Routing Traffic to Docker Services

By the end of this exercise, you should be able to:

Route traffic to a Docker service from within the swarm using dnsrr DNS lookups
Route traffic to a Docker service from outside the swarm using host port publishing

Configuring an Internally Routable Service

Create an overlay network my_overlay, or use the one you created in the last exercise.
Create a service out of the training/whoami-windows:latest image; containers created from this image will respond with their container ID when receiving a web request on their port 5000.
```
PS: node-0 Administrator> docker service create --name whoami `
    --replicas=4 --network my_overlay `
    --endpoint-mode dnsrr `
    training/whoami-windows:latest
```

Create another service attached to the same network, which we'll use to attempt to communicate with your first service:

PS: node-0 Administrator> docker service create --name pinger `
    --network my_overlay --replicas=4 `
    microsoft/nanoserver ping 8.8.8.8 -t

Launch a powershell in one of your pinger containers (it doesn't matter which one or on which node). Then, probe DNS resolution for your whoami service:

PS: node-0 Administrator> docker container exec -it <container ID> powershell
PS C:\> Resolve-DnsName -Name whoami

Name     Type   TTL   Section    IPAddress
----     ----   ---   -------    ---------
whoami   A      600   Answer     10.0.0.29
whoami   A      600   Answer     10.0.0.31
whoami   A      600   Answer     10.0.0.30
whoami   A      600   Answer     10.0.0.27

By launching the whoami service with the flag --endpoint-mode dnsrr, a DNS lookup on the service name whoami returns all the A-records corresponding to that service name, one for each replica of that service.

Try contacting a few of your whoami containers using the IPs you found above:

PS C:\> (Invoke-WebRequest http://<container IP>:5000).Content

I am 02FF20A20861

Try doing the same to resolve the DNS entry pinger:
```
PS C:\> Resolve-DnsName -Name pinger

Name     Type   TTL   Section    IPAddress
----     ----   ---   -------    ---------
pinger   A      600   Answer     10.0.0.32
```
The pinger service wasn't started with the --endpoint-mode dnsrr flag, so its DNS resolution defaults to vip, or the Virtual IP of the service as a whole (not of individual containers like we got for the whoami service). On Windows Server 1709+ (semi-annual Server channel), requests to a virtual IP such as this get load balanced by Docker to the containers managed by that service; this functionality is not available in Windows Server 2016, but is expected in Windows Server 2019.
Exit your container, and use docker service inspect pinger to confirm the single IP the DNS entry pinger resolved to is the virtual IP of the pinger service, and use docker container inspect on your whoami containers to confirm their IPs match the IPs returned in the DNS lookup above.

Configuring an Externally Routable Service

Start a service using host mode port publishing:
```
PS: node-0 Administrator> docker service create --name iis --replicas=4 `
    --publish mode=host,target=80 `
    microsoft/iis
```
This will map port 80 inside each container managed by this service onto a random available port on the host, so traffic from the outside network can be forwarded to your containerized application.
Do docker container ls on a node that has one of these iis containers running on it. Under the PORTS column in the resulting table, you should see something like 0.0.0.0:20931->80/tcp; in this case, Docker is forwarding traffic from the host port's 20931 to the container's port 80. Try visiting the iis landing page in your browser at :, and confirm that your containerized website is reachable.

Cleaning up

Remove all existing services, in preparation for future exercises:

PS: node-0 Administrator> docker service rm $(docker service ls -q)

Conclusion

In this exercise, we've explored the two main ways traffic gets routed to containerized applications on Windows: internal to the cluster, dnsrr routing resolves service names to a list of IPs of corresponding containers; it's up to you to load balance across these, and periodically check whether the list has changes (due to rescheduled containers or rescaled services). External to the cluster, host mode publishing maps a random host port onto a specified container port; it's up to you to then point your load balancer at the hosts and ports your application is running on.