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:

  1. % 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:

  1. % 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:

  1. % docker service create --name ruby-nats --network nats-cluster-example wallyqs/ruby-nats:ruby-2.3.1-nats-v0.8.0 -e '
  2.   NATS.on_error do |e|
  3.     puts "ERROR: #{e}"
  4.   end
  5.   NATS.start(:servers => ["nats://nats-cluster-node-1:4222"]) do |nc|
  6.     inbox = NATS.create_inbox
  7.     puts "[#{Time.now}] Connected to NATS at #{nc.connected_server}, inbox: #{inbox}"
  9.     nc.subscribe(inbox) do |msg, reply|
  10.       puts "[#{Time.now}] Received reply - #{msg}"
  11.     end
  13.     nc.subscribe("hello") do |msg, reply|
  14.       next if reply == inbox
  15.       puts "[#{Time.now}] Received greeting - #{msg} - #{reply}"
  16.       nc.publish(reply, "world")
  17.     end
  19.     EM.add_periodic_timer(1) do
  20.       puts "[#{Time.now}] Saying hi (servers in pool: #{nc.server_pool}"
  21.       nc.publish("hello", "hi", inbox)
  22.     end
  23.   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:

  1. % docker service create --network nats-cluster-example --name nats-cluster-node-2 nats:1.0.0 -cluster nats://0.0.0.0:6222 -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:

  1. % docker service scale ruby-nats=3

Then confirm the distribution on the Docker Swarm cluster:

  1. % docker service ps ruby-nats
  2. ID                         NAME         IMAGE                                     NODE    DESIRED STATE  CURRENT STATE          ERROR
  3. 25skxso8honyhuznu15e4989m  ruby-nats.1  wallyqs/ruby-nats:ruby-2.3.1-nats-v0.8.0  node-1  Running        Running 2 minutes ago  
  4. 0017lut0u3wj153yvp0uxr8yo  ruby-nats.2  wallyqs/ruby-nats:ruby-2.3.1-nats-v0.8.0  node-1  Running        Running 2 minutes ago  
  5. 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!

  1. [2016-08-15 12:51:52 +0000] Saying hi (servers in pool: [{:uri=>#<URI::Generic nats://10.0.1.3:4222>, :was_connected=>true, :reconnect_attempts=>0}]
  2. [2016-08-15 12:51:53 +0000] Saying hi (servers in pool: [{:uri=>#<URI::Generic nats://10.0.1.3:4222>, :was_connected=>true, :reconnect_attempts=>0}]
  3. [2016-08-15 12:51:54 +0000] Saying hi (servers in pool: [{:uri=>#<URI::Generic nats://10.0.1.3:4222>, :was_connected=>true, :reconnect_attempts=>0}]
  4. [2016-08-15 12:51:55 +0000] Saying hi (servers in pool: [{:uri=>#<URI::Generic nats://10.0.1.3:4222>, :was_connected=>true, :reconnect_attempts=>0}, {:uri=>#<URI::Generic nats://10.0.1.7:4222>, :reconnect_attempts=>0}, {:uri=>#<URI::Generic nats://10.0.1.6:4222>, :reconnect_attempts=>0}]

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

  1. [2016-08-15 16:06:26 +0000] Received reply - world
  2. [2016-08-15 16:06:26 +0000] Received reply - world
  3. [2016-08-15 16:06:27 +0000] Received greeting - hi - _INBOX.b8d8c01753d78e562e4dc561f1
  4. [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.

  1. % docker service create --network nats-cluster-example --name nats-cluster-node-3 nats:1.0.0 -cluster nats://0.0.0.0:6222 -routes nats://nats-cluster-node-1:6222 -DV