Bug Investigation - Scientific Method
Apply scientific methodology to investigate and resolve software bugs systematically.
Scientific Method Process
- Observe - Gather data about the problem
- Hypothesize - Form testable explanations (ranked by likelihood)
- Experiment - Test hypotheses with controlled changes
- Analyze - Interpret results objectively
- Conclude - Identify root cause and validate fix
Core Principles
- Context first - Understand the project before investigating
- Hypothesis-driven - Never jump to solutions without forming testable hypotheses
- Isolate variables - Change one thing at a time
- Reproduce reliably - Can't fix what you can't reproduce
- Root causes over symptoms - Dig deeper than surface fixes
- Validate rigorously - Confirm fix resolves issue without regressions
Investigation Workflow
Phase 1: Project Context (2-5 min)
Discover before investigating:
- Language & version (Python 3.11, Java 17, Go 1.21, etc.)
- Build system (Gradle, npm, Cargo, Make, etc.)
- Key dependencies & frameworks
- Architecture pattern (MVC, microservices, etc.)
- Testing setup
Quick discovery:
# Find package managers
view package.json / requirements.txt / Cargo.toml / pom.xml
# Check config files
view .env / config.yml / settings.py
# Identify entry points
view main.* / index.* / app.*Output: One-line context
Python 3.11, Flask API, PostgreSQL, pytest, DockerPhase 2: Problem Definition
Gather:
- Error messages (full text, codes)
- Stack traces / logs
- Steps to reproduce
- Expected vs actual behavior
- Environment (OS, version, config)
- Reproducibility (always/sometimes/rare)
Document:
Bug: [Short description]
Reproduces: [Always/Sometimes/Unable]
Error: [Key error message]
Steps:
1. [Action 1]
2. [Action 2]
3. [Failure occurs]
Expected: [What should happen]
Actual: [What happens]Phase 3: Hypotheses (Ranked)
Phase 3: Hypotheses (Ranked)
Form 2-4 testable hypotheses, ranked by likelihood.
H1: [Most likely cause]
- Evidence for: [Why this is likely]
- Test: [How to prove/disprove]
- If true: [Expected result]
- If false: [Expected result]
H2: [Alternative cause]
- Evidence for: [Supporting observations]
- Test: [Falsifiable experiment]
Common categories:
- Logic errors (off-by-one, wrong operator, incorrect condition)
- State issues (race condition, uninitialized, stale data)
- Type/data (null/nil, type mismatch, parsing error)
- Concurrency (data race, deadlock, thread safety)
- Integration (API mismatch, version incompatibility)
- Environment (config, platform-specific, resource limits)
Phase 4: Experiment
For each hypothesis:
Test H1: [Hypothesis name]
- Change: [One variable to modify]
- Measure: [What to observe]
- Method: [Specific steps]
- Result: [Actual outcome]
- Conclusion: [Validated/Invalidated]
Techniques:
- Add logging at key points
- Use debugger breakpoints
- Binary search (remove half the code)
- Minimal reproduction (strip to essentials)
- Diff working vs broken states
- Isolate components
Phase 5: Root Cause
Identified: [Clear statement of actual cause]
Evidence: [Chain from observation → hypothesis → validation]
Why it occurred:
- Immediate: [Technical reason]
- Contributing: [What enabled this]
- Systemic: [Deeper issue if any]
Phase 6: Solution & Validation
Fix: [Specific changes to make]
Why this works: [Explain causal connection]
Validation:
- Reproduce bug (confirm failure)
- Apply fix
- Retest (confirm success)
- Test edge cases
- Run test suite (no regressions)
- Add test for this bug
Prevention:
- [Test to add]
- [Assertion to include]
- [Pattern to avoid]
Investigation Techniques
Binary Search: Remove half the code, test if bug persists. Repeat on failing half until isolated.
Minimal Reproduction: Strip to <50 lines that reproduce issue. Removes noise.
Differential Testing: Compare working vs broken (commits, versions, configs).
Strategic Logging: Add prints at key decision points to trace execution flow.
Rubber Duck: Explain code line-by-line aloud. Often reveals logic errors.
Common Bug Patterns (Language-Agnostic)
Logic Errors:
- Off-by-one:
i < nvsi <= n - Wrong operator:
&&vs||,==vs= - Negation errors:
!conditionlogic flipped
State Issues:
- Race conditions: concurrent access without synchronization
- Uninitialized: using variable before setting value
- Stale state: using outdated cached data
Type/Data:
- Null/nil dereference
- Type coercion errors
- Integer overflow
- Floating-point precision
Concurrency:
- Deadlock: mutual waiting for locks
- Data race: unsynchronized shared access
- Thread safety: non-thread-safe code on multiple threads
Integration:
- API contract mismatch
- Version incompatibility
- Missing dependencies
- Incorrect configuration
Output Format
# Bug Investigation: [Name]
## Context
[One-line: Language, framework, architecture]
## Problem
Error: [Message]
Reproduces: [Always/Sometimes]
Steps: [1,2,3]
## Hypotheses
H1: [Most likely] - Test by [method]
H2: [Alternative] - Test by [method]
## Investigation
Tested H1: [Result - validated/invalidated]
[If needed] Tested H2: [Result]
## Root Cause
[One sentence explanation]
Evidence: [What confirmed it]
## Solution
Fix: [Specific change]
Why it works: [Explanation]
Validated: [Tested successfully]
## Prevention
- Test added: [Description]
- Warning signs: [What to watch for]Quick Decision Tree
Symptom → Likely Category:
- Intermittent failure → Concurrency/state
- Always fails same way → Logic error
- Null/nil crash → Type/data
- Specific environment only → Configuration
- Performance degradation → Resource/algorithm
- After dependency update → Integration
Critical Reminders
- Start with context discovery
- Form hypotheses before coding
- Change ONE variable at a time
- Reproduce before fixing
- Validate fix rigorously
- Add regression test
- Document root cause