Services in Kubernetes

In Kubernetes, a Service is an abstraction that defines a logical set of Pods and a policy to access them. Services  provide a stable endpoint to applications  running on Pods, even if the underlying Pods are dynamically created or destroyed.

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Key Concepts

1. Purpose of Services:

    - Pods in Kubernetes are ephemeral, meaning their IPs can change if the Pod is restarted. Services provide a consistent way to access Pods.

    - They enable communication between components of an application (e.g., frontend talking to backend).

    - They provide load balancing across multiple Pods.

2. Service Selector:

     - The set of Pods targeted by a Service is usually determined by a selector that you define

    - A Service uses selectors to identify the Pods it routes traffic to, based on Pod labels.

3. ClusterIP and Endpoints:

    - When you create a Service, Kubernetes assigns a ClusterIP to the Service, which acts as a virtual IP inside the cluster.

    - The Service points to a set of Endpoints, which are the IPs of the Pods matching the selector.

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There are 3 types of services mainly used in k8s :

a. ClusterIP (Default)-

• Exposes the Service on a cluster-internal IP. Choosing this value makes the Service only reachable from within the cluster.

• What it does:

    - Exposes the Service internally within the Kubernetes cluster.

    - Only accessible from within the cluster (e.g., by other pods).

• Use cases:

    - For internal communication between microservices.

    - Backend services like databases or caches that do not need to be exposed to the internet.

b. NodePort -(Not safe cause we expose the port)

• Exposes the Service on each Node's IP at a static port (the NodePort). To make the node port available

• What it does:

    - Exposes the Service externally on a static port (between 30000--32767) on each node in the cluster.

    - Accessible using <NodeIP>:<NodePort>.

• Use cases:

    - For development or debugging purposes where direct node access is acceptable.

    - For exposing services in environments without a LoadBalancer.

C. LoadBalancer-

• On cloud providers which support external load balancers, setting the type field to LoadBalancer provisions a load balancer for your Service.(if you have their services like EKS,GKE)

-  cloud-controller-manager component then configures the external load balancer to forward traffic to that assigned node port.

• What it does:

    - Exposes the Service externally using a cloud provider's load balancer (e.g., AWS ELB, Google Cloud LB).(for automatic load balancer -EKS )

    - Automatically provisions a load balancer and assigns an external IP.

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• Use cases:

    - For production environments where external traffic needs to reach the application.

    - Scenarios requiring high availability and load distribution.

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3. Core Concepts of Kubernetes Services

• Selectors: Define the pods the Service routes traffic to, based on labels.

• Endpoints: Represents the set of IPs and ports for the pods selected by the Service.

• TargetPort: The port on the pod where the Service forwards traffic.

• ClusterIP: Internal IP assigned to the Service for communication

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In Kubernetes, a Service provides a stable endpoint (IP address and port) to access a set of pods. A Service routes traffic to the correct pods based on labels and selectors, and the port configuration plays a key role in how traffic is handled within the cluster.  The ports in a Kubernetes Service configuration define the communication between different components.

Key Ports in Kubernetes Service

1. port

2. targetPort

3. nodePort (for NodePort services)

4. protocol

5. name

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1. port

• Definition: This is the port on which the Service is exposed within the Kubernetes cluster.

• Usage: The port is the port that other services or pods in the cluster use to access the Service.

• Example: If you define port: 80, any service or pod that communicates with your Service would connect to port 80.

• `ports:

     port: 80  # Exposed port inside the cluster`

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2. targetPort

• Definition: This is the port on the pods that the Service forwards traffic to. This is where your application is listening.(Expose used in docker image)

• Usage: The targetPort should match the port the container inside the pod is listening on. If this is not specified, it defaults to the same value as the port.

• Example: If you set targetPort: 8080, Kubernetes will route traffic that hits port 80 to port 8080 inside the pods.

• ports:

     port: 80       # Exposed port on the service

     targetPort: 8080  # Port where the pod is actually listening

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3. nodePort (only for NodePort Service type)

• Definition: This is the port on each node in the cluster that exposes the service externally (outside the Kubernetes cluster).(port of your host)

• Usage: The nodePort is used in NodePort type services. It allows you to access the service from outside the Kubernetes cluster by connecting to nodeIP:nodePort.

• Port Range: By default, the valid range for nodePort is from 30000 to 32767, but you can specify a custom range if needed.

• Example: If you set nodePort: 30080, traffic hitting port 30080 on any node's external IP will be forwarded to the service.

-  ports:

     port: 80         # service port (port on which service is exposed)

     targetPort: 8080  # To this port service forward traffic (application port)

     nodePort: 30080   # Port on each node forward's traffic to target port