How make an async RequestHandler in tornado python

Question

currently I'm working on my Backend websserver using tornado.

The problem i have right now:
- when a request is made and the server is processing the request all other request are blocked

My RequestHandler:

class UpdateServicesRequestHandler( RequestHandler ):

    @gen.coroutine
    def get( self ):

        update = ServiceUpdate()
        response = yield update.update_all( )

        if self.request.headers.get('Origin'):
            self.set_header( 'Access-Control-Allow-Origin', self.request.headers.get('Origin') )
        self.set_header( 'Content-Type', 'application/json')
        self.write( response )

My update_all():

@gen.coroutine
def update_all( self ):

    for service in self.port_list:
        response = yield self.update_service( str( service.get( 'port' ) ) )
        self.response_list.append( response )

    self.response = json.dumps( self.response_list )

    return self.response

My update_sevice():

process = Popen( [ command ], stdout=PIPE, stderr=PIPE, shell=True )
output, error = process.communicate()

The thing is, that I need the result of the update_all() method. So is there a possibility to make this request not block my whole server for requests?

Thank you!

Is update.update_all() a coroutine? Does it use non-blocking I/O to do its work? — dano
– dano, Commented Aug 14, 2015 at 15:31
Now we need to know what update_service looks like. :) Ultimately, we need to know if you're making a slow, blocking call somewhere inside update_all. — dano
– dano, Commented Aug 14, 2015 at 15:48
i'm using a subprocess process = Popen( [ command ], stdout=PIPE, stderr=PIPE, shell=True ) to run a generated command. Generally I'm calling 'git pull' commands on severall directories — fr4nk
– fr4nk, Commented Aug 14, 2015 at 15:57
Are you then waiting for the Popen command to finish? Because that will definitely block the event loop. — dano
– dano, Commented Aug 14, 2015 at 15:57

Ben Darnell · Accepted Answer · 2015-08-15 12:03:46Z

4

In addition to using tornado.process.Subprocess as dano suggests, you should use stdout=tornado.process.Subprocess.STREAM instead of PIPE, and read from stdout/stderr asynchronously. Using PIPE will work for small amounts of output, but you will deadlock in wait_for_exit() if you use PIPE and the subprocess tries to write too much data (used to be 4KB but the limit is higher in most modern linux systems).

process = Subprocess([command], 
    stdout=Subprocess.STREAM, stderr=Subprocess.STREAM,
    shell=True)
out, err = yield [process.stdout.read_until_close(),
    process.stderr.read_until_close()]

answered Aug 15, 2015 at 12:03

Ben Darnell

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2 Comments

Nicholas White Over a year ago

What's the idiomatic way of getting the return code and raising an exception if non-zero in this case? Thanks -

Ben Darnell Over a year ago

yield process.wait_for_exit(): tornadoweb.org/en/stable/…

dano · Accepted Answer · 2015-08-14 16:16:59Z

2

You need to use tornado's wrapper around subprocess.Popen to avoid blocking the event loop:

from tornado.process import Subprocess
from subprocess import PIPE
from tornado import gen

@gen.coroutine
def run_command(command):
    process = Subprocess([command], stdout=PIPE, stderr=PIPE, shell=True)
    yield process.wait_for_exit()  # This waits without blocking the event loop.
    out, err = process.stdout.read(), process.stderr.read()
    # Do whatever you do with out and err

answered Aug 14, 2015 at 16:16

dano

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2 Comments

fr4nk Over a year ago

This really helped me out! Thanks a lot!! :-)

osullivj Over a year ago

Consider wait_for_exit( raise_error=False), otherwise a non 0 return from the subprocess will raise an exception.

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