Posts Tagged ‘Roy Fielding’

Inside-Out, Outside-In

December 22, 2014 Leave a comment

There are two common perspectives on the process of architectural design, whether it be for buildings or for software. The first is that a designer starts with nothing—a blank slate, whiteboard, or drawing board—and builds-up an architecture from familiar components until it satisfies the needs of the intended system. The second is that a designer starts with the system needs as a whole, without constraints, and then incrementally identifies and applies constraints to elements of the system in order to differentiate the design space and allow the forces that influence system behavior to flow naturally, in harmony with the system. Where the first emphasizes creativity and unbounded vision, the second emphasizes restraint and understanding of the system context. – “RESTful” Roy Fielding

It might not be a correct interpretation, but BD00 associates Mr. Fielding’s two alternatives with “inside-out” and “outside-in” design.

The figure below illustrates the process of inside-out design. The designer iteratively composes a structure and “hopes” it will integrate smoothly downstream into the context for which it is intended. During the inside-out design process, the parts are king and the system context is secondary.

Parts First

The figure below depicts an outside-in design process. The designer iteratively composes a structure within the bounded constraints of the context (the “whole“) for which it is intended. During the outside-in design process, system context is king and the parts are secondary.

Context First

Because system contexts can vary widely from system to system and they’re usually vaguely defined, messy, and underspecified, designers often opt for the faster inside-out approach. BD00 uses the outside-in design process. What process do you use?

Properties Of Interest

Roy Fielding‘s famous PhD thesis, “Architectural Styles and the Design of Network-based Software Architectures” introduces the REST (REpresentational State Transfer) architectural style for large-scale, distributed, hypermedia-based network applications.

In his thesis, Mr. Fielding defines the non-functional property set that he uses to evaluate various architectural styles against one another:

Of course, there is no universal set of “ilities” definitions that technical stakeholders use to reason about, and evaluate, software architectures. Plus, depending on the application domain one is immersed in, some “ilities” are more important than others. Nevertheless, Mr. Fielding’s set is as good as any other that I’ve seen to date.

Whenever I see someone’s personal list of “ilities“, sometimes I discover at least one that I’ve never seen before. In Mr. Fielding’s list, the “visibility” property is one such “ility“. Here’s Mr. Fielding’s definition:

Visibility… refers to the ability of a component to monitor or mediate the interaction between two other components.

In a “distributed hypermedia systems” application like the www, visibility impacts several other properties as follows:

Visibility can enable improved performance via shared caching of interactions, scalability through layered services, reliability through reflective monitoring,
and security by allowing the interactions to be inspected by mediators (e.g., network firewalls).

I wonder what my next “ility” discovery will be?

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