ZeroMQ

What is ZeroMQ?

1. Just another MQ
2. Aha, so it's just plain old sockets.
3. But messaging, isn't that a solved problem? (Maybe with ZeroMQ)

• An asynchronous messaging library
• Acts like a concurrency framework
• Fast
• Used by banks for trading systems
• 0MQ, ZMQ, ØMQ, ZeroMQ
• Don't need no Message Broker
• The API is similar to the socket API
• The same people that drafted AMQP created ZeroMQ
• Bindings to most/all languages

Transport layer

• TCP
• PGM (multicast)
• IPC (inter-process)
• ITC/INPROC (inter-thread)

Funfact: Can have higher throughput than TCP because of message batching.

The point of a messaging system

$$$

Because a client may easily send work to more servers, you have a way of getting more horsepower without writing a lot of code.

Even poorly implemented code can have a new life with scaling by the assistance of the messaging middleware.

Design criteria in a messaging system

• Whether messages are transferred reliably
• Whether messages are guaranteed to be delivered in order
• Whether messages are passed
  • one-to-one (unicast)
  • one-to-many (multicast/broadcast)
  • many-to-one (client-server)
  • many-to-many
• Whether communication is synchronous or asynchronous.

Source: Wikipedia

Request-reply

RPC and classic client-server

Publish-subscribe

Data distribution

Push-pull (pipeline)

The Broker/Queue

Broker - Advantages

• Apps don't need to care about the whereabouts of other apps
• Sender and receiver lifetimes don't have to overlap
• The app can fail if the broker has received the message

Broker - Drawbacks

• Excessive amount of network communication
• Can turn in to a bottleneck, or SPOF

Brokers in ZeroMQ

Zero M Q

But that doesn't mean we can't create one.

Example 1 - request and reply

ctx = zmq.Context()
socket = ctx.socket(zmq.REQ)
socket.connect("tcp://0.0.0.0:12345")

while True:
    socket.send("Hello from %s" % sys.argv[1])
    print socket.recv() # Blocking
                        

client.py

Example 1 - request and reply

ctx = zmq.Context()
socket = ctx.socket(zmq.REP)
socket.bind("tcp://*:12345")

while True:
    request = socket.recv() # Blocking
    print 'Got message: %s' % request
    time.sleep(.5)
    socket.send('Hello world back at ya from the server')
                        

server.py

Example 2 - publish and subscribe

context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.bind("tcp://*:5556")

while True:
    socket.send("Arsenal:red card")

    time.sleep(1)
                        

publisher.py

Example 2 - publish and subscribe

context = zmq.Context()
socket = context.socket(zmq.SUB)

socket.connect ("tcp://localhost:5556")

socket.setsockopt(zmq.SUBSCRIBE, 'Arsenal')

while True:
    string = socket.recv()
    print string
                        

subscriber.py