Lecture 11: Debugging - After the System is Deployed

1 Reproduce the Bug

• Start by finding a small, repeatable test case that produces the failure.
• Once you’ve found a small test case, find and fix the bug using that smaller test case, and then go back to the original buggy input and confirm that you fixed the same bug.

2 Understand the Location and Cause of the Bug

Step1: Study the Data

• test input that causes the bug
• the incorrect results
• failed assertions
• stack trace from an exception.

Step2: Hypothesize

• Propose a hypothesis, consistent with all the data, about where the bug might be, or where it cannot be.
• It helps to think about your program as modules, or steps in an algorithm, and try to rule out whole sections of the program at once.

Step3: Experiment - Devise an experiment that tests your hypothesis

• Run a different test case
• Insert a print statement or assertion in the running program, to check something about its internal state.
• Set a breakpoint using a debugger, then single-step through the code and look at variable and object values.

2.1 Other Tips for the Location of the Bug

• Bug localization by binary search
• Prioritize your hypotheses
  
  • Different parts of the system have different likelihoods of failure.
  • Old, well-tested code is probably more trustworthy than recently-added code.
  • Java library code is probably more trustworthy than yours.
• Swap components.
  
  • If you suspect the Java runtime, run with a different version of Java.
  • If you suspect the operating system, run your program on a different OS.
  • If you suspect the hardware, run on a different machine.
  • If you suspect your binarySearch() implementation, then substitute a simpler linearSearch() instead.
• Make sure your source code and object code are up to date.
• Get help.
• Sleep on it, If you’re too tired, you won’t be an effective debugger. Trade latency for efficiency. :)

3 Fix the Bug

• Ask yourself whether the bug was a coding error, like a misspelled variable or interchanged method parameters, or a design error, like an underspecified or insufficient interface.
• Think also whether the bug has any relatives.

Reference

[1] 6.005 — Software Construction on MIT OpenCourseWare | OCW 6.005 Homepage at https://ocw.mit.edu/ans7870/6/6.005/s16/

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