ZeroMQ is a library used to implement messaging and communication systems between applications and processes - fast and asynchronously.
If you have past experience with other application messaging solutions such as RabbitMQ, it might come a little bit challenging to understand the exact position of ZeroMQ.
When compared to some much larger projects, which offer all necessary parts of enterprise messaging, ZeroMQ remains as just a lightweight and fast tool to craft your own.
Although technically not a framework, given its functionality and the key position it has for the tasks it solves, you can consider ZeroMQ to be the backbone for implementing the actual communication layer of your application.
In this article, we aim to offer you some examples to inspire you with all the things you can do.
Note:We will be working with the Python language and its classic interpreter (Python C interpreter) in our examples. After installing the necessary language bindings, you should be able to simply translate the code and use your favorite instead without any issues.
ZeroMQ as a library works through sockets by following certain network communication patterns. It is designed to work asynchronously, and that's where the MQ suffix to its name comes - from thread queuing messages before sending them.
ZeroMQ differs in the way its sockets work. Unlike the synchronous way the regular sockets work, ZeroMQ's socket implementation "present an abstraction of an asynchronous message queue".
The way these sockets work depend on the type of socket chosen. And flow of messages being sent depend on the chosen patterns, of which there are four:
Request/Reply Pattern:Used for sending a request and receiving subsequent replies for each one sent.
Publish/Subscribe Pattern:Used for distributing data from a single process (e.g. publisher) to multiple recipients (e.g. subscribers).
Pipeline Pattern:Used for distributing data to connected nodes.
Exclusive Pair Pattern:Used for connecting two peers together, forming a pair.
ZeroMQ offers four different types of transport for communication. These are:
In-Process (INPROC): Local (in-process) communication transport.
Inter-Process (IPC): Local (inter-process) communication transport.
TCP: Unicast communication transport using TCP.
PGM: Multicast communication transport using PGM.
ZeroMQ works differently than typical and traditional communication set ups. It can have either side of the link (i.e. either the server or the client) bind and wait for connections. Unlike standard sockets, ZeroMQ works by the notion of knowing that a connection might occur and hence, can wait for it perfectly well.
For structuring your client and server code, it would be for the best to decide and elect one that is more stable as the binding side and the other(s) as the connecting.
Example:
Server Application Client Application
---------------------[ < .. < .. < .. < .. ......................
Bound -> Port:8080 Connects <- Port:8080To solve the problems caused by both ends of the communication being in a dynamic (hence unstable) state, ZeroMQ provides networking devices (i.e. utensils out of the box). These devices connect to two different ports and route the connections across.
Example:
Server App. Device | Forward Client App.
............ > .. > . ]------------------[ < .. < .. .........
Connects 2 Port Binding ConnectsUsing our knowledge from the past section, we will now begin utilizing them to create simple applications.
Note:Below examples usually consist of applications running simultaneously. For example, for a client/server setup to work, you will need to have both the client and the server application running together. One of the ways to do this is by using the tool Linux Screen. To install screen on a CentOS system, remember that you can simply run:yum install -y screen.
In terms of communicating between applications, the request/reply pattern probably forms the absolute classic and gives us a good chance to start with the fundamental basics of ZeroMQ.
Use-cases:
For simple communications between a server and client(s).
Checking information and requesting updates.
Sendingchecksand updates to the server.
Echo or ping/pong implementations.
Socket type(s) used:
Create a "server.py" using nano (nano server.py) and paste the below self-explanatory contents.
import zmq
# ZeroMQ Context
context = zmq.Context()
# Define the socket using the "Context"
sock = context.socket(zmq.REP)
sock.bind("tcp://127.0.0.1:5678")
# Run a simple "Echo" server
while True:
message = sock.recv()
sock.send("Echo: " + message)
print "Echo: " + messageWhen you are done editing, save and e xit by pressing CTRL+X followed with Y.
Create a "client.py" using nano (nano client.py) and paste the below contents.
import zmq
import sys
# ZeroMQ Context
context = zmq.Context()
# Define the socket using the "Context"
sock = context.socket(zmq.REQ)
sock.connect("tcp://127.0.0.1:5678")
# Send a "message" using the socket
sock.send(" ".join(sys.argv[1:]))
print sock.recv()When you are done editing, save and exit by pressing CTRL+X followed with Y.
Note: When working with ZeroMQ library, remember that each thread used to send a message (i.e..send(..)) expects a .recv(..)to follow. Failing to implement the pair will cause exceptions.
Ourserver.pyis set to work as an "echoing" application. Whatever we choose to send to it, it will send it back (e.g. "Echo:message").
Run the server using your Python interpreter:
python server.pyOn another window, send messages using the client application:
python client.py hello world!
# Echo: hello world!Note: To shut down the server, you can use the key combination: Ctrl+C
In the case of publish/subscribe pattern, ZeroMQ is used to establish one or more subscribers, connecting to one or more publishers and receiving continuously what publisher sends (or seeds).
A choice to specify a prefix to accept only such messages beginning with it is available with this pattern.
Use-cases:
Publish/subscribe pattern is used for evenly distributing messages across various consumers. Automatic updates for scoreboards and news can be considered as possible areas to use this solution.
Socket type(s) used:
Create a "pub.py" using nano (nano pub.py) and paste the below contents.
import zmq
import time
# ZeroMQ Context
context = zmq.Context()
# Define the socket using the "Context"
sock = context.socket(zmq.PUB)
sock.bind("tcp://127.0.0.1:5680")
id = 0
while True:
time.sleep(1)
id, now = id+1, time.ctime()
# Message [prefix][message]
message = "1-Update! >> #{id} >> {time}".format(id=id, time=now)
sock.send(message)
# Message [prefix][message]
message = "2-Update! >> #{id} >> {time}".format(id=id, time=now)
sock.send(message)
id += 1When you are done editing, save and exit by pressing CTRL+X followed with Y.
Create a "sub.py" using nano (nano sub.py) and paste the below contents.
import zmq
# ZeroMQ Context
context = zmq.Context()
# Define the socket using the "Context"
sock = context.socket(zmq.SUB)
# Define subscription and messages with prefix to accept.
sock.setsockopt(zmq.SUBSCRIBE, "1")
sock.connect("tcp://127.0.0.1:5680")
while True:
message= sock.recv()
print messageWhen you are done editing, save and exit by pressing CTRL+X followed with Y.
Note: Using the .setsockopt(..)procedure, we are subscribing to receive messages starting with string 1. To receive all, leave it not set (i.e."").
Ourpub.pyis set to work as a publisher, sending two different messages - simultaneously - intended for different subscribers.
Run the publisher to send messages:
python pub.pyOn another window, see the print outs of subscribed content (i.e.1):
python sub.py!
# 1-Update! >> 1 >> Wed Dec 25 17:23:56 2013Note: To shut down the subscriber and the publisher applications, you can use the key combination: Ctrl+C
Very similar in the way it looks to the Publish/Subscribe pattern, the third in line Pipeline pattern comes as a solution to a different kind of problem: distributing messages upon demand.
Use-cases:
Pipelining pattern can be used in cases where are list of queued items need to be routed (i.e. pushed in line) for the one asking for it (i.e. those who pull).
Socket type(s) used:
Create a "manager.py" using nano (nano manager.py) and paste the below contents.
import zmq
import time
# ZeroMQ Context
context = zmq.Context()
# Define the socket using the "Context"
sock = context.socket(zmq.PUSH)
sock.bind("tcp://127.0.0.1:5690")
id = 0
while True:
time.sleep(1)
id, now = id+1, time.ctime()
# Message [id] - [message]
message = "{id} - {time}".format(id=id, time=now)
sock.send(message)
print "Sent: {msg}".format(msg=message)The file manager.py will act as a task allocator.
Create a "worker_1.py" using nano (nano worker_1.py) and paste the below contents.
import zmq
# ZeroMQ Context
context = zmq.Context()
# Define the socket using the "Context"
sock = context.socket(zmq.PULL)
sock.connect("tcp://127.0.0.1:5690")
while True:
message = sock.recv()
print "Received: {msg}".format(msg=message)The file worker_1.py will act as a task processes (consumer/worker).
Our manager.py is set to have a role of an allocator of tasks (i.e. a manager), PUSH ing the items. Likewise, worker_1.py set to work as a worker instance receives these items, when it's done processing by PULL ing down the list.
Run the publisher to send messages:
python manager.pyOn another window, see the print outs of subscribed content (i.e.1):
python worker_1.py!
# 1-Update! >> 1 >> Wed Dec 25 17:23:56 2013Note: To shut down the subscriber and the publisher applications, you can use the key combination: Ctrl+C
Exclusive pair pattern implies and allows establishing one-tone sort of communication channels using the zmq/PAIR socket type.
Create a "bind.py" using nano (nano bind.py) and paste the below contents.
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")When you are done editing, save and exit by pressing CTRL+X followed with Y.
Create a "connect.py" using nano (nano connect.py) and paste the below contents.
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")When you are done editing, save and exit by pressing CTRL+X followed with Y.
You can use the above example to create any bidirectional uni-connection communication applications.
Note: To shut down either, you can use the key combination: Ctrl+C
Article ID: 270
Created On: Sat, Jan 4, 2014 at 3:18 AM
Last Updated On: Sat, Jan 4, 2014 at 3:18 AM
Authored by: ASPHostServer Administrator [asphostserver@gmail.com]
Online URL: http://faq.asphosthelpdesk.com/article.php?id=270