Learn how to break a monolithic Node.js app into clean, scalable microservices using Express, MongoDB, and Docker

 As your application grows, a single backend often turns into a tangled mess — harder to scale, test, and deploy.

That’s where microservice architecture comes in.

Instead of one large codebase, you split your app into independent, loosely coupled services — each handling one specific business function (like users, products, or orders).

In this blog, we’ll build a simple microservice-based system using:

• 🟢 Node.js + Express — for APIs

• 🍃 MongoDB — as individual service databases

• 🐳 Docker Compose — to run them together

---

🧠 What Are Microservices?

Microservices are small, autonomous services that communicate over APIs.
Each service:

• Has its own database

• Runs independently

• Can be scaled or deployed separately

Example:
A shopping app might have:

• 👤 User Service → handles signup/login

• 📦 Product Service → manages items

• 🛒 Order Service → processes purchases

Instead of one app handling all three, you split them like this:

[User Service] ↔ [Order Service] ↔ [Product Service]
         ↘           ↘
        MongoDB     MongoDB

---

⚙️ Step 1 — Project Setup

Let’s create a folder structure:

microservices-demo/
│
├── user-service/
├── product-service/
├── order-service/
└── docker-compose.yml

Each service is its own Express + MongoDB app.

---

👤 Step 2 — Creating the User Service

Inside user-service/index.js:

import express from "express";
import mongoose from "mongoose";

const app = express();
app.use(express.json());

mongoose.connect("mongodb://localhost:27017/users");

const UserSchema = new mongoose.Schema({
  name: String,
  email: String,
});

const User = mongoose.model("User", UserSchema);

app.post("/users", async (req, res) => {
  const user = await User.create(req.body);
  res.json(user);
});

app.listen(4001, () => console.log("User Service running on port 4001"));

✅ What’s happening:

• A small API that creates users in a separate database.

• Runs on port 4001.

---

📦 Step 3 — Product Service

product-service/index.js:

import express from "express";
import mongoose from "mongoose";

const app = express();
app.use(express.json());

mongoose.connect("mongodb://localhost:27017/products");

const ProductSchema = new mongoose.Schema({
  name: String,
  price: Number,
});

const Product = mongoose.model("Product", ProductSchema);

app.post("/products", async (req, res) => {
  const product = await Product.create(req.body);
  res.json(product);
});

app.listen(4002, () => console.log("Product Service running on port 4002"));

Each service has its own database and API — completely independent.

---

🛒 Step 4 — Order Service & Service Communication

In order-service/index.js:

import express from "express";
import mongoose from "mongoose";
import axios from "axios";

const app = express();
app.use(express.json());

mongoose.connect("mongodb://localhost:27017/orders");

const OrderSchema = new mongoose.Schema({
  userId: String,
  productId: String,
  status: String,
});

const Order = mongoose.model("Order", OrderSchema);

app.post("/orders", async (req, res) => {
  const { userId, productId } = req.body;

  // Communicate with other services
  const user = await axios.get(`http://localhost:4001/users/${userId}`);
  const product = await axios.get(`http://localhost:4002/products/${productId}`);

  const order = await Order.create({
    userId,
    productId,
    status: "Created",
  });

  res.json({
    message: "Order created successfully",
    order,
    user: user.data,
    product: product.data,
  });
});

app.listen(4003, () => console.log("Order Service running on port 4003"));

✅ Key concept:
Services communicate via HTTP. Later, you could upgrade this to message queues (RabbitMQ, Kafka) for better scalability.

---

🔗 Step 5 — Adding an API Gateway (Optional but Recommended)

Instead of calling each service directly, create an API Gateway.

gateway/index.js:

import express from "express";
import { createProxyMiddleware } from "http-proxy-middleware";

const app = express();

app.use("/users", createProxyMiddleware({ target: "http://localhost:4001", changeOrigin: true }));
app.use("/products", createProxyMiddleware({ target: "http://localhost:4002", changeOrigin: true }));
app.use("/orders", createProxyMiddleware({ target: "http://localhost:4003", changeOrigin: true }));

app.listen(4000, () => console.log("API Gateway running on port 4000"));

Now, your frontend can just call:

http://localhost:4000/users
http://localhost:4000/products
http://localhost:4000/orders

---

🐳 Step 6 — Dockerize Everything

Create a Dockerfile for each service:

FROM node:18
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
CMD ["npm", "start"]

Then a docker-compose.yml at the root:

version: "3"
services:
  user-service:
    build: ./user-service
    ports:
      - "4001:4001"
  product-service:
    build: ./product-service
    ports:
      - "4002:4002"
  order-service:
    build: ./order-service
    ports:
      - "4003:4003"
  gateway:
    build: ./gateway
    ports:
      - "4000:4000"

Run it all with:

docker compose up --build

---

🔒 Step 7 — Authentication & Security (Next Steps)

To make this production-ready:

• Add JWT authentication via a dedicated Auth service.

• Use service-to-service tokens for internal requests.

• Secure your databases and API Gateway.

---

📈 Step 8 — Testing the System

Use Postman or Thunder Client:

1. Create a user → POST /users

2. Create a product → POST /products

3. Create an order → POST /orders (include userId + productId)

4. Verify responses and linked data

You’ll see independent services working together like a small distributed system.

---

🧭 Conclusion

Congratulations — you just built a microservice architecture using Node.js and MongoDB! 🎉

You learned:

• How to separate services by domain

• Communicate using REST APIs

• Deploy with Docker Compose

• Use an API Gateway for unified routing

💡 Tip: For larger projects, explore message queues (RabbitMQ, Kafka), Kubernetes, and service discovery for even more scalable setups.

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✍️ Final Thoughts

Microservices aren’t always necessary — for small apps, monoliths are faster to build.
But if you’re planning for growth, scalability, or team collaboration, microservices give you that flexibility.

“Build small, deploy often, scale independently.”