Exchanging Messages

The examples here all refer to the messenger module.

Send & receive a message

Process A

import networkzero as nw0

address = nw0.advertise("messenger1")

message = nw0.wait_for_message_from(address)
print("Message received: ", message)
nw0.send_reply_to(address, "Received: %s" % message)

Process B

import networkzero as nw0

service = nw0.discover("messenger1")

reply = nw0.send_message_to(service, "This is a message")
print("Reply: ", reply)

Discussion

The message exchange in networkzero works on a very simple message-reply model: one process waits for a message from another and sends a reply. The process sending the original message waits for the reply before carrying on. This is less flexible that network communications usually are (the sending process must wait for a reply before carrying on) but cuts out the complications which arise when several processes are sending messages to the same listening process.

Send a message without expecting a reply

Process A

import networkzero as nw0

address = nw0.advertise("messenger2")

message = nw0.wait_for_message_from(address, autoreply=True)
print("Message received: ", message)

Process B

import networkzero as nw0

service = nw0.discover("messenger2")

nw0.send_message_to(service, "This is a command")

Discussion

The message exchange in networkzero always requires a reply to any message. But sometimes, no reply is needed, for example when sending commands to a robot. It would be possible for the process which receives a message to send a dummy reply and for the process which sent the message to ignore any reply.

This situation is so common that there is a special parameter autoreply to the wait_for_message_from function. Behind the scenes, networkzero completes the message-reply contract for you by sending an empty reply. The process which sent the message can simply ignore any reply.

Send news

Process A

import networkzero as nw0
import random
import time

address = nw0.advertise("news1")

#
# In lieu of an actual temperature sensor!
#
temperatures = range(15, 20)
while True:
    temperature = random.choice(temperatures) + random.random()
    nw0.send_news_to(address, "temperature", temperature)
    time.sleep(1.0)

Process B and C and D...

import networkzero as nw0

address = nw0.discover("news1")

while True:
    topic, temperature = nw0.wait_for_news_from(address)
    print("Temperature is:", temperature)

Discussion

One process can send news to any process which is listening. It doesn’t matter if no-one is listening or if 100 other machines are listening. This could be used, for example, to have one machine linked directly to a temperature sensor outside a classroom which broadcasts the temperature once a minute.

Send news on a specific topic

Process A

import networkzero as nw0
import random
import time

address = nw0.advertise("news2")

#
# In lieu of an actual temperature sensor!
#
temperatures = range(15, 20)
humidities = range(0, 100)
while True:
    temperature = random.choice(temperatures) + random.random()
    humidity = random.choice(humidities)
    nw0.send_news_to(address, "temperature", temperature)
    time.sleep(random.random())
    nw0.send_news_to(address, "humidity", humidity)
    time.sleep(random.random())

Process B

import networkzero as nw0

address = nw0.discover("news2")

while True:
    topic, temperature = nw0.wait_for_news_from(address, "temperature")
    print("Temperature is:", temperature)

Process C

import networkzero as nw0

address = nw0.discover("news2")

while True:
    topic, humidity = nw0.wait_for_news_from(address, "humidity")
    print("Humidity is:", humidity)

Discussion

One process can send news on a variety of topics. Other processes can choose to listen to a specific topic or to all topics. In this example, Process A is sending news on temperature and humidity. Process B is listening to the temperature feed while process C is listening to the data about humidity.