How to Decompose Monolith to Microservices: Step-by-Step Guide

To decompose a monolith into microservices, start by identifying distinct business domains or modules within the monolith. Then, extract these modules into independent services with clear APIs, ensuring each service owns its data and can be deployed separately.

Syntax

Decomposing a monolith involves these key steps:

• Identify Domains: Find logical parts of the app that can work independently.
• Define Service Boundaries: Decide what each microservice will do.
• Extract Code: Move code and data related to each domain into separate services.
• Establish Communication: Use APIs or messaging for services to interact.
• Deploy Independently: Each service runs and scales on its own.

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Step 1: Identify domains
Step 2: Define service boundaries
Step 3: Extract code and data
Step 4: Setup communication (REST, messaging)
Step 5: Deploy services independently

Example

This example shows a simple monolith decomposed into two microservices: UserService and OrderService. Each service has its own API and database.

python

from flask import Flask, jsonify, request

# UserService
user_app = Flask('UserService')
users = {1: {'name': 'Alice'}, 2: {'name': 'Bob'}}

@user_app.route('/users/<int:user_id>')
def get_user(user_id):
    user = users.get(user_id)
    if user:
        return jsonify(user)
    return jsonify({'error': 'User not found'}), 404

# OrderService
order_app = Flask('OrderService')
orders = {1: {'user_id': 1, 'item': 'Book'}, 2: {'user_id': 2, 'item': 'Pen'}}

@order_app.route('/orders/<int:order_id>')
def get_order(order_id):
    order = orders.get(order_id)
    if order:
        return jsonify(order)
    return jsonify({'error': 'Order not found'}), 404

if __name__ == '__main__':
    from threading import Thread
    Thread(target=lambda: user_app.run(port=5001)).start()
    Thread(target=lambda: order_app.run(port=5002)).start()

Output

Running two Flask apps on ports 5001 (UserService) and 5002 (OrderService). Access /users/1 returns {"name": "Alice"}, /orders/1 returns {"user_id": 1, "item": "Book"}.

Common Pitfalls

Common mistakes when decomposing monoliths include:

• Breaking services too small: Leads to complex communication and overhead.
• Sharing database between services: Causes tight coupling and data conflicts.
• Ignoring domain boundaries: Results in unclear service responsibilities.
• Lack of automation: Manual deployment slows down releases.

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Wrong approach:
# Two services sharing one database table
# Leads to coupling and data issues

Right approach:
# Each service owns its own database
# Communicate via APIs or events

Quick Reference

Summary tips for decomposing monoliths:

• Start small: Extract one service at a time.
• Use domain-driven design to find boundaries.
• Keep services loosely coupled with clear APIs.
• Automate testing and deployment pipelines.
• Monitor services independently for issues.

Key Takeaways

Identify clear business domains before splitting the monolith.

Each microservice should own its data and run independently.

Avoid sharing databases between services to reduce coupling.

Use APIs or messaging for service communication.

Automate deployment and monitoring for smooth operation.