== Signal handling

In general, signals need only be sent to the master process.  However,
the signals Rainbows! uses internally to communicate with the worker
processes are documented here as well.  With the exception of TTIN/TTOU,
signal handling matches the behavior of and {nginx}[http://nginx.net/]
so it should be possible to easily share process management scripts
between \Rainbows!, unicorn and nginx.

=== Master Process

* HUP - reload config file, app, and gracefully restart all workers
  If the "preload_app" directive is false (the default), then workers
  will also pick up any application code changes when restarted.  If
  "preload_app" is true, then application code changes will have no
  effect; USR2 + QUIT (see below) must be used to load newer code in
  this case.  When reloading the application, +Gem.refresh+ will
  be called so updated code for your application can pick up newly
  installed RubyGems.  It is not recommended that you uninstall
  libraries your application depends on while Rainbows! is running,
  as respawned workers may enter a spawn loop when they fail to
  load an uninstalled dependency.

* INT/TERM - quick shutdown, kills all workers immediately

* QUIT - graceful shutdown, waits for workers to finish their
  current request before finishing.  This currently does not
  wait for requests deferred to a separate thread when using
  EventMachine (when app.deferred?(env) => true)

* USR1 - reopen all logs owned by the master and all workers
  See Unicorn::Util.reopen_logs for what is considered a log.

* USR2 - reexecute the running binary.  A separate QUIT
  should be sent to the original process once the child is verified to
  be up and running.

* WINCH - gracefully stops workers but keep the master running.
  This will only work for daemonized processes.

* TTIN - increment the number of worker processes by one

* TTOU - decrement the number of worker processes by one

=== Worker Processes

Sending signals directly to the worker processes should not normally be
needed.  If the master process is running, any exited worker will be
automatically respawned.

* INT/TERM - Quick shutdown, immediately exit.
  Unless WINCH has been sent to the master (or the master is killed),
  the master process will respawn a worker to replace this one.

* QUIT - Gracefully exit after finishing the current request.
  Unless WINCH has been sent to the master (or the master is killed),
  the master process will respawn a worker to replace this one.
  This currently does not wait for requests deferred to a separate
  thread when using EventMachine (when app.deferred?(env) => true)

* USR1 - Reopen all logs owned by the worker process.
  See Unicorn::Util.reopen_logs for what is considered a log.
  Unlike unicorn, log files are reopened immediately in \Rainbows!
  since worker processes are likely to be serving multiple clients
  simutaneously, we can't wait for all of them to finish.

=== Procedure to replace a running rainbows executable

You may replace a running instance of rainbows with a new one without
losing any incoming connections.  Doing so will reload all of your
application code, unicorn/Rainbows! config, Ruby executable, and all
libraries.  The only things that will not change (due to OS limitations)
are:

1. The path to the rainbows executable script.  If you want to change to
   a different installation of Ruby, you can modify the shebang
   line to point to your alternative interpreter.

The procedure is exactly like that of nginx:

1. Send USR2 to the master process

2. Check your process manager or pid files to see if a new master spawned
   successfully.  If you're using a pid file, the old process will have
   ".oldbin" appended to its path.  You should have two master instances
   of rainbows running now, both of which will have workers servicing
   requests.  Your process tree should look something like this:

     rainbows master (old)
     \_ rainbows worker[0]
     \_ rainbows worker[1]
     \_ rainbows worker[2]
     \_ rainbows worker[3]
     \_ rainbows master
        \_ rainbows worker[0]
        \_ rainbows worker[1]
        \_ rainbows worker[2]
        \_ rainbows worker[3]

3. You can now send WINCH to the old master process so only the new workers
   serve requests.  If your rainbows process is bound to an
   interactive terminal, you can skip this step.  Step 5 will be more
   difficult but you can also skip it if your process is not daemonized.

4. You should now ensure that everything is running correctly with the
   new workers as the old workers die off.

5. If everything seems ok, then send QUIT to the old master.  You're done!

   If something is broken, then send HUP to the old master to reload
   the config and restart its workers.  Then send QUIT to the new master
   process.
