Saturday, February 27, 2016

Single Responsibility or Changing Growing Responsibilities Principle

Architecture is full of examples of items designed to do more than one thing.  Your smart phone is a flash light.  From my last examples, a chair is also a clothes hanger.  A seat cushion is a flotation device. 

Robert Martin said in his book Agile Software Development, Principles, Patterns, and Practice (,  that every class should have a single responsibility over a part of the function.  The function-part should be encapsulated and the class should do that and only that. 

I understand the need for this as it makes the class easy to understand.  It also could reduce the introduction of bugs.  Have you ever worked with "spaghetti code"?  You make a change and it causes a bug in another area you couldn’t have imagined in your wildest dreams.   If a class has two different things it does, and you change one thing, it could affect the other piece and thus break something.  Martin gives an example a class that compiles AND prints a report.    If the content of the report changes, the class must change.  If the format of the print must change, then the class changes.  Martin argues that there is no reason for these two things to be coupled because if the format of the print changes, the content does not change and thus should not need to be recompiled and tested. Martin says a class should have only one reason to change (not two, like in this example).  

In an imperfect world, needs of software change over time.  An example: My manager is crazy over this particular module we have to is used to access and configure accounting data.  Over time it has evolved to access security data as well.  This drives him crazy because it violates the single responsibility principle (although he doesn't call it that and doesn't know that Robert Martin invented the term).  I however, think this is OK, and have argued this fact with him.   Given the schedule pressures we were under to combine these two pieces, which are closely related in our system, is not horrific.  Not perfect but not catastrophic either.  


  1. that's a good one. As far as I remember, Grady Booch once defined software architecture as everything that is costly to change.
    Architect Engineer in Pakistan

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