Mr. Lott has been involved in over 70 software development projects in a career that spans 30 years. He has worked in the capacity of internet strategist, software architect, project leader, DBA, programmer. Since 1993 he has been focused on data warehousing and the associated e-business architectures that make the right data available to the right people to support their business decision-making. Steven is a DZone MVB and is not an employee of DZone and has posted 139 posts at DZone. You can read more from them at their website. View Full User Profile

The Wrapper vs. Library vs. Aspect Problem

12.10.2010
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Imagine that we've got a collection of applications used by customers to provide data, a collection of applications we use to collect data from vendors. We've got a third collection of analytical tools. Currently, they share a common database, but the focus, use cases, and interfaces are different. Okay so far? Three closely-related groups or families of applications.

We need to introduce a new cross-cutting capability. Let's imagine that it's something central like using celery to manage long-running batch jobs. Clearly, we don't want to just hack celery features into all three families of applications. Do we?

Choices

It appears that we have three choices.

  1. A "wrapper" application that unifies all the application families and provides a new central application. Responsibilities shift to the new application.
  2. A site-specific library that layers some common features so that our various families of applications can be more consistent. This involves less of a responsibility shift.
  3. An "aspect" via Aspect-Oriented programming techniques. Perhaps some additional decorators added to the various applications to make them use the new functionality in a consistent way.
Lessons LearnedAdding a new application to be an overall wrapper turned out to be a bad idea. After implementing it, it was difficult to extend. We had two dimensions of extension.
  1. The workflows in the "wrapper" application needed constant tweaking as the other applications evolved. Every time we wanted to add a step, we had to update the real application and also update the wrapper. Python has a lot of introspection, but these aren't technical changes, these are user-visible workflow changes.
  2. Introducing a new data types and file formats was painful. The responsibility for this is effectively split between the wrapper and the underlying applications. The wrapper merely serves to dilute the responsibilities.
Libraries/AspectsIt appears that new common features are almost always new aspects of existing applications.What makes this realization painful is the process of retrofitting a supporting library into multiple, existing applications. It seems like a lot of cut-and-paste to add the new import statements, add the new decorators and lines of code. However, it's a pervasive change. The point is to add the common decorator in all the right places. Trying to "finesse" a pervasive change by introducing a higher-level wrapper isn't a very good idea. A pervasive change is simply a lot of changes and regression tests. Okay, I'm over it.

From http://slott-softwarearchitect.blogspot.com/2010/12/wrapper-vs-library-vs-aspect-problem.html

Published at DZone with permission of Steven Lott, author and DZone MVB.

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