How to know if load balancing works in Docker Swarm? - docker

Question

I created a service called accountservice and replicated it 3 times after. In my service I get IP address of the producing service instance and populate it in JSON response. The question is everytime I run curl $manager-ip:6767/accounts/10000 the returned IP is the same as before (I tried 100 times)
manager-ip environment variable:
set -x manager-ip (docker-machine ip swarm-manager-1)
Here's my Dockerfile:
FROM iron/base
EXPOSE 6767
ADD accountservice-linux-amd64 /
ADD healthchecker-linux-amd64 /
HEALTHCHECK --interval=3s --timeout=3s CMD ["./healthchecker-linux-amd64", "-port=6767"] || exit 1
ENTRYPOINT ["./accountservice-linux-amd64"]
And here's my automation script to build and run service:
#!/usr/bin/env fish
set -x GOOS linux
set -x CGO_ENABLED 0
set -x GOBIN ""
eval (docker-machine env swarm-manager-1)
go get
go build -o accountservice-linux-amd64 .
pushd ./healthchecker
go get
go build -o ../healthchecker-linux-amd64 .
popd
docker build -t azbshiri/accountservice .
docker service rm accountservice
docker service create \
--name accountservice \
--network my_network \
--replicas=1 \
-p 6767:6767 \
-p 6767:6767/udp \
azbshiri/accountservice
And here's the function I call to get the IP:
package common
import "net"
func GetIP() string {
addrs, err := net.InterfaceAddrs()
if err != nil {
return "error"
}
for _, addr := range addrs {
ipnet, ok := addr.(*net.IPNet)
if ok && !ipnet.IP.IsLoopback() {
if ipnet.IP.To4() != nil {
return ipnet.IP.String()
}
}
}
panic("Unable to determine local IP address (non loopback). Exiting.")
}
And I scale the service using the command below:
docker service scale accountservice=3

Answer 1

A few things:
Your results are normal. By default, a Swarm service has a VIP (virtual IP) in front of the service tasks to act as a load balancer. Trying to reach that service from inside the virtual network will only show that IP.
If you want to use a round-robin approach and skip the VIP, you could create a service with --endpoint-mode=dnsrr that would then return a different service task for each DNS request (but your client might be caching DNS names, causing that to show the same IP, which is why VIP is usually better).
If you wanted to get a list of IP's for task replicas, do a dig tasks.<servicename> inside the service's network.
If you wanted to test something easy, have your service create a random string, or use hostname on startup and return that so you can tell the different replicas when accessing. A easy example is to run one service using image elasticsearch:2 which will return JSON on port 9200 with a different random name per container.