Integrating External Services

Kubernetes, and by extension OpenShift OKD, gives a lot of flexibility regarding network use cases. One of the uses cases is the one that allows to use external services, like databases, transparently in a OpenShift project. Other of the use cases would be to to have a network proxy between OpenShift installations. This could be used as a temporal solution to ease migration periods.

Proxy between clusters

In the example above we are redirecting traffic from Rahti 1 to Rahti 2. This is obtained by creating two routes, a service and an Endpoint.

Procedure

First deploy an application in Rahti 2. Any application is suitable for this test.
Install and login with OC in Rahti 2.
Create a special Route in Rahti 2:
```
echo 'apiVersion: route.openshift.io/v1
kind: Route
metadata:
  name: test-route
spec:
  host: <test>.rahtiapp.fi
  port:
    targetPort: http
  to:
    kind: Service
    name: <Service-name>
    weight: 100
  wildcardPolicy: None
status: {}' | oc create -f -
```
Replace <test> so the URL is the same as in Rahti 1. Rahti 1 will be later configured to relay the request to Rahti 2, when the request reaches Rahti 2, the Loadbalancer needs to know to which application to send the request, the host parameter is used (Using HTTPD headers). The <service-name> must be the one corresponding to the application you deployed. Double check which ports is <Service-name> exporting and adapt the Route in accordance to that.

You can test the setup so far:
```
curl <test>.rahtiapp.fi -vL --resolve <test>.rahtiapp.fi:80:195.148.21.61
```
The command above uses --resolve to change the ip associated to a DNS that corresponds to Rahti 2.
Install and login with OC in Rahti 1.
Create a project in Rahti 1, or use an existing one.

Then create an EndPoint in Rahti 1:

echo 'kind: Endpoints
apiVersion: v1
metadata:
  name: proxy-service
subsets: 
- addresses:
  - ip: 195.148.21.61
  ports:
  - port: 80
    name: http' | oc create -f -

The IP in the example, is the one behind router-default.apps.2.rahti.csc.fi.

Then a Service, also in Rahti 1:

echo 'kind: Service
apiVersion: v1
metadata:
  name: proxy-service
spec:
  ports:
  -  name: http
     protocol: TCP
     port: 80
     targetPort: 80 
     nodePort: 0
selector: {}' | oc create -f -

The name of the Service and the EndPoint must be the same.

Finally the Route in Rahti 1:

echo 'apiVersion: route.openshift.io/v1
kind: Route
metadata:
  name: test-route
spec:
  host: <test>.rahtiapp.fi
  port:
    targetPort: http
  tls:
    insecureEdgeTermination: Redirect
    termination: edge
  to:
    kind: Service
    name: proxy-service
    weight: 100
  wildcardPolicy: None
status: {}' | oc create -f -

The URL must be the same as in step 3.

Final considerations

In the tutorial above we have setup a proxy from Rahti 1 to Rahti 2, every request will go first to Rahti 1. This means two things, first that the performance will be affected by this proxy, secondly that when Rahti 1 is retired (or is down due to an incident or a planned maintenance) this setup will stop working. This is just a temporal fix.

It is possible to "proxy" more than one URL, the only necessary step is to create the Routes in Rahti 1 and 2 with the same URL in each service. It is not necessary to replicate the Service and Endpoint in Rahti 1.

You need to also consider that some applications will not work transparently with this setup. In some cases, applications need to know from which URL they are working. This means that the URL is hardwired in some assets like CSS or images, the result is the look of a "broken webpage".

For more information and other network proxy options:

https://docs.openshift.com/dedicated/3/dev_guide/integrating_external_services.html