How can I debug my Rack app while it's running?

Question

I wrote a rack app ( here ) which locks up about once or twice per day, and I need to debug the app.

So, I tried following this: http://robots.thoughtbot.com/using-gdb-to-inspect-a-running-ruby-process . I tried debugging against a simple test script:

class MyTest
  def initialize
    @a ||= 1
  end

  def something
    @a += 1
    puts @a
  end
end

test = MyTest.new

while true do
  sleep 3
  test.something
end

However, I get this:

(gdb) redirect_stdout
No symbol "rb_eval_string" in current context.
(gdb) ruby_eval('Kernel.caller')
No symbol "rb_p" in current context.

Any idea what I need to do different? Or is there an alternative way to debug? I want to avoid using "puts" statements and writing to logs, as I've tried that already, and it's very hit or miss.

Answer 1

You can use a number of different OS tools like gdb or dtrace . Here's some articles about debugging ruby processes:

• http://blog.newrelic.com/2013/04/29/debugging-stuck-ruby-processes-what-to-do-before-you-kill-9/
• http://crypt.codemancers.com/posts/2013-04-16-profile-ruby-apps-dtrace-part1/
• http://tenderlovemaking.com/2011/06/29/i-want-dtrace-probes-in-ruby.html

Answer 2

Looks like it was not attaching to the process properly. Running in root mode let me attach and debug. I ran call rb_backtrace() , and that gave some useful output.