productpage cannot be accessed and return 503 or 000

Problem:
curl -o /dev/null -s -w "%{http_code}\n" http://${GATEWAY_URL}/productpage

503 or 000

# When visit the GATEWAY_URL
no healthy upstream

Reason:
Istio Containers / BookInfo Pods were initializing.

Solution: 
#check pods and wait untill all pods running
kubectl get pods -n istio-system
kubectl get pods