Basic communication patterns

Push-Pull

Example

Now that we understand the basics of how proxies, agents and name servers work, let us jump into a more interesting example.

As mentioned before, a multi-agent system only makes sense if agents are connected with each other and share some information using message passing.

In this first example, we will create two agents: Alice and Bob, and we will make alice send messages to Bob using a simple push-pull communication pattern.

import time
from osbrain import run_agent
from osbrain import run_nameserver


def log_message(agent, message):
    agent.log_info('Received: %s' % message)


if __name__ == '__main__':

    # System deployment
    run_nameserver()
    alice = run_agent('Alice')
    bob = run_agent('Bob')

    # System configuration
    addr = alice.bind('PUSH', alias='main')
    bob.connect(addr, handler=log_message)

    # Send messages
    while True:
        time.sleep(1)
        alice.send('main', 'Hello, Bob!')

So, in this case, we are doing some more stuff. After we spawn Alice and Bob, we connect them.

First, we make Alice bind:

addr = alice.bind(‘PUSH’, alias=’main’)

There are three things to remark in that line:

  1. The first parameter 'PUSH' represents the communication pattern we want to use. In this case we are using a simple push-pull (unidirectional) pattern to allow Alice to send messages to Bob.
  2. The second parameter is, once again, an alias. We can use this alias to refer to this communication channel in an easier way.
  3. The binding, as you already guessed, takes place in the remote agent, but it actually returns a value, which is the address the agent binded to. This address is serialized back to us so we can use it to connect other agents to it.

The next interesting line of code is the one in which Bob connects to Alice:

bob.connect(addr, handler=log_message)

There are two things to remark in here:

  1. Calling connect() from an agent requires, first, an address. This address is, in this case, the one we got after binding Alice. This method will automatically select the appropriate communication pattern to connect to this pattern ('PULL' in this case).
  2. Bob will be receiving messages from Alice, so we must set a handler function that will be executed when a message from Alice is received. This handler will be serialized and stored in the remote agent to be executed there when needed.

The handler function, in its most basic form, accepts two parameters:

def handler(agent, message):
    ...
  1. The actual agent (can be named self as well, in an OOP way).
  2. The message that is received.

In the example above, the handler simply logs the message received.

List of handlers

When using push-pull communication patterns we are allowed to set multiple handlers using a list. In example:

agent.connect('PULL', handler=[handler1, handler2, handler3])

Note that in this case all handlers will be executed in sequence.

Request-Reply

Example

Another common communication pattern is the request-reply, in which a requester sends a message to the replier and expects always a reply back. It is sometimes useful, specially when some kind of synchronization is required.

from osbrain import run_agent
from osbrain import run_nameserver


def reply(agent, message):
    return 'Received ' + str(message)


if __name__ == '__main__':

    run_nameserver()
    alice = run_agent('Alice')
    bob = run_agent('Bob')

    addr = alice.bind('REP', alias='main', handler=reply)
    bob.connect(addr, alias='main')

    for i in range(10):
        bob.send('main', i)
        reply = bob.recv('main')
        print(reply)

The main difference with respect to the push-pull pattern is that, in this case, Bob must run the recv method in order to get the reply back from Alice.

Note

Although the requester is not required to immediately await for the reply (i.e.: can do other stuff after sending the request and before receiving the response), it is required to receive a reply back before making another request through the same communication channel. Multiple requests can be made from the same agent as long as it uses different communication channels for each request.

Return versus yield

The easiest way to reply to a request is to return a value from the handler, as seen in Request-Reply:

def reply(agent, message):
    return 'Received ' + str(message)

However, using return the agent can only send a response after executing the handler. Instead, an agent can use yield to reply earlier if needed:

def reply(agent, message):
    yield 'Received' + str(message)  # Reply now
    agent.log_info('Already sent a reply back!')   # Do some stuff later

Publish-Subscribe

Example

One of the most useful communication patterns between agents is the publish and subscribe pattern. The publisher will send messages to all subscribed agents.

Here is an example in which Alice is the publisher and Bob and Eve subscribe to Alice. This way, when Alice sends a message, both Bob and Eve will receive it:

import time
from osbrain import run_agent
from osbrain import run_nameserver


def log_message(agent, message):
    agent.log_info('Received: %s' % message)


if __name__ == '__main__':

    # System deployment
    run_nameserver()
    alice = run_agent('Alice')
    bob = run_agent('Bob')
    eve = run_agent('Eve')

    # System configuration
    addr = alice.bind('PUB', alias='main')
    bob.connect(addr, handler=log_message)
    eve.connect(addr, handler=log_message)

    # Send messages
    while True:
        time.sleep(1)
        alice.send('main', 'Hello, all!')

Note the similarities between this example and the Sender-Receiver example. The only differences are that Alice is now binding using the 'PUB' pattern and that, instead of having just Bob connecting to Alice, we now have Eve as well connecting to Alice.

This communication pattern allows for easy filtering. Refer to the Filtering section in the tutorial for more details.

Filtering

The publish-subscribe pattern is very useful, but it is also very powerful when combined with filtering.

Any time we publish a message from an agent, a topic can be specified. If a topic is specified, then only the agents that are subscribed to that topic will receive the message. This filtering is done in the publisher side, meaning that the network does not suffer from excessive message passing.

In the following example we have Alice publishing messages using topic a or b at random. Then we have Bob subscribed to both topics, Eve subscribed to topic a only and Dave subscribed to topic b only.

import time
import random
from osbrain import run_agent
from osbrain import run_nameserver


def log_a(agent, message):
    agent.log_info('Log a: %s' % message)


def log_b(agent, message):
    agent.log_info('Log b: %s' % message)


if __name__ == '__main__':

    # System deployment
    run_nameserver()
    alice = run_agent('Alice')
    bob = run_agent('Bob')
    eve = run_agent('Eve')
    dave = run_agent('Dave')

    # System configuration
    addr = alice.bind('PUB', alias='main')
    bob.connect(addr, handler={'a': log_a, 'b': log_b})
    eve.connect(addr, handler={'a': log_a})
    dave.connect(addr, handler={'b': log_b})

    # Send messages
    while True:
        time.sleep(1)
        topic = random.choice(['a', 'b'])
        message = 'Hello, %s!' % topic
        alice.send('main', message, topic=topic)

Note how we can specify different handlers for different topics when subscribing agents.