Regression Testing

One-liner: Testing to ensure new code changes don't break existing functionality.

🎯 What Is It?

Regression Testing is a type of software testing performed to verify that recent code changes, bug fixes, or new features haven't negatively impacted existing functionality. When developers modify code, there's a risk of introducing new bugs into previously working features—regression testing catches these "regressions" before they reach production.

This is a critical component of the Testing phase in Software Development Lifecycle (SDLC) and is essential for maintaining software quality in iterative development environments like Agile and DevSecOps.

🤔 Why It Matters

• Prevent breaking changes: Catch unintended side effects of new code
• Maintain quality: Ensure existing features continue working as expected
• Enable rapid iteration: Developers can change code confidently knowing tests will catch issues
• Reduce production bugs: Find problems before users do
• Cost savings: Fixing bugs during testing is 100x cheaper than in production
• Developer Velocity: Automated regression tests enable faster development cycles
• Continuous delivery: Critical for CI/CD pipelines to deploy safely

🔬 How It Works

Core Principles

1. When to Run Regression Tests

• After bug fixes (ensure fix didn't break other features)
• After new feature additions (verify existing features still work)
• Before releases (validate entire application)
• After configuration changes (ensure environment updates are safe)
• During CI/CD pipeline execution (automated on every commit)

2. Regression Testing Process

1. Developer makes code change (bug fix or new feature)
         ↓
2. Run regression test suite
         ↓
3a. All tests pass → Merge code ✓
3b. Tests fail → Identify regression, fix, retest

3. Types of Regression Testing

Technical Deep-Dive

Manual vs. Automated Regression Testing

Manual Regression Testing:

• ❌ Slow and time-consuming
• ❌ Human error prone
• ❌ Expensive (QA engineer time)
• ❌ Can't run on every commit
• ✅ Good for exploratory edge cases
• Use case: Rare, complex scenarios that are hard to automate

Automated Regression Testing:

• ✅ Fast (runs in minutes)
• ✅ Consistent and repeatable
• ✅ Runs on every commit (CI/CD)
• ✅ Increases Developer Velocity
• ❌ Requires initial investment to write tests
• Use case: Core functionality, API endpoints, critical user flows

Impact on Developer Velocity:

From Software Development Lifecycle (SDLC) writeup: "Developer velocity is a metric that helps us understand and estimate how much development our team can perform in a given timeframe."

Example:

• Without automation: QA manually tests for 2 days per release → 2 releases/month
• With automation: Regression tests run in 20 minutes → 10+ releases/month
• Result: 5x increase in Developer Velocity

Test Execution in SDLC

As developers make fixes during the Testing phase of SDLC:

1. Developer fixes bug A
2. Regression tests run to ensure bug fix didn't introduce bug B
3. If regression found → fix bug B, retest
4. Repeat until all tests pass

This iterative process is why testing and development are sometimes merged in modern SDLC models (especially DevSecOps).

Regression Testing Example

Scenario: E-commerce application

• Change: Developer updates checkout page to support new payment method
• Regression test coverage:
  • ✅ Existing payment methods still work
  • ✅ Cart calculations still accurate
  • ✅ Order confirmation emails still sent
  • ✅ Inventory still updates after purchase
  • ✅ User account history still displays orders

Without regression testing: Updating checkout could accidentally break cart totals, and users would see incorrect prices—only discovered in production.

With regression testing: Automated tests catch broken cart totals immediately, developer fixes before merge.

🎤 Interview Angles

Common Questions

• "What is regression testing?" → Testing to ensure new changes don't break existing functionality
• "When do you run regression tests?" → After every code change, ideally automated in CI/CD pipeline
• "Manual or automated regression testing?" → Automated for core functionality, manual for exploratory edge cases
• "How does regression testing relate to CI/CD?" → Automated regression tests are critical gates in deployment pipelines

STAR Story

Situation: Web application had frequent production bugs where new features broke unrelated functionality. QA manually tested before releases, but issues still slipped through.
Task: Reduce production regression bugs and increase Developer Velocity.
Action: Implemented automated regression test suite covering 80% of core user flows (login, checkout, search, profile). Integrated tests into CI/CD pipeline—pull requests couldn't merge if regression tests failed. Set up test coverage reporting to track gaps.
Result: Production regression bugs dropped 85% (from ~8/month to 1/month). Developer Velocity increased 40% because developers no longer waited 2 days for manual QA—automated tests ran in 15 minutes. Test failures caught issues immediately, with clear error messages pointing to exact problem.

Q: What's the difference between regression testing and unit testing?

Unit testing: Tests individual functions in isolation (e.g., "does calculateTax() return correct value?")
Regression testing: Tests that changes to codebase don't break existing features across the entire system (e.g., "does the full checkout flow still work end-to-end?")

Regression tests often include unit tests but also integration and system tests.

✅ Best Practices

• Automate early: Build regression test suite from day one, not after bugs pile up
• Prioritize critical paths: Test most-used features first (login, payments, core workflows)
• Fast execution: Keep regression suite under 20-30 minutes so it doesn't block Developer Velocity
• Run on every commit: Integrate with CI/CD pipeline as a mandatory gate
• Maintain test suite: Update tests when requirements change (flaky tests erode trust)
• Clear failure messages: Tests should indicate exactly what broke and where
• Track coverage: Monitor test coverage to identify untested code paths
• Fail fast: Stop test execution on first failure to get quicker feedback

❌ Common Misconceptions

• "Regression testing is only before releases" — Should run continuously throughout development
• "Manual testing is sufficient" — Manual testing can't keep up with modern deployment frequency
• "Regression tests slow down development" — Opposite: automation increases Developer Velocity by catching bugs early
• "We don't need regression tests for small changes" — Small changes can have big side effects
• "100% test coverage eliminates all bugs" — Tests reduce bugs but can't catch everything (business logic errors, edge cases)

🔗 Related Concepts

• Software Development Lifecycle (SDLC) — Regression testing is part of Testing phase
• CI/CD — Automated regression tests run in deployment pipelines
• Developer Velocity — Regression automation significantly increases velocity
• DevSecOps — Extends regression testing to include security tests
• Test-Driven Development (TDD) — Write tests before code to prevent regressions
• Unit Testing — Foundation for regression test suites
• Integration Testing — Broader regression testing across system components
• Quality Assurance (QA) — Team responsible for testing strategy

📚 References

• TryHackMe SDLC Room: https://tryhackme.com/room/sdlc (Testing phase)
• "Continuous Delivery" by Jez Humble and David Farley (automated testing)
• ISTQB (International Software Testing Qualifications Board) - Regression Testing Guide
• Martin Fowler: "Refactoring: Improving the Design of Existing Code" (importance of regression tests)