NATS - Ruby Client

A Ruby client for the NATS messaging system.

License Apache 2.0 Build Status Gem Version Yard Docs

Supported Platforms

This gem and the client are known to work on the following Ruby platforms:

  • MRI 2.3.0, 2.4.0, 2.5.0
  • JRuby,,

If you're looking for a non-EventMachine alternative, check out the nats-pure gem.

Getting Started

gem install nats

nats-sub foo &
nats-pub foo 'Hello World!'

Basic Usage

require "nats/client"

NATS.start do

  # Simple Subscriber
  NATS.subscribe('foo') { |msg| puts "Msg received : '#{msg}'" }

  # Simple Publisher
  NATS.publish('', 'Hello World!')

  # Unsubscribing
  sid = NATS.subscribe('bar') { |msg| puts "Msg received : '#{msg}'" }

  # Requests
  NATS.request('help') { |response| puts "Got a response: '#{response}'" }

  # Replies
  NATS.subscribe('help') { |msg, reply| NATS.publish(reply, "I'll help!") }

  # Stop using NATS.stop, exits EM loop if NATS.start started the loop


Wildcard Subscriptions

# "*" matches any token, at any level of the subject.
NATS.subscribe('foo.*.baz') { |msg, reply, sub| puts "Msg received on [#{sub}] : '#{msg}'" }
NATS.subscribe('*') { |msg, reply, sub| puts "Msg received on [#{sub}] : '#{msg}'" }
NATS.subscribe('*.bar.*')   { |msg, reply, sub| puts "Msg received on [#{sub}] : '#{msg}'" }

# ">" matches any length of the tail of a subject and can only be the last token
# E.g. 'foo.>' will match '', '', ''
NATS.subscribe('foo.>') { |msg, reply, sub| puts "Msg received on [#{sub}] : '#{msg}'" }

Queues Groups

# All subscriptions with the same queue name will form a queue group
# Each message will be delivered to only one subscriber per queue group, queuing semantics
# You can have as many queue groups as you wish
# Normal subscribers will continue to work as expected.
NATS.subscribe(subject, :queue => 'job.workers') { |msg| puts "Received '#{msg}'" }

Clustered Usage

NATS.start(:servers => ['nats://', 'nats://']) do |nc|
  puts "NATS is connected to #{nc.connected_server}"

  nc.on_reconnect do
    puts "Reconnected to server at #{nc.connected_server}"

  nc.on_disconnect do |reason|
    puts "Disconnected: #{reason}"

  nc.on_close do
    puts "Connection to NATS closed"

opts = {
  :dont_randomize_servers => true,
  :reconnect_time_wait => 0.5,
  :max_reconnect_attempts => 10,
  :servers => ['nats://', 'nats://', 'nats://']

NATS.connect(opts) do |c|
  puts "NATS is connected!"

Auto discovery

The client also auto discovers new nodes announced by the server as they attach to the cluster. Reconnection logic parameters such as time to back-off on failure and max attempts apply the same to both discovered nodes and those defined explicitly on connect:

opts = {
  :dont_randomize_servers => true,
  :reconnect_time_wait => 0.5,
  :max_reconnect_attempts => 10,
  :servers => ['nats://', 'nats://'],
  :user => 'secret',
  :pass => 'deadbeef'

NATS.connect(opts) do |c|
  # Confirm number of available servers in cluster.
  puts "Connected to NATS! Servers in pool: #{c.server_pool.count}"

Advanced Usage

# Publish with closure, callback fires when server has processed the message
NATS.publish('foo', 'You done?') { puts 'msg processed!' }

# Timeouts for subscriptions
sid = NATS.subscribe('foo') { received += 1 }
NATS.timeout(sid, TIMEOUT_IN_SECS) { timeout_recvd = true }

# Timeout unless a certain number of messages have been received
NATS.timeout(sid, TIMEOUT_IN_SECS, :expected => 2) { timeout_recvd = true }

# Auto-unsubscribe after MAX_WANTED messages received
NATS.unsubscribe(sid, MAX_WANTED)

# Multiple connections
NATS.subscribe('test') do |msg|
  puts "received msg"

  # Gracefully disconnect from NATS after handling
  # messages that have already been delivered by server.

# Form second connection to send message on
NATS.connect { NATS.publish('test', 'Hello World!') }

See examples and benchmarks for more information..


Advanced customizations options for setting up a secure connection can be done by including them on connect:

options = {
  :servers => [
  :max_reconnect_attempts => 10,
  :reconnect_time_wait => 2,
  :tls => {
    :private_key_file => './spec/configs/certs/key.pem',
    :cert_chain_file  => './spec/configs/certs/server.pem'
    # Can enable verify_peer functionality optionally by passing
    # the location of a ca_file.
    # :verify_peer => true,
    # :ca_file => './spec/configs/certs/ca.pem'

# Set default callbacks
NATS.on_error do |e|
  puts "Error: #{e}"

NATS.on_disconnect do |reason|
  puts "Disconnected: #{reason}"

NATS.on_reconnect do |nats|
  puts "Reconnected to NATS server at #{nats.connected_server}"

NATS.on_close do
  puts "Connection to NATS closed"

NATS.start(options) do |nats|
  puts "Connected to NATS at #{nats.connected_server}"

  nats.subscribe("hello") do |msg|
    puts "Received: #{msg}"

  nats.flush do
    nats.publish("hello", "world")


Requests without a callback can be made to work synchronously and return the result when running in a Fiber. For these type of requests, it is possible to set a timeout of how long to wait for a single or multiple responses.

NATS.start {

  NATS.subscribe('help') do |msg, reply|
    puts "[Received]: <<- #{msg}"
    NATS.publish(reply, "I'll help! - #{msg}")

  NATS.subscribe('slow') do |msg, reply|
    puts "[Received]: <<- #{msg}"
    EM.add_timer(1) { NATS.publish(reply, "I'll help! - #{msg}") }

  10.times do |n|
    NATS.subscribe('hi') do |msg, reply|
      NATS.publish(reply, "Hello World! - id:#{n}")
  end do
    # Requests work synchronously within the same Fiber
    # returning the message when done.
    response = NATS.request('help', 'foo')
    puts "[Response]: ->> '#{response}'"

    # Specifying a custom timeout to give up waiting for
    # a response.
    response = NATS.request('slow', 'bar', timeout: 2)
    if response.nil?
      puts "No response after 2 seconds..."
      puts "[Response]: ->> '#{response}'"

    # Can gather multiple responses with the same request
    # which will then return a collection with the responses
    # that were received before the timeout.
    responses = NATS.request('hi', 'quux', max: 10, timeout: 1)
    responses.each_with_index do |response, i|
      puts "[Response# #{i}]: ->> '#{response}'"

    # If no replies then an empty collection is returned.
    responses = NATS.request('nowhere', '', max: 10, timeout: 2)
    if responses.any?
      puts "Got #{responses.count} responses"
      puts "No response after 2 seconds..."


  # Multiple fibers can make requests concurrently
  # under the same Eventmachine loop. do
    10.times do |n|
      response = NATS.request('help', "help.#{n}")
      puts "[Response]: ->> '#{response}'"


Unless otherwise noted, the NATS source files are distributed under the Apache Version 2.0 license found in the LICENSE file.