+        },
+        "description": "Check whether the server is running."
+      },
+      "response": []
+    },
+    {
+      "name": "Create Book",
+      "request": {
+        "method": "POST",
+        "header": [
+          {
+            "key": "Content-Type",
+            "value": "application/json"
+          }
+        ],
+        "body": {
+          "mode": "raw",
+          "raw": "{\n  \"title\": \"Clean Code\",\n  \"author\": \"Robert C. Martin\",\n  \"status\": \"available\",\n  \"description\": \"A book about writing readable code.\"\n}"
+        },
+        "url": {
+          "raw": "{{baseUrl}}/books",
+          "host": [
+            "{{baseUrl}}"
+          ],
+          "path": [
+            "books"
+          ]
+        },
+        "description": "Create a new book."
+      },
+      "response": []
+    },
+    {
+      "name": "List Books",
+      "request": {
+        "method": "GET",
+        "header": [],
+        "url": {
+          "raw": "{{baseUrl}}/books",
+          "host": [
+            "{{baseUrl}}"
+          ],
+          "path": [
+            "books"
+          ]
+        },
+        "description": "List books without filters."
+      },
+      "response": []
+    },
+    {
+      "name": "List Books With Filters",
+      "request": {
+        "method": "GET",
+        "header": [],
+        "url": {
+          "raw": "{{baseUrl}}/books?status=available&author=robert&q=clean&limit=10&offset=0",
+          "host": [
+            "{{baseUrl}}"
+          ],
+          "path": [
+            "books"
+          ],
+          "query": [
+            {
+              "key": "status",
+              "value": "available"
+            },
+            {
+              "key": "author",
+              "value": "robert"
+            },
+            {
+              "key": "q",
+              "value": "clean"
+            },
+            {
+              "key": "limit",
+              "value": "10"
+            },
+            {
+              "key": "offset",
+              "value": "0"
+            }
+          ]
+        },
+        "description": "List books with status, author, search, pagination filters."
+      },
+      "response": []
+    },
+    {
+      "name": "Get Book By ID",
+      "request": {
+        "method": "GET",
+        "header": [],
+        "url": {
+          "raw": "{{baseUrl}}/books/{{bookId}}",
+          "host": [
+            "{{baseUrl}}"
+          ],
+          "path": [
+            "books",
+            "{{bookId}}"
+          ]
+        },
+        "description": "Get a single book by ID. Set the collection variable `bookId` first."
+      },
+      "response": []
+    },
+    {
+      "name": "Update Book",
+      "request": {
+        "method": "PATCH",
+        "header": [
+          {
+            "key": "Content-Type",
+            "value": "application/json"
+          }
+        ],
+        "body": {
+          "mode": "raw",
+          "raw": "{\n  \"status\": \"borrowed\",\n  \"description\": \"Checked out by a reader.\"\n}"
+        },
+        "url": {
+          "raw": "{{baseUrl}}/books/{{bookId}}",
+          "host": [
+            "{{baseUrl}}"
+          ],
+          "path": [
+            "books",
+            "{{bookId}}"
+          ]
+        },
+        "description": "Update fields of an existing book. Set the collection variable `bookId` first."
+      },
+      "response": []
+    },
+    {
+      "name": "Delete Book",
+      "request": {
+        "method": "DELETE",
+        "header": [],
+        "url": {
+          "raw": "{{baseUrl}}/books/{{bookId}}",
+          "host": [
+            "{{baseUrl}}"
+          ],
+          "path": [
+            "books",
+            "{{bookId}}"
+          ]
+        },
+        "description": "Delete a book by ID. Set the collection variable `bookId` first."
+      },
+      "response": []
+    }
+  ]
+}
diff --git a/src/problem5/server.ts b/src/problem5/server.ts
new file mode 100644
index 0000000000..024a7da3ef
--- /dev/null
+++ b/src/problem5/server.ts
@@ -0,0 +1,105 @@
+import express, { NextFunction, Request, Response } from "express";
+import {
+  createBook,
+  deleteBook,
+  getBookById,
+  listBooks,
+  parseBookFilters,
+  parseCreateBookInput,
+  parseUpdateBookInput,
+  updateBook,
+} from "./bookService";
+
+const app = express();
+const port = Number(process.env.PORT) || 3000;
+
+app.use(express.json());
+
+function getRouteId(request: Request): string {
+  const id = request.params.id;
+  return Array.isArray(id) ? id[0] : id;
+}
+
+app.get("/health", (_request: Request, response: Response) => {
+  response.json({ status: "ok" });
+});
+
+app.post("/books", async (request: Request, response: Response, next: NextFunction) => {
+  try {
+    const input = parseCreateBookInput(request.body);
+    const book = await createBook(input);
+
+    response.status(201).json(book);
+  } catch (error) {
+    next(error);
+  }
+});
+
+app.get("/books", async (request: Request, response: Response, next: NextFunction) => {
+  try {
+    const filters = parseBookFilters(request.query);
+    const result = await listBooks(filters);
+
+    response.json({
+      ...result,
+      limit: filters.limit,
+      offset: filters.offset,
+    });
+  } catch (error) {
+    next(error);
+  }
+});
+
+app.get("/books/:id", async (request: Request, response: Response, next: NextFunction) => {
+  try {
+    const book = await getBookById(getRouteId(request));
+
+    if (book === undefined) {
+      response.status(404).json({ message: "Book not found." });
+      return;
+    }
+
+    response.json(book);
+  } catch (error) {
+    next(error);
+  }
+});
+
+app.patch("/books/:id", async (request: Request, response: Response, next: NextFunction) => {
+  try {
+    const input = parseUpdateBookInput(request.body);
+    const book = await updateBook(getRouteId(request), input);
+
+    if (book === undefined) {
+      response.status(404).json({ message: "Book not found." });
+      return;
+    }
+
+    response.json(book);
+  } catch (error) {
+    next(error);
+  }
+});
+
+app.delete("/books/:id", async (request: Request, response: Response, next: NextFunction) => {
+  try {
+    const deleted = await deleteBook(getRouteId(request));
+
+    if (!deleted) {
+      response.status(404).json({ message: "Book not found." });
+      return;
+    }
+
+    response.status(204).send();
+  } catch (error) {
+    next(error);
+  }
+});
+
+app.use((error: Error, _request: Request, response: Response, _next: NextFunction) => {
+  response.status(400).json({ message: error.message });
+});
+
+app.listen(port, () => {
+  console.log(`Problem 5 server is running on http://localhost:${port}`);
+});
diff --git a/src/problem5/types.ts b/src/problem5/types.ts
new file mode 100644
index 0000000000..52c616792f
--- /dev/null
+++ b/src/problem5/types.ts
@@ -0,0 +1,28 @@
+export type BookStatus = "available" | "borrowed" | "archived";
+
+export type Book = {
+  id: string;
+  title: string;
+  author: string;
+  status: BookStatus;
+  description?: string;
+  createdAt: string;
+  updatedAt: string;
+};
+
+export type CreateBookInput = {
+  title: string;
+  author: string;
+  status?: BookStatus;
+  description?: string;
+};
+
+export type UpdateBookInput = Partial<CreateBookInput>;
+
+export type BookFilters = {
+  status?: BookStatus;
+  author?: string;
+  q?: string;
+  limit?: number;
+  offset?: number;
+};
diff --git a/src/problem6/README.md b/src/problem6/README.md
new file mode 100644
index 0000000000..61f440fd56
--- /dev/null
+++ b/src/problem6/README.md
@@ -0,0 +1,341 @@
+# Problem 6: Live Scoreboard Module Specification
+
+## Purpose
+
+This document specifies a backend module that:
+
+- stores and updates user scores
+- exposes the top 10 leaderboard
+- pushes live leaderboard updates to connected clients
+- prevents unauthorized or replayed score increases.
+
+The intended audience is the backend engineering team that will implement the
+module inside the API service.
+
+## Scope
+
+### In scope
+
+- API contract for score updates and leaderboard reads
+- realtime update flow
+- authorization and anti-abuse controls
+- persistence model
+
+## Requirements Mapping
+
+| Product requirement               | Module responsibility                                                 |
+| --------------------------------- | --------------------------------------------------------------------- |
+| Show top 10 scores                | Provide `GET /v1/scoreboard`                                          |
+| Live updates                      | Push leaderboard changes over SSE                                     |
+| Action completion increases score | Accept score-award requests from a trusted workflow                   |
+| API call updates score            | Provide `POST /v1/score-events`                                       |
+| Prevent cheating score increases  | Require auth, validate action proof, avoid cheating api, audit events |
+
+## Design Summary
+
+The client is never allowed to submit a score directly. The client only
+submits proof that a server-recognized action was completed. The API service
+validates the caller, validates that the action belongs to that caller and is
+eligible for scoring, writes a single immutable score event, updates the user's
+total score in a transaction, recomputes the top 10 leaderboard, and publishes a
+realtime update to subscribers.
+
+Deduplication happens at two different levels:
+
+- retry safety via `Idempotency-Key`,
+- business-level exactly-once scoring via unique `actionCompletionId`.
+
+For the first implementation:
+
+- use HTTP + JSON for command and query APIs
+- use Server-Sent Events for one-way live updates
+- use a relational database as the source of truth
+- use Redis Pub/Sub only when the API runs on multiple instances.
+
+## Architecture
+
+### Module boundary
+
+The module owns:
+
+- score-award request handling
+- score event deduplication
+- user score mutation
+- leaderboard query
+- realtime leaderboard publication.
+
+The module depends on:
+
+- authentication middleware
+- a trusted action validation source
+- the primary SQL database
+- an optional cross-instance pub/sub layer.
+
+### Components
+
+#### 1. Score Event API
+
+Receives score-award requests from authenticated users or trusted internal
+callers, validates request shape, and delegates to the application service.
+
+#### 2. Score Application Service
+
+Core orchestration layer. It:
+
+- verifies the action proof
+- resolves the reward amount from server-side rules
+- enforces idempotency
+- writes the score event
+- updates the aggregate user score
+- fetches the latest top 10
+- emits a leaderboard-updated event.
+
+#### 3. Action Validation Gateway
+
+Reads from the trusted action system and answers:
+
+- did this action completion exist
+- does it belong to this user
+- is it eligible for score award
+- has it already been consumed for scoring.
+
+This can be implemented by reading an internal table, calling another internal
+service, or validating a server-signed completion token.
+
+#### 4. Leaderboard Query Service
+
+Reads the current top 10 ranking from persisted score totals.
+
+#### 5. Realtime Publisher
+
+Pushes `scoreboard.updated` events to connected clients through SSE. In a
+multi-instance deployment, each API node subscribes to a shared pub/sub topic
+and fans out the same event to its own clients.
+
+## Execution Flow
+
+Mermaid source: [scoreboard-flow-flow.png](../problem6/scoreboard-flow.png)
+
+## API Contract
+
+### 1. Submit score event
+
+```http
+POST /v1/score-events
+Authorization: Bearer <token>
+Idempotency-Key: <uuid>
+Content-Type: application/json
+```
+
+Request body:
+
+```json
+{
+  "actionCompletionId": "act_01JXYZ..."
+}
+```
+
+Rules:
+
+- caller must be authenticated
+- `Idempotency-Key` is required and is an opaque client-generated retry token
+- `actionCompletionId` must reference a trusted completed action
+- the action must belong to the authenticated user
+- the action must not have been scored before
+- the server resolves the score delta; the client never sends it.
+
+Successful response:
+
+```http
+201 Created
+```
+
+```json
+{
+  "eventId": "evt_01JXYZ...",
+  "user": {
+    "id": "usr_123",
+    "score": 1250
+  },
+  "scoreAdded": 10,
+  "scoreboard": [
+    {
+      "rank": 1,
+      "userId": "usr_123",
+      "displayName": "Alice",
+      "score": 1250
+    }
+  ],
+  "processedAt": "2026-04-17T00:00:00.000Z"
+}
+```
+
+Idempotency behavior:
+
+- `Idempotency-Key` is not a payload hash; it identifies one logical mutation
+  attempt so safe retries return the same result
+- server should persist a canonical `request_hash` alongside the
+  `Idempotency-Key` to detect the same key being reused with a different payload
+- same `Idempotency-Key` + same payload for the same user must return the
+  original success response
+- same `Idempotency-Key` + different payload must return `409`
+- separate idempotency keys for the same `actionCompletionId` must still be
+  blocked by a database uniqueness rule.
+
+Example:
+
+- first request: `Idempotency-Key = k1`, `actionCompletionId = act_123` ->
+  score awarded
+- retry request with `k1` and the same payload -> return the original success
+  response, do not award score again
+- request with `k1` but a different `actionCompletionId` -> return `409`
+- request with a new key `k2` but the same `actionCompletionId = act_123` ->
+  reject because that business action has already been consumed.
+
+### 2. Get leaderboard
+
+```http
+GET /v1/scoreboard?limit=10
+```
+
+Successful response:
+
+```json
+{
+  "items": [
+    {
+      "rank": 1,
+      "userId": "usr_123",
+      "displayName": "Alice",
+      "score": 1250
+    }
+  ],
+  "updatedAt": "2026-04-17T00:00:00.000Z"
+}
+```
+
+Rules:
+
+- default `limit` is `10`,
+- maximum supported `limit` is `100`,
+- website scoreboard should always request `10`.
+
+### 3. Subscribe to live leaderboard
+
+```http
+GET /v1/scoreboard/stream
+Accept: text/event-stream
+Authorization: Bearer <token>
+```
+
+SSE event:
+
+```text
+event: scoreboard.updated
+data: {"items":[{"rank":1,"userId":"usr_123","displayName":"Alice","score":1250}],"updatedAt":"2026-04-17T00:00:00.000Z"}
+```
+
+Realtime rules:
+
+- send current leaderboard immediately after subscribe,
+- push only after a committed score update,
+- clients should reconnect automatically,
+- if an event is missed, client recovers by refetching `GET /v1/scoreboard`.
+
+## Data Model
+
+### `users`
+
+| Column         | Type          | Notes                               |
+| -------------- | ------------- | ----------------------------------- |
+| `id`           | uuid / string | primary key                         |
+| `display_name` | varchar       | leaderboard-safe public name        |
+| `score`        | bigint        | current total score                 |
+| `created_at`   | timestamp     | record creation time                |
+| `updated_at`   | timestamp     | last score change or profile change |
+
+Indexes:
+
+- primary key on `id`
+- leaderboard index on `(score DESC, updated_at ASC, id ASC)`
+
+### `score_events`
+
+Immutable audit log of every accepted score increase.
+
+| Column                 | Type          | Notes                                             |
+| ---------------------- | ------------- | ------------------------------------------------- |
+| `id`                   | uuid / string | primary key                                       |
+| `user_id`              | uuid / string | target user                                       |
+| `action_completion_id` | string        | trusted action reference                          |
+| `idempotency_key`      | string        | client-generated retry token                      |
+| `score_delta`          | integer       | awarded score                                     |
+| `request_hash`         | string        | server-computed hash of canonical request payload |
+| `created_at`           | timestamp     | insertion time                                    |
+
+Constraints:
+
+- unique `(user_id, action_completion_id)`
+- unique `(user_id, idempotency_key)`
+- `score_delta > 0`
+
+Notes:
+
+- `idempotency_key` protects against duplicate processing caused by retries,
+- `action_completion_id` protects the business rule that one completed action
+  can award score only once,
+- `request_hash` is optional but strongly recommended to detect misuse of the
+  same `idempotency_key` for different payloads.
+
+### `action_rewards`
+
+| Column        | Type      | Notes                   |
+| ------------- | --------- | ----------------------- |
+| `action_type` | string    | reward rule key         |
+| `score_delta` | integer   | points awarded          |
+| `is_active`   | boolean   | operational kill switch |
+| `updated_at`  | timestamp | config change time      |
+
+## Transaction and Consistency Rules
+
+The score mutation path must be transactionally safe.
+
+Required sequence:
+
+1. validate auth outside the transaction,
+2. resolve action validity and reward amount,
+3. begin database transaction
+4. insert into `score_events`
+5. update `users.score`
+6. read the top 10 leaderboard
+7. commit
+8. publish realtime event after commit.
+
+Hard requirements:
+
+- no score change without a persisted `score_events` row
+- duplicate action processing must fail even under concurrent requests
+- leaderboard responses must reflect only committed score updates
+- publish failure must not roll back the committed score update.
+
+## Security Requirements
+
+### Trust model
+
+The client is untrusted. The server must treat all score-related user input as
+claims to be verified, never as facts.
+
+### Required controls
+
+- require authenticated access for score updates
+- get `userId` from auth context only
+- validate action completion against a trusted source
+- enforce one-time consumption of each action completion
+- require idempotency keys on mutating requests
+- rate limit by user ID and by IP address
+
+## Additional Comments and Improvements
+
+- Should leaderboard reads be public or require authentication?
+- If users have the same score in the leaderboard, how do we do?
+- In the future, the leaderboard maybe more than 10.
diff --git a/src/problem6/scoreboard-flow.png b/src/problem6/scoreboard-flow.png
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diff --git a/tsconfig.json b/tsconfig.json
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--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,13 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "CommonJS",
+    "rootDir": "src",
+    "outDir": "dist",
+    "strict": true,
+    "esModuleInterop": true,
+    "forceConsistentCasingInFileNames": true,
+    "skipLibCheck": true
+  },
+  "include": ["src/**/*.ts"]
+}