NATS cluster on Docker Swarm

The official documentation explains how build nats-server cluster on docker swam. But it is Manual/Script method instead using docker-compose.yml and prevents scaling nats cluster itself.

Ideas

• Use "service name" to specify "seed" server address. --routes $SEED
• Use "hostname" of node server address. --cluster_advertise ${HOSTNAME}
  • I tried entrypoint: tag to specify generated values, but it does not support environment variable support.
  • So I added a bootstrap script and used $NATS_SEED_ADDRESS environment variable to determine if work as seed or not.

if [ -z $NATS_SEED_ADDRESS ]; then
# this is seed node : no --routes parameter.
    nats-server \
    -m 8222 \
    --cluster nats://0.0.0.0:6222 \
    --cluster_advertise $HOSTNAME \
    --connect_retries 20
else
# this is NOT seed node : join the cluster using --routes parameter.
    nats-server \
    -m 8222 \
    --cluster nats://0.0.0.0:6222 \
    --routes $NATS_SEED_ADDRESS \
    --cluster_advertise $HOSTNAME \
    --connect_retries 20
fi

Running cluster

$ docker stack deploy -c docker-compose.yml nats

Scaling cluster size

Initial cluster size is 3 (Seed 1, node 2). You can scale out the node.

$ docker service scale nats_server=4

Client connection addresses

• There are 2 kind(Seed, Node) of nats-server in the network. On client it is natural to specify both of them ['nats://seed:4222', 'nats://server:4222'].
• However in my use case I just only specified 'nats://server:4222'. Seed wasted at first and will be used after a node down and client try to reconnect.

Docker Image

I pushed my image here. https://hub.docker.com/r/rushprint/nats-cluster-on-docker-swarm.