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Exposing containers

Exposing containers

  • kubectl expose creates a service for existing pods
  • A service is a stable address for pod(s)
  • If we want to connect to pod(s) we need a service
  • CoreDNS (part of control plane) allows us to resolve services by name
  • There are different types of services
    • ClusterIP (default)
      • Single, internal virtual IP allocated
      • Only reachable from within cluster (nodes and pods)
      • Pods can reach service on apps port number
      • Always available in Kubernetes
    • NodePort
      • High port allocated on each node
      • Port is open on every node's IP
      • Anyone can connect (if they can reach node)
      • Other pods need to be updated to this port
      • Always available in Kubernetes
    • LoadBalancer
      • Controls LB endpoint external to the cluster
      • Only available when infrastructure provider gives you a load balancer (AWS ELB, etc)
      • Creates NodePort + ClusterIP services, tells external load balancer to send traffic to NodePort
    • ExternalName
      • Adds CNAME DNS records to CoreDNS only
      • Not used for pods, but for giving pods a DNS name to use for something outside Kubernetes

Creating a ClusterIP service

  • kubectl get pods -w
  • kubectl create deployment httpenv --image=bretfisher/httpenv
  • kubectl scale deployment/httpenv --replicas=5
  • kubectl expose deployment/httpenv --port 8888
  • kubectl get service
  • kubectl run --generator run-pod/v1 tmp-shell --rm -it --image=bretfisher/netshoot -- bash
  • curl httpenv:8888
{
"HOME": "/root",
"HOSTNAME": "httpenv-7cc9888d59-s9sj6",
"KUBERNETES_PORT": "tcp://10.96.0.1:443",
"KUBERNETES_PORT_443_TCP": "tcp://10.96.0.1:443",
"KUBERNETES_PORT_443_TCP_ADDR": "10.96.0.1",
"KUBERNETES_PORT_443_TCP_PORT": "443",
"KUBERNETES_PORT_443_TCP_PROTO": "tcp",
"KUBERNETES_SERVICE_HOST": "10.96.0.1",
"KUBERNETES_SERVICE_PORT": "443",
"KUBERNETES_SERVICE_PORT_HTTPS": "443",
"PATH": "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
}

Creating a NodePort service

  • kubectl expose deployment/httpenv --port 8888 --name httpenv-np --type NodePort
  • kubectl get services
NAME
TYPE
CLUSTER-IP
EXTERNAL-IP
PORT(S)
AGE
httpenv
ClusterIP
10.107.92.157
none
8888/TCP
24h
httpenv-np
NodePort
10.109.25.172
none
8888:31642/TCP
8s
kubernetes
ClusterIP
10.96.0.1
none
443/TCP
2d13h
  • 8888:31642/TCP - from port 8888 inside the cluster to port 31642 externally accessible
  • Default port ranges in the cluster 30000-32767
  • These 3 service types are additive, each one creates one above it:
    • ClusterIP
    • NodePort
    • LoadBalancer
  • curl localhost:31642 (on Linux - goes directly to NodePort, on Mac/Windows goes through Docker VPNKit)

Creating a LoadBalancer service

  • Docker Desktop provides a built-in LoadBalancer that publishes the port on localhost
  • kubectl expose deployment/httpenv --port 8888 --name httpenv-lb --type LoadBalancer
  • curl localhost:8888
NAME
TYPE
CLUSTER-IP
EXTERNAL-IP
PORT(S)
AGE
httpenv
ClusterIP
10.107.92.157
none
8888/TCP
24h
httpenv-np
NodePort
10.109.25.172
none
8888:31642/TCP
8s
httpenv-lb
LoadBalancer
10.98.39.13
localhost
8888:30695/TCP
19s
kubernetes
ClusterIP
10.96.0.1
none
443/TCP
2d13h

Cleanup

  • kubectl delete service/httpenv service/httpenv-np service/httpenv-lb deployment/httpenv

Kubernetes Service DNS

  • Starting with 1.11, internal DNS is provided by CoreDNS
  • Like Swarm, this is DNS-based service discovery
  • Accessing services by hostname works only if they're in the same namespace
  • kubectl get namespaces
  • Services also have a FQDN
    curl <hostname>.<namespace>.svc.cluster.local
​
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Last modified 3yr ago