How do applications running in Kubernetes interact with each other?

Application interaction in Kubernetes primarily occurs through network services.

1. Service Discovery: Applications do not connect directly to pods via their IP addresses, as pod IPs are ephemeral and may change upon recreation. Instead, they use Service Discovery, which provides a stable DNS name and IP address for a group of pods performing the same function.

   • ClusterIP: The default service type, providing an internal IP address accessible only within the cluster.
   • NodePort: Opens a static port on each cluster node, forwarding traffic to the ClusterIP service.
   • LoadBalancer: Creates an external load balancer (if supported by the cloud provider).
   • ExternalName: Allows the service to refer to an external DNS name.

2. DNS: Kubernetes provides an internal DNS server (e.g., CoreDNS), which automatically creates DNS names for services. An application can refer to another service by the name service-name.namespace.svc.cluster.local or the shortened service-name.

3. Network Policies: Define how groups of pods can interact with each other and with external network endpoints. They are used to isolate network traffic for security reasons.

   apiVersion: networking.k8s.io/v1
   kind: NetworkPolicy
   metadata:
     name: deny-all-ingress
     namespace: default
   spec:
     podSelector: {} # Apply to all pods in the default namespace
     policyTypes:
     - Ingress
     ingress: [] # Deny all incoming traffic
   

4. Ingress: Manages external access to services in the cluster, usually via HTTP/HTTPS. An Ingress controller (e.g., Nginx Ingress, Traefik) routes traffic from the external network to the appropriate internal services based on rules defined in the Ingress resource.

5. Service Mesh: (e.g., Istio, Linkerd) Adds an additional layer of abstraction and functionality for service interaction. It provides features such as mutual TLS, request tracing, metrics, traffic management (A/B testing, canary releases) at the proxy level (sidecar containers) added to each pod.

Thus, the main interaction mechanisms are Service Discovery via DNS and various Service types, complemented by Network Policies for security and Ingress/Service Mesh for more complex traffic management and observability.