Kubernetes how to access application in one namespace from another

I have the following components up and running in a kubernetes cluster

• A GoLang Application writing data to a mongodb statefulset replicaset in namespace app1
• A mongodb replicaset (1 replica) running as a statefulset in the namespace ng-mongo

What I need to do is, I need to access the mongodb database by the golang application for write/read opeations, so what I did was;

1. Create a headless service for the mongodb in the ng-mongo namespace as below:

# Source: mongo/templates/svc.yaml
apiVersion: v1
kind: Service
metadata:
  name: mongo
  namespace: ng-mongo
  labels:
    app: mongo
spec:
  ports:
  - port: 27017
    targetPort: 27017
    name: mongo
  clusterIP: None
  selector:
    role: mongo

2. And then I deployed the mongodb statefulset and initialized the replicaset as below:

kubectl exec -it mongo-0 -n ng-mongo mongosh
rs.initiate({_id: "rs0",members: [{_id: 0, host: "mongo-0"}]})


// gives output 
{ ok: 1 }

3. Then I created an ExternalName service in the app1 namespace linking the above mongo service in step 1, look below:

# Source: app/templates/svc.yaml
kind: Service
apiVersion: v1
metadata:
  name: app1
  namespace: app1
spec:
  type: ExternalName
  externalName: mongo.ng-mongo.svc.cluster.local
  ports:
  - port: 27017

4. And at last, I instrumented my golang application as follows;

// Connection URI 
const mongo_uri = "mongodb://app1" <-- Here I used the app1, as the ExternalName service's name is `app1`

<RETRACTED-CODE>

And then I ran the application, and checked the logs. Here is what I found:

> 2022/11/22 12:49:47 server selection error: server selection timeout, current topology: { Type: ReplicaSetNoPrimary, Servers: [{ Addr: mongo-0:27017, Type: Unknown, Last error: connection() error occurred during connection handshake: dial tcp: lookup mongo-0 on 10.96.0.10:53: no such host }, ] }

Update: I haven't set any usernames or passwords for the mongodb

Can someone help me why this is happening?

After some digging, I was able to find the issue.

When specifying the host entry for the rs.initiate({}), I should provide the FQDN of the relevant mongodb instance (in my case it is the mongo-0 pod). Therefore, my initialisation command should look like this;

rs.initiate({_id: "rs0",members: [{_id: 0, host: "mongo-0.mongo.ng-mongo.svc.cluster.local:27017"}]})

From my understanding of what you are trying to do,

Your Pod(golang application) and app1 Service are already in the same namespace.

However, looking at the log,

> 2022/11/22 12:49:47 server selection error: server selection timeout, current topology: { Type: ReplicaSetNoPrimary, Servers: [{ Addr: mongo-0:27017, Type: Unknown, Last error: connection() error occurred during connection handshake: dial tcp: lookup mongo-0 on 10.96.0.10:53: no such host }, ] }

The log means that the domain named 'mongo-0' could not be found in DNS. (Note that 10.96.0.10 IP is probably kube-dns)

Your application tries to connect to the domain mongo-0, but the domain mongo-0 does not exist in DNS (It means there is no service named mongo-0 on app1 namespace).

What is the 'mongo-0' that your Application trying to access?
(Obviously the log shows an attempt to access the domain mongo-0 and your golang applications mongo_uri indicates mongodb://app1)

Finding out why your application are trying to connect to the mongo-0 domain will help solve the problem.

Hope this helps you.