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Characterising Architectural Styles II – State

My last post explored some distinguishing characteristics of three common architectural styles in an attempt to understand better how they differ and therefore how they may apply in different contexts. A fourth distinguishing characteristic of these architectural styles is the way in which state is managed during the execution of a business process.

There are three aspects of state that I want to consider:

  • Management – how state change is initiated or managed through a business process.
  • Monitoring – how state is monitored or accessed or derived during the execution of a business process.
  • Consistency – how state is made consistent across different systems involved in a business process.

A summary of the state characteristics of the different architectural styles is listed in the following table along with the other characteristics discussed in my last post.


In EAI, state is managed within one application and then synchronised to other applications, usually after the business process has completed. This means that in some cases state may be inconsistent across the organisation or its systems. That may or may not be a problem (see ‘Eventually Consistent‘) but is a common side effect when a business process is executed within one system rather than across systems in an independent process layer. During the execution of an EAI process, there is often no monitoring of the state. I.e. the process may simply replicate data changes to other systems without explicily tracking state. A limited form of state monitoring may exist in the sense that the local application or associated middleware may check the status of data synchronization and error out in the event of an exception. I refer to this as ‘local’ state monitoring. So under the EAI architectural style, state is managed locally, monitored locally (if at all) and is eventually consistent.

Under SOA, the driver of a business process is BPM orchestration in an independent process layer. In this case, we can say that state is managed centrally (in the BPM layer) and monitored centrally (also in the BPM layer). State in end-systems is updated progressively through the business process (via service calls) and so we could say that state is ‘progressively’ consistent as opposed to ‘eventually’ consistent.

Under EDA, there is no central or even local management of state. Instead, events signify distributed actions which together may be used to infer the state of a system. To the extent that any ‘management’ occurs, we could say that state is managed in a distributed fashion – one or more agents each acting on their own. Perhaps ¬†it is more accurate to say that state is manifested globally. Converesely, state is monitored centrally within an Event Processing (CEP) layer which correlates events to infer system state. Under EDA, state is progressively consistent because the system is progressively reacting to events which are a by-product of a hidden or implicit business process.

Characterising Architectural Styles

I’ve recently been thinking about simple ways to characterise the different architectural approaches we use in distributed systems today. Three simple architectural characteristics I have come up with are:

  • Asset – this is a core capability or “thing” that must be built, procured, maintained and managed in a corporate IT “inventory”.
  • Element – these are the “atomic” building blocks used in the process of Composition.
  • Composition – this is the mechanism which allows the different assets to work together to support a business requirement.

If we apply these characteristics to three common architectural approaches then we get the results in the following table.


EAI, although  unfashionable is still prevalent Рeven dominant in the industry. For EAI, we are primarily concerned with applications (usually COTS) which embody and support the requirements of different parts of the business. Multiple applications must be coordinated to support the whole business. The primary coordination mechanism under EAI is synchronization of state between the different applications Рprimarily via data integration. The composition element is the API.

SOA is characterised by Services as the key asset. Services are acquired or built to execute business operations. Elemental Services are composed to support business processes – a sequence of operations which results in a business outcome. BPM (or process orchestration) is the composition mechanism within SOA. The underlying functionality of a Service may reside in one or more applications, but from an SOA perspective this is of secondary importance…SOA is concerned with the Service, not necessarily its implementation.

EDA (Event Driven Architecture) is characterised by Event Services as the key asset which represent an asynchronous notification of an important event associated with the business. Elemental Events are correlated and further processed to derive higher-order business intelligence which may in turn trigger other Events. The primary composition mechanism within EDA is Event Processing (or CEP). An important part of this composition mechanism is the ability to manage or track system state.

This characterization gives more clarity to the difference between JABOWS and true SOA. Many so-called SOA projects have simply involved the “bottom up” exposure of application APIs using web-services standards – resulting in “Just a Bunch of Web Services” which don’t realise the business value of a true SOA. JABOWS is EAI because the applications are the core “asset”. A true SOA has a “top down” process-centric architecture with Services as the core asset.

Emergent Architecture

Dion Hinchcliffe has written a nice blog entry “pragmatic new models for enterprise architecture take shape“. This resonates well with some of the things I’ve been saying about how enterprise architecture needs to be enabling rather than blocking:

“In recent years enterprise architecture has been moving from a discipline that provides top-down, a priori technology blueprints to the business side to one that articulates key, strategic possibilities and only the most critical high-level constraints (such as security standards) and then operates as a conductor, promoter, problem solver, and evangelist across the organization through the vehicle of a cohesive community to co-develop needed solutions.”

(my emphasis added). And:

“Invariably, the best architecture I see comes naturally from self-organizing thought leaders in an organization that seek each other out and collaborate on common solutions to their problems. Rather than the us vs. them mentality of old-world enterprise architecture, there is only an us mentality. Instead of prescribed standards, designs, technologies, and tools there is real [two-way] consensus and immediate buy-in.”

And as with all Dion’s posts there is the cool graphic. I especially like the two downward facing arrows – “community leadership” and “guides”:

The article is well worth reading but I think there is a large element of wishful thinking here. It paints a good picture of how enterprise architecture should be, but I fear we are a long way from this nirvana. Much of the vision depends on a fundamental change in organizational culture. Businesses that recognise the value of an “emergent culture” will naturally have an “emergent enterprise architecture.” I don’t think it will go the other way around. Emergent enterprise architecture will not survive in or be able to change a top-down corporation.

In the long term, those companies with the right culture will support emergent enterprise architecture and thrive on IT success. Those companies that don’t will cede their IT resources to others. Those who can do IT will do it on behalf of those who can’t. Cloud anyone?

EA and Agile Projects

A recent comment on my post about the value of enterprise architecture suggested that enterprise architects should be directly involved in Scrum projects. In that post I suggested that the primary value of EA is “continuity” and “self-correction” across all the IT initiatives. If these qualities represent the value of EA, then the mechanism for realising that value must involve communication.

Communication is the key to ensuring that IT project deliverables provide cohesion across organizational, geographical and temporal boundaries. I have previously written that communication and governance are effectively the same thing. The problem is that “governance” needs to overcome the image of heavy-handed, top-down, hierarchical “rules” handed down by “out of touch” enterprise architects – like commandments on tablets of stone. EA must be seen as an enabler, not a roadblock.

But communication and governance must also be relevant to the organizational culture. So if the culture embraces agile development methods such as Scrum, then on the face of it the governance structure ought to conform with agile practices which favour direct verbal communication over documentation. If it is true that an embedded business representative can communicate requirements more effectively than a written requirements document, then perhaps it is also true that an embedded enterprise architect can “govern” more effectively than a written EA document.

However, on reflection, I don’t think the analogy of EA as a “customer” of the project is quite as strong as I first thought. There are a couple of differences between “business requirements” and “EA requirements”.

First, business requirements for a project are communicated primarily to that project. Few if any other IT projects would be interested in these particular business requirements which we are trying to encode into an IT solution. However, EA requirements must be communicated consistently across multiple projects and consistency demands that the EA requirements – the “governance principals” – need to be encoded in some written form, independent of an individual architect or even team of architects.

Second, business requirements change very frequently within the timespan of an IT project. Indeed the development process itself can change business requirements. This is why agile methods value verbal communication methods over written – so as to allow for frequent change in business requirements. EA requirements will not change as rapidly. Even though the Enterprise Architecture will evolve over time, it should not change over the course of a single project. Indeed if your Enterprise Architecture undergoes sudden rapid shifts (e.g. adopting an overnight SOA mandate) then you have a recipe for IT disaster. Hence the agile “insight” that written business requirements cannot keep up with reality, is not quite as relevant when it comes to EA requirements.

So my response to Paul’s comment is still: Yes! I agree that an EA representation on a Scrum project would be a good idea – to communicate, elucidate and interpret EA requirements to the project and importantly to take new lessons learned back from the project into the governance structures. However, this is no substitute for EA “documentation” – artefact-based governance – which is necessary to ensure consistency and continuity.

The Value of Enterprise Architecture

There has lately been a lot of discussion about Enterprise Architecture (hereafter EA) in my neighbourhood of the blogosphere. Perhaps this is a sign of adolescence for EA. Its existence is apparent, and now EA needs to decide who it should hang out with and what it wants to be when it grows up. As part of a wider discussion, Richard Veryard has posted some excellent thoughts on the importance of a value proposition for EA. I totally agree with Richard’s view that EA is not just about making projects successful and since this is my blog, I’ll put my view on what I think is the value of EA. EA provides two perspectives which are vital in any long-lived and complex system such as an enterprise.

Continuity: EA provides continuity across organizational boundaries, geographies and – most importantly – the span of time. This is something that any individual project – successful or not – could not do. During the lifetime of an enterprise there are multitudes of projects executed in different places, by different people – outsourced and insourced. Ultimately all these projects must pull in approximately the same direction to support the business strategy.

To take a hypothetical example: “The data warehouse project in Bangalore being built by Tatosys must support the new marketing system being built in Sydney by Bluehair Consulting. And both are critical components for the mobile commerce platform slated for 2012 go-live by either PDQ Global Services or IBS (an HQ company) depending on who bids the lowest fixed price. And by-the-way the core transaction systems run on a mainframe that we are phasing out in the next three years as part of our 2002 strategic plan.”

Each of these projects is a major undertaking in its own right. Even if every project is 100% successful, you cannot be certain they will all fit together in a coherent and efficient manner. Making them fit is more than just a business problem. It goes beyond getting the functionality right and encompasses deeper technical layers of standards, frameworks and systems partitioning which are neither the domain of the business nor of business analysts.

Self Correction: The second value proposition of EA that I see is the role of “external observer and governor”. Enterprises exist in a constant flux of technologies, fashions, methodologies and business requirements. Someone needs to have the ability to evaluate new practices and make the decision to incorporate those that are valuable and relevant. This is not something that individual projects can do, although they may have a role in trialing new practices.

Each of these areas of change exist in the domain of different parts of the organization. The business is hopefully across changes in business requirements, the Project Management Office is (perhaps) across changes in software development methodologies, and the IT department always want to try out the cool new tools. But normally these parts of the organization don’t interact outside the constraints of an individual project. You need some constituency that is across all these areas and helping to guide the amalgamation of new “best practices” into the enterprise. I think that EA is the natural venue for this interaction.

So is this just the “old fashioned” value proposition of “EA-as-IT-planning”, instead of the cool new view of “EA-as-business-strategy”? I see it as something midway between the two. Certainly “EA-as-IT-strategy” is close, but also “EA-as-trusted-advisor-to-business-strategy”. Leave the business strategy to the business, but when it comes to the mechanics of implementing business strategy in terms of systems, processes and technologies, the strategy-makers should be able to rely on EA to guide them as to what works, what doesn’t – and who might take them for a ride.

Richard mentions that he’s “not convinced that the EA value proposition is understood by its customers”. I would go so far as to say I don’t think the EA value proposition is even trusted by its customers. EA needs to engage with its customers, figure out the value proposition and then execute.