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

Food in NYC • Subs • Pizza • Omakase 

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

CPU-bound web app Client Server Clients • Throughput bound by computation
 • No async 

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

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

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

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

What’s async for? Minimizes resources per connection. 

C10K kegel.com/c10k.html 

Why is async hard to code? Backend Client Server request response store state request response time 

Why is async hard to code? Backend Client Server websocket event store state register for events event time 

Ways to store state: Coding difficulty Threads Callbacks Memory per connection 

Ways to store state: • Threads • Callbacks ! ... and: • Coroutines • Greenlets • Fibers • Erlang processes • and so on.... 

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

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

17 

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

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

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? 

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()! 

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 

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 

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? 

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()! 

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 

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

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. 

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

A. Jesse Jiryu Davis 
 @jessejiryudavis 
 MongoDB