Python’s asynchronous frameworks, like asyncio, Tornado, and Twisted, are increasingly important for writing high-performance web applications. Even if you’re an experienced web programmer, you may lack a rigorous understanding of how these frameworks work and when to use them. Let’s see how asyncio's event loop works, and learn how to efficiently handle very large numbers of concurrent connections.

1. What Is Async,
How Does It Work,
A. Jesse Jiryu Davis
@jessejiryudavis
MongoDB
When Should I Use It?
&

2. Food in NYC
• Subs
• Pizza
• Omakase

3. Subs
Counter
⚇!
client
⚇!
sandwich!
maker
pool
☺

4. CPU-bound web app
Client ServerClients
• Throughput bound by computation
• No async

5. Pizza
5
Counter
⚇!
client
⚇!
pizza
cook
☺
oven
🍕
⚇⚇⚇

6. Normal web app
Client Server
• Throughput bound by memory
• Async
Backend
Database,
OAuth service,
etc.
Clients

7. Omakase
7
Counter
⚇!
waiter
kitchen
⚇⚇⚇!
clients

8. Websocket application
Client Server
sockets
Events
sockets
Clients
• Number of clients bound by memory
• Async

9. What’s async for?
Minimizes resources per connection.

10. C10K
kegel.com/c10k.html

11. Why is async hard to code?
BackendClient Server
request
response
store state
request
response
time

12. Why is async hard to code?
BackendClient Server
websocket
event
store state
register for events
event
time

13. Ways to store state:
Coding difficulty
Threads
Callbacks
Memoryperconnection

14. Ways to store state:
• Threads
• Callbacks
!
... and:
!
• Coroutines
• Greenlets
!
and so on....

15. So, what is async?
• Single-threaded
• I/O concurrency
• Non-blocking sockets
• epoll / kqueue
• Event loop

16. asyncio
• AKA “Tulip”
• Python 3.4 standard library
• Implements PEP 3156
• Standard event loop
• Coroutines

17. 17

18. Layers
18
Application
Protocol
Transport
Event Loop
Selectors
asyncio {
autobahn websockets
example.py

19. from autobahn import (WebSocketServerProtocol,!
WebSocketServerFactory)!
example.py

20. clients = set()!
!
class ChatProtocol(WebSocketServerProtocol):!
def onConnect(self):!
clients.add(self)!
!
def onMessage(self, msg):!
for c in clients:!
if c is not self:!
c.sendMessage(msg)!
!
def onClose(self):!
clients.remove(self)!
example.py
How is this called?

21. Let’s look at this
example.py
factory = WebSocketServerFactory(!
"ws://localhost:8888")!
!
factory.protocol = ChatProtocol!
!
loop = asyncio.get_event_loop()!
asyncio.Task(!
loop.create_server(factory, '127.0.0.1', 8888))!
!
loop.run_forever()!

22. class BaseEventLoop(events.AbstractEventLoop):!
def create_server(!
self, protocol_factory, host, port):!
!
sock = socket.socket(...)!
sock.bind(...)!
sock.listen()!
sock.setblocking(False)!
!
fd = sock.fileno()!
self._selector.register(!
fd,!
selectors.EVENT_READ,!
(self._accept_connection, None))!
Magic
Let’s look at this
asyncio
reader, writer

23. asyncio
class BaseEventLoop(events.AbstractEventLoop):!
def _accept_connection(!
self, protocol_factory, sock):!
conn, addr = sock.accept()!
conn.setblocking(False)!
!
protocol = protocol_factory()!
_SelectorSocketTransport(!
self, conn, protocol)!
class _SelectorSocketTransport(_SelectorTransport):!
def __init__(self, loop, sock, protocol):!
super().__init__(loop, sock, protocol)!
self._protocol.connection_made(self)!
ChatProtocol
This was our goal

24. asyncio
class BaseEventLoop(events.AbstractEventLoop):!
def _accept_connection(!
self, protocol_factory, sock):!
conn, addr = sock.accept()!
conn.setblocking(False)!
!
protocol = protocol_factory()!
_SelectorSocketTransport(!
self, conn, protocol)!
But how exactly is this called?

25. Let’s look at this
example.py
factory = WebSocketServerFactory(!
"ws://localhost:8888")!
!
factory.protocol = ChatProtocol!
!
loop = asyncio.get_event_loop()!
asyncio.Task(!
loop.create_server(factory, '127.0.0.1', 8888))!
!
loop.run_forever()!

26. asyncio
magic
class BaseEventLoop(events.AbstractEventLoop):!
def run_forever(self):!
while True:!
event_list = self._selector.select()!
!
for fd, mask, data in event_list:!
reader, writer = data!
!
if reader and mask & EVENT_READ:!
self._ready.append(reader)!
!
if writer and mask & EVENT_WRITE:!
self._ready.append(writer)!
!
ntodo = len(self._ready)!
for i in range(ntodo):!
callback = self._ready.popleft()!
callback()!
accept_connection

27. Application asyncio’s event loop
start_server()
register(fd,!
accept_connection)
accept_connection()
run_forever()
onConnect()

28. Review
• asyncio uses non-blocking sockets.
!
• Event loop tracks sockets, and the
callbacks waiting for them.
!
• selectors: wait for network events.
!
• Event loop runs callbacks.

29. Should I Use It?
Yes:
• Slow backend
• Websockets
• Many connections
29
No:
• CPU-bound
• No async driver
• No async expertise
🍕

30. A. Jesse Jiryu Davis
@jessejiryudavis
MongoDB