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Docker Swarm

Step 1:

Create an overlay network for the cluster (in this example, nats-cluster-example), and instantiate an initial NATS server.
First create an overlay network:
docker network create --driver overlay nats-cluster-example
Next instantiate an initial "seed" server for a NATS cluster listening for other servers to join route to it on port 6222:
docker service create --network nats-cluster-example --name nats-cluster-node-1 nats:1.0.0 -cluster nats:// -DV

Step 2:

The 2nd step is to create another service which connects to the NATS server within the overlay network. Note that we connect to to the server at nats-cluster-node-1:
docker service create --name ruby-nats --network nats-cluster-example wallyqs/ruby-nats:ruby-2.3.1-nats-v0.8.0 -e '
NATS.on_error do |e|
puts "ERROR: #{e}"
NATS.start(:servers => ["nats://nats-cluster-node-1:4222"]) do |nc|
inbox = NATS.create_inbox
puts "[#{}] Connected to NATS at #{nc.connected_server}, inbox: #{inbox}"
nc.subscribe(inbox) do |msg, reply|
puts "[#{}] Received reply - #{msg}"
nc.subscribe("hello") do |msg, reply|
next if reply == inbox
puts "[#{}] Received greeting - #{msg} - #{reply}"
nc.publish(reply, "world")
EM.add_periodic_timer(1) do
puts "[#{}] Saying hi (servers in pool: #{nc.server_pool}"
nc.publish("hello", "hi", inbox)

Step 3:

Now you can add more nodes to the Swarm cluster via more docker services, referencing the seed server in the -routes parameter:
docker service create --network nats-cluster-example --name nats-cluster-node-2 nats:1.0.0 -cluster nats:// -routes nats://nats-cluster-node-1:6222 -DV
In this case, nats-cluster-node-1 is seeding the rest of the cluster through the autodiscovery feature. Now NATS servers nats-cluster-node-1 and nats-cluster-node-2 are clustered together.
Add in more replicas of the subscriber:
docker service scale ruby-nats=3
Then confirm the distribution on the Docker Swarm cluster:
docker service ps ruby-nats
25skxso8honyhuznu15e4989m ruby-nats.1 wallyqs/ruby-nats:ruby-2.3.1-nats-v0.8.0 node-1 Running Running 2 minutes ago
0017lut0u3wj153yvp0uxr8yo ruby-nats.2 wallyqs/ruby-nats:ruby-2.3.1-nats-v0.8.0 node-1 Running Running 2 minutes ago
2sxl8rw6vm99x622efbdmkb96 ruby-nats.3 wallyqs/ruby-nats:ruby-2.3.1-nats-v0.8.0 node-2 Running Running 2 minutes ago
The sample output after adding more NATS server nodes to the cluster, is below - and notice that the client is dynamically aware of more nodes being part of the cluster via auto discovery!
[2016-08-15 12:51:52 +0000] Saying hi (servers in pool: [{:uri=>#<URI::Generic nats://>, :was_connected=>true, :reconnect_attempts=>0}]
[2016-08-15 12:51:53 +0000] Saying hi (servers in pool: [{:uri=>#<URI::Generic nats://>, :was_connected=>true, :reconnect_attempts=>0}]
[2016-08-15 12:51:54 +0000] Saying hi (servers in pool: [{:uri=>#<URI::Generic nats://>, :was_connected=>true, :reconnect_attempts=>0}]
[2016-08-15 12:51:55 +0000] Saying hi (servers in pool: [{:uri=>#<URI::Generic nats://>, :was_connected=>true, :reconnect_attempts=>0}, {:uri=>#<URI::Generic nats://>, :reconnect_attempts=>0}, {:uri=>#<URI::Generic nats://>, :reconnect_attempts=>0}]
Sample output after adding more workers which can reply back (since ignoring own responses):
[2016-08-15 16:06:26 +0000] Received reply - world
[2016-08-15 16:06:26 +0000] Received reply - world
[2016-08-15 16:06:27 +0000] Received greeting - hi - _INBOX.b8d8c01753d78e562e4dc561f1
[2016-08-15 16:06:27 +0000] Received greeting - hi - _INBOX.4c35d18701979f8c8ed7e5f6ea

And so forth...

From here you can experiment adding to the NATS cluster by simply adding servers with new service names, that route to the seed server nats-cluster-node-1. As you've seen above, clients will automatically be updated to know that new servers are available in the cluster.
docker service create --network nats-cluster-example --name nats-cluster-node-3 nats:1.0.0 -cluster nats:// -routes nats://nats-cluster-node-1:6222 -DV