- Now we know how to manage Pods using
ReplicaSet.
- You know that
ReplicaSet make sure that desired number of Pods are always alive.
- One very important thing we should remember is that Pods are disposable, they should be easily replaceable.
- Another thing we need to remember is that, there can be multiple Pods that serves the same content.
- How do we keep tract of Pods IP. Pods can scale up / down, how to know new IP of pods when they are added.
- If Pods is unhealthy, they are replaced. They may or may not get the same IP.
- Service is a resource that makes a single, constant point of entry to a group of pods providing the same service.
- Each service is assigned a IP and port that never change while the service exists.
- Using service IP we can communicate with Pods that service manages.
- Service also manages Pods using label selector.
- To create a Service, you create a resource with
kind: Service
apiVersion: v1
kind: Service # 1
metadata:
name: v1-web-svc # 2
spec:
type: NodePort # 3
ports:
- port: 80 # 4
targetPort: 8080 # 5
nodePort: 30000 # 6
selector: # 7
env: prod
version: v1
#1: We are defining the type of resource to Service.#3: Type of Service. Default is ClusterIP but if we use ClusterIP it be visible only inside cluster.
NodePort will expose set of Pod it manages to external client.
#4: Expose external endpoint to port 80. Type NodePort will ignore this properties.#5: targetPort is the port container serves the app.#6: nodePort will use this IP to expose the service to external client. It must be range from 30000-32767.#7: This is the label selector that is used by Service to select the Pods.minikube doesn't support LoadBalancer as of now.
- You can create a service by using
kubectl create -f svc.yaml.
+----------------------------------+
+--------+ | +-----------+ |
|External+----------------->| Service | |
| Client | | +-----+-----+ |
+--------+ | | |
| +------------------------+ |
| | | | |
| +-v-+ +-v-+ +-v-+ |
| |Pod| |Pod| |Pod| |
| +---+ +---+ +---+ |
+----------------------------------+
- To list the services in cluster you use
kubectl get svc, where svc is short form for service.
- Now that service is running, we can access our app from outside.
- If you try
localhost:30000/test it will not work. Why?
- Remember Kubernetes is running inside minikube cluster, which is running inside Virtual Machine.
- We can use
minikube service @service_name to easily access your services through your browser.
minikube ip echos the minikube IP.minikube service @service_name --url echos the IP with port instead of opening browser.