Question

I have a server where I have implemented a child of the NetstringReceiver protocol. I want it to perform an asynchronous operation (using txredisapi) based on the client’s request and then respond with the results of the operation. A generalization of my code:

class MyProtocol(NetstringReceiver):
  def stringReceived(self, request):
    d = async_function_that_returns_deferred(request)
    d.addCallback(self.respond)
    # self.sendString(myString)

  def respond(self, result_of_async_function):
    self.sendString(result_of_async_function)

In the above code, the client connecting to my server does not get a response. However, it does get myString if I uncomment

# self.sendString(myString)

I also know that result_of_async_function is a non-empty string because I print it to stdout .

What can I do that will allow me to respond to the client with the result of the asynchronous function?

Update: Runnable source code

from twisted.internet import reactor, defer, protocol
from twisted.protocols.basic import NetstringReceiver
from twisted.internet.task import deferLater

def f():
  return "RESPONSE"

class MyProtocol(NetstringReceiver):
  def stringReceived(self, _):
    d = deferLater(reactor, 5, f)
    d.addCallback(self.reply)
    # self.sendString(str(f())) # Note that this DOES send the string.

  def reply(self, response):
    self.sendString(str(response)) # Why does this not send the string and how to fix?

class MyFactory(protocol.ServerFactory):
  protocol = MyProtocol

def main():
  factory = MyFactory()
  from twisted.internet import reactor
  port = reactor.listenTCP(8888, factory, )
  print 'Serving on %s' % port.getHost()
  reactor.run()

if __name__ == "__main__":
  main()

Answer 1

There’s one specific feature about NetstringReceiver:

The connection is lost if an illegal message is received

Are you sure that your messages conform djb’s Netstring protocol?

Obviously the client sends illegal string that could not be parsed, and connection is lost by protocol conditions. Everything else looks good in your code.

If you don’t need that specific protcol, you’d better inherit LineReceiver instead of NetstringReceiver.