Microservices in 12 steps: A Short Guide with Docker for Monolithic Code Base Migrations (with some code)
Tags: webdev, devops, microservices, programming
Published: 2023-04-25 17:14:20

## Intro

Modern software development practices are shifting towards microservices architecture, which offers benefits such as improved scalability, flexibility, and maintainability. If you have a monolithic code base and are looking to embrace microservices, Docker can be a powerful tool to help with the migration process. 

In this article, we will provide you with a step-by-step guide on how to migrate a monolithic code base to a microservices architecture using Docker. With Docker's containerization capabilities, you can encapsulate individual microservices, ensure consistency in packaging, and achieve portability across different environments. So, let's dive in and learn how to make this transition in a structured and efficient manner.

## Step 1: Understand the Current Monolithic Code Base

Gain a deep understanding of the existing monolithic code base, its architecture, dependencies, and functionalities. Identify the different components or modules that can be decoupled and isolated as microservices.

## Step 2: Define a Microservices Architecture

Design the target microservices architecture, including the desired granularity of microservices, communication patterns, data management, and deployment strategies. This includes defining the boundaries and interfaces of each microservice.

Example, including defining the boundaries and interfaces of each microservice:

### 1. Defining the Microservice Boundaries and Interfaces:

```python
# Example of defining the boundaries and interfaces of a User microservice in Python using FastAPI

# user.py - User microservice

from fastapi import FastAPI

app = FastAPI()

@app.get("/users/{user_id}")
async def get_user(user_id: int):
    # Logic to fetch user data from database
    return {"user_id": user_id, "name": "John", "age": 30}

@app.post("/users")
async def create_user(user_data: dict):
    # Logic to create a new user in the database
    return {"user_id": 1, "name": user_data["name"], "age": user_data["age"]}
```

### 2. Communication Patterns between Microservices:

```python
# Example of communication patterns between User and Order microservices using RESTful APIs in Node.js with Express

// user-service.js - User microservice

const express = require('express');
const app = express();

app.get('/users/:userId', (req, res) => {
  // Logic to fetch user data from database
  res.json({ userId: req.params.userId, name: 'John', age: 30 });
});

// order-service.js - Order microservice

const express = require('express');
const app = express();

app.post('/orders', (req, res) => {
  // Logic to create a new order in the database
  // and communicate with User microservice to fetch user data
  // Example of making API call to User microservice
  axios.get(`http://user-service/users/${req.body.userId}`)
    .then(response => {
      // Process user data and create order
      res.json({ orderId: 1, userId: req.body.userId, productName: req.body.productName });
    })
    .catch(error => {
      // Handle error
      res.status(500).json({ error: 'Failed to create order' });
    });
});
```

### 3. Data Management in Microservices:

```python
// Example of data management in microservices using a message queue (RabbitMQ) in Java with Spring Boot

// user-service - User microservice

@RestController
public class UserController {

  @Autowired
  private UserService userService;

  @GetMapping("/users/{userId}")
  public User getUser(@PathVariable("userId") int userId) {
    // Logic to fetch user data from database
    return userService.getUserById(userId);
  }
}

// order-service - Order microservice

@RestController
public class OrderController {

  @Autowired
  private OrderService orderService;

  @Autowired
  private RabbitTemplate rabbitTemplate;

  @PostMapping("/orders")
  public Order createOrder(@RequestBody OrderDto orderDto) {
    // Logic to create a new order in the database
    // Publish order data to RabbitMQ message queue for processing
    rabbitTemplate.convertAndSend("order-exchange", "order.create", orderDto);
    return orderService.createOrder(orderDto);
  }
}

@Component
public class OrderConsumer {

  @RabbitListener(queues = "order-queue")
  public void processOrder(OrderDto orderDto) {
    // Logic to process order data, communicate with User microservice, and create order
  }
}
```


## Step 3: Containerize Microservices with Docker
Use Docker to containerize the microservices. Create Docker images for each microservice, which encapsulate the application code, runtime dependencies, and configuration. Docker containers provide consistency in packaging and portability across different environments.

Example:

```
# Example of deployment strategy using Docker Compose for a microservices architecture with multiple services

version: '3'
services:
  user-service:
    build: ./user-service
    ports:
      - "8001:8001"
    networks:
      - my-network
  order
```

## Step 4: Set Up Docker Infrastructure

Set up the Docker infrastructure, including Docker Engine, Docker Compose, and Kubernetes, depending on the desired deployment approach. Docker Compose can be used for local development and testing, while Kubernetes can be used for container orchestration in a production environment.

Examples:

### 1. Docker Engine setup:

```bash
# Example of installing Docker Engine on Ubuntu using Docker's official installation script

# Update package index
sudo apt update

# Install dependencies
sudo apt install apt-transport-https ca-certificates curl software-properties-common

# Add Docker repository
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /etc/apt/trusted.gpg.d/docker.gpg
sudo add-apt-repository "deb [arch=$(dpkg --print-architecture signed-by /etc/apt/trusted.gpg.d/docker.gpg)] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

# Update package index again
sudo apt update

# Install Docker
sudo apt install docker-ce docker-ce-cli containerd.io

# Start and enable Docker service
sudo systemctl start docker
sudo systemctl enable docker

# Verify Docker installation
docker --version
```

### 2. Docker Compose setup:

```yaml
# Example of Docker Compose configuration for local development and testing

version: '3'
services:
  user-service:
    build: ./user-service
    ports:
      - "8001:8001"
    networks:
      - my-network
  order-service:
    build: ./order-service
    ports:
      - "8002:8002"
    networks:
      - my-network

networks:
  my-network:
```

### 3. Kubernetes setup:

```bash
# Example of installing Kubernetes using Minikube for container orchestration

# Install dependencies
sudo apt update
sudo apt install curl

# Install kubectl
sudo snap install kubectl --classic

# Install Minikube
curl -LO https://storage.googleapis.com/minikube/releases/latest/minikube-linux-amd64
sudo install minikube-linux-amd64 /usr/local/bin/minikube

# Start Minikube cluster
minikube start

# Verify Kubernetes installation
kubectl version
```

Detailed setup instructions for Docker Engine, Docker Compose, and Kubernetes may vary depending on the operating system and environment you are using. Please refer to official documentation for accurate installation steps.

## Step 5: Develop and Test Microservices

Refactor and rewrite the monolithic code into individual microservices. Develop and test each microservice in isolation using Docker containers to ensure they are functioning correctly and communicate effectively with each other.

## Step 6: Implement Service Discovery

Implement a service discovery mechanism to allow microservices to discover and communicate with each other dynamically. Tools such as Consul, etcd, or Kubernete's built-in DNS-based service discovery can be used for this purpose.

Examples:

### 1. Refactor and rewrite monolithic code into microservices:

Example of monolithic code before refactoring:
```python
# Monolithic code for an e-commerce application

def process_order(order):
    # Process order logic here
    # ...
    return result

def get_customer_info(customer_id):
    # Get customer info logic here
    # ...
    return customer_info

def calculate_shipping_cost(order):
    # Calculate shipping cost logic here
    # ...
    return shipping_cost

# Other functionalities...
```

Example of microservices after refactoring:
```python
# Microservice 1: Order Service

def process_order(order):
    # Process order logic here
    # ...
    return result

# Other functionalities specific to order service...

# Microservice 2: Customer Service

def get_customer_info(customer_id):
    # Get customer info logic here
    # ...
    return customer_info

# Other functionalities specific to customer service...

# Microservice 3: Shipping Service

def calculate_shipping_cost(order):
    # Calculate shipping cost logic here
    # ...
    return shipping_cost

# Other functionalities specific to shipping service...

# Each microservice is a separate module or application that can be developed, tested, and deployed independently.
```

### 2. Develop and test each microservice using Docker containers:

Example of Dockerfile for a microservice:
```Dockerfile
# Dockerfile for Order Service microservice

# Use a base image
FROM python:3.9

# Set working directory
WORKDIR /app

# Copy source code into the container
COPY . .

# Install dependencies
RUN pip install --no-cache-dir -r requirements.txt

# Expose necessary ports
EXPOSE 8001

# Run the microservice
CMD ["python", "app.py"]
```

Example of Docker Compose configuration for local development and testing:

```yaml
# Docker Compose configuration for local development and testing

version: '3'
services:
  order-service:
    build: ./order-service
    ports:
      - "8001:8001"
    networks:
      - my-network

# Other services and networks...
```

The above examples are simplified and may not cover all aspects of developing and testing microservices with Docker.

## Step 7: Set Up Logging and Monitoring

Implement logging and monitoring mechanisms to collect and analyze logs, metrics, and traces from microservices. Tools such as ELK Stack (Elasticsearch, Logstash, and Kibana), Prometheus, or Zipkin can be used for this purpose.

Examples:

### 1. Implement logging using ELK Stack (Elasticsearch, Logstash, and Kibana):

Example of logging configuration in a microservice using Logstash:
```yaml
# Logstash configuration file

input {
  beats {
    port => 5044
  }
}

filter {
  # Filter logic here
}

output {
  elasticsearch {
    hosts => ["elasticsearch:9200"]
    index => "my-microservice-%{+YYYY.MM.dd}"
  }
}
```

Example of logging code in a microservice using a logging library like Log4j2 (Java):
```java
// Log4j2 logging code in a Java microservice

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class MyMicroservice {
  private static final Logger LOGGER = LogManager.getLogger(MyMicroservice.class);

  public void processOrder(Order order) {
    // Process order logic here

    // Log an info level message
    LOGGER.info("Order processed successfully: {}", order);
  }
}
```

### 2. Implement monitoring using Prometheus:

Example of monitoring code in a microservice using Prometheus client library (Python):

```python
# Prometheus monitoring code in a Python microservice

from prometheus_client import Counter, start_http_server

# Define a counter for tracking order requests
ORDER_REQUESTS = Counter('order_requests_total', 'Total number of order requests')

def process_order(order):
    # Process order logic here

    # Increment the order requests counter
    ORDER_REQUESTS.inc()

    # ...
```

### 3. Implement distributed tracing using Zipkin:

Example of distributed tracing code in a microservice using OpenZipkin library (Java):

```java
// Zipkin distributed tracing code in a Java microservice

import brave.Span;
import brave.Tracer;

public class MyMicroservice {
  private final Tracer tracer;

  public MyMicroservice(Tracer tracer) {
    this.tracer = tracer;
  }

  public void processOrder(Order order) {
    // Process order logic here

    // Start a new Zipkin span
    Span span = tracer.newTrace().name("processOrder").start();

    try {
      // ... Processing logic ...
    } finally {
      // End the Zipkin span
      span.finish();
    }
  }
}
```

Actual implementation may vary depending on the specific tools, libraries, and frameworks used in your microservices architecture.

## Step 8: Implement Deployment and Scaling Strategies

Define and implement deployment and scaling strategies for microservices using Kubernete, such as rolling updates, blue-green deployments, or canary releases. This allows for seamless deployment and scaling of microservices in a distributed environment.

## Step 9: Implement Fault Tolerance and Resiliency

Implement fault tolerance and resiliency measures in microservices to handle failures gracefully. This may include retry mechanisms, circuit breakers, and fallback strategies to ensure the overall system's reliability.

## Step 10: Monitor and Optimize

Continuously monitor and optimize the microservices architecture using Docker monitoring and logging tools, and make adjustments as needed to improve performance, scalability, and reliability.

## Step 11: Gradual Rollout

Plan for a gradual rollout of microservices in production, starting with less critical services and gradually moving towards more critical ones. Monitor the system's performance and stability during the rollout, and make necessary adjustments as needed.

## Step 12: Train and Educate Teams

Provide training and education to development and operations teams on Docker, microservices, and the new architecture. Ensure that teams are proficient in using Docker and understand the best practices for developing, deploying, and managing microservices.

## Conclusion:

Migrating from a monolithic code base to a microservices architecture using Docker can be a challenging but rewarding journey.

By following the step-by-step guide provided in this article, you can effectively decouple dependencies, containerize microservices, implement service discovery, and scale your applications with Kubernetes.

It's important to carefully plan, test, and monitor the migration process to ensure a smooth transition. With the right approach, Docker can be a valuable tool to enable the adoption of microservices architecture and unlock the benefits of improved scalability, flexibility, and maintainability in your software development practices. So, get started with Docker and embark on the path towards [a modern and efficient microservices architecture](https://www.squash.io/mastering-microservices-a-comprehensive-guide-to-building-scalable-and-agile-applications/) for your applications.