Connect to Kubernetes mongo db in different namespace - mongodb

Question

Can anyone point out how to connect to the mongo db instance using mongo client using either command line client or from .net core programs with connection strings?
We have created a sample cluster in digitalocean with a namespace, let's say mongodatabase.
We installed the mongo statefulset with 3 replicas. We are able to successfully connect with the below command
kubectl --kubeconfig=configfile.yaml -n mongodatabase exec -ti mongo-0 mongo
But when we connect from a different namespace or from default namespace with the pod names in the below format, it doesn't work.
kubectl --kubeconfig=configfile.yaml exec -ti mongo-0.mongo.mongodatabase.cluster.svc.local mongo
where mongo-0.mongo.mongodatabase.cluster.svc.local is in pod-0.service_name.namespace.cluster.svc.local (also tried pod-0.statfulset_name.namespace.cluster.svc.local and pod-0.service_name.statefulsetname.namespace.cluster.svc.local) etc.,
Can any one help with the correct dns name/connection string to be used while connecting with mongo client in command line and also from the programs like java/.net core etc.,?
Also should we use kubernetes deployment instead of statefulsets here?

Answer 1

You need to reference the mongo service by namespaced dns. So if your mongo service is mymongoapp and it is deployed in mymongonamespace, you should be able to access it as mymongoapp.mymongonamespace.
To test, I used the bitnami/mongodb docker client. As follows:
From within mymongonamespace, this command works
$ kubectl config set-context --current --namespace=mymongonamespace
$ kubectl run mongodbclient --rm --tty -i --image bitnami/mongodb --command -- mongo --host mymongoapp
But when I switched to namespace default it didn't work
$ kubectl config set-context --current --namespace=default
$ kubectl run mongodbclient --rm --tty -i --image bitnami/mongodb --command -- mongo --host mymongoapp
Qualifying the host with the namespace then works
$ kubectl run mongodbclient --rm --tty -i --image bitnami/mongodb --command -- mongo --host mymongoapp.mymongonamespace

Answer 2

This is how you can get inside mongo-0 pod
kubectl --kubeconfig=configfile.yaml exec -ti mongo-0 sh

Answer 3

I think you are looking for this DNS for Services and Pods.
You can have a fully qualified domain name (FQDN) for a Services or for a Pod.
Also please have a look at this kubernetes: Service located in another namespace, as I think it will provide you with answer on how to access it from different namespace.
An example would look like this:
apiVersion: v1
kind: Service
metadata:
name: default-subdomain
spec:
selector:
name: busybox
clusterIP: None
ports:
- name: foo # Actually, no port is needed.
port: 1234
targetPort: 1234
---
apiVersion: v1
kind: Pod
metadata:
name: busybox1
labels:
name: busybox
spec:
hostname: busybox-1
subdomain: default-subdomain
containers:
- image: busybox:1.28
command:
- sleep
- "3600"
name: busybox
---
apiVersion: v1
kind: Pod
metadata:
name: busybox2
labels:
name: busybox
spec:
hostname: busybox-2
subdomain: default-subdomain
containers:
- image: busybox:1.28
command:
- sleep
- "3600"
name: busybox
If there exists a headless service in the same namespace as the pod and with the same name as the subdomain, the cluster’s KubeDNS Server also returns an A record for the Pod’s fully qualified hostname. For example, given a Pod with the hostname set to “busybox-1” and the subdomain set to “default-subdomain”, and a headless Service named “default-subdomain” in the same namespace, the pod will see its own FQDN as “busybox-1.default-subdomain.my-namespace.svc.cluster.local”. DNS serves an A record at that name, pointing to the Pod’s IP. Both pods “busybox1” and “busybox2” can have their distinct A records.
The Endpoints object can specify the hostname for any endpoint addresses, along with its IP.
Note: Because A records are not created for Pod names, hostname is required for the Pod’s A record to be created. A Pod with no hostname but with subdomain will only create the A record for the headless service (default-subdomain.my-namespace.svc.cluster.local), pointing to the Pod’s IP address. Also, Pod needs to become ready in order to have a record unless publishNotReadyAddresses=True is set on the Service.

Answer 4

Your question about Deployments vs StatefulSets should be a different question. But the answer is that the StatefulSet is used when you want "Stable Persistent Storage" kubernetes.io.
Also from the same page "stable is synonymous with persistence across Pod (re)scheduling". So basically your mongo instance is backed by a PeristentVolume and you want the volume reattached after the pod is rescheduled.