Asynchronous MongoDB with Python & Tornado A.  Jesse  Jiryu  Davis [email protected] emptysquare.net Tuesday, July 24, 12

Agenda • Async  is  good • Why  is  async  hard? • What  is  Tornado  and  how  does  it  work? • Using  Tornado  with  PyMongo • Motor,  my  experimental  driver Tuesday, July 24, 12

CPU-­‐bound  web  service Client Server sockets •  No  need  for  async •  Just  spawn  one  process  per  core Clients Tuesday, July 24, 12

Normal  web  service Client Server sockets •  Assume  backend  is  slow  but  high-­‐throughput •  Service  is  bound  by  memory Backend (DB,  web  service, SAN,  …) socket Clients Tuesday, July 24, 12

What’s  async  for? • Minimize  resources  per  connecTon • I.e.,  wait  for  backend  as  cheaply  as  possible Tuesday, July 24, 12

CPU-­‐  vs.  Memory-­‐bound Crypto Chat Most  web  services? • Memory-­‐bound CPU-­‐bound Tuesday, July 24, 12

HTTP  long-­‐polling  (“COMET”) • E.g.,  chat  server,  Twi[er  client • Async’s  killer  app • Short-­‐polling  is  CPU-­‐bound:  tradeoff  between   latency  and  load • Long-­‐polling  is  memory  bound • “C10K  problem”:  kegel.com/c10k.html • Tornado  was  invented  for  this Tuesday, July 24, 12

Why  is  async  hard  to  code? Backend Client Server request response store  state request response Tme Tuesday, July 24, 12

Ways  to  store  state Coding  difficulty MulTthreading Tornado,  Node.js Memory  per  connecTon Tuesday, July 24, 12

Threads: # pseudo-Python sock = listen() request = parse_http(sock.recv()) mongo_data = db.collection.find() response = format_response(mongo_data) sock.sendall(response) Tuesday, July 24, 12

Tornado: class MainHandler(tornado.web.RequestHandler): @tornado.web.asynchronous def get(self): AsyncHTTPClient().fetch( ! ! "http://example.com", callback=self.on_response) def on_response(self, response): formatted = format_response(response) self.write(formatted) self.finish() Tuesday, July 24, 12

Tornado  IOStream class IOStream(object): def read_bytes(self, num_bytes, callback): self.read_bytes = num_bytes self.read_callback = callback io_loop.add_handler( self.socket.fileno(), ! ! ! ! self.handle_events, ! ! ! ! events=READ) def handle_events(self, fd, events): data = self.socket.recv(self.read_bytes) self.read_callback(data) Tuesday, July 24, 12

Tornado  IOLoop class IOLoop(object): def add_handler(self, fd, handler, events): self._handlers[fd] = handler # _impl is epoll or kqueue or ... self._impl.register(fd, events) def start(self): while True: event_pairs = self._impl.poll() for fd, events in event_pairs: self._handlers[fd](fd, events) Tuesday, July 24, 12

Python,  MongoDB,  &  concurrency • Threads  work  great  with  PyMongo – to  the  extent  they  work  in  Python  at  all • Tornado  works  so-­‐so – AsyncMongo • No  replica  sets,  only  first  batch,  no  SON  manipulators,  no   document  classes,  … – PyMongo • OK  if  all  your  queries  are  fast • Use  extra  Tornado  processes Tuesday, July 24, 12

Introducing:  “Motor” • Mongo  +  Tornado • Experimental • Might  be  official  in  a  few  months • Uses  Tornado  IOLoop  and  IOStream • Presents  standard  Tornado  callback  API • Stores  state  internally  with  greenlets h[p://emptysquare.net/motor Tuesday, July 24, 12

Motor db = motor.MotorConnection().open_sync().my_database class MainHandler(tornado.web.RequestHandler): @tornado.web.asynchronous def get(self): db.collection.insert( {'x':1}, callback=self.inserted) def inserted(self, result, error): if error: raise error self.write('OK') self.finish() Tuesday, July 24, 12

Motor  (with  Tornado  Tasks!) db = motor.MotorConnection().open_sync().mydb class MainHandler(tornado.web.RequestHandler): @tornado.web.asynchronous @gen.engine def get(self): yield motor.Op( db.collection.insert, {'x':1}) self.write('OK') self.finish() Tuesday, July 24, 12

Demo:  “Chirp” h[ps://github.com/ajdavis/chirp • Using  PyMongo • Using  Motor Tuesday, July 24, 12

QuesTons? A.  Jesse  Jiryu  Davis [email protected] emptysquare.net Tuesday, July 24, 12