Service Discovery

Apps need two things to be able to send requests to other apps: 1. A way to know the name of the other app (the name of its Service) 2. A way to convert the name into an IP address

srvDiscovery

Every Kubernetes cluster has a built-in cluster DNS that it uses as its service registry. It’s a Kubernetes-native application running on the control plane of every Kubernetes cluster as two or more Pods managed by a Deployment and fronted by a Service. The Deployment is usually called coredns or kube-dns, and the Service is always called kube-dns.

cluster_dns

kubectl get pods -n kube-system -l k8s-app=kube-dns
kubectl get deploy -n kube-system -l k8s-app=kube-dns
kubectl get svc -n kube-system -l k8s-app=kube-dns

svc_reg_flow

You post a new Service resource manifest to the API server, where it’s authenticated and authorised. Kubernetes allocates it a ClusterIP and persists its configuration to the cluster store. The cluster DNS observes the new Service and registers the appropriate DNS A and SRV records. Associated EndpointSlice objects are created to hold the list of healthy Pod IPs that match the Service’s label selector. Every node runs a kube-proxy that observes the new objects and creates local routing rules so that requests to the Service’s ClusterIP get routed to Pods.

srvDiscovery2

Debug

A common way to test if the cluster DNS is working is to use nslookup to resolve the kubernetes Service.

kubectl run -it dnsutils \
--image registry.k8s.io/e2e-test-images/jessie-dnsutils:1.7