Backend Architecture

The Monkeytype backend is a RESTful API built with Express.js and TypeScript, designed for high performance, type safety, and security.

Technology Overview

Core Stack

Technology	Version	Purpose
Express.js	5.2.0	HTTP server framework
TypeScript	6.0.0-beta	Type safety
ts-rest	3.52.1	Type-safe API contracts
MongoDB	6.3.0	Primary database
Redis (ioredis)	4.28.5	Caching and leaderboards
Firebase Admin	12.0.0	Authentication
BullMQ	1.91.1	Background job queues

Source location: backend/src/

Project Structure

backend/
├── src/
│   ├── api/                   # API layer
│   │   ├── controllers/       # Business logic
│   │   ├── routes/            # Route definitions
│   │   ├── ts-rest-adapter.ts # ts-rest integration
│   │   └── types.ts           # API types
│   ├── dal/                   # Data Access Layer
│   │   ├── user.ts            # User database operations
│   │   ├── result.ts          # Result database operations
│   │   ├── leaderboards.ts    # Leaderboard operations
│   │   ├── config.ts          # Config operations
│   │   └── *.ts               # Other DAL modules
│   ├── middlewares/           # Express middleware
│   │   ├── auth.ts            # Authentication
│   │   ├── rate-limit.ts      # Rate limiting
│   │   ├── context.ts         # Request context
│   │   ├── error.ts           # Error handling
│   │   └── *.ts               # Other middleware
│   ├── init/                  # Initialization
│   │   ├── db.ts              # MongoDB connection
│   │   ├── redis.ts           # Redis connection
│   │   ├── firebase-admin.ts  # Firebase setup
│   │   └── configuration.ts   # Live config
│   ├── services/              # Business services
│   ├── utils/                 # Utility functions
│   ├── queues/                # BullMQ queues
│   ├── workers/               # Background workers
│   ├── jobs/                  # Cron jobs
│   ├── constants/             # Constants
│   ├── app.ts                 # Express app setup
│   ├── server.ts              # Server boot
│   └── version.ts             # Version info
├── redis-scripts/             # Lua scripts for Redis
├── docker/                    # Docker configs
└── package.json

Application Bootstrap

The server initializes through a carefully orchestrated sequence:

backend/src/server.ts:23

async function bootServer(port: number): Promise<Server> {
  try {
    Logger.info(`Starting server version ${version}`);
    
    // 1. Connect to MongoDB
    Logger.info("Connecting to database...");
    await db.connect();
    Logger.success("Connected to database");
    
    // 2. Initialize Firebase Admin SDK
    Logger.info("Initializing Firebase app instance...");
    initFirebaseAdmin();
    
    // 3. Fetch live configuration
    Logger.info("Fetching live configuration...");
    await getLiveConfiguration();
    await updateFromConfigurationFile();
    
    // 4. Initialize email client
    Logger.info("Initializing email client...");
    await EmailClient.init();
    
    // 5. Connect to Redis
    Logger.info("Connecting to redis...");
    await RedisClient.connect();
    
    if (RedisClient.isConnected()) {
      // 6. Initialize BullMQ queues
      Logger.info("Initializing queues...");
      queues.forEach((queue) => queue.init(connection));
      
      // 7. Start workers
      Logger.info("Initializing workers...");
      workers.forEach(async (worker) => {
        await worker(connection).run();
      });
    }
    
    // 8. Start cron jobs
    Logger.info("Starting cron jobs...");
    jobs.forEach((job) => job.start());
    
    // 9. Set up database indices
    await leaderboardDbSetup();
    await blocklistDbSetup();
    await connectionsDbSetup();
    
  } catch (error) {
    Logger.error("Failed to boot server");
    return process.exit(1);
  }
  
  // 10. Start listening
  return app.listen(port, () => {
    Logger.success(`API server listening on port ${port}`);
  });
}

Express Application Setup

Core Middleware Pipeline

backend/src/app.ts:1

import express from "express";
import cors from "cors";
import helmet from "helmet";

function buildApp(): express.Application {
  const app = express();
  
  // 1. Body parsing
  app.use(express.urlencoded({ extended: true }));
  app.use(express.json());
  
  // 2. Security
  app.use(cors({ exposedHeaders: [COMPATIBILITY_CHECK_HEADER] }));
  app.use(helmet());
  
  // 3. Trust proxy (for rate limiting)
  app.set("trust proxy", 1);
  
  // 4. Custom middleware
  app.use(compatibilityCheckMiddleware);  // API version check
  app.use(contextMiddleware);              // Request context
  app.use(badAuthRateLimiterHandler);      // Auth attempt limiting
  app.use(rootRateLimiter);                // Global rate limiting
  
  // 5. ETag generation
  app.set("etag", createETagGenerator({ weak: true }));
  
  // 6. API routes
  addApiRoutes(app);
  
  // 7. Error handling (must be last)
  app.use(errorHandlingMiddleware);
  
  return app;
}

export default buildApp();

Middleware Execution Order

Request
  ↓
1. Body Parser (express.json)
  ↓
2. CORS (cors)
  ↓
3. Security Headers (helmet)
  ↓
4. Compatibility Check (compatibilityCheckMiddleware)
  ↓
5. Request Context (contextMiddleware)
  ↓
6. Rate Limiting (rootRateLimiter)
  ↓
7. ts-rest Routing
  ↓
8. Authentication (authenticateTsRestRequest)
  ↓
9. Rate Limiting (rateLimitRequest)
  ↓
10. Configuration Check (verifyRequiredConfiguration)
  ↓
11. Permissions (verifyPermissions)
  ↓
12. Request Validation (ts-rest schema validation)
  ↓
13. Controller Logic
  ↓
14. Error Handler (errorHandlingMiddleware)
  ↓
Response

API Architecture (ts-rest)

Type-Safe Contracts

Monkeytype uses ts-rest for end-to-end type safety:

@monkeytype/contracts

import { initContract } from "@ts-rest/core";
import { z } from "zod";

const c = initContract();

export const contract = c.router({
  users: {
    getProfile: {
      method: "GET",
      path: "/users/:uid/profile",
      responses: {
        200: z.object({
          data: UserProfileSchema,
        }),
        404: z.object({
          message: z.string(),
        }),
      },
      summary: "Get user profile",
    },
  },
});

Route Registration

backend/src/api/routes/index.ts:49

import { initServer } from "@ts-rest/express";
import { contract } from "@monkeytype/contracts";

const s = initServer();

// Register all route modules
const router = s.router(contract, {
  admin,
  apeKeys,
  configs,
  presets,
  psas,
  public: publicStats,
  leaderboards,
  results,
  configuration,
  dev,
  users,
  quotes,
  webhooks,
  connections,
});

// Attach to Express app
createExpressEndpoints(contract, router, app, {
  jsonQuery: true,
  globalMiddleware: [
    authenticateTsRestRequest(),
    rateLimitRequest(),
    verifyRequiredConfiguration(),
    verifyPermissions(),
  ],
});

Controller Example

backend/src/api/controllers/user.ts

import { initServer } from "@ts-rest/express";
import { contract } from "@monkeytype/contracts";
import * as UserDAL from "../../dal/user";

const s = initServer();

export const userController = s.router(contract.users, {
  getProfile: async ({ params, req }) => {
    // req.ctx contains: decodedToken, configuration, etc.
    const { uid } = params;
    
    // Call Data Access Layer
    const user = await UserDAL.getUser(uid, "get user profile");
    
    if (!user) {
      return {
        status: 404,
        body: { message: "User not found" },
      };
    }
    
    return {
      status: 200,
      body: {
        data: {
          name: user.name,
          addedAt: user.addedAt,
          // ... other profile data
        },
      },
    };
  },
});

Authentication System

Authentication Flow

backend/src/middlewares/auth.ts:44

export function authenticateTsRestRequest() {
  return async (req, res, next) => {
    const { authorization } = req.headers;
    const options = getMetadata(req).authenticationOptions;
    
    let token: DecodedToken;
    
    if (options.isPublic) {
      // Public endpoint - no auth required
      token = { type: "None", uid: "", email: "" };
    } else if (authorization?.startsWith("Bearer ")) {
      // Firebase Bearer token
      const bearerToken = authorization.slice(7);
      const decoded = await verifyIdToken(bearerToken);
      token = {
        type: "Bearer",
        uid: decoded.uid,
        email: decoded.email,
      };
    } else if (authorization?.startsWith("ApeKey ")) {
      // API Key authentication
      const apeKey = authorization.slice(7);
      const validated = await validateApeKey(apeKey);
      token = {
        type: "ApeKey",
        uid: validated.uid,
        email: validated.email,
      };
    } else {
      throw new MonkeyError(401, "Unauthorized");
    }
    
    req.ctx.decodedToken = token;
    next();
  };
}

Authentication Types

1. Bearer Token (Firebase)

GET /users/123/profile
Authorization: Bearer eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9...

Used for user actions
Verified via Firebase Admin SDK
Short-lived tokens (1 hour)

2. ApeKey (API Key)

GET /results
Authorization: ApeKey mk_live_abc123...

Used for programmatic access
Hashed and stored in database
User-generated, long-lived

3. GitHub Webhook

POST /webhooks/github
X-Hub-Signature-256: sha256=...

HMAC signature verification
Used for automated deployments

Rate Limiting

Monkeytype implements multi-tiered rate limiting:

Global Rate Limiter

backend/src/middlewares/rate-limit.ts

import rateLimit from "express-rate-limit";

export const rootRateLimiter = rateLimit({
  windowMs: 60 * 1000, // 1 minute
  max: 120,             // 120 requests per minute
  message: "Too many requests, please try again later.",
});

Endpoint-Specific Limiting

import { RateLimiterRedis } from "rate-limiter-flexible";

const limiter = new RateLimiterRedis({
  storeClient: redisClient,
  points: 30,           // 30 requests
  duration: 60,         // per minute
  keyPrefix: "rl:results",
});

// In controller
await limiter.consume(req.ctx.decodedToken.uid);

Bad Auth Rate Limiter

// Stricter limits on failed auth attempts
export const badAuthRateLimiter = rateLimit({
  windowMs: 5 * 60 * 1000, // 5 minutes
  max: 10,                  // 10 attempts
  skipSuccessfulRequests: true,
});

Data Access Layer (DAL)

The DAL provides a clean abstraction over database operations:

User DAL Example

backend/src/dal/user.ts:79

import { Collection, ObjectId } from "mongodb";
import * as db from "../init/db";

export const getUsersCollection = (): Collection<DBUser> =>
  db.collection<DBUser>("users");

export async function addUser(
  name: string,
  email: string,
  uid: string
): Promise<void> {
  const newUserDocument: Partial<DBUser> = {
    name,
    email,
    uid,
    addedAt: Date.now(),
    personalBests: {
      time: {},
      words: {},
      quote: {},
      zen: {},
      custom: {},
    },
    testActivity: {},
  };
  
  await getUsersCollection().insertOne(newUserDocument);
}

export async function getUser(
  uid: string,
  stack: string
): Promise<DBUser> {
  const user = await getUsersCollection().findOne({ uid });
  if (!user) throw new MonkeyError(404, "User not found", stack);
  return user;
}

Result DAL Example

backend/src/dal/result.ts:1

export async function addResult(
  uid: string,
  result: DBResult
): Promise<{ insertedId: ObjectId }> {
  const { data: user } = await tryCatch(getUser(uid, "add result"));
  
  if (!user) throw new MonkeyError(404, "User not found");
  
  result.uid ??= uid;
  const res = await getResultCollection().insertOne(result);
  
  return { insertedId: res.insertedId };
}

export async function getResults(
  uid: string,
  opts?: GetResultsOpts
): Promise<DBResult[]> {
  const { onOrAfterTimestamp, offset, limit } = opts ?? {};
  
  let query = getResultCollection()
    .find({ uid, timestamp: { $gte: onOrAfterTimestamp } })
    .sort({ timestamp: -1 });
  
  if (limit) query = query.limit(limit);
  if (offset) query = query.skip(offset);
  
  return await query.toArray();
}

Background Jobs

BullMQ Queues

backend/src/queues/

import { Queue } from "bullmq";
import { Redis } from "ioredis";

class EmailQueue {
  public queueName = "email";
  private queue?: Queue;
  
  init(connection: Redis): void {
    this.queue = new Queue(this.queueName, { connection });
  }
  
  async sendVerificationEmail(email: string, token: string) {
    await this.queue?.add("verification", { email, token });
  }
}

export default new EmailQueue();

Workers

backend/src/workers/

import { Worker } from "bullmq";

export default function emailWorker(connection: Redis) {
  return new Worker("email", async (job) => {
    const { email, token } = job.data;
    await sendEmail(email, "Verify your email", generateHtml(token));
  }, { connection });
}

Cron Jobs

backend/src/jobs/

import { CronJob } from "cron";

const cleanupJob = new CronJob(
  "0 0 * * *", // Every day at midnight
  async () => {
    await cleanupExpiredSessions();
    await updateLeaderboards();
  },
  null,
  false,
  "UTC"
);

export default [cleanupJob];

Redis Integration

Connection Setup

backend/src/init/redis.ts:75

import IORedis from "ioredis";

let connection: IORedis.Redis;

export async function connect(): Promise<void> {
  const { REDIS_URI } = process.env;
  
  connection = new IORedis(REDIS_URI, {
    maxRetriesPerRequest: null,
    enableReadyCheck: false,
    lazyConnect: true,
  });
  
  await connection.connect();
  
  // Load custom Lua scripts
  loadScripts(connection);
}

function loadScripts(client: IORedis.Redis): void {
  const scriptFiles = fs.readdirSync(REDIS_SCRIPTS_DIRECTORY_PATH);
  
  scriptFiles.forEach((scriptFile) => {
    const scriptSource = fs.readFileSync(scriptPath, "utf-8");
    const scriptName = kebabToCamelCase(scriptFile.split(".")[0]);
    
    client.defineCommand(scriptName, { lua: scriptSource });
  });
}

Custom Lua Scripts

Redis operations are optimized with Lua scripts:

-- redis-scripts/add-result.lua
-- Atomically add result to leaderboard
local scoresKey = KEYS[1]
local resultsKey = KEYS[2]
local uid = ARGV[1]
local score = ARGV[2]
local data = ARGV[3]

redis.call('ZADD', scoresKey, score, uid)
redis.call('HSET', resultsKey, uid, data)
return redis.call('ZRANK', scoresKey, uid)

Error Handling

Custom Error Class

backend/src/utils/error.ts

export default class MonkeyError extends Error {
  constructor(
    public statusCode: number,
    public message: string,
    public stack?: string
  ) {
    super(message);
    this.name = "MonkeyError";
  }
}

Error Middleware

backend/src/middlewares/error.ts

export default function errorHandlingMiddleware(
  err: Error,
  req: Request,
  res: Response,
  next: NextFunction
): void {
  if (err instanceof MonkeyError) {
    res.status(err.statusCode).json({
      message: err.message,
      data: null,
    });
  } else {
    Logger.error(err);
    Sentry.captureException(err);
    res.status(500).json({
      message: "Internal server error",
    });
  }
}

Testing

Unit Tests

pnpm test              # Run unit tests
pnpm integration-test  # Run integration tests
pnpm test-coverage     # Generate coverage report

Integration Tests (Testcontainers)

import { MongoDBContainer } from "testcontainers";
import { describe, it, beforeAll } from "vitest";

describe("User DAL", () => {
  let mongoContainer: MongoDBContainer;
  
  beforeAll(async () => {
    mongoContainer = await new MongoDBContainer().start();
    await db.connect(mongoContainer.getConnectionString());
  });
  
  it("creates user", async () => {
    await addUser("test", "[email protected]", "uid123");
    const user = await getUser("uid123", "test");
    expect(user.name).toBe("test");
  });
});

Next Steps

Database Architecture - MongoDB and Redis data models
Frontend Architecture - Client-side architecture

Contributing

Architecture

API Reference

Backend Architecture

Technology Overview

Core Stack

Project Structure

Application Bootstrap

Express Application Setup

Core Middleware Pipeline

Middleware Execution Order

API Architecture (ts-rest)

Type-Safe Contracts

Route Registration

Controller Example

Authentication System

Authentication Flow

Authentication Types

Rate Limiting

Global Rate Limiter

Endpoint-Specific Limiting

Bad Auth Rate Limiter

Data Access Layer (DAL)

User DAL Example

Result DAL Example

Background Jobs

BullMQ Queues

Workers

Cron Jobs

Redis Integration

Connection Setup

Custom Lua Scripts

Error Handling

Custom Error Class

Error Middleware

Testing

Unit Tests

Integration Tests (Testcontainers)

Next Steps

Build docs developers (and LLMs) love

Contributing

Architecture

API Reference

​Technology Overview

​Core Stack

​Project Structure

​Application Bootstrap

​Express Application Setup

​Core Middleware Pipeline

​Middleware Execution Order

​API Architecture (ts-rest)

​Type-Safe Contracts

​Route Registration

​Controller Example

​Authentication System

​Authentication Flow

​Authentication Types

​Rate Limiting

​Global Rate Limiter

​Endpoint-Specific Limiting

​Bad Auth Rate Limiter

​Data Access Layer (DAL)

​User DAL Example

​Result DAL Example

​Background Jobs

​BullMQ Queues

​Workers

​Cron Jobs

​Redis Integration

​Connection Setup

​Custom Lua Scripts

​Error Handling

​Custom Error Class

​Error Middleware

​Testing

​Unit Tests

​Integration Tests (Testcontainers)

​Next Steps

Build docs developers (and LLMs) love

Technology Overview

Core Stack

Project Structure

Application Bootstrap

Express Application Setup

Core Middleware Pipeline

Middleware Execution Order

API Architecture (ts-rest)

Type-Safe Contracts

Route Registration

Controller Example

Authentication System

Authentication Flow

Authentication Types

Rate Limiting

Global Rate Limiter

Endpoint-Specific Limiting

Bad Auth Rate Limiter

Data Access Layer (DAL)

User DAL Example

Result DAL Example

Background Jobs

BullMQ Queues

Workers

Cron Jobs

Redis Integration

Connection Setup

Custom Lua Scripts

Error Handling

Custom Error Class

Error Middleware

Testing

Unit Tests

Integration Tests (Testcontainers)

Next Steps