How often do you see vendor literature or system or application descriptions that claim extensibility simply because of a heavy reliance on XML? I find it amazing how common the claim is and how prevalent are the logical fallacies surrounding this notion.

Don’t get me wrong. As a data exchange format, eXtensible Markup Language (XML) does provide data representation extensibility. This contribution is great, with widespread adoption a major factor in its own right helping to bring down the Tower of Babel. But the simple use of XML is insufficient alone to provide extensibility.

Fully extensible systems need to have at least these capabilities:

• Extensible data representation so that any data type and form can be transmitted between two disparate systems. XML and its other structured cousins such as RDF and OWL perform this role. Note, however, that standard data exchange formats have been an active topic of research and adoption for at least 20 years, with other notable formats such as ASN.1, CDF, EDI, etc., also performing the task now largely being overtaken by XML
• Extensible semantics, since once more than one source of data is brought into an extended environment it likely introduces new semantics and heterogeneities. These mismatches fall into the classic challenge areas of data federation. The key point, however, is that simply being able to ingest extended data does nothing if the meaning of that data is not also captured. Semantic extensibilitiy requires more structured data representations (RDF-S or OWL, for example), reference vocabularies and ontologies, and utilities and means to map the meanings between different schema
• Extensible data management. Though native XML data bases and other extensions to conventional data systems have been attempted, truly extensible data management systems have not yet been developed that: 1) perform at scale; 2) can be extended without re-architecting the schema; 3) can be extended without re-processing the original source data; and 4) perform efficiently. Until extensible infrastructure with these capabilities is available, extensibility will not become viable at the enterprise level and will remain an academic or startup curiosity, and
• Extensible capabilities through extendable and interoperable applications or tools. Though we are now moving up the stack into the application layer, real extensibility comes from true interoperability. Service-oriented architectures (SOAs) and other approaches allow the registry and message brokering amongst extended apps and services. But central v. decentralized systems, inclusion or not of business process interoperabilty, and even the accommodation of the other extensible imperatives above make this last layer potentially fiendishly difficult.

These challenges are especially daunting in a completely decentralized, chaotic, distributed enviornment such as the broader Internet. This environment requires peer-to-peer protocols, significant error checking and validation, and therefore the inefficiencies due to excessive protocol layering. Moreover, there are always competing standards and few incentives and fewer rewards for gaining compliance or adherence.

Thus it is likely that whatever progress is made on these extensibility and interoperabilkity fronts will show themselves soonest in the enterprise. Enterprises can better enforce and reward centralized standards. Yet even in this realm, while perhaps virtually all of the extensible building blocks and nascent standards exist, pulling them together into a cohesive whole, in which the standards themselves are integrated and cohesive, is the next daunting challenge.

Thus, the next time you hear about a system with its amazing extensibilitiy, look more closely at it in terms of these threshold criteria. The claims will likely fail. And, even if they do appear to work in a demo setting, make sure you look around carefully for the wizard’s curtain.