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

Transcript

1. What Is Async,
   How Does It Work,
   A. Jesse Jiryu Davis
   

   @jessejiryudavis
   

   MongoDB
   When Should I Use It?
   &
   

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2. Food in NYC
   • Subs
   • Pizza
   • Omakase
   

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

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4. CPU-bound web app
   Client Server
   Clients
   • Throughput bound by computation

   • No async
   

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5. Pizza
   5
   Counter
   ⚇!
   client
   ⚇!
   pizza

   cook
   ☺
   oven
   
   ⚇⚇⚇
   

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6. Normal web app
   Client Server
   • Throughput bound by memory

   • Async
   Backend
   Database,
   OAuth service,
   etc.
   Clients
   

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

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

   • Async
   

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

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10. C10K
    kegel.com/c10k.html
    

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

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

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

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

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15. So, what is async?
    • Single-threaded
    • I/O concurrency

    • Non-blocking sockets
    • epoll / kqueue
    • Event loop
    

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

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

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

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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?
    

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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()!
    

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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
    

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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
    

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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?
    

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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()!
    

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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
    

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

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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.
    

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

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30. A. Jesse Jiryu Davis
    

    @jessejiryudavis
    

    MongoDB
    

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