Kubernetes : What is Internal and External services in Kubernetes ? Is service attached to a pod?

 What is Internal and External services in Kubernetes ? Is service attached to a pod? Is same Service acts as a routing when there are multiple nodes and traffic is routed to node depending on the traffic to a specific node? Is this like a load balancer? If the services acts as load balancer, then what is role of Load balancers provided by cloud providers ? 

Let’s break it down clearly:

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🔹 Internal vs External Services in Kubernetes

• Internal Service (ClusterIP)

• Default type.

• Exposes the service only within the cluster (accessible by other pods).

• Gets a virtual IP (cluster IP).

• Useful for microservice-to-microservice communication.

• External Service

• Exposes the service outside the cluster.

• Types:

• NodePort → Exposes service on a port of every node (e.g. <NodeIP>:<NodePort>).

• LoadBalancer → Integrates with cloud provider’s load balancer (AWS ELB, GCP LB, Azure LB).

• ExternalName → Maps service to an external DNS name.

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🔹 Is a Service attached to a Pod?

• Not exactly.

• A Service is attached to a set of Pods (selected via labels and selectors).

• Pods are ephemeral (they can die, restart, or move to other nodes), but the Service stays stable and always points to healthy pods.

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🔹 Does the same Service act as routing across multiple nodes?

✅ Yes.

• Service routes traffic to pods regardless of which node they are on.

• Kubernetes uses kube-proxy (iptables or IPVS) to distribute traffic across pods.

• From the client’s perspective, you just talk to the Service — Kubernetes handles finding the right Pod across nodes.

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🔹 Is this like a Load Balancer?

• Internally, yes → Kubernetes Service acts as a layer-4 load balancer across pods.

• But, externally, a Service of type LoadBalancer just provisions a cloud provider’s load balancer to expose your service outside the cluster.

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🔹 Then what is the role of Cloud Provider Load Balancers?

• Kubernetes internal Service load balancing happens inside the cluster only.

• Cloud provider external load balancers are needed to:

• Expose the service to the internet or external clients.

• Provide global load balancing, SSL termination, WAF, health checks, autoscaling integration, etc.

• Example: In AWS → an ELB distributes traffic to your Kubernetes nodes, then kube-proxy forwards traffic to the right pod.

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✅ In summary:

• Kubernetes Services = Internal cluster service discovery + load balancing across pods.

• Cloud Load Balancers = External exposure, internet traffic routing, enterprise-grade LB features.

• They work together: cloud LB sends traffic into cluster → Service distributes across pods.

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Would you like me to also give you a diagram (ASCII or image) that shows Cloud LB → Node → Service → Pods so you can visualize the full traffic flow?

Below is sequence diagram describing these 

sequenceDiagram

    participant User as External User

    participant CloudLB as Cloud Provider Load Balancer (AWS ELB / GCP LB / Azure LB)

    participant Node1 as Kubernetes Node 1

    participant Node2 as Kubernetes Node 2

    participant Service as Kubernetes Service (ClusterIP / NodePort)

    participant PodA as Pod A (on Node1)

    participant PodB as Pod B (on Node2)

    User->>CloudLB: HTTP Request to app.example.com

    CloudLB->>Node1: Forward traffic (round-robin / LB logic)

    CloudLB->>Node2: Or forward traffic (depending on LB)

    Node1->>Service: kube-proxy routes request

    Node2->>Service: kube-proxy routes request

    Service->>PodA: Forward traffic (if Pod selected)

    Service->>PodB: Forward traffic (if Pod selected)

    Note over Service,PodA: Service load-balances traffic <br> internally across Pods

    Note over CloudLB,Service: Cloud LB exposes service <br> to internet, forwards to cluster