Development

0MQ: A new approach to messaging

January 20, 2010

This article was contributed by Martin Sustrik & Martin Lucina

BSD sockets have been used in thousands of applications over the years, but they suffer from some limitations. The low-level nature of the socket API leads developers to reimplementing the same functionality on top of sockets over and over again. Alternatives exist in the form of various "I/O frameworks" and "enterprise messaging systems" but both of these approaches have their own set of drawbacks. The former are generally bound to a certain programming languages or paradigms, while the latter tend to be bloated, proprietary solutions with resident daemons that hog system resources.

0MQ ("Zero-Em-Queue") is a messaging system that tackles these issues by taking a different approach. Instead of inventing new APIs and complex wire protocols, 0MQ extends the socket API, eliminating the learning curve and allowing a network programmer to master it in a couple of hours. The wire protocols are simplistic, even trivial. Performance matches and often exceeds that of raw sockets.

A client/server example

Let's have a look at a trivial example of 0MQ usage. Say we want to implement an SQL server. The networking part of the code-base is fairly complex; we have to manage multiple connections in a non-blocking fashion, a pool of worker threads is needed to send large result sets in the background allowing the SQL engine to process new requests in the meantime, and so on.

Here's how we can accomplish this using the 0MQ C language bindings:

    #include <zmq.h>

    int main () 
    {
        void *ctx, *s;
        zmq_msg_t query, resultset;

        /* Initialise 0MQ context, requesting a single application thread
           and a single I/O thread */
        ctx = zmq_init (1, 1, 0);

        /* Create a ZMQ_REP socket to receive requests and send replies */
        s = zmq_socket (ctx, ZMQ_REP);

        /* Bind to the TCP transport and port 5555 on the loopback interface */
        zmq_bind (s, "tcp://lo:5555");

        while (1) {
            /* Allocate an empty message to receive a query into */
            zmq_msg_init (&query);

            /* Receive a message, blocks until one is available */
            zmq_recv (s, &query, 0);

            /* Allocate a message for sending the resultset */
            zmg_msg_init (&resultset);

            /* TODO: Process the query here and fill in the resultset */
            
            /* Deallocate the query */
            zmq_msg_close (&query);

            /* Send back our canned response */
            zmq_send (s, &resultset, 0);
            zmq_msg_close (&resultset);
        }
    }

This example shows us several basic principles of 0MQ:

0MQ transports data as messages, represented in this example by zmq_msg_t. 0MQ considers a message to be a discrete unit of transport, and message data as an opaque blob. This is considered a deliberate improvement compared to systems like CORBA with their 1000+ pages of core specification. Developers can always use a third-party library such as Google protocol buffers if they do not wish to write custom serialization code.
0MQ supports different socket types which are specified at socket creation time and implement different messaging patterns. In this example we use ZMQ_REP which stands for the replier pattern, meaning that we wish to receive requests from many sources and send replies back to the original requester.
zmq_init() has two arguments related to threads. The first argument is the maximum number of application threads that will access the 0MQ API. The second argument specifies the size of the I/O thread pool 0MQ will create and use to retrieve and send messages in the background. For instance, when sending a large result set the send() function will return immediately and the actual work of pushing the data to the network will be offloaded to an I/O thread in the background.
For the sake of brevity, error handling has been omitted in this example. The 0MQ C binding uses standard POSIX conventions, so most functions return 0 on success and -1 on failure, with the actual error code being stored in errno. 0MQ also provides a zmq_strerror() function to handle it's specific error codes.

The client code is equally simple (C++):

    #include <string.h>
    #include <stdio.h>
    #include <zmq.hpp>

    int main ()
    {
        try {
            // Initialise 0MQ context with one application and one I/O thread
            zmq::context_t ctx (1, 1);

            // Create a ZMQ_REQ socket to send requests and receive replies
            zmq::socket_t s (ctx, ZMQ_REQ);

            // Connect it to port 5555 on localhost using the TCP transport
            s.connect ("tcp://localhost:5555");

            // Construct an example message containing our query
            const char *query_string = "SELECT * FROM mytable";
            zmq::message_t query (strlen (query_string) + 1);
            memcpy (query.data (), query_string, strlen (query_string) + 1);

            // Send the query
            s.send (query);

            // Receive the result
            zmq::message_t resultset;
            s.recv (&resultset);

            // TODO: Process the resultset here
        }
        catch (std::exception &e) {
            // 0MQ throws standard exceptions just like any other C++ API
            printf ("An error occurred: %s\n", e.what());
            return 1;
        }

        return 0;
    }

This example uses the ZMQ_REQ socket type, which specifies the requester messaging pattern, meaning that we wish to send requests, which may be possibly load-balanced to all peers listening for such requests, and we wish to receive replies to our requests. The example also nicely shows how the 0MQ C++ language bindings map to the native language features and allow us to use exceptions for error handling.

One socket, many endpoints: A publish/subscribe example

Now, let's have a look at another common pattern used in messaging, where one application (the publisher) publishes a stream of messages while other interested applications (subscribers) receive and process the messages.

The publisher application (Java):

    import org.zmq.*;

    class publisherApp
    {
        public static void main (String [] args)
        {
            // Initialise 0MQ with a single application and I/O thread
            org.zmq.Context ctx = new org.zmq.Context (1, 1, 0);

            // Create a PUB socket for port 5555 on the eth0 interface
            org.zmq.Socket s = new org.zmq.Socket (ctx, org.zmq.Socket.PUB);
            s.bind ("tcp://eth0:5555");

            for (;;) {
                // Create a new, empty, 10 byte message
                byte msg [] = new byte [10];

                // TODO: Fill in the message here

                // Publish our message   
                s.send (msg, 0);
            }
        }
    }

The subscriber application (Python):

    import libpyzmq

    def main ():
        
        # Initialise 0MQ with a single application and I/O thread
        ctx = libpyzmq.Context (1, 1)

        # Create a SUB socket ...
        s = libpyzmq.Socket (ctx, libpyzmq.SUB)

        # ... ask to subscribe to all messages ...
        s.setsockopt (libpyzmq.SUBSCRIBE , "")

        # ... request the tcp transport with the endpoint myserver.lan:5555
        s.connect ("tcp://myserver.lan:5555")

        while True:
            # Receive one message
            msg = s.recv ()

            # TODO: Process the message here
     
    if __name__ == "__main__":
        main ()

As with our previous examples, we have deliberately omitted error handling and processing code for the sake of brevity. Error handling in both the Java and Python bindings is implemented using native exceptions.

These examples demonstrate the following:

Message subscriptions using the setsockopt() call. Using this call the subscriber indicates that it is only interested in the subset of messages sent by the publisher starting with the specified string. For example, to subscribe to only those messages beginning with ABC we would use the call:
```
    s.setsockopt (libpyzmq.SUBSCRIBE , "ABC")
```
To boost performance 0MQ subscriptions are simple plain strings rather than regular expressions, however, you can use them for simple prefix-style matching where a subscription to animals. would match messages such as animals.dogs and animals.cats.

Now, let's look at a more complex scenario such as one that is often encountered in the stock trading business. We want to send a high volume message feed of stock prices to multiple applications, some of which are located on our local LAN and others which are located at branch offices connected via slow and expensive WAN links. The message load is so high that sending the feed to each receiver on our LAN, individually using TCP, would exhaust our LAN bandwidth.

For the subscribers located on our local LAN, the ideal solution would be to use a multicast transport. 0MQ supports a reliable multicast protocol known as Pragmatic General Multicast (PGM) which suits this purpose ideally. Many LWN readers may not have heard of PGM — without going into too much detail here we can say that it's an industry standard protocol specified in RFC 3208 and implemented mostly by proprietary messaging and operating system vendors such as Tibco, IBM, and Microsoft. Luckily, the excellent Open Source OpenPGM implementation exists and is used by 0MQ.

Back to our stock trading example: While using a PGM transport is fine on our local LAN, multicast won't work too well for our overseas offices, so we really want to be able to talk to those using plain old TCP. In terms of code, we want something like this to bind our sending socket to both a TCP port as well as a multicast group:

    s.bind ("tcp://eth0:5555");
    s.bind ("pgm://eth0;224.0.0.1:5555");

This example shows off two major features of 0MQ. First, the ability to bind or connect a socket to multiple endpoints by simply calling bind() and/or connect() more than once. Second, the use of different underlying transports for a socket. 0MQ supports several transports, of which the most important are tcp, pgm, and inproc (optimized for sending messages between threads within the same application). Using the same API for in-process and remote communication allows us to easily move computations from local worker threads to remote hosts and vice versa.

While the example above works, it results in each overseas receiver creating a separate TCP connection to the publisher in our main office which may result in messages being transferred multiple times over the WAN link. What we would like instead is a proxy application that would run at each branch office (ideally directly on it's edge router to minimize the number of network hops, thus reducing latency to a minimum) connecting to the publisher and re-distributing the messages on the branch office LAN via reliable multicast.

For the scenario described above we can use zmq_forwarder which is part of the 0MQ distribution. Running zmq_forwarder with a simple XML configuration file that describes the inbound and outbound connections is all that is needed.

Performance

Of course, none of this is any good if your code runs slowly. Assuming recent and well-tuned hardware, the end to end latency for transferring a small message (64 bytes or less) on a 1GbE LAN using 0MQ is approximately 30-40 microseconds. As for throughput, when transferring a single stream of one byte messages 0MQ achieves rates of up to 3-4 million messages per second.

Observant readers will note that achieving these throughput figures with raw BSD sockets is impossible. The 0MQ approach is to use "message batching" (i.e. sending messages in batches rather than one by one) thus avoiding frantic up-and-down traversal of the networking stack. Thanks to smart batching algorithms, message batching has almost no impact on latency.

The memory footprint of 0MQ will be particularly interesting to embedded developers, as it is much smaller that of conventional messaging systems. For instance on Linux/x86, the core code occupies only a couple of pages in resident memory.

Conclusion

While 0MQ is still a young project which is evolving rapidly, it is an interesting and powerful alternative for those who prefer a messaging system which emphasizes simplicity, efficiency, and low footprint over the complex bells and whistles of most current enterprise messaging systems. Particularly noteworthy is 0MQ's extension of the well known socket APIs and it's ambition to be "just a transport" which gets out of the way rather than yet another bloated messaging or RPC framework.

0MQ is licensed under the LGPL and you can download it on the project website. API documentation is provided in the form of traditional UNIX man pages and help can be found on the community mailing list or IRC channel. The project also has a GIT repository and is accepting contributions from the wider community.

As of this writing, 0MQ runs on Linux, Windows, Solaris, and many other POSIX platforms. Language bindings available include C, C++, Common Lisp, Java, Python, Ruby, and more. Unfortunately, packaging is lagging behind, so community contributions in this area would be very helpful. Due to the closed nature of stock trading systems it's hard to get a handle on actual adoption of 0MQ in the wild, but there is one case study available.

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A client/server example

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Conclusion

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