We Keep Coding

How to handle a bidirectional grpc stream asynchronously

I have a game or for that matter any remote user interface with a server and multiple clients which should communicate via network.
Both client and server should be able to send updates asynchronously.
This seems to be a very natural service definition, which let's grpc manage sessions.
syntax = "proto3";
package mygame;
service Game {
rpc participate(stream ClientRequest) returns (ServerResponse);
}
message ClientRequest {
// Fields for the initial request and further updates
}
message ServerResponse {
// Game updates
}
Implementing the client is trivial (although the following code is obviously incomplete and simplified).
class Client:
def __init__(self):
self.channel = grpc.insecure_channel("localhost:50051")
self.stub = game_pb2_grpc.GameStub(channel)
self.output_queue = queue.Queue()
def output_iter(self):
while True:
client_output_msg = self.output_queue.get()
self.output_queue.task_done()
yield client_output_msg
def do_work(self):
for response in self.stub.participate(self.output_iter()):
print(response) # handle update
with grpc.insecure_channel("localhost:50051") as channel:
client = Client()
client.do_work()
What seems hard is implementing the server without blocking.
class Game(game_pb2_grpc.GameServicer):
def __init__(self):
self.pending_events = queue.Queue()
def participate(self, request_iter, context):
for client_update in request_iter:
print(client_update)
# !!!
# The next bit won't happen if the client has no updates
# !!!
try:
while True:
server_update = self.pending_events.get_nowait()
yield server_update
except queue.Empty:
pass
server = grpc.server(ThreadPoolExecutor(max_workers=100))
game_pb2_grpc.add_GameServicer_to_server(Game(), server)
server.add_insecure_port("[::]:50051")
server.start()
server.wait_for_termination()
As commented in the code, the client won't receive updates if it doesn't constantly send requests.
Maybe a async approach would be better, which might also solve other problems in this design.
PS: This issue has been solved with grpc in go here, however i don't see how to translate this to pythons grpc implementations.
I would be very happy about any help!

I was finally able to get it working using the python asynio api.
The basic idea is to decouple read and write into two coroutines using asyncio.create_task.
For anybody interested, here is a solution.
class Game(game_pb2_grpc.GameServicer):
async def read_client_requests(self, request_iter):
async for client_update in request_iter:
print("Recieved message from client:", client_update, end="")
async def write_server_responses(self, context):
for i in range(15):
await context.write(game_pb2.ServerResponse(dummy_value=str(i)))
await asyncio.sleep(0.5)
async def participate(self, request_iter, context):
read_task = asyncio.create_task(self.read_client_requests(request_iter))
write_task = asyncio.create_task(self.write_server_responses(context))
await read_task
await write_task
async def serve():
server = grpc.aio.server()
game_pb2_grpc.add_GameServicer_to_server(Game(), server)
server.add_insecure_port("[::]:50051")
await server.start()
await server.wait_for_termination()
if __name__ == "__main__":
asyncio.run(serve())
Note that instead of the write coroutine, a yield would also be sufficient.

asyncio with synchronous code

I have a module which makes blocking network requests to some TCP server and receive responses. I must integrate it into asyncio application. My module looks like this:
import socket
# class providing transport facilities
# can be implemented in any manner
# for example it can manage asyncio connection
class CustomTransport:
def __init__(self, host, port):
self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.host = host
self.port = port
def write(self, data):
left = len(data)
while left:
written = self.sock.send(data.encode('utf-8'))
left = left - written
data = data[written:]
def read(self, sz):
return self.sock.recv(sz)
def open(self):
self.sock.connect((self.host, self.port))
def close(self):
self.sock.shutdown(2)
# generated. shouldn't be modified
# however any transport can be passed
class HelloNetClient:
def __init__(self, transport):
self.transport = transport
def say_hello_net(self):
self.transport.write('hello')
response = self.transport.read(5)
return response
# can be modified
class HelloService:
def __init__(self):
# create transport for connection to echo TCP server
self.transport = CustomTransport('127.0.0.1', 6789)
self.hello_client = HelloNetClient(self.transport)
def say_hello(self):
print('Saying hello...')
return self.hello_client.say_hello_net()
def __enter__(self):
self.transport.open()
return self
def __exit__(self,exc_type, exc_val, exc_tb):
self.transport.close()
Usage:
def start_conversation():
with HelloService() as hs:
answer = hs.say_hello()
print(answer.decode('utf-8'))
if __name__ == "__main__":
start_conversation()
Now I see that only way to turn my module to be compatible with asyncio is to convert everything to coroutines and replace regular socket with asyncio-provided transport. But I don't want to touch generated code (HelloNetClient). Is it possible?
P.S. I want it to be used like this:
async def start_conversation():
async with HelloService() as hs:
answer = await hs.say_hello()
print(answer.decode('utf-8'))
if __name__ == "__main__":
loop = asyncio.get_event_loop()
loop.run_until_complete(start_conversation())

HelloService will probably need to use run_in_executor (which manages a thread pool) to run HelloNetClient methods in the background. For example:
async def say_hello(self):
print('Saying hello...')
loop = asyncio.get_event_loop()
return await loop.run_in_executor(None, self.hello_client.say_hello_net)
This is not an idiomatic use of asyncio, and you're missing out on some of its features - for example, you won't be able to create thousands of clients that all work in parallel, and you won't get reliable cancellation (ability to cancel() any task you wish). Nonetheless, simple usage will work just fine.
Unfortunately the ability to provide custom transports is not of help here because the middle tier, HelloNetClient, expects synchronous behavior. Even if you were to write a custom transport that hooked into asyncio, methods like say_hello_net would still wait for as long as it takes for the response to arrive, so HelloService would have to schedule them in a separate thread. For this reason your best bet is to use the default transport and connect the code with asyncio with the code in the service as shown above.

Python 3 websockets - send message before closing connection

I'm new to Stack Overflow (although have been a long-term "stalker"!) so please be gentle with me!
I'm trying to learn Python, in particular Asyncio using websockets.
Having scoured the web for examples/tutorials I've put together the following tiny chat application, and could use some advice before it gets bulkier (more commands etc) and becomes difficult to refactor.
My main question, is why (when sending the DISCONNECT command) does it need the asyncio.sleep(0) in order to send the disconnection verification message BEFORE closing the connection?
Other than that, am I on the right tracks with the structure here?
I feel that there's too much async/await but I can't quite wrap my head around why.
Staring at tutorials and S/O posts for hours on end doesn't seem to be helping at this point so I thought I'd get some expert advice directly!
Here we go, simple WS server that responds to "nick", "msg", "test" & "disconnect" commands. No prefix required, i.e "nick Rachel".
import asyncio
import websockets
import sys
class ChatServer:
def __init__(self):
print("Chat Server Starting..")
self.Clients = set()
if sys.platform == 'win32':
self.loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(self.loop)
else:
self.loop = asyncio.get_event_loop()
def run(self):
start_server = websockets.serve(self.listen, '0.0.0.0', 8080)
try:
self.loop.run_until_complete(start_server)
print("Chat Server Running!")
self.loop.run_forever()
except:
print("Chat Server Error!")
async def listen(self, websocket, path):
client = Client(websocket=websocket)
sender_task = asyncio.ensure_future(self.handle_outgoing_queue(client))
self.Clients.add(client)
print("+ connection: " + str(len(self.Clients)))
while True:
try:
msg = await websocket.recv()
if msg is None:
break
await self.handle_message(client, msg)
except websockets.exceptions.ConnectionClosed:
break
self.Clients.remove(client)
print("- connection: " + str(len(self.Clients)))
async def handle_outgoing_queue(self, client):
while client.websocket.open:
msg = await client.outbox.get()
await client.websocket.send(msg)
async def handle_message(self, client, data):
strdata = data.split(" ")
_cmd = strdata[0].lower()
try:
# Check to see if the command exists. Otherwise, AttributeError is thrown.
func = getattr(self, "cmd_" + _cmd)
try:
await func(client, param, strdata)
except IndexError:
await client.send("Not enough parameters!")
except AttributeError:
await client.send("Command '%s' does not exist!" % (_cmd))
# SERVER COMMANDS
async def cmd_nick(self, client, param, strdata):
# This command needs a parameter (with at least one character). If not supplied, IndexError is raised
# Is there a cleaner way of doing this? Otherwise it'll need to reside within all functions that require a param
test = param[1][0]
# If we've reached this point there's definitely a parameter supplied
client.Nick = param[1]
await client.send("Your nickname is now %s" % (client.Nick))
async def cmd_msg(self, client, param, strdata):
# This command needs a parameter (with at least one character). If not supplied, IndexError is raised
# Is there a cleaner way of doing this? Otherwise it'll need to reside within all functions that require a param
test = param[1][0]
# If we've reached this point there's definitely a parameter supplied
message = strdata.split(" ",1)[1]
# Before we proceed, do we have a nickname?
if client.Nick == None:
await client.send("You must choose a nickname before sending messages!")
return
for each in self.Clients:
await each.send("%s says: %s" % (client.Nick, message))
async def cmd_test(self, client, param, strdata):
# This command doesn't need a parameter, so simply let the client know they issued this command successfully.
await client.send("Test command reply!")
async def cmd_disconnect(self, client, param, strdata):
# This command doesn't need a parameter, so simply let the client know they issued this command successfully.
await client.send("DISCONNECTING")
await asyncio.sleep(0) # If this isn't here we don't receive the "disconnecting" message - just an exception in "handle_outgoing_queue" ?
await client.websocket.close()
class Client():
def __init__(self, websocket=None):
self.websocket = websocket
self.IPAddress = websocket.remote_address[0]
self.Port = websocket.remote_address[1]
self.Nick = None
self.outbox = asyncio.Queue()
async def send(self, data):
await self.outbox.put(data)
chat = ChatServer()
chat.run()

Your code uses infinite size Queues, which means .put() calls .put_nowait() and returns immediately. (If you do want to keep these queues in your code, consider using 'None' in the queue as a signal to close a connection and move client.websocket.close() to handle_outgoing_queue()).
Another issue: Consider replacing for x in seq: await co(x) with await asyncio.wait([co(x) for x in seq]). Try it with asyncio.sleep(1) to experience a dramatic difference.
I believe a better option will be dropping all outbox Queues and just relay on the built in asyncio queue and ensure_future. The websockets package already includes Queues in its implementation.

I want to point out that the author of websockets indicated in a post on July 17 of 2017 that websockets used to return None when the connection was closed but that was changed at some point. Instead he suggests that you use a try and deal with the exception. The OP's code shows both a check for None AND a try/except. The None check is needlessly verbose and apparently not even accurate since with the current version, websocket.recv() doesn't return anything when the client closes.
Addressing the "main" question, it looks like a race condition of sorts. Remember that asyncio does it's work by going around and touching all the awaited elements in order to nudge them along. If your 'close connection' command is processed at some point ahead of when your queue is cleared, the client will never get that last message in the queue. Adding the async.sleep adds an extra step to the round robin and probably puts your queue emptying task ahead of your 'close connection'.
Addressing the amount of awaits, it's all about how many asynchronous things you need to have happen to accomplish the goal. If you block at any point you'll stop all the other tasks that you want to keep going.

What should I do when I have a blocking thing in the on_message tornado.websocket?

I'm new to tornado.I am trying to build a chat server proxy with tornado,I got the message from the web client,normally it just need to send it back,however,i need to send those message to another server first,here comes the problem,it costs a lot of time to wait the other server response,i need to make it no-blocking,but when i use the anonymous methods of the tornado,it doesn't work at all,help me,thank you very much!
That's the part of my pseudo code：
class ClientWSConnectienter(websocket.WebSocketHandler):
_thread_pool = ThreadPoolExecutor(20)
def initialize(self, room_handler):
#chat room initiate
self.__rh = room_handler
#run_on_executor(executor='_thread_pool')
def worker(self,msg):
#send the msg to another server
pmessage=send_msg_to_server(msg)
return pmessage
#tornado.web.asynchronous
#tornado.gen.coroutine
def on_message(self, message):
#this will blocking for too much time,and I want make it no-blocking
pmessage=yeild worker(msg)
#send the recive pmessage to others client
room.write_message(pmessage)
self.finish()
obviously,it doesn't work,I got something like this:
error:websocket cannot use this method
So,what should I do? thanks a lot
But after I reedit my code,it still blocks in the task part.I don't know why,this is still part of my code
Re_edit:
class ClientWSConnection(websocket.WebSocketHandler):
def initialize(self, room_handler):
self.queue = tornado.queues.Queue()
def open(self, client_id):
IOLoop.current().spawn_callback(self.loop)
def on_message(self, message):
self.queue.put(msg)
def on_close(self):
self.queue.put(None)
#coroutine
def loop(self):
while 1:
msg=yield self.queue.get()
if msg is None:
return
msg=yield self.worker(msg)
pmessage = msg
room.write_message(pmessage)
#coroutine
def worker(self,msg):
#need to send the other server,blocking here
time.sleep(10)
raise Return(msg)

I think that error message is coming from your call to finish(), which is not meaningful for websockets (did you mean close()?). (Also, there's no need to use both #asynchronous and #coroutine; #coroutine alone is sufficient)
But there's a bigger problem: Remember that when overriding methods defined in a superclass, you can only make them a coroutine if the documentation says you can (because coroutines are called differently from regular methods). WebSocketHandler.on_message does not currently (as of Tornado 4.3) support coroutines.
So you need to use a queue to hand this off to another task. Something like this:
class MyHandler(WebSocketHandler):
def initialize(self):
self.queue = tornado.queues.Queue()
def on_open(self):
IOLoop.current().spawn_callback(self.loop)
def one_message(self, msg):
self.queue.put(msg)
def on_connection_close(self):
self.queue.put(None)
#coroutine
def loop(self):
while True:
msg = yield self.queue.get()
if msg is None:
return
pmessage = yield self.worker(msg)
self.write_message(pmessage)

Communicate between asyncio protocol/servers

I'm trying to write a Server Side Events server which I can connect to with telnet and have the telnet content be pushed to a browser. The idea behind using Python and asyncio is to use as little CPU as possible as this will be running on a Raspberry Pi.
So far I have the following which uses a library found here: https://pypi.python.org/pypi/asyncio-sse/0.1 which uses asyncio.
And I have also copied a telnet server which uses asyncio as well.
Both work separately, but I have no idea how to tie both together. As I understand it, I need to call send() in the SSEHandler class from inside Telnet.data_received, but I don't know how to access it. Both of these 'servers' need to be running in a loop to accept new connections, or push data.
Can anyone help, or point me in another direction?
import asyncio
import sse
# Get an instance of the asyncio event loop
loop = asyncio.get_event_loop()
# Setup SSE address and port
sse_host, sse_port = '192.168.2.25', 8888
class Telnet(asyncio.Protocol):
def connection_made(self, transport):
print("Connection received!");
self.transport = transport
def data_received(self, data):
print(data)
self.transport.write(b'echo:')
self.transport.write(data)
# This is where I want to send data via SSE
# SSEHandler.send(data)
# Things I've tried :(
#loop.call_soon_threadsafe(SSEHandler.handle_request());
#loop.call_soon_threadsafe(sse_server.send("PAH!"));
def connection_lost(self, esc):
print("Connection lost!")
telnet_server.close()
class SSEHandler(sse.Handler):
#asyncio.coroutine
def handle_request(self):
self.send('Working')
# SSE server
sse_server = sse.serve(SSEHandler, sse_host, sse_port)
# Telnet server
telnet_server = loop.run_until_complete(loop.create_server(Telnet, '192.168.2.25', 7777))
#telnet_server.something = sse_server;
loop.run_until_complete(sse_server)
loop.run_until_complete(telnet_server.wait_closed())

Server side events are a sort of http protocol; and you may have any number of concurrent http requests in flight at any given moment, you may have zero if nobody is connected, or dozens. This nuance is all wrapped up in the two sse.serve and sse.Handler constructs; the former represents a single listening port, which dispatches each separate client request to the latter.
Additionally, sse.Handler.handle_request() is called once for each client, and the client is disconnected once that co-routine terminates. In your code, that coroutine terminates immediately, and so the client sees a single "Working" event. So, we need to wait, more-or-less forever. We can do that by yield froming an asyncio.Future().
The second issue is that we'll somehow need to get a hold of all of the separate instances of a SSEHandler() and use the send() method on each of them, somehow. Well, we can have each one self-register in their handle_request() methods; by adding each one to a dict which maps the individual handler instances to the future they are waiting on.
class SSEHandler(sse.Handler):
_instances = {}
#asyncio.coroutine
def handle_request(self):
self.send('Working')
my_future = asyncio.Future()
SSEHandler._instances[self] = my_future
yield from my_future
Now, to send an event to every listening we just visit all of the SSEHandler instances registered in the dict we created and using send() on each one.
class SSEHandler(sse.Handler):
#...
#classmethod
def broadcast(cls, message):
for instance, future in cls._instances.items():
instance.send(message)
class Telnet(asyncio.Protocol):
#...
def data_received(self, data):
#...
SSEHandler.broadcast(data.decode('ascii'))
lastly, your code exits when the telnet connection closes. that's fine, but we should clean-up at that time, too. Fortunately, that's just a matter of setting a result on all of the futures for all of the handlers
class SSEHandler(sse.Handler):
#...
#classmethod
def abort(cls):
for instance, future in cls._instances.items():
future.set_result(None)
cls._instances = {}
class Telnet(asyncio.Protocol):
#...
def connection_lost(self, esc):
print("Connection lost!")
SSEHandler.abort()
telnet_server.close()
here's a full, working dump in case my illustration is not obvious.
import asyncio
import sse
loop = asyncio.get_event_loop()
sse_host, sse_port = '0.0.0.0', 8888
class Telnet(asyncio.Protocol):
def connection_made(self, transport):
print("Connection received!");
self.transport = transport
def data_received(self, data):
SSEHandler.broadcast(data.decode('ascii'))
def connection_lost(self, esc):
print("Connection lost!")
SSEHandler.abort()
telnet_server.close()
class SSEHandler(sse.Handler):
_instances = {}
#classmethod
def broadcast(cls, message):
for instance, future in cls._instances.items():
instance.send(message)
#classmethod
def abort(cls):
for instance, future in cls._instances.items():
future.set_result(None)
cls._instances = {}
#asyncio.coroutine
def handle_request(self):
self.send('Working')
my_future = asyncio.Future()
SSEHandler._instances[self] = my_future
yield from my_future
sse_server = sse.serve(SSEHandler, sse_host, sse_port)
telnet_server = loop.run_until_complete(loop.create_server(Telnet, '0.0.0.0', 7777))
loop.run_until_complete(sse_server)
loop.run_until_complete(telnet_server.wait_closed())