Why amqp-contract?

Working with RabbitMQ and AMQP messaging is powerful and flexible, but it comes with significant challenges when building TypeScript applications. amqp-contract solves these problems by bringing a contract-first, type-safe approach to AMQP messaging.

The Problem

Traditional AMQP development in TypeScript lacks type safety and validation, leading to several issues:

1. No Type Safety

Without types, you're working blind:

// ❌ Traditional approach - no type safety
channel.publish(
  "orders",
  "order.created",
  Buffer.from(
    JSON.stringify({
      orderId: "ORD-123", // What fields are required? What types?
    }),
  ),
);

channel.consume("order-processing", (msg) => {
  const data = JSON.parse(msg.content.toString()); // unknown type
  console.log(data.orderId); // No autocomplete, no type checking
  // Is it orderId or order_id? Did the field change?
});

2. Manual Validation Everywhere

You must validate messages manually at every boundary:

// ❌ Validation scattered throughout the codebase
function validateOrder(data: unknown): Order {
  if (!data || typeof data !== "object") {
    throw new Error("Invalid data");
  }
  const order = data as any;
  if (typeof order.orderId !== "string") {
    throw new Error("orderId must be a string");
  }
  // ... dozens more checks ...
  return order as Order;
}

3. Runtime Errors from Wrong Data

Without validation, invalid messages cause runtime failures:

// ❌ No validation - crashes at runtime
channel.consume("orders", (msg) => {
  const order = JSON.parse(msg.content.toString());
  processPayment(order.amount); // TypeError: amount is undefined
});

4. Scattered Message Definitions

Message schemas are duplicated across services:

// ❌ Duplicated schemas in multiple files
// publisher.ts
interface OrderEvent {
  orderId: string;
  amount: number;
}

// consumer.ts
interface OrderEvent {
  // Same schema, different file
  orderId: string;
  amount: number;
}

5. Difficult Refactoring

Changing a message schema means hunting through multiple files:

// ❌ Change orderId to id - must update everywhere manually
// No compile-time checks to catch all usages

The Solution

amqp-contract transforms AMQP development with a contract-first approach:

1. End-to-End Type Safety

Define your contract once, get types everywhere:

import {
  defineContract,
  defineExchange,
  defineQueue,
  definePublisherFirst,
  defineMessage,
} from "@amqp-contract/contract";
import { TypedAmqpClient } from "@amqp-contract/client";
import { TypedAmqpWorker } from "@amqp-contract/worker";
import { z } from "zod";

// 1. Define resources
const ordersExchange = defineExchange("orders", "topic", { durable: true });
const orderProcessingQueue = defineQueue("order-processing", { durable: true });

// 2. Define message schema
const orderMessage = defineMessage(
  z.object({
    orderId: z.string(),
    customerId: z.string(),
    amount: z.number().positive(),
  }),
);

// 3. Publisher-first pattern for event-oriented messaging
const { publisher: orderCreatedPublisher, createConsumer: createOrderCreatedConsumer } =
  definePublisherFirst(ordersExchange, orderMessage, { routingKey: "order.created" });

// 4. Create consumer from event (ensures schema consistency)
const { consumer: processOrderConsumer, binding: orderBinding } =
  createOrderCreatedConsumer(orderProcessingQueue);

// 5. Create contract
const contract = defineContract({
  exchanges: { orders: ordersExchange },
  queues: { orderProcessing: orderProcessingQueue },
  bindings: { orderBinding },
  publishers: {
    orderCreated: orderCreatedPublisher,
  },
  consumers: {
    processOrder: processOrderConsumer,
  },
});

// 4. Publisher gets full type safety
const clientResult = await TypedAmqpClient.create({
  contract,
  urls: ["amqp://localhost"],
});

const client = clientResult.get();
const result = await client.publish("orderCreated", {
  orderId: "ORD-123", // ✅ TypeScript knows these fields!
  customerId: "CUST-456", // ✅ Autocomplete works!
  amount: 99.99, // ✅ Type checked at compile time!
});

// 5. Consumer gets fully typed messages
const workerResult = await TypedAmqpWorker.create({
  contract,
  handlers: {
    processOrder: async (message) => {
      console.log(message.orderId); // ✅ Fully typed!
      console.log(message.customerId); // ✅ Autocomplete!
      console.log(message.amount); // ✅ Type safe!
    },
  },
  urls: ["amqp://localhost"],
});

2. Automatic Validation

Schema validation happens automatically at network boundaries:

// ✅ Validation happens automatically
const result = await client.publish("orderCreated", {
  orderId: "ORD-123",
  customerId: "CUST-456",
  amount: -10, // ❌ Validation error: amount must be positive
});

result.match({
  Ok: () => console.log("Published"),
  Error: (error) => console.error("Validation failed:", error),
});

3. Compile-Time Checks

TypeScript catches errors before runtime:

// ❌ TypeScript error at compile time
await client.publish("orderCreated", {
  orderId: "ORD-123",
  // Missing customerId and amount - TypeScript error!
});

// ❌ TypeScript error for wrong types
await client.publish("orderCreated", {
  orderId: 123, // Error: orderId must be string
  customerId: "CUST-456",
  amount: 99.99,
});

4. Single Source of Truth

Your contract is the single source of truth:

// ✅ One contract definition using publisher-first pattern
const { publisher: orderCreatedPublisher, createConsumer: createOrderCreatedConsumer } =
  definePublisherFirst(ordersExchange, orderMessage, { routingKey: "order.created" });

const { consumer: processOrderConsumer, binding: orderBinding } =
  createOrderCreatedConsumer(orderProcessingQueue);

const contract = defineContract({
  // Define once, use across all publishers and consumers
  // Publisher and consumer guaranteed to use same schema
  exchanges: { orders: ordersExchange },
  queues: { orderProcessing: orderProcessingQueue },
  bindings: { orderBinding },
  publishers: {
    orderCreated: orderCreatedPublisher,
  },
  consumers: {
    processOrder: processOrderConsumer,
  },
});

5. Safe Refactoring

Refactoring is safe and guided by TypeScript:

// Change the schema
const orderMessage = defineMessage(
  z.object({
    id: z.string(), // Changed from orderId to id
    customerId: z.string(),
    amount: z.number().positive(),
  }),
);

// TypeScript immediately shows all places that need updates:
// - Publisher calls
// - Consumer handlers
// - Type definitions

Key Benefits

Better Developer Experience

• Autocomplete - Your IDE knows all message fields and types
• Inline Documentation - Hover over fields to see schemas
• Refactoring Support - Rename fields safely across the codebase
• Jump to Definition - Navigate from usage to contract definition

Compile-Time Safety

• Catch Errors Early - TypeScript catches issues before runtime
• Type Inference - No manual type annotations needed
• Exhaustive Checks - Ensure all consumers are implemented

Runtime Safety

• Automatic Validation - Zod, Valibot, or ArkType validate messages
• Explicit Error Handling - Result types for predictable error handling
• No Surprises - Invalid messages are caught at boundaries

Maintainability

• Single Source of Truth - Contract defines everything
• Documentation - AsyncAPI generation for API docs
• Version Control - Track contract changes in git
• Clear Boundaries - Well-defined publisher/consumer interfaces

Inspired By

This project adapts the contract-first approach from:

• tRPC - End-to-end type safety for RPC
• oRPC - Contract-first RPC with OpenAPI
• ts-rest - Type-safe REST APIs

We've brought their excellent ideas to the world of RabbitMQ and AMQP messaging.

When to Use amqp-contract

Perfect for:

• ✅ TypeScript projects using RabbitMQ/AMQP
• ✅ Microservices with message-based communication
• ✅ Projects requiring strong type safety
• ✅ Teams that value developer experience
• ✅ Applications with complex message schemas

Consider alternatives if:

• ❌ You're not using TypeScript
• ❌ You need extremely low overhead (though validation overhead is minimal)
• ❌ You prefer dynamic, untyped messaging

Next Steps

Ready to get started?

• Getting Started → - Install and create your first contract
• Core Concepts → - Understand the fundamentals
• Examples → - See real-world usage patterns