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?

CodePudding user response：

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.

CodePudding user response：

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"}]})