What makes a good design?

Tuesday, December 23rd, 2008

One of my core job responsibilities for the past several years has been to conduct technical design and implementation (code) reviews during various phases of the software development life cycle. This is typically a highly collaborative process whereas myself and an individual engineer, or the team as a whole will begin by performing a detailed analysis of business requirements in order to gain an initial understanding of the specific component(s) being developed. Once an understanding of the requirements has been reached a brainstorming session ensues which ultimately leads to various creative, technical solutions. After discussing the pros and cons of each the best solutions quickly begin to reveal themselves, at which point it is simply a process of elimination until the most appropriate solution has surfaced.

The next step is to translate the requirements into the proposed technical solution in the form of a design document. The design is specified on a high level and is only intended to provide an overview of the appropriate technical road map which is to be implemented. This typically consists of higher level UML Sequence and Class diagrams, either in the form of actual diagrams produced in a UML editor, or could simply be a picture captured from UML drawn out during a whiteboarding session. The formality of the documented design is less important, what is important is that the design is captured in some form before it is implemented. Implementation specific details such as exact class and method signatures and so forth are intentionally left out as they are to be considered outside the scope of the design. See Let Design Guide, not Dictate for more on this subject. Once the design is documented it is reviewed and changes are made if needed. This process is repeated until all business and technical requirements have been satisfied, at which point the “all clear” is given to move forward with implementing the design.

But what exactly constitutes a good design? How does one determine a good design from a bad one? In reality it could vary significantly based on a number of factors, however in my experience I have found a design can almost always be judged according to three fundamental criterion: Correctness, Cohesion / Coupling and Scalability. For the most part everything falls into one of these three categories. Below is a brief description of the specific design questions each category sets out to determine.

  • Correctness
    Does the design solve the problems described in the requirements and discussed by the team? This is Correctness in the form of satisfying business requirements. Are the patterns implemented in the design appropriate, or are additional patterns being used just for the sake of using the pattern? This is Correctness in the form of technical requirements. A good design is well focused and only strives to provide a solution which meets the requirements specified by the business owners, client etc; it does not attempt to be overly clever.
  • Cohesion / Coupling
    Has a highly cohesive, loosely coupled design been achieved? Have the classes, interfaces and APIs been logically organized? Does each provide a specific, well-defined set of functionality? Is composition used over inheritance where applicable? Has related functionality been properly abstracted? Does changing this break that, does adding that break this, etc.
  • Scalability
    Does the solution scale well? Is it flexible? A good design strives to facilitate change with confidence, and with as little risk as possible. A good design also achieves transparency at some level in the areas where it is most applicable.

The concepts outlined above are crucial to achieving a good design, however they are often overlooked or misunderstood to some degree. Throughout the years I have began to recognize some commonality in the design mistakes I find in Object Oriented Designs in general, and within Flex projects in particular. Many of which typically can be attributed to violations of basic MVC principles, but most commonly the design mistakes appear to be a negation of Separation of Concerns (SoC).

There are close relationships between Correctness, Cohesion / Coupling and Scalability, each of which plays a very significant role in the resulting design as a whole.

So lets start with Correctness, which is by far the single most important facet of design, for if the design does not provide a solution which satisfies the requirements specified then it has failed – all other aspects of the design are for the most part, details.

It is important to understand that Correctness has a dependency on Flexibility. For example, as architects and developers our understanding of the problem domain is constantly evolving as we gain experience in the domain. Additionally, as requirements may change significantly as a product is being developed, our designs must be able to adapt to these changes as well. Although this poses some challenges it is wrong to suggest that requirements need to be locked down completely before the design phase begins, but rather requirements need only be clearly defined to the extent that the designer is aware of what is required at that point in time and how it fits into the “big picture”. A competent designer understand this well and makes careful considerations before committing to any design decisions. This is where the importance of Flexibility comes into play. In order for a design to be conceptually and technically correct it needs to be flexible enough to support change. This is why good design is so important – to easily facilitate change. As such the flexibility to allow change should be evident throughout the design. A good example might be where the middle-tier has not decided which service layer implementation will be used (e.g. XML:80, WSDL, REST etc.), or the Information Architects have not decided what the constraints of each user role will be. A good design should be flexible enough to allow for changes such as these as well as others with confidence and more importantly, little risk to other parts of the application; after all, you shouldn’t have to tear down the house just to renovate the bathroom – in addition to Correctness and Scalability, this is where Cohesion and Coupling come into play.

High Cohesion is vital to achieving a good design as it ensures related functionality and responsibilities are logically grouped together, encapsulated and abstracted. We have all seen the dreaded, all encompassing class which assumes multiple responsibilities. Classes such as these have low cohesion and are a sign of future challenges if not addressed immediately. On a higher level, if high cohesion had not been achieved it is easy to notice as there will typically only be one class which comprises an entire API, however quite often low cohesion in classes may be a bit more subtle than one might expect and a code review will reveal areas where low cohesion has been implemented.

For example, consider the following Logging facility which is intended to provide a very simple logging implementation:

The above example is such a classic case of low cohesion. I see this kind of thing all the time. The problem here is that the Logger class has low cohesion because it is assuming the responsibility of creating and formatting a time stamp, this functionality is outside of the responsibilities of the Logging API. The creating and formatting of a time stamp is not a concern of the Logger, but rather would be the responsibility of a separate DateFormatting utility whose sole purpose is to provide an API for formatting Date objects. Removing the Date formatting functionality from the Logging API to a class which is responsible for formatting Date objects would facilitate code reuse across many APIs, reduce redundancy and testing as well as allow the Logger class to only define operations which are directly related to Logging. A good design must achieve high cohesion if it is to be successful.

Coupling is essential in determining a good design. A good way to think of coupling is like this: Think back to when you were a kid playing with blocks, you could easily take any number of different blocks and rearrange them to build whatever you like – that’s loose coupling. Now compare that to a crossword puzzle or a jigsaw puzzle, the pieces only fit together in a very specific way – that’s tight coupling. A good design strives to achieve loosely coupled APIs in order to facilitate change as well as reuse. A classic, yet less commonly mentioned example tight coupling is in the packaging of APIs. Often, many times designers will achieve loosely coupled APIs however the APIs themselves are tightly coupled to the application namespace.

Consider the of Logging API example from above, note that the API is defined under the package com.somedomain.someproject.logging. Even if the example were to be refactored to achieve high cohesion it would still be tightly coupled to the project specific namespace. This is a bad design as in the event another product should need to use the Logging API it would first need to be refactored to a common namespace. A better design would be to define the Logging API under the less specific namespace of com.somedomain.logging. This is important as the Logging facility itself should be generic in that it could be used across multiple projects. Something as simple as proper packaging of generic and specific components plays a key role in a good design. A better design for the above example would be as follows, this design achieves both high cohesion and loose coupling:

As with all design, technical design is subjective. Architects and Engineers can spend an infinite amount of time debating the various points of design. In my experience it really comes down to organization and efficiency, that is, organization of responsibilities and concerns, and the efficiency of their implementation both individually and as a whole.

It may sound cliche’ however before you begin a new design, or review an existing one, consider the following quote before doing so – it pretty much sums up what good design is:

“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.”
– Antoine de Saint-Exupery

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