I'm trying to create a simple microservice, where a JQuery app in one Docker container uses this code to get a JSON object from another (analytics) app that runs in a different container:
<script type="text/javascript">
$(document).ready(function(){
$('#get-info-btn').click(function(){
$.get("http://localhost:8084/productinfo",
function(data, status){
$.each(data, function(i, obj) {
//some code
});
});
});
});
</script> The other app uses this for the Deployment containerPort.
ports:
- containerPort: 8082and these for the Service ports.
type: ClusterIP
ports:
- targetPort: 8082
port: 8084 The 'analytics' app is a golang program that listens on 8082.
func main() {
http.HandleFunc("/productinfo", getInfoJSON)
log.Fatal(http.ListenAndServe(":8082", nil))
}
When running this on Minikube, I encountered issues with CORS, which was resolved by using this in the golang code when returning a JSON object as a response:
w.Header().Set("Access-Control-Allow-Origin", "*")
w.Header().Set("Access-Control-Allow-Headers", "Content-Type") All this worked fine on Minikube (though in Minikube I was using localhost:8082). The first app would send a GET request to http://localhost:8084/productinfo and the second app would return a JSON object.
But when I tried it on a cloud Kubernetes setup by accessing the first app via <IP address of node>:<nodePortNumber>, when I open the browser console, I keep getting the error Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:8084/productinfo.
Question:
Why is it working on Minikube but not on the cloud Kubernetes worker nodes? Is using localhost the right way to access another container? How can I get this to work? How do people who implement microservices use their GET and POST requests across containers? All the microservice examples I found are built for simple demos on Minikube, so it's difficult to get a handle on this nuance.
@P.... is absolutely right, I just want to provide some more details about DNS for Services and communication between containers in the same Pod.
As we can find in the documentation, Kubernetes Services are assigned a DNS A (or AAAA) record, for a name of the form <serviceName>.<namespaceName>.svc.<cluster-domain>. This resolves to the cluster IP of the Service.
"Normal" (not headless) Services are assigned a DNS A or AAAA record, depending on the IP family of the service, for a name of the form my-svc.my-namespace.svc.cluster-domain.example. This resolves to the cluster IP of the Service.
Let's break down the form <serviceName>.<namespaceName>.svc.<cluster-domain> into individual parts:
<serviceName> - The name of the Service you want to connect to.
<namespaceName> - The name of the Namespace in which the Service to which you want to connect resides.
svc - This should not be changed - svc stands for Service.
<cluster-domain> - cluster domain, by default it's cluster.local.
We can use <serviceName> to access a Service in the same Namespace, however we can also use <serviceName>.<namespaceName> or <serviceName>.<namespaceName>.svc or FQDN <serviceName>.<namespaceName>.svc.<cluster-domain>.
If the Service is in a different Namespace, a single <serviceName> is not enough and we need to use <serviceName>.<namespaceName> (we can also use: <serviceName>.<namespaceName>.svc or <serviceName>.<namespaceName>.svc.<cluster-domain>).
In the following example, app-1 and app-2 are in the same Namespace and app-2 is exposed with ClusterIP on port 8084 (as in your case):
$ kubectl run app-1 --image=nginx
pod/app-1 created
$ kubectl run app-2 --image=nginx
pod/app-2 created
$ kubectl expose pod app-2 --target-port=80 --port=8084
service/app-2 exposed
$ kubectl get pod,svc
NAME READY STATUS RESTARTS AGE
pod/app-1 1/1 Running 0 45s
pod/app-2 1/1 Running 0 41s
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
service/app-2 ClusterIP 10.8.12.83 <none> 8084/TCP 36sNOTE: The app-2 is in the same Namespace as app-1, so we can use <serviceName> to access it from app-1, you can also notice that we got the FQDN for app-2 (app-2.default.svc.cluster.local):
$ kubectl exec -it app-1 -- bash
root@app-1:/# nslookup app-2
Server: 10.8.0.10
Address: 10.8.0.10#53
Name: app-2.default.svc.cluster.local
Address: 10.8.12.83NOTE: We need to provide the port number because app-2 is listening on 8084:
root@app-1:/# curl app-2.default.svc.cluster.local:8084
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
...
Let's create app-3 in a different Namespace and see how to connect to it from app-1:
$ kubectl create ns test-namespace
namespace/test-namespace created
$ kubectl run app-3 --image=nginx -n test-namespace
pod/app-3 created
$ kubectl expose pod app-3 --target-port=80 --port=8084 -n test-namespace
service/app-3 exposedNOTE: Using app-3 (<serviceName>) is not enough, we also need to provide the name of the Namespace in which app-3 resides (<serviceName>.<namespaceName>):
# nslookup app-3
Server: 10.8.0.10
Address: 10.8.0.10#53
** server can't find app-3: NXDOMAIN
# nslookup app-3.test-namespace
Server: 10.8.0.10
Address: 10.8.0.10#53
Name: app-3.test-namespace.svc.cluster.local
Address: 10.8.12.250
# curl app-3.test-namespace.svc.cluster.local:8084
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
...
We can use localhost to communicate with other containers, but only within the same Pod (Multi-container pods).
I've created a simple multi-container Pod with two containers: nginx-container and alpine-container:
$ cat multi-container-app.yml
apiVersion: v1
kind: Pod
metadata:
name: multi-container-app
spec:
containers:
- image: nginx
name: nginx-container
- image: alpine
name: alpine-container
command: ["sleep", "3600"]
$ kubectl apply -f multi-container-app.yml
pod/multi-container-app created
We can connect to the alpine-container container and check if we can access the nginx web server located in the nginx-container with localhost:
$ kubectl exec -it multi-container-app -c alpine-container -- sh
/ # netstat -tulpn
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN -
tcp 0 0 :::80 :::* LISTEN -
/ # curl localhost
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
...More information on communication between containers in the same Pod can be found here.