1   Using otrace with the Tornado web server

Contents

• 1   Using otrace with the Tornado web server
  • 1.1   Setup
  • 1.2   Holds
  • 1.3   /osh/web

This brief tutorial illustrates the use of otrace with the asynchronous Tornado web server using the demo program, tornademos/torna_trace.py, included in the distribution. It is assumed that Tornado is installed in the system (or in the tornademos directory).

1.1   Setup

For use with Tornado, otrace requires a couple of additional options, hold_wrapper and eventloop_callback:

# Initialize OShell instance (to run on separate thread)
IO_loop = tornado.ioloop.IOLoop.instance()
trace_shell = OShell(locals_dict=locals(), globals_dict=globals(), allow_unsafe=True,
                     init_file="torna_trace.trc", new_thread=True,
                     hold_wrapper=tornado.gen.Task,
                     eventloop_callback=IO_loop.add_callback)

try:
    # Start oshell
    trace_shell.loop()

    # Start tornado event loop
    IO_loop.start()
except KeyboardInterrupt:
    trace_shell.shutdown()

1.2   Holds

Since Tornado is a single-threaded asynchronous server, breakpoints would halt the server. However, using the asynchronous request handling supported by Tornado, one can hold the execution of a single request using a statement of the form:

@tornado.web.asynchronous
@tornado.gen.engine
...
yield traceassert(number != "77", label="hold_check", action="hold")

instead of setting a breakpoint. The hold trace event occurs when the trace condition is false. The resume command is used to resume execution from a hold. Here’s an example where a hold is triggered when a user enters the number 77 in the form:

tornademos$ ./torna_trace.py
Listening on port 8888
  ***otrace object shell (v0.30)*** (type 'help' for info)
globals> set trace_active True
trace_active = True
> rootW path=/?number=1
rootW path=/?number=77
GetHandler.get:hd-hold_check:21-06-24
> ls ~~
/osh/recent/holds/GetHandler.get/hd-hold_check/21-06-24
> cd ~~
GetHandler..06-24> ls
__down __trc  number self
GetHandler..06-24> ls -l
__down = {__trc, __down, __up}
__trc  = {thread, framestack, frame, related, funcname, context, argvalues, where, id, argstr}
number = u'77'
self   = <__main__.GetHandler object at 0x10ac0d950>
GetHandler..06-24> resume
globals>

Note that we need to activate tracing explicitly by setting parameter trace_active to True to trace traceassert calls. (This step is not needed when the trace command is used for tracing, because tracing is automatically activated.)

1.3   /osh/web

The /osh/web feature, used with Websockets and some Javascript “glue code”, allows the otrace server console to execute arbitrary Javacript commands on the client-side browser. The demo program tornademos/chat_websock.py is a modified version of the chat demo program distributed with Tornado, using websockets. If you run the program, and have one or more users chatting, you can execute javascript commands in any user’s browser via the /osh/web/username directory:

tornademos$ ./chat_websock.py
chat_websock: Listening on 127.0.0.1:8888
  ***otrace object shell (v0.30)*** (type 'help' for info)
> cd /osh/web
web> ls
user1
web> set safe_mode False
safe_mode = False
web> cd user1
web..user1> $("body")
[object Object]
web..user1> $("body").css("background","red") // Change background to red
[object Object]

In chat_websock.py, user input is handled by the input element with id message. The repeat command can be used to monitor user’s typing in real time:

web..user1> $("#message")
[object Object]
web..user1> repeat "User is typing: "+$("#message").val()

Last modified: 2016-03-31