Kubernetes Orchestration for Clojure Microservices

20.7.2 Orchestration with Kubernetes

As we delve into the world of microservices, one of the key challenges is managing the deployment, scaling, and operation of these services. Kubernetes, an open-source platform designed to automate deploying, scaling, and operating application containers, is a powerful tool for orchestrating microservices, including those written in Clojure. In this section, we’ll explore how to leverage Kubernetes to manage Clojure microservices effectively.

Understanding Kubernetes

Kubernetes, often abbreviated as K8s, is a container orchestration platform that provides a robust framework for running distributed systems resiliently. It takes care of scaling and failover for your application, provides deployment patterns, and more.

Key Concepts

• Pods: The smallest deployable units in Kubernetes, which can contain one or more containers.
• Nodes: Machines (physical or virtual) that run your applications and are managed by Kubernetes.
• Clusters: A set of nodes that run containerized applications managed by Kubernetes.
• Deployments: Define the desired state for your application, such as the number of replicas.
• Services: An abstraction that defines a logical set of Pods and a policy by which to access them.
• Ingress: Manages external access to the services, typically HTTP.

Setting Up a Kubernetes Cluster

Before deploying your Clojure microservices, you need a Kubernetes cluster. You can set up a local cluster using Minikube or use cloud providers like Google Kubernetes Engine (GKE), Amazon Elastic Kubernetes Service (EKS), or Azure Kubernetes Service (AKS).

Minikube Setup

Minikube is a tool that lets you run Kubernetes locally. It creates a virtual machine on your local machine and deploys a simple cluster containing only one node.

1. Install Minikube: Follow the official installation guide.
2. Start Minikube: Run minikube start to create a local Kubernetes cluster.
3. Verify Installation: Use kubectl get nodes to ensure your cluster is running.

Deploying Clojure Microservices

Let’s deploy a simple Clojure microservice to our Kubernetes cluster. We’ll use a basic Clojure application packaged as a Docker container.

Dockerizing a Clojure Application

First, we need to containerize our Clojure application using Docker. Here’s a simple Dockerfile for a Clojure application:

# Use the official Clojure image as a parent image
FROM clojure:openjdk-11-tools-deps

# Set the working directory
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Install the application dependencies
RUN clojure -M:deps

# Run the application
CMD ["clojure", "-M:run"]

Build the Docker image:

docker build -t clojure-app .

Creating a Kubernetes Deployment

A Deployment in Kubernetes is responsible for managing a set of identical Pods. It ensures that the desired number of Pods are running and can update them in a controlled manner.

Here’s a YAML file for a Kubernetes Deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: clojure-app-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: clojure-app
  template:
    metadata:
      labels:
        app: clojure-app
    spec:
      containers:
      - name: clojure-app
        image: clojure-app:latest
        ports:
        - containerPort: 8080

Apply the Deployment:

kubectl apply -f clojure-app-deployment.yaml

Exposing the Application with a Service

To make your application accessible, you need to define a Service. A Service in Kubernetes is an abstraction that defines a logical set of Pods and a policy by which to access them.

Here’s a YAML file for a Kubernetes Service:

apiVersion: v1
kind: Service
metadata:
  name: clojure-app-service
spec:
  selector:
    app: clojure-app
  ports:
  - protocol: TCP
    port: 80
    targetPort: 8080
  type: LoadBalancer

Apply the Service:

kubectl apply -f clojure-app-service.yaml

Managing External Access with Ingress

Ingress in Kubernetes manages external access to services, typically HTTP. It provides load balancing, SSL termination, and name-based virtual hosting.

Here’s a YAML file for a Kubernetes Ingress:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: clojure-app-ingress
spec:
  rules:
  - host: clojure-app.local
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: clojure-app-service
            port:
              number: 80

Apply the Ingress:

kubectl apply -f clojure-app-ingress.yaml

Monitoring and Scaling

Kubernetes provides built-in tools for monitoring and scaling your applications. You can use Horizontal Pod Autoscaler to automatically scale the number of Pods in a deployment based on observed CPU utilization or other select metrics.

Horizontal Pod Autoscaler

Here’s how you can define a Horizontal Pod Autoscaler:

apiVersion: autoscaling/v1
kind: HorizontalPodAutoscaler
metadata:
  name: clojure-app-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: clojure-app-deployment
  minReplicas: 1
  maxReplicas: 10
  targetCPUUtilizationPercentage: 50

Apply the Autoscaler:

kubectl apply -f clojure-app-hpa.yaml

Best Practices for Kubernetes Orchestration

• Use Namespaces: Organize your resources using namespaces to avoid conflicts and manage resources efficiently.
• Resource Requests and Limits: Define resource requests and limits for your containers to ensure fair resource allocation and avoid resource contention.
• ConfigMaps and Secrets: Use ConfigMaps and Secrets to manage configuration and sensitive data.
• Network Policies: Implement network policies to control traffic flow between Pods.

Try It Yourself

Experiment with the Kubernetes setup by modifying the number of replicas in the Deployment or changing the resource limits. Observe how Kubernetes handles these changes seamlessly.

Summary

Kubernetes provides a powerful platform for orchestrating Clojure microservices, offering features like automated deployment, scaling, and management. By leveraging Kubernetes, you can ensure your applications are resilient, scalable, and easy to manage.

Further Reading

• Kubernetes Official Documentation
• Clojure Official Documentation
• Docker Official Documentation

Exercises

1. Deploy a Clojure microservice with a different configuration and observe the behavior.
2. Implement a Horizontal Pod Autoscaler for a different metric, such as memory usage.
3. Set up a monitoring solution using Prometheus and Grafana to visualize metrics from your Clojure microservices.

Kubernetes Orchestration Quiz for Clojure Microservices