Coupling is one of the most fundamental measures of “quality” for an information system. The concepts of coupling and cohesion appear in software design best practices for at least a couple of decades. And these concepts are also vital to the development of distributed systems. As core as the concept of coupling is, it is difficult to find a real definition in the distributed systems context. Coupling is like obscenity – we can’t define it, but we know it when we see it.

Which is why I was pleased to see Ian Robinson’s post which presented coupling as lying on two dimensions – temporal and behavioural and even put in place some characteristics which helps you put a rough measure on the degree of coupling. Coincidently, I had drafted my own version of this some time ago, but it had never made it to publication.

Like Ian, I was trying to quantify coupling so that we can understand what constitutes a tightly or a loosely coupled system and we can have some approach to measure it and therefore have a method to decide between design trade-offs in satisfying the various requirements of our distributed systems. While Ian presents a conceptually clean two-dimensional picture, I felt the true story involves multiple interacting dimensions.

While I was researching this, I happened to find a book extract which covers what I wanted to say and more. The full extract is well worth reading, but is summarised in the following table:

Here we have no less than seven dimensions to the coupling equation.

The final paragraph of this article highlights the costs of loose-coupling (and only some of the benefits).

However, in most cases, the increased flexibility achieved through loose coupling comes at a price, due to the increased complexity of the system. Additional efforts for development and higher skills are required to apply the more sophisticated concepts of loosely coupled systems. Furthermore, costly products such as queuing systems are required. However, loose coupling will pay off in the long term if the coupled systems must be rearranged quite frequently.

I think this understates the benefit. “Rearranged frequently” seems to only cover design-changes. But it should also cover “runtime rearrangement” such as partitioning across redundant components for the purpose of load-balancing and fault-tolerance. In such cases, “loose-coupling” provides significant value in higher uptime and scalability of distributed systems.

Update May 14, 2009: Richard Veryard has pointed me to his paper “Component Based Service Engineering” (subscription required) which discusses an even wider range of coupling beyond the technical layers into Process, Organizational and Business layers. The CBDi Wiki has a table summarizing all the coupling dimensions identified in Richard’s paper.

One section of the paper struck a chord with me:

How can we have loose coupling and hard-wiring at the same time? The answer comes as soon as we recognize that coupling is multidimensional or multilayered. My head is connected (coupled) to the rest of my body in several different ways. Even if I could introduce some technology to decouple the nervous system, that doesn’t allow me to remove my head….With Web Services, SOAP simply removes one set of the hard-wired connections. Other forms of coupling remain.

This was written in 2003 and proves quite prescient in that many SOA projects in the interim have failed to achieve their goals by simply adopting out-of-the-box “web services” which only address one or two of the many dimensions of coupling.

In my last post on this topic I talked about the concept of an ESB. Here I talk about why you would want one.

There are plenty of whitepapers, analyst reports and vendor statements about the features and functions of the various ESB products. In my experience, the key advantages of using an ESB are less about features and functions and more about how you use it.

Standardization

One of the primary advantages of an ESB is that it gives you a standardized platform for integration. When everyone is using the same tools you can develop enterprise-wide frameworks, patterns and best practices for building re-usable services. Without a unifying platform, you get a divergence of integration methods which leads to inconsistency and higher cost of management and change. So an ESB platform helps with design-time governance. Note that this is not the same as standardization in the sense of using web-services standards. The important thing is that you use the ESB to support your own enterprise standards. These may be based on external standards – but that may be of secondary importance.

Loose Coupling

The bus architecture of an ESB encourages you toward a loosely coupled architecture.

• Physically decoupled by making use of message passing mechanisms (e.g. JMS) versus direct network connections (e.g. HTTP).
• Semantically decoupled by use of message transformation so that services are exposed in a system-neutral format which reduces application lock-in and reduces the cost of change.

Scalability and Reliability

Physical loose-coupling provides scalability advantages such as high-availability, fault-tolerance and load balancing. The messaging layer in the ESB directs messages between service endpoints to the appropriate instance of the endpoint. For example, in the event of a service provider failure, messages will be redirected to a backup provider – thus supporting high availability. In the case of load balancing, messages are distributed between redundant providers (or consumers) to handle high volumes of message traffic. You could say that physical loose-coupling supports change at the “micro” level where short term changes in the system topology can be compensated for via real-time message redirection.

Routing and mediation

Message routing supports scalability and fault tolerance. An ESB can also be used to support business-level routing and mediation. For example content-based routing allows services to be invoked based on the content of a service request. A business example would be routing of a customer enquiry to the branch where that customer account is located. A technical example would be the routing of a service request based on the version of the service being invoked.

Complex message exchange patterns

Traditional HTTP-based services support only one-to-one request-reply MEPs. An ESB supports more complex MEPs such as asynchronous one-way messaging and to multiple subscribers using topic-based messaging. Asynchronous publish and subscribe mechanisms support new ways of intermediating service consumers and subscribers – such as auditing, service monitoring – which are extremely useful for runtime management and governance of your services. Beyond mere governance, higher level business functions such as complex event processing (CEP) and business activity monitoring (BAM) are supported by this ability to “listen in” to service traffic on the ESB.

The benefits of an ESB that I’ve described above stem largely from the architecture of an ESB and in particular from the use of a message bus as the primary underlying transport. But it is important to understand that these benefits don’t automatically come “out of the box”. Your solution architecture (and your architects) must recognise and utilise the architectural principles underlying the ESB.

A question I often get is “what is an ESB and why do I need one”? This question is motivated by a number of concerns; non-technical people have heard the term but don’t understand the concept, semi-technical people are trying to figure out conflicting vendor definitions, and technical people are confused by the debate between different service enablement approachs – RESTful versus ws-* versus middleware-supported hybrids.

The Elevator Pitch

A service bus provides a uniform and consistent platform to allow service providers and service consumers to interoperate. An ESB provides benefits such as:

• standardization
• loose coupling
• resilience and high availability
• monitoring and intermediation

Hardware

I’m not sure of the provenance of the word “bus” as it is applied in the technical domain (I’m sure there is some interesting etymology there) but you can confidently trace it back to the concept of a computer hardware bus. The idea of a hardware bus (or backplane) is that hardware components – such as sound-cards, video cards, floating point accelerators, tape-drives, barcode scanners etc – can all slot into and interoperate through a shared infrastructure. By supporting a standard hardware interface and a standard software protocol, the hardware bus abstracts the details of each individual hardware component. The key features of the harwdare bus are:

• standardized hardware connectivity to the backplane
• standardized software protocol between each component and the backplane
• hardware components can operate independently without having to know details about each other
• a single infrastructure replaces multiple point-to-point connections between components (i.e. does away with a lot of ad hoc soldering).

Software

Networked systems arrived in the seventies and grew out of control in the eighties. Early network infrastructures such as Unix sockets were hard-wired point-to-point affairs with little or no abstraction of the the two programs that were working together.

The idea of a software bus is that software components can work together – yet independently – via a standardized message passing mechanism that would abstract away the need to create individual network connections between components. The software bus would take care of routing messages to the required location and also take care of all that hard stuff like quality-of-service, reliability and scalability. This is equivalent to standardizing the “hardware connectivity” in the hardware bus. TIBCO’s predecessor – Teknekron – articulated the concept of the software bus in the early nineties

The Service Bus

So the hardware bus standardizes hardware connectivity and the software bus standardizes software connectivity. The Service Bus has refined the concept of the software bus by taking a more service-oriented approach and adding support for the XML stack underlying web services and transport connectivity (e.g. bridging HTTP to JMS).

So why do you need an ESB? More on that anon…