Editing Cfengine3 Messaging (section)

= 0mq =
CFEngine Enterprise offers messaging to some degree. Nodes share information they hold with the Policy Hub. This information can be
* Patches of installed software
* State of the Nodes with regards to promises fulfilled
* Configuration information (CMDB)
Outside this context, however, there is no method available to the agent to share any information it is willing to share, with a monitoring system for instance.<br/>
There are many messaging systems around and CFEngine should be able to interface with any of them, but in the initial stages the need is for a very light weight messaging system. The search for a suitable messaging mechanism eventually led to 0mq, just a library not an application. Could it be less in weight?<br/>
For reasons of simplicity the code examples, so far are in Python. CFEngine is independent of anything but libc and thus the end results of CFEngine messaging  have to be developed in C.
== Information sources ==
* http://nichol.as/zeromq-an-introduction
* https://github.com/zeromq/pyzmq/issues/132
* Author: Lev Givon <lev(at)columbia(dot)edu>
* http://blog.pythonisito.com/2012/08/distributed-systems-with-zeromq.html
* http://blog.scottlogic.com/2015/03/20/ZeroMQ-Quick-Intro.html
* http://lists.zeromq.org/pipermail/zeromq-dev/2011-February/009490.html

== Basic patterns ==
ZeroMQ comes with five basic patterns
* Synchronous Request/Response
* Asynchronous Request/Response
* Publish/Subscribe
* Push/Pull
* Exclusive Pair
Clients and servers can take the role of server, being the one that is listening on a certain socket, IP address and port number, to incoming requests. When it comes to exchanging messages the most stable one is chosen to be the listener.


== Synchronous Request/Response ==
This is the client server model and the most basic pattern. The client sends a request to the server and the server replies to the request. Other names are Request Reply 
=== Sample Message ===

=== Example Code ===
Client Requests
<pre>
socket = context.socket(zmq.REQ)
socket.connect("tcp://wbhs-pkg.webhuis.nl:8080")

#  Do 10 requests, waiting each time for a response
print("Sending request %s ..." % request)
socket.send(b"Hello")

#  Get the reply.
message = socket.recv()
print("Received reply %s [ %s ]" % (request, message))
</pre>
Server Replies
<pre>
socket = context.socket(zmq.REP)
socket.bind("tcp://10.68.71.184:8080")

#  Wait for next request from client
message = socket.recv()
print("Received request: %s" % message)

#  Send reply back to client
socket.send(b"World")
</pre>

== Asynchronous Request/Response ==

=== Example Code ===

== Publish/Subscribe ==
This is comparable to broadcasting.
=== Example Code ===
Server publishes
<pre>
ctx = zmq.Context()
publisher = ctx.socket(zmq.PUB)

publisher.bind("tcp://10.68.71.184:5556")

time.sleep(5)

sequence = 0
id = 0
data =1000

while sequence < 20:
  id += 1;
  data += 1000;
  publisher.send("%i %i %i" % (sequence, id, data));
  sequence += 1
</pre>
Client subscriber receives
<pre>
context = zmq.Context()
socket = context.socket(zmq.SUB)

print("Collecting updates from server...")
socket.connect("tcp://10.68.71.184:5556")

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

for update_nbr in range(5):
    string = socket.recv_string()
    print string
</pre>
Output
<pre>
0 1 2000
1 2 3000
2 3 4000
3 4 5000
4 5 6000
</pre>

== Push/Pull ==
In the official 0mq documentation this pattern is referred to as Parallel Pipeline.
=== Example Code ===



<pre>
# The ventilator
# Binds PUSH socket to tcp://localhost:5557
# Sends batch of tasks to workers via that socket

import zmq
import random
import time

context = zmq.Context()

# Socket to send messages on
sender = context.socket(zmq.PUSH)
sender.bind("tcp://10.68.71.184:5557")

# Socket with direct access to the sink: used to syncronize start of batch
sink = context.socket(zmq.PUSH)
sink.connect("tcp://10.68.71.184:5558")

try:
    raw_input
except NameError:
    # Python 3
    raw_input = input

print("Press Enter when the workers are ready: ")
_ = raw_input()
print("Sending tasks to workers...")

# The first message is "0" and signals start of batch
sink.send(b'0')

# Initialize random number generator
random.seed()

# Send 100 tasks
total_msec = 0
for task_nbr in range(100):

    # Random workload from 1 to 100 msecs
    workload = random.randint(1, 100)
    total_msec += workload

    sender.send_string(u'%i' % workload)

print("Total expected cost: %s msec" % total_msec)

# Give 0MQ time to deliver
time.sleep(1)

</pre>
<pre>
# Worker
# Connects PULL socket to tcp://10.68.71.184:5557
# Collects workloads from ventilator via that socket
# Connects PUSH socket to tcp://10.68.71.184:5558
# Sends results to sink via that socket

import sys
import time
import zmq


context = zmq.Context()

# Socket to receive messages on
receiver = context.socket(zmq.PULL)
receiver.connect("tcp://10.68.71.184:5557")

# Socket to send messages to
sender = context.socket(zmq.PUSH)
sender.connect("tcp://10.68.71.184:5558")

# Process tasks forever
while True:
    s = receiver.recv()

    # Simple progress indicator for the viewer
    sys.stdout.write('.')
    sys.stdout.flush()

    # Do the work
    time.sleep(int(s)*0.001)

    # Send results to sink
    sender.send(b'')

</pre>
<pre>
# Sink
# Binds PULL socket to tcp://10.68.71.184:5558
# Collects results from worker via that socket

import sys
import time
import zmq


context = zmq.Context()

# Socket to receive messages on
receiver = context.socket(zmq.PULL)
receiver.bind("tcp://10.68.71.184:5558")

# Wait for start of batch
s = receiver.recv()

# Start our clock now
tstart = time.time()

# Process 100 confirmations
total_msec = 0
for task_nbr in range(100):
    s = receiver.recv()
    if task_nbr % 10 == 0:
        sys.stdout.write(':')
    else:
        sys.stdout.write('.')
    sys.stdout.flush()

# Calculate and report duration of batch
tend = time.time()
print("Total elapsed time: %d msec" % ((tend-tstart)*1000))
</pre>

== Exclusive Pair ==

=== Example Code ===
<pre>
exclusive-bind
import zmq

# ZeroMQ Context
context = zmq.Context()

# Define the socket using the "Context"
socket = context.socket(zmq.PAIR)
socket.bind("tcp://127.0.0.1:5696")
</pre>
<pre>
import zmq

# ZeroMQ Context
context = zmq.Context()

# Define the socket using the "Context"
socket = context.socket(zmq.PAIR)
socket.connect("tcp://127.0.0.1:5696")
</pre>
<hr>
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