Implementing the Sidecar Pattern in .NET Core

sidecar service handle tasks like logging, monitoring, or any cross-cutting concerns

Microservices architecture has revolutionized the way we design scalable, maintainable, and resilient applications. One design pattern that stands out in this space is the Sidecar Pattern. In this blog, we’ll dive into how you can implement the Sidecar Pattern using .NET Core, bringing in an Item API as an example.

The goal is to have a sidecar service handle tasks like logging, monitoring, or any cross-cutting concerns, allowing the main service to stay clean and focused. Let’s make this fun and interactive!

1. What Exactly is the Sidecar Pattern?

Imagine you’re running a main service, say an Item API, and you want to offload some of its extra responsibilities. Instead of clogging the API with logging, monitoring, or even service discovery logic, you add a helper: a sidecar service. This sidecar runs alongside the main service, sharing the same resources (like network, storage), but operates independently to perform tasks that support the main service.

Interactive Thought:

Think of the sidecar as a wingman in a flight operation. The wingman ensures smooth functioning by handling secondary tasks, while the lead pilot (main service) focuses on the mission!

2. Why the Sidecar Pattern?

• Decoupling Concerns: Keep your business logic separate from operational concerns like logging or metrics.

• Modular Design: Your sidecar can be reused by different services.

• Scalability: Scale your main service independently from the sidecar.

3. Building an Item API with a Sidecar in .NET Core

Let’s build our Item API and a logging sidecar. The Item API will handle requests for fetching items, while the sidecar will log every request to a shared log file.

Step 1: Create the Item API (Main Service)

We’ll start with a basic Item API that has endpoints to retrieve item details.

[ApiController]
[Route("api/[controller]")]
public class ItemController : ControllerBase
{
    [HttpGet("{id}")]
    public IActionResult GetItem(int id)
    {
        var item = new Item { Id = id, Name = $"Item {id}" };
        
        // Log the request (log file will be shared with sidecar)
        Console.WriteLine($"Item {id} retrieved at {DateTime.Now}");
        
        return Ok(item);
    }
}

public class Item
{
    public int Id { get; set; }
    public string Name { get; set; }
}

This API does the heavy lifting of managing item retrieval. Whenever an item is fetched, it writes a log entry to the console. Our sidecar will take care of these logs.

4. Sidecar Service: Logging

The sidecar will be a simple service that reads the logs from the main service and forwards them to a centralized logging system.

Step 2: Create the Logging Sidecar

In this case, the sidecar will be a small console application that reads from a shared log file and sends the logs to a central location.

public class LoggingSidecar
{
    private const string LogFilePath = "/app/logs/item-api-log.txt";
    
    public static void Main(string[] args)
    {
        Console.WriteLine("Starting Sidecar...");

        while (true)
        {
            if (File.Exists(LogFilePath))
            {
                var logContent = File.ReadAllText(LogFilePath);
                if (!string.IsNullOrEmpty(logContent))
                {
                    // Send logs to a central system or simply output them
                    Console.WriteLine($"Forwarding logs: {logContent}");
                    
                    // Clear the log file after forwarding
                    File.WriteAllText(LogFilePath, string.Empty);
                }
            }
            Thread.Sleep(5000); // Check every 5 seconds
        }
    }
}

This small sidecar service reads logs from a file that’s shared with the main service. Every 5 seconds, it checks the log file, reads its content, forwards it, and clears the file.

5. Kubernetes: The Magic Pod

When running these two services (the Item API and the logging sidecar), you want them to share the same resources but run as separate processes. Kubernetes Pods are perfect for this!

Step 3: Deploying to Kubernetes

We will create a Kubernetes Pod that runs both containers—the Item API and the logging sidecar.

apiVersion: v1
kind: Pod
metadata:
  name: item-api-pod
spec:
  containers:
    - name: item-api
      image: item-api-image
      volumeMounts:
        - name: log-volume
          mountPath: /app/logs
    - name: logging-sidecar
      image: logging-sidecar-image
      volumeMounts:
        - name: log-volume
          mountPath: /app/logs
  volumes:
    - name: log-volume
      emptyDir: {}

Here, both the Item API and the logging sidecar share the same log directory (/app/logs). The logs generated by the Item API will be stored in a file, and the sidecar will read from that file.

6. Why is This Awesome?

• Independent Scaling: If logging becomes heavy, you can scale the sidecar without affecting the main service.

• Loose Coupling: The main service doesn’t even know the sidecar exists, and vice versa. They work independently but share data.

• Improved Maintainability: You can update the sidecar without touching the main service code, and vice versa.

Interactive Insight:

Think about it—what if you wanted to add more functionality like metrics collection? You could spin up another sidecar for monitoring without touching your core business logic!

7. Wrapping Up

The Sidecar Pattern is a brilliant way to decouple cross-cutting concerns from the main service, making your microservices architecture more modular, scalable, and maintainable. By running a sidecar like we did with logging, you can let your main service focus solely on business logic.

In this example, we used .NET Core to create a simple Item API and a sidecar logging service. Then, we deployed both services in the same Kubernetes pod, allowing them to share a log file.

Have you used the Sidecar Pattern in your projects? What other responsibilities would you offload to a sidecar service? Let’s discuss!

With this hands-on example, you're ready to start decoupling your services using the Sidecar Pattern. Try it out in your microservices architecture and see how it simplifies your development workflow!