From: "Ara.T.Howard" <ahoward@...>
Date: 2003-10-05T01:54:58+09:00
Subject: Re: webrick, threads, and i/o

On Sat, 4 Oct 2003, NAKAMURA, Hiroshi wrote:

> You open fifo with blocking mode and read here.  Blocking entire process
> here is expected behavior if I understand correctly.

o.k. - that is what i was trying to confirm: that one must be careful using
webrick when doing something in a servlet that could block a process in the
kernel...  good (and not unexpected) to know.

> I think the process is blocked at the second access.  Does third
> access works?

no!?  i think it's a case of thread starvation here.  one servlet is blocked
opening the fifo for reading, but the other cannot open it for writing since
the entire process has halting on the first open...

> You can open fifo with File::RDONLY|File::NONBLOCK for read side and
> File::WRONLY|File::NONBLOCK for write side.  I think it should work.

strangley, this does not work:

----CUT----
#!/usr/local/ruby-1.8.0/bin/ruby
require 'webrick'
include WEBrick

# or set this up by hand before!
system 'mkfifo fifo && chmod 777 fifo' rescue nil

port = (ARGV[0] or 2003).to_i
s = HTTPServer.new( :Port => port )

class TimeServlet < HTTPServlet::AbstractServlet
  def do_GET(req, res)
    res['Content-Type'] = "text/plain"
    res.body = Time.now.to_s
  end
end

class ReadServlet < HTTPServlet::AbstractServlet
  def do_GET(req, res)
    res['Content-Type'] = "text/plain"
    res.body =
      begin
        fifo = open 'fifo', File::RDWR|File::NONBLOCK
        begin
          fifo.read
        rescue Errno::EWOULDBLOCK
          'NO DATA'
        end
      rescue Exception => e
        e.to_s << "\n" << e.backtrace.join("\n")
      end
  end
end

class WriteServlet < HTTPServlet::AbstractServlet
  def do_GET(req, res)
    res['Content-Type'] = "text/plain"
    res.body =
      begin
        fifo = open 'fifo', File::RDWR|File::NONBLOCK
        begin
          msg = "%s:%s" % [self.class, Time.now.to_s]
          fifo.puts msg
          fifo.flush
          "WROTE <%s>" % [msg]
        rescue Errno::EWOULDBLOCK
          'COULD NOT WRITE'
        end
      rescue Exception => e
        e.to_s << "\n" << e.backtrace.join("\n")
      end
  end
end

s.mount("/write", WriteServlet)
s.mount("/read", ReadServlet)
s.mount("/time", TimeServlet)
trap("INT"){ s.shutdown }
s.start
----CUT----


while this does:
----CUT----
#!/usr/local/ruby-1.8.0/bin/ruby

system 'mkfifo fifo && chmod 777 fifo' rescue nil
r = open 'fifo', File::RDWR|File::NONBLOCK
w = open 'fifo', File::RDWR|File::NONBLOCK
w.puts 'hi'
w.flush
puts r.gets
----CUT----

so i think the logic in my servlets is correct.  this is very strange?!

what's really strange about the above is that the WriteServlet DOES work, only
the ReadServlet hangs in the browser.  however, even when it hanging the other
servlets (Write, Time) continue to work, so it is not as if the _process_ is
blocked, only the ReadServlet request thread!?

> In other words, if you want the server never blocks no matter what servlets
> do, you should use other process forking WWW server application, or should
> make your server using WEBrick as a toolkit.

well - the first would require NOT using ruby so that's NOT an option! ;-)

as for the second, i'm unsure as to the best way to accomplish blocking
tasking from servlets:

  a) have a dedicated worker thread so all, IO for example, is done carefully
  there...

  b) fire up a Drb object (in another process) and use it to perform, for
  example, io.  (does Drb work on windows?)

??

any other ideas?


thanks very much.

-a
  ====================================
  | Ara Howard
  | NOAA Forecast Systems Laboratory
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