Coherent Architectures

In his paper, Urban and Enterprise Architectures: A Cross-Disciplinary Look at Complexity, Roger Sessions talks about Coherent architectures that are:

• Easier to implement
• Easier to maintain
• Easier to adapt
• Are less complex

By gleaning into architecture of buildings, Sessions finds four attributes of architecture that make an architecture coherent. He contends these attributes are equally applicable to Enterprise architecture.

1. Scale Range - There must be a range of architectural scales ranging from the very small to very large. For example, in buildings bricks make up the walls, walls make up the rooms, and rooms make up the building. Similarly, in an Enterprise architecture, there is a scale range of artifacts: data rows, tables, databases, programs, APIs, services, components, sub-systems, systems
2. Scale Hierarchy - There is an obvious  hierarchy of elements in a building. Buildings contain rooms, rooms contain walls, and walls contain bricks. Similarly, in Enterprise systems, raw data is contained in data rows, data rows are contained in tables, tables in databases, and databases are usually accessed through a well-defined programmatic interface. In a Coherent architecture, this hierarchy is clearly decipherable.
3. Scale Connections - Couplings are between elements of like scale. For example, bricks couple with bricks to form a wall. Walls couple with other walls to form a room. In an enterprise system, for example, Web Services collaborate with other Web Services, but not with the underlying database of another system.
4. Scale Tightness - The higher up you go in the hierarchy, the weaker the connection strength and there are fewer connection points. For example, two autonomous systems are likely to communicate through an asynchronous message rather than a synchronous method invocation. Furthermore, connection points will be fewer between the two autonomous systems than between a data access layer of a subsystem and its database.

Reference: http://www.objectwatch.com/whitepapers/SessionsSalingaros01.pdf