singleton_app

Socket-based singleton application class in Ruby.

singleton_app provides the class SingletonApp, which represents a singleton application that only allows one copy of itself to be running. The class enforces singleness by binding either a named Unix socket or a TCP socket. This eliminates some of the platform-specific hassles of process lock files, and also provides an easy means for subsequent invocations of the application to pass commandline arguments or other data to the running singleton.

To use it in its simplest form, inherit from SingletonApp and override the method #start_application.

Basic Example:

require './singleton_app'
    
class MyApp < SingletonApp
  def start_application
    20.times do
      puts "MyApp is doing something."
      sleep(1)
    end
  end
end
 
# Run the application, Unix-only version
MyApp.new("/tmp/my_app_socket")

# Cross-platform version. Notice how the port is specified.
MyApp.new('127.0.0.1',:port=>5024)

Data-passing example

To allow data to be passed from subsequent invocations back to the singleton, create the class with the option :listen=>true and override the methods #send_data_to_singleton and #handle_data_from_client in addition to #start_application, as in the example below.

When a second instance of this example is run, it sends the string 'Sending data to singleton' to the singleton instance. The singleton instance reads the string and prints it to STDOUT.

require './singleton_app'

class MyApp < SingletonApp
  def start_application
    puts "Starting singleton application"
    20.times do
      puts "MyApp is doing something."
      sleep(1)
      # If we were to meet a condition that would cause us to want to
      # exit here, we would call
      #stop_application
    end
  end

  def send_data_to_singleton(connection)
    puts "Starting client application"
    connection.puts('Sending data to singleton')
  end

  def handle_data_from_client(connection)
    data = connection.read
    # Do something with the data...
    puts "Got: #{data}"
    # And then return either success or failure
    return true
  end
end
  
# Run the application, Unix-only version; notice the :listen option
MyApp.new("/tmp/my_app_socket",:listen=>true)
# Cross-platform version
MyApp.new('127.0.0.1',:port=>5024,:listen=>true)

The following table represents two separate terminal sessions running the above code from the file myapp.rb. Time flows from top to bottom

    |Terminal 1 (First instance)       | Terminal 2 (Second instance)     |
    |----------------------------------|----------------------------------|
    | $ ruby myapp.rb                  |                                  |
    | Starting singleton application   |                                  |
    | MyApp is doing something.        |                                  |
    | MyApp is doing something.        | $ ruby myapp.rb                  |
    | MyApp is doing something.        | Starting client application      |
    | Got: Sending data to singleton   |                                  |
    | MyApp is doing something.        | #second instance has exited      |
    | MyApp is doing something.        |                                  |
    | MyApp is doing something         |                                  |
    | ...etc...                        |                                  |
    |----------------------------------|----------------------------------|