An abstract base class for a server, with the following properties:

  • The server has exactly one client, and is connected to that client at all times. The server will quit when the connection closes.
  • The server‘s main loop may be run in a child process (and so is asynchronous from the main process).
  • One can communicate with the server through discrete messages (as opposed to byte streams).
  • The server can pass file descriptors (IO objects) back to the client.

A message is just an ordered list of strings. The first element in the message is the _message name_.

The server will also reset all signal handlers (in the child process). That is, it will respond to all signals in the default manner. The only exception is SIGHUP, which is ignored. One may define additional signal handlers using define_signal_handler().

Before an AbstractServer can be used, it must first be started by calling start(). When it is no longer needed, stop() should be called.

Here‘s an example on using AbstractServer:

 class MyServer < Passenger::AbstractServer
    def initialize
       define_message_handler(:hello, :handle_hello)

    def hello(first_name, last_name)
       send_to_server('hello', first_name, last_name)
       reply, pointless_number = recv_from_server
       puts "The server said: #{reply}"
       puts "In addition, it sent this pointless number: #{pointless_number}"

    def handle_hello(first_name, last_name)
       send_to_client("Hello #{first_name} #{last_name}, how are you?", 1234)

 server =
 server.hello("Joe", "Dalton")
Included Modules
Classes and Modules
Class Passenger::AbstractServer::ServerAlreadyStarted
Class Passenger::AbstractServer::ServerError
Class Passenger::AbstractServer::ServerNotStarted
Class Passenger::AbstractServer::UnknownMessage
Public Class methods
    # File lib/passenger/abstract_server.rb, line 91
91:         def initialize
92:                 @done = false
93:                 @message_handlers = {}
94:                 @signal_handlers = {}
95:                 @orig_signal_handlers = {}
96:         end
Public Instance methods

Return the PID of the started server. This is only valid if start() has been called.

     # File lib/passenger/abstract_server.rb, line 216
216:         def server_pid
217:                 return @pid
218:         end

Start the server. This method does not block since the server runs asynchronously from the current process.

You may only call this method if the server is not already started. Otherwise, a ServerAlreadyStarted will be raised.

Derived classes may raise additional exceptions.

     # File lib/passenger/abstract_server.rb, line 105
105:         def start
106:                 if started?
107:                         raise ServerAlreadyStarted, "Server is already started"
108:                 end
110:                 @parent_socket, @child_socket = UNIXSocket.pair
111:                 before_fork
112:                 @pid = fork do
113:                         begin
114:                                 STDOUT.sync = true
115:                                 STDERR.sync = true
116:                                 @parent_socket.close
118:                                 # During Passenger's early days, we used to close file descriptors based
119:                                 # on a white list of file descriptors. That proved to be way too fragile:
120:                                 # too many file descriptors are being left open even though they shouldn't
121:                                 # be. So now we close file descriptors based on a black list.
122:                                 file_descriptors_to_close = [0, 1, 2, @child_socket.fileno]
123:                                 NativeSupport.close_all_file_descriptors(file_descriptors_to_close)
124:                                 # In addition to closing the file descriptors, one must also close
125:                                 # the associated IO objects. This is to prevent IO.close from
126:                                 # double-closing already closed file descriptors.
127:                                 close_all_io_objects_for_fds(
129:                                 # At this point, RubyGems might have open file handles for which
130:                                 # the associated file descriptors have just been closed. This can
131:                                 # result in mysterious 'EBADFD' errors. So we force RubyGems to
132:                                 # clear all open file handles.
133:                                 Gem.clear_paths
135:                                 start_synchronously(@child_socket)
136:                         rescue Interrupt
137:                                 # Do nothing.
138:                         rescue SignalException => signal
139:                                 if signal.message == SERVER_TERMINATION_SIGNAL
140:                                         # Do nothing.
141:                                 else
142:                                         print_exception(self.class.to_s, signal)
143:                                 end
144:                         rescue Exception => e
145:                                 print_exception(self.class.to_s, e)
146:                         ensure
147:                                 exit!
148:                         end
149:                 end
150:                 @child_socket.close
151:                 @parent_channel =
152:         end

Start the server, but in the current process instead of in a child process. This method blocks until the server‘s main loop has ended.

socket is the socket that the server should listen on. The server main loop will end if the socket has been closed.

All hooks will be called, except before_fork().

     # File lib/passenger/abstract_server.rb, line 161
161:         def start_synchronously(socket)
162:                 @child_socket = socket
163:                 @child_channel =
164:                 begin
165:                         reset_signal_handlers
166:                         initialize_server
167:                         begin
168:                                 main_loop
169:                         ensure
170:                                 finalize_server
171:                         end
172:                 ensure
173:                         revert_signal_handlers
174:                 end
175:         end

Return whether the server has been started.

     # File lib/passenger/abstract_server.rb, line 211
211:         def started?
212:                 return !@parent_channel.nil?
213:         end

Stop the server. The server will quit as soon as possible. This method waits until the server has been stopped.

When calling this method, the server must already be started. If not, a ServerNotStarted will be raised.

     # File lib/passenger/abstract_server.rb, line 182
182:         def stop
183:                 if !started?
184:                         raise ServerNotStarted, "Server is not started"
185:                 end
187:                 @parent_socket.close
188:                 @parent_channel = nil
190:                 # Wait at most 3 seconds for server to exit. If it doesn't do that,
191:                 # we kill it. If that doesn't work either, we kill it forcefully with
192:                 # SIGKILL.
193:                 begin
194:                         Timeout::timeout(3) do
195:                                 Process.waitpid(@pid) rescue nil
196:                         end
197:                 rescue Timeout::Error
198:                         Process.kill(SERVER_TERMINATION_SIGNAL, @pid) rescue nil
199:                         begin
200:                                 Timeout::timeout(3) do
201:                                         Process.waitpid(@pid) rescue nil
202:                                 end
203:                         rescue Timeout::Error
204:                                 Process.kill('SIGKILL', @pid) rescue nil
205:                                 Process.waitpid(@pid, Process::WNOHANG) rescue nil
206:                         end
207:                 end
208:         end
Protected Instance methods

A hook which is called when the server is being started, just before forking a new process. The default implementation does nothing, this method is supposed to be overrided by child classes.

     # File lib/passenger/abstract_server.rb, line 223
223:         def before_fork
224:         end

Return the communication channel with the client (i.e. the parent process that started the server). This is a MessageChannel object.

     # File lib/passenger/abstract_server.rb, line 268
268:         def client
269:                 return @child_channel
270:         end
define_message_handler(message_name, handler)

Define a handler for a message. message_name is the name of the message to handle, and handler is the name of a method to be called (this may either be a String or a Symbol).

A message is just a list of strings, and so handler will be called with the message as its arguments, excluding the first element. See also the example in the class description.

     # File lib/passenger/abstract_server.rb, line 243
243:         def define_message_handler(message_name, handler)
244:                 @message_handlers[message_name.to_s] = handler
245:         end
define_signal_handler(signal, handler)

Define a handler for a signal.

     # File lib/passenger/abstract_server.rb, line 248
248:         def define_signal_handler(signal, handler)
249:                 @signal_handlers[signal.to_s] = handler
250:         end

A hook which is called when the server is being stopped. This is called in the child process, after the main loop has been left. The default implementation does nothing, this method is supposed to be overrided by child classes.

     # File lib/passenger/abstract_server.rb, line 235
235:         def finalize_server
236:         end

A hook which is called when the server is being started. This is called in the child process, before the main loop is entered. The default implementation does nothing, this method is supposed to be overrided by child classes.

     # File lib/passenger/abstract_server.rb, line 229
229:         def initialize_server
230:         end

Tell the main loop to stop as soon as possible.

     # File lib/passenger/abstract_server.rb, line 273
273:         def quit_main
274:                 @done = true
275:         end

Return the communication channel with the server. This is a MessageChannel object.

Raises ServerNotStarted if the server hasn‘t been started yet.

This method may only be called in the parent process, and not in the started server process.

     # File lib/passenger/abstract_server.rb, line 259
259:         def server
260:                 if !started?
261:                         raise ServerNotStarted, "Server hasn't been started yet. Please call start() first."
262:                 end
263:                 return @parent_channel
264:         end