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://0.0.0.0:6222 -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}"
  end
  NATS.start(:servers => ["nats://nats-cluster-node-1:4222"]) do |nc|
    inbox = NATS.create_inbox
    puts "[#{Time.now}] Connected to NATS at #{nc.connected_server}, inbox: #{inbox}"

    nc.subscribe(inbox) do |msg, reply|
      puts "[#{Time.now}] Received reply - #{msg}"
    end

    nc.subscribe("hello") do |msg, reply|
      next if reply == inbox
      puts "[#{Time.now}] Received greeting - #{msg} - #{reply}"
      nc.publish(reply, "world")
    end

    EM.add_periodic_timer(1) do
      puts "[#{Time.now}] Saying hi (servers in pool: #{nc.server_pool}"
      nc.publish("hello", "hi", inbox)
    end
  end'

Step 3:

Now you can add more nodes to the Swarm cluster via more docker services, referencing the seed server in the -routes parameter:

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:

Then confirm the distribution on the Docker Swarm cluster:

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!

Sample output after adding more workers which can reply back (since ignoring own responses):

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.

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