principles:murphy_s_law
Differences
This shows you the differences between two versions of the page.
Both sides previous revisionPrevious revisionNext revision | Previous revisionNext revisionBoth sides next revision | ||
principles:murphy_s_law [2020-10-12 12:40] – old revision restored (2013-08-10 10:58) 159.69.186.191 | principles:murphy_s_law [2021-08-11 17:51] – old revision restored (2021-05-11 22:12) 82.80.249.158 | ||
---|---|---|---|
Line 3: | Line 3: | ||
===== Variants and Alternative Names ===== | ===== Variants and Alternative Names ===== | ||
- | * Design for Errors(({{page> | + | * Design for Errors((Alan M. Davis: //201 Principles |
===== Context ===== | ===== Context ===== | ||
/* fill in contexts here: */ | /* fill in contexts here: */ | ||
* [[contexts: | * [[contexts: | ||
- | * [[contexts: | + | |
- | * [[contexts: | + | |
- | * [[contexts: | + | |
===== Principle Statement ===== | ===== Principle Statement ===== | ||
Line 25: | Line 23: | ||
It is not always possible to design a system in such a way. But as systems are built and used by humans, one should strive for such " | It is not always possible to design a system in such a way. But as systems are built and used by humans, one should strive for such " | ||
- | There are different kinds of possible errors that can and according to ML eventually will occur in some way: Replicated data can get out of sync, invariants can be broken, preconditions can be violated, interfaces can be misunderstood, | + | Note that Murphy' |
- | + | ||
- | Note that Murphy' | + | |
Line 36: | Line 32: | ||
This means the fewer possibilities there are that a mistake is made, the fewer there will be. As mistakes are generally undesirable, | This means the fewer possibilities there are that a mistake is made, the fewer there will be. As mistakes are generally undesirable, | ||
- | Note that ML does //not// claim that everything constantly fails unless there is no possibility to do so. It simply says that statistically in the long run a system will fail if it can. | + | |
===== Strategies ===== | ===== Strategies ===== | ||
Line 42: | Line 38: | ||
* Make use of static typing, so the compiler will report faults | * Make use of static typing, so the compiler will report faults | ||
- | * Make the design simple, so there will be fewer implementation defects (see [[Keep It Simple Stupid|KISS]]) | + | * Make the design simple, so there will be fewer implementation defects (see [[Keep It Simple Stupid]]) |
* Use automatic testing to find defects | * Use automatic testing to find defects | ||
- | * Avoid duplication and manual tasks, so necessary changes are not forgotten (see [[Don' | + | * Avoid duplication and manual tasks, so necessary changes are not forgotten (see [[Don' |
* Use polymorphism instead of repeated switch statements | * Use polymorphism instead of repeated switch statements | ||
- | * Use the same mechanisms wherever reasonably possible (see [[Uniformity Principle|UP]]) | + | * Use the same mechanisms wherever reasonably possible (see [[Uniformity Principle]]) |
- | * Use consistent naming and models throughout the design (see [[Model Principle|MP]]) | + | * Use consistent naming and models throughout the design (see [[Model Principle]]) |
- | * Avoid Preconditions and Invariants (see [[Invariant Avoidance Principle|IAP]]) | + | |
- | * Use assertions to detect problems early. | + | |
* ... | * ... | ||
- | |||
- | |||
- | ===== Caveats ===== | ||
- | |||
- | See section [[#contrary principles]]. | ||
Line 72: | Line 61: | ||
/* * [[wiki: | /* * [[wiki: | ||
- | Furthermore every defect in any system is a manifestation of ML. If there is a fault then obviously something went wrong. The correlation between the number of possibilities for introducing defects and the actual defect count can be regarded trivially intuitive. | + | |
===== Relations to Other Principles ===== | ===== Relations to Other Principles ===== | ||
Line 82: | Line 71: | ||
* [[Easy to Use and Hard to Misuse]] (EUHM): Because of ML an interface should be crafted so it is easy to use and hard to misuse. EUHM is the application of ML to interfaces. | * [[Easy to Use and Hard to Misuse]] (EUHM): Because of ML an interface should be crafted so it is easy to use and hard to misuse. EUHM is the application of ML to interfaces. | ||
* [[Uniformity Principle]] (UP): A typical source of mistakes are differences. If similar things work similarly, they are more understandable. But if there are subtle differences in how things work, it is likely that someone will make the mistake to mix this up. | * [[Uniformity Principle]] (UP): A typical source of mistakes are differences. If similar things work similarly, they are more understandable. But if there are subtle differences in how things work, it is likely that someone will make the mistake to mix this up. | ||
- | * [[Invariant Avoidance Principle]] (IAP): Invariants are statements that have t be true in order to keep a module in a consistent state. ML states that eventually an invariant will be broken resulting in a hard to detect defect. IAP states that invariants should therefore be avoided. So IAP is the application of ML to invariants. | ||
==== Contrary Principles ==== | ==== Contrary Principles ==== | ||
- | * **[[Keep It Simple Stupid]] (KISS)**: On the one hand a simpler design is less prone to implementation errors. In this aspect KISS is similar to ML. On the other hand it is sometimes more complicated to make a design " | + | * **[[Keep It Simple Stupid]] (KISS)**: On the one hand a simpler design is less prone to implementation errors. In this aspect KISS is similar to ML. On the other hand it is sometimes more complicated to make a design " |
==== Complementary Principles ==== | ==== Complementary Principles ==== | ||
- | * **[[Fail Fast]] (FF)**: Sometimes it is impossible to actually prevent an error. In such a case it is advisable to fail fast so the error is recognized early. | + | * **[[Fail Fast]] (FF)**: Sometimes it is impossible to actually prevent an error. In such a case it is advisable to fail fast to the mistake |
Line 98: | Line 86: | ||
- | ===== Examples | + | ===== Example |
==== Example 1: Parameters ==== | ==== Example 1: Parameters ==== | ||
Line 106: | Line 94: | ||
The following method signatures are a bad choice: | The following method signatures are a bad choice: | ||
<code java> | <code java> | ||
- | replaceFirst(String pattern, | + | replaceFirst(String pattern, |
- | replaceAll(String replacement, | + | replaceAll(String replacement, |
</ | </ | ||
Eventually someone will mix up the order of the parameters leading to a fault in the software which is not detectable by the compiler. | Eventually someone will mix up the order of the parameters leading to a fault in the software which is not detectable by the compiler. | ||
Line 113: | Line 101: | ||
So it is better to make parameter lists consistent: | So it is better to make parameter lists consistent: | ||
<code java> | <code java> | ||
- | replaceFirst(String pattern, | + | replaceFirst(String pattern, |
- | replaceAll(String pattern, | + | replaceAll(String pattern, |
</ | </ | ||
This is less error prone. When for example a call to '' | This is less error prone. When for example a call to '' | ||
Line 120: | Line 108: | ||
But here still one could mix up the two string parameters. Although this is less likely, as having the substring to look for first is " | But here still one could mix up the two string parameters. Although this is less likely, as having the substring to look for first is " | ||
<code java> | <code java> | ||
- | replaceFirst(Pattern pattern, | + | replaceFirst(Pattern pattern, |
- | replaceAll(Pattern pattern, | + | replaceAll(Pattern pattern, |
</ | </ | ||
Here both methods expect a '' | Here both methods expect a '' | ||
Line 158: | Line 146: | ||
Note that the typecast is rather a symptom than the actual problem here. The problem is, that the '' | Note that the typecast is rather a symptom than the actual problem here. The problem is, that the '' | ||
- | ==== Example 3: Date, Mutability/ | ||
- | |||
- | In Java the classes [[http:// | ||
- | |||
- | <code java> | ||
- | Date date1 = new Date(2013, 01, 12); | ||
- | Date date2 = date1; | ||
- | System.out.println(date1); | ||
- | System.out.println(date2); | ||
- | date1.setMonth(2); | ||
- | System.out.println(date1); | ||
- | System.out.println(date2); | ||
- | </ | ||
- | |||
- | Furthermore as can be seen in the code above, the month value counterintuitively is zero-based, which results in 1 meaning February. This obviously is another source for mistakes. Also the order of the parameters can be mixed up easily. And lastly this does not refer to a date in 2013 but to one in 3913! The year value is meant to be " | ||
- | |||
- | Because of these and several other flaws in the design of the Java date API, most of the methods in '' | ||
===== Description Status ===== | ===== Description Status ===== | ||
Line 188: | Line 159: | ||
* [[wiki> | * [[wiki> | ||
- | ===== Discussion ===== | ||
- | |||
- | Discuss this wiki article and the principle on the corresponding [[talk: |
principles/murphy_s_law.txt · Last modified: 2021-10-20 21:18 by christian