Complete Guide to Kubernetes Load Balancer - Types, Examples & Best Practices

June 2, 2024

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Priyansh Khodiyar

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In this guide, we will talk about Kubernetes Load Balancer, its types, examples, best practices, and more.

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A Kubernetes LoadBalancer is a service that provides external network access to a set of pods running within a Kubernetes cluster. Kubernetes allows you to manage and scale containerized applications efficiently.

Still, when you have multiple instances of your application (pods) running within a cluster, you often need a way to distribute incoming network traffic across these pods to ensure high availability, load distribution, and reliability.

This is where a LoadBalancer service comes into play.

What is Kubernetes Load Balancer?

A LoadBalancer service exposes your application to the external world, allowing traffic from the internet or other networks to reach your application's pods. It evenly distributes incoming network traffic to the pods that make up the service, ensuring that no single pod becomes overwhelmed with requests.

LoadBalancers provide redundancy and failover support. If one node in the cluster fails, traffic is redirected to healthy nodes, ensuring that your application remains available.

They are typically dynamic and can adapt to changes in your cluster. When you scale the number of pods up or down, the LoadBalancer updates its routing accordingly.

Types of Load Balancers in Kubernetes & When Should You Use Them

In Kubernetes, there are two primary types of Load Balancers:

External Load Balancers
Internal Load Balancers

Each of them is used for different purposes and is used in various scenarios based on your application's architecture and requirements.

External Load Balancers

External Load Balancers are used when you want to expose your Kubernetes services to external, public network traffic, typically from the Internet.

They distribute incoming traffic from external clients to the pods running within your cluster.

Use Cases of External Load Balancers

Public-Facing Applications: When you have web applications, APIs, or services that must be accessible to users or clients over the internet.
High Availability: To ensure high availability and fault tolerance by distributing traffic among multiple pods.
E-commerce Websites: For applications where reliability and scalability are crucial, such as e-commerce platforms.
Content Delivery: To distribute content like media files, images, or videos across multiple pods.

For instance, an e-commerce website uses an External Load Balancer to distribute incoming web traffic to multiple web server pods for scalability and reliability.

Use an External Load Balancer when you have services that need to be accessible from the public internet. This is appropriate for applications where external users or clients need to reach your services.

Examples include web applications, APIs, public-facing services, and any system that needs to handle external traffic.

Internal Load Balancers

Internal Load Balancers are used to route network traffic within a Kubernetes cluster or between different parts of a microservices application.

They do not expose services to external clients but are designed for communication within the cluster.

Use Cases of Internal Load Balancers

Microservices Communication: When different microservices or components of your application need to communicate with each other, and you want to ensure load balancing among them.
Internal APIs: For managing and routing traffic between various parts of a complex application, like databases, message queues, and microservices.
Security: To isolate and secure communication between internal components of your application.

For example, consider a multi-tiered application where an Internal Load Balancer is used to distribute traffic between the frontend and backend components or to balance traffic between database replicas.

Use an Internal Load Balancer when you need to balance traffic within your cluster, but do not want to expose your services to external clients.

This is suitable for applications with complex microservices architectures.

Examples include microservices-based applications, internal APIs, and communication between different parts of a multi-tiered application.

Configuring Load Balancer in Kubernetes

Installing Load Balancer typically involves setting up the necessary components and services to ensure that network traffic is efficiently distributed to your application's pods.

Here's a detailed guide on how to configure it on Kubernetes.

1. Create a Kubernetes Deployment

Before you configure a Load Balancer, you need to have some of your applications running as pods.

You can create pods using a Kubernetes Deployment YAML manifest, which manages the desired number of replicas of your application.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: lb-app-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: lb-app
  template:
    metadata:
      labels:
        app: lb-app
    spec:
      containers:
      - name: lb-app-container
        image: lbapp-image:latest

Deploy this configuration using

kubectl apply -f deployment.yaml

2. Create a Kubernetes Service

Next, we can create a Kubernetes Service to manage the network traffic to your pods. This is necessary for load balancing as the Load Balancer is a networking component.

Depending on your needs, you can use either the LoadBalancer or NodePort type.

For an external Load Balancer, use the LoadBalancer type.

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

The type LoadBalancer tells Kubernetes to request a Load Balancer from your cloud provider.

Deploy this service configuration using

kubectl apply -f service.yaml

3. Wait for Load Balancer Provisioning

Once you deploy the LoadBalancer service, Kubernetes communicates with your cloud provider to provision the actual Load Balancer.

This process might take some time, depending on your provider. You can check them in the Kubernetes master components section.

4. Check Load Balancer Status

To check the status of your Load Balancer and get the external IP or hostname assigned to it, you can use kubectl get services.

Wait until the EXTERNAL-IP field is populated, indicating that the Load Balancer is ready to route traffic.

kubectl get services lb-app-service

5. Access Your Application

Now that your Load Balancer is provisioned and traffic is being distributed to your pods, you can access your application using the external IP or hostname assigned to the Load Balancer.

6. Scaling and Maintenance

You can scale your application by adjusting the number of replicas in your Deployment.

Use kubectl scale deployment my-app-deployment --replicas=5 to scale to five replicas, for example.

Maintenance and updates to your application can be done by updating the Deployment to use a new container image.

After the update, Kubernetes can perform rolling updates to ensure zero-downtime deployments.

7. Monitoring and Autoscaling

You can set up monitoring and autoscaling based on traffic or resource utilization to ensure that your application can handle varying loads efficiently.

Kubernetes supports Horizontal Pod Autoscaling (HPA) for this purpose.

Follow the above steps properly as it is essential to accurately configure and manage your Load Balancer to ensure that your application remains highly available and scalable.

Traffic Distribution Strategies - Load Balancer in Kubernetes

Load balancers use traffic distribution strategies to evenly distribute network traffic to application pods. These strategies ensure high availability and reliability within your application.

Here is a list of methods that you can go through to understand how the traffic is distributed within a Load Balancer with a complex infrastructure.

1. Round Robin

In a round-robin strategy, the load balancer distributes incoming requests sequentially to each backend pod in the rotation. Once a pod receives a request, it goes to the end of the queue, and the next request is sent to the next pod in the list.

This strategy is straightforward but may not be ideal for all scenarios, especially if your pods have varying capacities or resource utilization.

2. Least Connections

The Least Connections strategy routes traffic to the pod with the fewest active connections. It aims to distribute traffic based on the current load on the pods.

Pods with lower connection counts receive more traffic until they become more heavily loaded.

This strategy can be more efficient than Round Robin, especially when pods have different loads.

3. IP Hash

In an IP Hash strategy, the load balancer computes a hash based on the source IP address of the incoming request. It then maps this hash to a specific pod.

This approach can be useful when you want to maintain session persistence, ensuring that requests from the same client always go to the same pod. It's commonly used for stateful applications.

4. Weighted Traffic

Weighted traffic distribution allows you to assign different weights to different pods. The load balancer then uses these weights to determine how traffic should be distributed. Pods with higher weights receive more traffic.

Weighted traffic distribution is valuable when you have pods with varying capacities or resources. You can allocate more traffic to pods with more resources.

5. Session Affinity (Sticky Sessions)

Also known as sticky sessions, ensures that all requests from a specific client are directed to the same pod. This is achieved by associating a session identifier or cookie with a particular pod.

It is useful for applications that require stateful communication, such as those using web sessions or user logins. Session affinity can help maintain the user's session state across multiple requests.

6. Path-Based Routing (Ingress Controllers)

Path-based routing is a strategy used with Ingress controllers in Kubernetes. It routes traffic based on the path of the URL in the HTTP request. Different paths can be mapped to specific services or pods.

This is helpful when you want to create microservices-based applications with different services handling different parts of your application based on the URL path.

The choice of a traffic distribution strategy in Kubernetes depends on your application's needs, such as its architecture, scalability requirements, and the nature of the traffic it receives.

Understanding these strategies and selecting the most appropriate one is essential for optimizing the performance, reliability, and scalability of your Kubernetes-based applications.

Best Practices for Kubernetes Load Balancer

1. Implement health checks

Implement health checks to ensure that the Load Balancer directs traffic only to healthy pods. Health checks regularly monitor the status of your pods and exclude unhealthy ones from receiving traffic.

Example: Define a health check probe in your Kubernetes Service configuration.

apiVersion: v1
kind: Service
metadata:
  name: my-app-service
spec:
  selector:
    app: my-app
  ports:
  - protocol: TCP
    port: 80
    targetPort: 80
  type: LoadBalancer
  healthCheckNodePort: 30000
  healthCheckPath: /healthz

2. Use Session Affinity

Use session affinity (sticky sessions) when your application requires maintaining a user session state across multiple requests. This ensures that a user's requests go to the same pod.

Example: Enable session affinity in your Kubernetes Service configuration.

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

3. Set Resource Limits

Set resource limits and requests for your pods to ensure they don't consume all available resources. This prevents overloading pods and degrades performance.

Example: Define resource limits and requests in your Deployment configuration.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app-deployment
spec:
  replicas: 3
  template:
    spec:
      containers:
      - name: my-app-container
        image: my-app-image:latest
        resources:
          requests:
            memory: "64Mi"
            cpu: "250m"
          limits:
            memory: "128Mi"
            cpu: "500m"

4. Security & RBAC

Configure proper security and access control for your Load Balancer, services, and pods using Kubernetes RBAC and network policies.

Summary of Kubernetes Load Balancer

In conclusion, load balancers play a pivotal role in modern application deployment and scalability. They act as traffic guardians, efficiently distributing requests to ensure high availability, fault tolerance, and optimal resource utilization.

Implementing the best practices outlined here, such as health checks, session persistence, resource management, scaling, monitoring, and security measures, can help you harness the full potential of load balancers within your Kubernetes environment.

By adhering to these practices, you can build and maintain resilient, high-performing applications that meet the demands of today's dynamic and ever-evolving digital landscape.