Extensibility (sometimes confused with forward compatibility) is a system design principle in software engineering where the implementation takes into consideration future growth. For example, the data exchange format, eXtensible Markup Language (XML) was named such because of its inherent design to enable extensibility.
But simply using XML does not make a system extensible. 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.
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