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Services

Service's abbreviation is svc. (You can type service or services or svc in your command.)

Pods can be moved accross nodes (thus assigned a different IP), or even replicated (the set of pods' IPs at one time may be different to another).

A service solves this issue by exposing multiple pods at a single constant IP-and-port pair (Lukša). Instead of connecting to the pods directly, you should connect to the service using its static IP.

A service exposes a constant IP and port pair.
A service exposes a constant IP and port pair. (Source: Lukša)
A service knows which pods handle the service through the use of labels.
A service knows which pods handle the service through the use of labels. (Source: Lukša)

Service descriptor

Mention selector (pod label selector, i.e., which pods handle the service) AND ports (ports mapping):

apiVersion: v1
kind: Service
metadata:
  name: kubia
spec:
  # ClusterIP is the default type of services,
  # so you don't have to specify it:
  # type: ClusterIP
  selector:
    app: kubia
  ports:
    - port: 80
      targetPort: 8080

Without selectors

You can create a Service descriptor without the selector field, but you will need to manage your own Endpoints.

Using named ports

You can target named ports in your pod's descriptor.

If those port number changes, you don't need to change the references to those ports which use the ports' name.

apiVersion: v1
kind: Pod
spec:
  containers:
    - name: kubia
      ports:
        # here's a pod named http in the container
        - name: http
          containerPort: 8080
        # here's a pod named https in the container
        - name: https
          containerPort: 8443

apiVersion: v1
kind: Service
spec:
  ports:
    # target the pods' "http" port
    - name: service-http-port
      port: 80
      targetPort: http
    # target the pods' "https" port
    - name: service-https-port
      port: 443
      targetPort: https

Discovering services

For pods to connect to a service, you don't always need to create the service first and then manually look up its IP and port.

Kubernetes provides some methods for service discovery.

Through environment variables

When a pod is started, all preexisting services' ports and IPs are exposed through environment variables.

The services' names will be converted to SNAKE_CASE. If your service's name is xxx, you will see these environment variables:

  • XXX_SERVICE_HOST — telling you the service's host
  • XXX_SERVICE_PORT — telling you the service's port
kubectl exec kubia-3inly env
# PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
# HOSTNAME=kubia-3inly
# KUBERNETES_SERVICE_HOST=10.111.240.1
# KUBERNETES_SERVICE_PORT=443
# ...
# KUBIA_SERVICE_HOST=10.111.249.153
# KUBIA_SERVICE_PORT=80
# ...

Through DNS (only for the IP)

When you list pods in the kube-system namespace, you will see a Kubernetes DNS pod, named either:

  • kube-dns-<something> for Kube-DNS, or
  • coredns-<something> for Core DNS.

You can perform a DNS query (in the form of fully qualified domain name, aka FQDN) to get a service's IP address (you still need to know the port yourself).

Example

Using DNS resolution allows you to do stuff like this (note that you don't need to know the IP of the kubia service in the pod's SSH session):

kubectl exec -it kubia-3inly -- bash
# root@kubia-3inly:/# curl http://kubia.default.svc.cluster.local
# You've hit kubia-5asi2

Service FQDN format

A sample FQDN is as follows (you can configure the .cluster.local suffix if you want):

# <serviceName>.<serviceNamespaceName>.svc.cluster.local
backend-database.default.svc.cluster.local

Configuring DNS lookup behavior

Kubernetes sets the default DNS lookup behavior in each container by writing a /etc/resolv.conf file (e.g., it sets the IP of the DNS server).

You can modify this behavior by changing the resolv.conf file yourself. For example, setting the search field will allow you to automatically expand queries like data to data.test.svc.cluster.local:

# /etc/resolv.conf
# 'data' queries can be expanded to 'data.test.svc.cluster.local', etc.
# see https://www.man7.org/linux/man-pages/man5/resolv.conf.5.html
nameserver 10.32.0.10
search test.svc.cluster.local svc.cluster.local cluster.local
options ndots:5

Excluding pods for DNS query

Set the dnsPolicy field in each pod's spec.

Working with services

Creating a Service

You can use a descriptor file:

kubectl create -f <filename>

Or the kubectl expose command:

kubectl expose pod <podName> --type=LoadBalancer --name <serviceName>

Listing Services

kubectl get svc
# NAME         CLUSTER-IP       EXTERNAL-IP   PORT(S)   AGE
# kubernetes   10.111.240.1     <none>        443/TCP   30d
# kubia        10.111.249.153   <none>        80/TCP    6m

Describing a Service

kubectl describe svc <serviceName>

References