A Kubernetes service is a logical collection of pods in a Kubernetes cluster. We can define a K8s service as an abstract way to load balance across the pods and expose an application deployed on a set of Pods. Moreover, using the inbuilt service mechanism in Kubernetes eliminates the need for implementing a separate service discovery mechanism.
What are Kubernetes services?
A Kubernetes service can be used to easily expose an application deployed on a set of pods using a single endpoint. Service is both a REST object and an abstraction that defines:
- A set of pods
- A policy to access them
Pods in a Kubernetes deployment are regularly created and destroyed, causing their IP addresses to change constantly. It will create discoverability issues for the deployed, application making it difficult for the application frontend to identify which pods to connect.
This is where the strengths of Kubernetes services come into play: services keep track of the changes in IP addresses and DNS names of the pods and expose them to the end-user as a single IP or DNS.
Kubernetes services utilize selectors to target a set of pods:
- For native Kubernetes applications, the endpoint API will be updated whenever there are changes to the pods in the service.
- Non-native applications can use virtual-IP-based bridge or load balancer implementation methods offered by Kubernetes to direct traffic to the backend pods.
Attributes of a Kubernetes service
Here are general attributes of a service:
- A service is assigned an IP address (“cluster IP”), which the service proxies use.
- A service can map an incoming port to any target port. (By default, the targetPort is set to the same value of the port field, and it can be defined as a string.)
- The port number assigned to each name can vary in each backend pod. (For example, you can change the port number that pods expose in the next version of your backend software without breaking clients.)
- Services support TCP (default), UDP, and SCTP for protocols.
- Services can be defined with or without a selector.
- Services support a variety of port definitions
Defining a service with a selector
In the following example, we have defined a simple service exposing port 80 in the service while targeting port 8080 in the Pods with the selector label “app=webserver”.
apiVersion: v1 kind: Service metadata: name: nginx-service spec: selector: app: webserver ports: - protocol: TCP port: 80 targetPort: 8080
Kubernetes will automatically assign an IP address (“Cluster IP”) which will then be used by service proxies to route the traffic. The controller for the selector will consistently monitor for Pods matching the defined label.
Some applications will require multiple ports to be exposed via the service. Kubernetes facilitates this using multi-port services where a user can define multiple ports in a single service object. When defining a multi-port service, the main requirement is to provide names for each port exposed so that they are unambiguous.
In this example, we have exposed both ports 80 and 443 to target ports 8080 and 8090 to route HTTP and HTTPS traffic to underlying pods using the selector “app=webserver-multiport”
apiVersion: v1 kind: Service metadata: name: nginx-service spec: selector: app: webserver-multiport ports: - name: http protocol: TCP port: 80 targetPort: 8080 - name: https protocol: TCP port: 443 targetPort: 8090