I had the pleasure of attending two conferences in recent weeks - the Australian Architecture Forum in Melbourne, and JAOO in Sydney. Both conferences held some surprises and in particular I was struck by the very different attitudes toward SOA.

The Architecture Forum was of course aimed at IT Architects and as a result it was about SOA, SOA and SOA…with a bit of SOA as well. Even when you tried to not talk about SOA (as one bold presenter attempted)…the conversation still swung around to SOA. By contrast, at JAOO, there was nary a mention of SOA. You don’t have to look far to understand why…JAOO’s by-line is “by Developers for Developers”…so the target audience and attendees were - Developers. Hence much of the content was around languages, platforms and agile methodologies.

What I enjoyed most about JAOO was the chance to meet and talk with the great lineup of speakers. Kudos to Dave Thomas for wrangling these people to this unfashionable side of the planet. Among the highlights were: Gregor Hohpe talking about the challenges of distributed systems, Martin Fowler expounding on the virtues of simplicity in design, Jim Webber trying to reconcile his two loves in life and Steve Vinoski on how a portfolio of languages helps us develop better solutions. All of this gave me the opportunity to consider the perspectives of architects versus developers. Gregor Hohpe captured the tone with his statement that an “architects dream is often a developers nightmare”.

An Enterprise Architect is generally focussed on the larger components in the IT infrastructure - how they are owned, sourced, maintained, combined and deprecated. And an architect considers not only machines and software but also data, processes and organizational structure. A key consideration for architects is how these things work together and therefore standards are important.

Developers care about delivering solutions for a particular business requirement. They care about the languages and platforms and methodologies that support their needs for correctioness, quality and maintainability. Developers have evolved techniques to deliver these qualities which are sometimes at odds with the needs of Architects and associated standards. From a developers perspective, many of the web-services standards conflict with their best practices. For example, XML is not really a language and cannot be completely treated like a language. Source-code management systems often have difficulty with XML. XML-based “languages” such as WSDL and BPEL are so complex that they require visual interfaces and complex IDEs that are at a higher abstraction than the XML “code”. The ability of these visual interfaces to “diff”, “merge” and debug are often pretty poor. Recently there has been a welcome move away from the masses of XML configuration files in J2EE in favour of java annotations. And perhaps there is further value in domain specific languages like Integration Flow Language in project Fuji.

Architects and Developers are in a symbiotic relationship which is definitely not frictionless. A good understanding on both sides of the needs and drivers of the other party will help to get everyone working in the same direction. Each should know the languages, techniques and platforms used by the other and a common understanding will lead to better ways of achieving what is ultimately a common goal.

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…