How to connect MongoDB which is running on Kubernetes cluster through UI tools like MongoDB Compass or RoboMongo?

You can use kubectl port-forward to connect to MongoDB from outside the cluster.

Run kubectl port-forward << name of a mongodb pod >> --namespace << mongodb namespace >> 27018:27018.
Now point your UI tool to localhost:27018 and kubectl will forward all connections to the pod inside the cluster.

Starting with Kubernetes 1.10+ you can also use this syntax to connect to a service (you don't have to find a pod name first):
kubectl port-forward svc/<< mongodb service name >> 27018:27018 --namespace << mongodb namespace>>

If it is not your production database you can expose it through a NodePort service:

# find mongo pod namekubectl get podskubectl expose pod <<pod name>> --type=NodePort# find new mongo servicekubectl get services

Last command will output something like

mongodb-0   10.0.0.45    <nodes>       27017:32151/TCP   30s

Now you can access your mongo instance with mongo <<node-ip>>:32151

If not resolved, expose your mongo workload as a load balancer and use the IP provided by the service. Copy the LB IP and use the same in the robo3T. If it requires authentication, check my YAML file below:

apiVersion: apps/v1kind: Deploymentmetadata:    name: mongodb    labels:        app: mongodbspec:    replicas: 1    selector:        matchLabels:            app: mongodb    template:        metadata:            labels:                app: mongodb        spec:            containers:                - name: mongodb                  image: mongo                  volumeMounts:                      - name: data                        mountPath: "/data/db"                        subPath: "mongodb_data"                  ports:                      - containerPort: 27017                        protocol: TCP                  env:                      - name: MONGO_INITDB_ROOT_USERNAME                        value: xxxx                      - name: MONGO_INITDB_ROOT_PASSWORD                        value: xxxx            imagePullSecrets:                - name: xxxx            volumes:                - name: data                  persistentVolumeClaim:                      claimName: xxx

Set the same values in the authentication tab in ROBO3T

NOTE: I haven't mentioned the service section in the YAML since I directly exposed as an LB in the GCP UI itself.