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MSc Software MaintenanceMS Viðhald hugbúnaðar
Fyrirlestur 42Maintainability Index Revisited
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Case StudyDæmisaga
ReferenceMaintainability Index Revisited – position paper -, Tobias
Kuipers and Joost Visser. CSRM2007/SQM2007http://www.cs.vu.nl/csmr2007/workshops/SQM07_paper3.pdf
Problems with the Maintainability Index
• “We have used the Maintainability Index in our consultancy practice over the last four years and found a number of problems with it.”
• “Although we see a clear use for determining the maintainability of the source code in one (or a few) simple to understand metrics, we have a hard time using the Maintainability Index to the desired effect.”
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Root cause analysis
• Because MI is a composite metric it is very difficult to know what a value for means.
• When the MI has a low value, it is not immediately clear what should be done to increase it.
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Average complexity
• The complexity metric is flawed.• Because of all the getters and setters in Java
which have a complexity of 1, the average complexity is low.– a class with 9 getters and setters and 1 method with a
complexity of 10 will have an average complexity of 1.9
• Anecdotal evidence suggests maintainance problems occur in a few outliers that have a complexity of over 100.
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Computability
• There is no formal definition of what constitutes an operator or operand for Java or C#.
• The Halstead Volume metric “is not widely accepted within the software engineering community”.
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Comments• “... we find that counting the number of lines... in
general, has no relation with maintainability whatsoever.”
• Source code which has been “commented out” gets counted.
• Comments sometimes are not kept up-to-date and refer to a previous version.
• The comments part of the MI is optional.– it is possible drop the term containing PerCM
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171 5.2 ( ) 0.23 ( ) 16.2 ln( ) 50sin( 2.4 )MI ln avgV avgV g avgLOC avgPerCM
Understandability
• Developers feel they have a lack of control over the value of the MI and this “makes them dismissive of the MI for quality assessment purposes”.
• The developers´ attitude directly influences management´s acceptance of the value.
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The SIG Maintainability Model
• The model is under development
• Five easy-to-calculate metrics
• Metrics are not composed into one unifying metric like MI
• “From discussions with developers of dozens of industrial systems we learn that the metrics are well accepted, or acceptable.”
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Software Improvement Group
1. Total Size
• Source lines of code (SLOC).– excluding comments and blank lines
• A larger system requires a larger effort to maintain. So a smaller system is better.– a simple, intuitive idea that everyone understands
• No correction is applied for the expressiveness of a programming language.– a 1000-line Java program is considered to be more
easily maintained than the functionally equivalent 1200-line C program
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Five easy-to-calculate metrics
2. Number of Modules
• The ratio between the number of modules and the total lines of code is “a measure of how well a system is decomposed”.– “as an initial estimate it turns out to be rather
useful”
• A module in Java or C# is a class.
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Five easy-to-calculate metrics
Andy asks: What is a reasonable ratio?
3. Number of units
• A module is decomposed into units.
• In Java or C#, a unit is a method.
• A unit is the smallest piece of code that can be executed (tested) individually.
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Five easy-to-calculate metrics
Andy asks: What is a reasonable number of units per module?
4. Cyclomatic Complexity above X
• Calculate complexity of units.
• Do not calculate average values.
• Complexity is expressed as the percentage of lines of code of the system that are in units which have a higher complexity than some threshold X.– X is currently taken to be 20 though McCabe
suggests that X should be 10
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Five easy-to-calculate metrics
Andy asks: What is a reasonable % for complexity?
5. Duplication• “... measuring code duplication gives a fairly
simple estimate of how much larger a system is than it needs to be.”
• Exact string matching duplication.• Duplication is measured as the percentage of all
code that occurs more than once in equal code blocks of at least X lines.– “we take X to be 6”
• “we see a duplication percentage of around 3% for well managed systems”
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Five easy-to-calculate metrics
Discussion and Conclusions
• The SIG Maintainability Model is easily explained to technical personnel and managers.
• How the source code influences the five metrics is clear.
• “We are currently putting this model to work in our consultancy practice.”
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Andy asks: what about use of OO language features such as inheritance?
keep DOI < 5?
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Case StudyDæmisaga
ReferenceThe Software Maintainability Index Revisited Kurt D.
Welker. CROSSTALK The Journal of Defense Software Engineering, August 2001, pp18-21 http://www.stsc.hill.af.mil/crosstalk/2001/08/welker.html
Comments in code
• Comments that are out-of-date can actually hinder maintenance.
• Sometimes identifier names are used instead of comments.– distanceMetres– distance //measured in metres
• “A man in the maintainability assessment loop is essential both in how to measure comments in the source code (which MI equation) and then in determining the meaning of the results.”
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Figure 1: The Twelve Days of Christmas
Andy says: we agree this code is maintainable...