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ZeroMQ: The Design of Messaging Middleware


It turns out that shutting down a fully asynchronous system in a clean way is a dauntingly complex task. Trying to shut down a thousand moving parts, some of them working, some idle, some in the process of being initiated, some of them already shutting down by themselves, is prone to all kinds of race conditions, resource leaks and similar. The shutdown subsystem is definitely the most complex part of ØMQ. A quick check of the bug tracker indicates that some 30--50% of reported bugs are related to shutdown in one way or another.

Lesson learned: When striving for extreme performance and scalability, consider the actor model; it's almost the only game in town in such cases. However, if you are not using a specialized system like Erlang or ØMQ itself, you'll have to write and debug a lot of infrastructure by hand. Additionally, think, from the very beginning, about the procedure to shut down the system. It's going to be the most complex part of the codebase and if you have no clear idea how to implement it, you should probably reconsider using the actor model in the first place.

Lock-Free Algorithms

Lock-free algorithms have been in vogue lately. They are simple mechanisms for inter-thread communication that don't rely on the kernel-provided synchronization primitives, such as mutexes or semaphores; rather, they do the synchronization using atomic CPU operations, such as atomic compare-and-swap (CAS). It should be understood that they are not literally lock-free — instead, locking is done behind the scenes at the hardware level.

ØMQ uses a lock-free queue in pipe objects to pass messages between the user's threads and ØMQ's worker threads. There are two interesting aspects to how ØMQ uses the lock-free queue.

First, each queue has exactly one writer thread and exactly one reader thread. If there's a need for 1-to-N communication, multiple queues are created (Figure 8). Given that this way the queue doesn't have to take care of synchronizing the writers (there's only one writer) or readers (there's only one reader) it can be implemented in an extra-efficient way.

OMQ
Figure 8: Queues.

Second, we realized that while lock-free algorithms were more efficient than classic mutex-based algorithms, atomic CPU operations are still rather expensive (especially when there's contention between CPU cores) and doing an atomic operation for each message written and/or each message read was slower than we were willing to accept.

The way to speed it up was batching once again. Imagine you had 10 messages to be written to the queue. It can happen, for example, when you receive a network packet containing 10 small messages. Receiving a packet is an atomic event; you cannot get half of it. This atomic event results in the need to write 10 messages to the lock-free queue. There's not much point in doing an atomic operation for each message. Instead, you can accumulate the messages in a "pre-write" portion of the queue that's accessed solely by the writer thread, and then flush it using a single atomic operation.

The same applies to reading from the queue. Imagine the 10 messages above were already flushed to the queue. The reader thread can extract each message from the queue using an atomic operation. However, it's overkill; instead, it can move all the pending messages to a "pre-read" portion of the queue using a single atomic operation. Afterwards, it can retrieve the messages from the "pre-read" buffer one by one. "Pre-read" is owned and accessed solely by the reader thread and thus no synchronization whatsoever is needed in that phase.

The arrow on the left of Figure 9 shows how the pre-write buffer can be flushed to the queue simply by modifying a single pointer. The arrow on the right shows how the whole content of the queue can be shifted to the pre-read by doing nothing but modifying another pointer.

OMQ
Figure 9: Lock-free queue.

Lesson learned: Lock-free algorithms are hard to invent, troublesome to implement and almost impossible to debug. If at all possible, use an existing proven algorithm rather than inventing your own. When extreme performance is required, don't rely solely on lock-free algorithms. While they are fast, the performance can be significantly improved by doing smart batching on top of them.

API

The user interface is the most important part of any product. It's the only part of your program visible to the outside world and if you get it wrong, the world will hate you. In end-user products it's either the GUI or the command line interface. In libraries it's the API.

In early versions of ØMQ the API was based on AMQP's model of exchanges and queues. (See the AMQP specification.) From a historical perspective it's interesting to have a look at the white paper from 2007 that tries to reconcile AMQP with a brokerless model of messaging. I spent the end of 2009 rewriting it almost from scratch to use the BSD Socket API instead. That was the turning point; ØMQ adoption soared from that point on. While before it was a niche product used by a bunch of messaging experts, afterwards it became a handy commonplace tool for anybody. In a year or so, the size of the community increased tenfold, some 20 bindings to different languages were implemented, etc.

The user interface defines the perception of a product. With basically no change to the functionality — just by changing the API — ØMQ changed from an "enterprise messaging" product to a "networking" product. In other words, the perception changed from "a complex piece of infrastructure for big banks" to "hey, this helps me to send my 10-byte-long message from application A to application B".

Lesson learned: Understand what you want your project to be and design the user interface accordingly. Having a user interface that doesn't align with the vision of the project is a 100% guaranteed way to fail.

One of the important aspects of the move to the BSD Sockets API was that it wasn't a revolutionary freshly invented API, but an existing and well-known one. Actually, the BSD Sockets API is one of the oldest APIs still in active use today; it dates back to 1983 and 4.2BSD Unix. It's been widely used and stable for literally decades.

The above fact brings a lot of advantages. Firstly, it's an API that everybody knows, so the learning curve is very short. Even if you've never heard of ØMQ, you can build your first application in couple of minutes thanks to the fact that you are able to reuse your BSD Sockets knowledge.

Additionally, using a widely implemented API enables integration of ØMQ with existing technologies. For example, exposing ØMQ objects as "sockets" or "file descriptors" allows for processing TCP, UDP, pipe, file and ØMQ events in the same event loop. Another example: the experimental project to bring ØMQ-like functionality to the Linux kernel turned out to be pretty simple to implement. By sharing the same conceptual framework it can re-use a lot of infrastructure already in place.

Most importantly, the fact that the BSD Sockets API has survived almost three decades despite numerous attempts to replace it means that there is something inherently right in the design. BSD Sockets API designers have — whether deliberately or by chance — made the right design decisions. By adopting the API we can automatically share those design decisions without even knowing what they were and what problem they were solving.

Lesson learned: While code reuse has been promoted from time immemorial and pattern reuse joined in later on, it's important to think of reuse in an even more generic way. When designing a product, have a look at similar products. Check which have failed and which have succeeded; learn from the successful projects. Don't succumb to Not Invented Here syndrome. Reuse the ideas, the APIs, the conceptual frameworks, whatever you find appropriate. By doing so you are allowing users to reuse their existing knowledge. At the same time you may be avoiding technical pitfalls you are not even aware of at the moment.

Messaging Patterns

In any messaging system, the most important design problem is that of how to provide a way for the user to specify which messages are routed to which destinations. There are two main approaches, and I believe this dichotomy is quite generic and applicable to basically any problem encountered in the domain of software.

One approach is to adopt the UNIX philosophy of "do one thing and do it well". What this means is that the problem domain should be artificially restricted to a small and well-understood area. The program should then solve this restricted problem in a correct and exhaustive way. An example of such approach in the messaging area is MQTT. It's a protocol for distributing messages to a set of consumers. It can't be used for anything else (say for RPC) but it is easy to use and does message distribution well.

The other approach is to focus on generality and provide a powerful and highly configurable system. AMQP is an example of such a system. Its model of queues and exchanges provides the user with the means to define programmatically almost any routing algorithm they can think of. The trade-off, of course, is a lot of options to take care of.

ØMQ opts for the former model because it allows the resulting product to be used by basically anyone, while the generic model requires messaging experts to use it. To demonstrate the point, let's have a look how the model affects the complexity of the API. What follows is an implementation of an RPC client on top of a generic system (AMQP):

connect ("192.168.0.111")
exchange.declare (exchange="requests", type="direct", passive=false,
    durable=true, no-wait=true, arguments={})
exchange.declare (exchange="replies", type="direct", passive=false,
    durable=true, no-wait=true, arguments={})
reply-queue = queue.declare (queue="", passive=false, durable=false,
    exclusive=true, auto-delete=true, no-wait=false, arguments={})
queue.bind (queue=reply-queue, exchange="replies",
    routing-key=reply-queue)
queue.consume (queue=reply-queue, consumer-tag="", no-local=false,
    no-ack=false, exclusive=true, no-wait=true, arguments={})
request = new-message ("Hello World!")
request.reply-to = reply-queue
request.correlation-id = generate-unique-id ()
basic.publish (exchange="requests", routing-key="my-service",
    mandatory=true, immediate=false)
reply = get-message ()

On the other hand, ØMQ splits the messaging landscape into so-called "messaging patterns". Examples of the patterns are "publish/subscribe", "request/reply" or "parallelized pipeline". Each messaging pattern is completely orthogonal to other patterns and can be thought of as a separate tool.

What follows is the re-implementation of the above application using ØMQ's request/reply pattern. Note how all the option tweaking is reduced to the single step of choosing the right messaging pattern ("REQ"):

s = socket (REQ)
s.connect ("tcp://192.168.0.111:5555")
s.send ("Hello World!")
reply = s.recv ()

Up to this point, we've argued that specific solutions are better than generic solutions. We want our solution to be as specific as possible. However, at the same time, we want to provide our customers with as wide a range of functionality as possible. How can we solve this apparent contradiction?

The answer consists of two steps:

  1. Define a layer of the stack to deal with a particular problem area (transport, routing, presentation, etc.).
  2. Provide multiple implementations of the layer. There should be a separate non-intersecting implementation for each use case.

Let's have a look at the example of the transport layer in the Internet stack. It is meant to provide services such as transferring data streams, applying flow control, providing reliability, etc., on the top of the network layer (IP). It does so by defining multiple non-intersecting solutions: TCP for connection-oriented reliable stream transfer, UDP for connectionless unreliable packet transfer, SCTP for transfer of multiple streams, DCCP for unreliable connections and so on.

Note that each implementation is completely orthogonal: a UDP endpoint cannot speak to a TCP endpoint. Neither can a SCTP endpoint speak to a DCCP endpoint. It means that new implementations can be added to the stack at any moment without affecting the existing portions of the stack. Conversely, failed implementations can be forgotten and discarded without compromising the viability of the transport layer as a whole.

The same principle applies to messaging patterns as defined by ØMQ. Messaging patterns form a layer (the so-called "scalability layer") on top of the transport layer (TCP and friends). Individual messaging patterns are implementations of this layer. They are strictly orthogonal — the publish/subscribe endpoint can't speak to the request/reply endpoint, etc. Strict separation between the patterns, in turn, means that new patterns can be added as needed and that failed experiments with new patterns won't hurt the existing patterns.

Lesson learned: When solving a complex and multi-faceted problem, it may turn out that a monolithic general-purpose solution may not be the best way to go. Instead, we can think of the problem area as an abstract layer and provide multiple implementations of this layer, each focused on a specific well-defined use case. When doing so, delineate the use case carefully. Be sure about what is in the scope and what is not. By restricting the use case too aggressively the application of your software may be limited. If you define the problem too broadly, however, the product may become too complex, blurry, and confusing for the users.

Conclusion

As our world becomes populated with lots of small computers connected via the Internet — mobile phones, RFID readers, tablets and laptops, GPS devices, etc. — the problem of distributed computing ceases to be the domain of academic science and becomes a common everyday problem for every developer to tackle. The solutions, unfortunately, are mostly domain-specific hacks. This article summarizes our experience with building a large-scale distributed system in a systematic manner. It focuses on problems that are interesting from a software architecture point of view, and I hope that designers and programmers in the open source community will find it useful.


Martin Sústrik is an expert in the field of messaging middleware. He participated in the creation and reference implementation of the AMQP standard and has been involved in various messaging projects in the financial industry. He is a founder of the ØMQ project, and currently is working on integration of messaging technology with operating systems and the Internet stack. This article is excerpted and adapted from The Architecture of Open Source Applications: Volume II.


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