How to know if load balancing works in Docker Swarm?

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.