How can interaction between applications in different namespaces be limited?

Use Network Policies.

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-from-namespace
  namespace: default # Namespace where the service is located, which interaction we are restricting
spec:
  podSelector: {} # Apply to all pods in this namespace
  ingress:
  - from:
    - namespaceSelector:
        matchLabels:
          project: internal # Allow traffic only from namespaces with the label project: internal
    ports:
    - protocol: TCP
      port: 80 # Allow traffic only on port 80

Other approaches include:

• Service Mesh (Istio, Linkerd): Provide more granular control over traffic, including mutual authentication (mTLS).
• Firewalls: At the node level (iptables) or cloud provider.
• Built-in application mechanisms: For example, authorization at the API Gateway level.