Add How to stop Puma on Heroku using plugins to the example directory

examples/plugins/redis_stop_puma.rb

@@ -0,0 +1,46 @@
+require 'puma/plugin'
+require 'redis'
+
+# How to stop Puma on Heroku
+# - You can't use normal methods because the dyno is not accessible
+# - There's no file system, no way to send signals
+# but ...
+# - You can use Redis or Memcache; any network distributed key-value
+#   store
+
+# 1. Add this plugin to your 'lib' directory
+# 2. In the `puma.rb` config file add the following lines
+#    === Plugins ===
+#    require './lib/puma/plugins/redis_stop_puma'
+#    plugin 'redis_stop_puma'
+# 3. Now, when you set the redis key "puma::restart::web.1", your web.1 dyno
+#    will restart
+# 4. Sniffing the Heroku logs for R14 errors is application (and configuration)
+#    specific. I use the Logentries service, watch for the pattern and the call
+#    a webhook back into my app to set the Redis key. YMMV
+
+# You can test this locally by setting the DYNO environment variable when
+# when starting puma, e.g. `DYNO=pants.1 puma`
+
+Puma::Plugin.create do
+  def start(launcher)
+
+    hostname = ENV['DYNO']
+    return unless hostname
+
+    redis = Redis.new(url: ENV.fetch('REDIS_URL', nil))
+    return unless redis.ping == 'PONG'
+
+    in_background do
+      while true
+        sleep 2
+        if message = redis.get("puma::restart::#{hostname}")
+          redis.del("puma::restart::#{hostname}")
+          $stderr.puts message
+          launcher.stop
+          break
+        end
+      end
+    end
+  end
+end