Ingress configuration for k8s in different namespaces

Outside traffic comes through ingress controller service that is responsible for routing the traffic based on the defined routing rules or what we call ingress rules in k8s world.

In other words, ingress resources are just routing rules (think of it in away that's similar to DNS records) so when you define an ingress resource you just defined a rule for ingress controller to work on and route traffic based on such defined rules.

Solution:

1. Since Ingress are nothing but routing rules, you could define such rules anywhere in the cluster (in any namespace) and controller should pick them up as it monitors creation of such resources and react accordingly.

   Here's how to create ingress easily using kubectl

   kubectl create ingress <name> -n namespaceName --rule="host/prefix=serviceName:portNumber"

   Note: Add --dry-run=client -oyaml to generate yaml manifest file

2. Or you may create a service of type ExternalName in the same namespace where you have defined your ingress. such external service can point to any URL (a service that lives outside namespace or even k8s cluster)

   Here's an example that shows how to create an ExternalName service using kubectl:

   kubectl create service externalname ingress-ns -n namespaceName --external-name=serviceName.namespace.svc.cluster.local --tcp=80:80 --dry-run=client -oyaml

this should generate something similar to the following:

kind: Service
apiVersion: v1
metadata:
  name: nginx
  namespace: ingress-ns
spec:
  type: ExternalName
  externalName: serviceName.namespace.svc.cluster.local #or any external svc
  ports:
  - port: 80 #specify the port of service you want to expose 
    targetPort: 80 #port of external service 

As described above, create an ingress as below: kubectl create ingress <name> -n namespaceName --rule="host/prefix=serviceName:portNumber"

Note: Add --dry-run=client -oyaml to generate yaml manifest file

I would like to simplify the answer a bit further for those who are reletively new to Kubernetes and its ingress options in particular. There are 2 separate things that need to be present for ingress to work:

1. Ingress Controller(essentially a separate Pod/Deployment along with a Service that can be used to utilize routing and proxying. Based on nginx container for example);
2. Ingress rules(a separate Kubernetes resourse with kind: Ingress. Will only take effect if Ingress Controller is already deployed)

Now, Ingress Controller can be deployed in any namespace and is, in fact, usually deployed in a namespace separate from your app services. It can out-of-the-box see Ingress rules in all namespaces in the cluster and will pick them up.
The Ingress rules, however, must reside in the namespace where the app that they configure reside.

There are some workarounds for that, but this is the most common approach.

Instead of creating the ingress app-ingress in ingress-nginx namespace you should create it in the namespace where you have the service api-sandand the pod.

Alternatively there is way to achieve ingress in one namespace and service in another namespace via externalName.Checkout Kubernetes Cross Namespace Ingress Network

Here is an example referred from here.

kind: Service
apiVersion: v1
metadata:
  name: my-service
spec:
  type: ExternalName
  externalName: test-service.namespacename.svc.cluster.local

---

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: example-ingress
  annotations:
    kubernetes.io/ingress.class: "nginx"
spec:
  rules:
  - host: example.com
    http:
      paths:
      - path: /
        backend:
          serviceName: my-service
          servicePort: 80

It's possible actually, you can define ingress and a service with ExternalName type in namespace A, while the ExternalName points to DNS of the service in namespace B. For further details, please refer to this answer: https://stackoverflow.com/a/51899301/2995449