Kubernetes/Services and networking

https://kubernetes.io/docs/concepts/services-networking/ gives a deep dive into the various capabilities around exposing services and the networking models in Kubernetes.

What is a Kubernetes Service?

Service is a top-level resource in the Kubernetes REST API. You can find more details about the Service API object

Example

Service Config to load balance traffic across all Pods with the app=nginx label. Receives on and sends to port 80. Exposes an externally accessible endpoint.

kind: Service
apiVersion: v1
metadata:
  # Unique key of the Service instance
  name: service-example
spec:
  ports:
    # Accept traffic sent to port 80
    - name: http
      port: 80
      targetPort: 80
  selector:
    # Loadbalance traffic across Pods matching
    # this label selector
    app: nginx
  # Create an HA proxy in the cloud provider
  # with an External IP address - *Only supported
  # by some cloud providers*
  type: LoadBalancer

Service Type

(quoted from the documentation )

type determines how the Service is exposed. Defaults to ClusterIP. Valid options are ExternalName, ClusterIP, NodePort, and LoadBalancer. "ClusterIP" allocates a cluster-internal IP address for load-balancing to endpoints. Endpoints are determined by the selector or if that is not specified, by manual construction of an Endpoints object or EndpointSlice objects. If clusterIP is "None", no virtual IP is allocated and the endpoints are published as a set of endpoints rather than a virtual IP. "NodePort" builds on ClusterIP and allocates a port on every node which routes to the same endpoints as the clusterIP. "LoadBalancer" builds on NodePort and creates an external load-balancer (if supported in the current cloud) which routes to the same endpoints as the clusterIP. "ExternalName" aliases this service to the specified externalName. Several other fields do not apply to ExternalName services. More info: https://kubernetes.io/docs/concepts/services-networking/service/#publishing-services-service-types

The official documentation is at https://kubernetes.io/docs/concepts/services-networking/service/#publishing-services-service-types

type: ClusterIP

: expose pods only inside your kubernetes cluster. You can make them public using an Ingress or a Gateway

type: NodePort

expose services on a static port

type: LoadBalancer

Just what it sounds like. You can create both internal and external load balancers in cases where you need Split Horizon DNS

type: ExternalName

map a Service to a DNS name, not to a typical selector such as my-service or cassandra

This Service definition, for example, maps the my-service Service in the prod namespace to my.database.example.com:

apiVersion: v1
kind: Service
metadata:
  name: my-service
  namespace: prod
spec:
  type: ExternalName
  externalName: my.database.example.com

When looking up the host my-service.prod.svc.cluster.local, the cluster DNS Service returns a CNAME record with the value my.database.example.com. Accessing my-service works in the same way as other Services but with the crucial difference that redirection happens at the DNS level rather than via proxying or forwarding. Should you later decide to move your database into your cluster, you can start its Pods, add appropriate selectors or endpoints, and change the Service's type.

Note: although you can use an IPv4 address string of digits as the name, don't do it.

Consider using headless services for that use case.

Application Programming Interface

A Pod (as in a pod of whales or pea pod) is a group of one or more containers , with shared storage/network resources, and a specification for how to run the containers. https://kubernetes.io/docs/concepts/workloads/pods/

Internet Protocol. An IP Address is a host or device name in numeric form.

Domain Name System. An Named Host to IP Address lookup system

Internet Protocol version 4