Fundamental Concepts of Business Applications III: Locator Definitions

In the previous article, we argued that a Locator is neither a UI control nor a search mechanism. It is an architectural concept that resolves business references within a specific business context and transforms them into canonical business identities. If that is true, however, an obvious question immediately follows. How can a Locator be described? Not how it is implemented. Not how it is executed. But how it can be described independently of any particular implementation. This question is more important than it may seem at first glance. If two different development teams decide to implement a Locator, how can they be sure they are implementing the same concept? How can we discuss a Locator without referring to a particular library, programming language, or framework? The answer is that, like any mature architectural concept, a Locator must first be separated from its implementation. This distinction appears repeatedly throughout the history of software engineering. Relational theory existed before relational database systems. SQL does not describe how a query will be executed. It describes only the result we want to obtain. The choice of indexes, execution plans, and optimization algorithms is the responsibility of the query optimizer. Exactly the same principle applies here. A Locator should not be defined by the mechanism that implements it. It should be possible to describe it independently of that mechanism. This naturally leads to three distinct levels of abstraction. Locator Pattern │ ▼ Locator Definition │ ▼ Execution Engine The Locator Pattern is the architectural concept itself. The Locator Definition is the declarative description of a particular resolution process. The Execution Engine is the component responsible for interpreting that description and performing the actual resolution. Each level has a different responsibility, and they should never be confused with one another. This means that a Locator Definition is not an implementation artifact. It