Regression Testing

Overview

Regression Testing validates that existing functionality continues to work after changes, updates, or bug fixes. It ensures new code doesn't break previously working features through automated test suites run regularly.

🔄 Continuous Validation

Regression Testing ensures:

• Previous fixes stay fixed

• New code doesn't break existing features

• Changes don't have unintended side effects

• Quality maintained across releases

• Quick feedback on breakage

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When to Use Regression Testing

✅ Good Use Cases

Scenario	Why Regression
After bug fixes	Ensure fix doesn't break elsewhere
New features	Don't break existing functionality
Version updates	Libraries/frameworks update
Refactoring	Code restructuring shouldn't change behavior
Before releases	Quality gate before deployment
Continuous integration	Every commit checked

❌ Anti-Patterns (Don't Do)

• ❌ Only test new changes (miss regressions)

• ❌ Run regression tests rarely (too late to fix)

• ❌ Include too many tests (too slow)

• ❌ Manual regression testing (not scalable)

• ❌ Run full suite for minor changes (overkill)

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Test Suite Strategies

Quick Smoke Suite (15 minutes)

Coverage: 20% of features
Focus: Critical paths

Tests:
- User login/logout
- Core feature works
- Payment processing
- Critical integrations

When to run:
✓ Every commit
✓ Multiple times per day
✓ Fast feedback
✓ Catch obvious breaks

Standard Suite (45 minutes)

Coverage: 60% of features
Focus: Important features

Tests:
- Core workflows
- Common scenarios
- Important integrations
- Critical edge cases

When to run:
✓ Before merge
✓ Multiple times per day
✓ Pre-release
✓ Comprehensive check

Full Suite (2+ hours)

Coverage: 90%+ of features
Focus: Everything

Tests:
- All major features
- Most edge cases
- Integration scenarios
- Performance expectations

When to run:
✓ Pre-release
✓ Nightly runs
✓ Complete verification
✓ Final quality gate

Risk-Based Suite (30 minutes)

Coverage: Areas with recent changes
Focus: Highest risk

Analyze:
- Recent commits
- Modified code paths
- Related features

Tests:
- Changed features
- Dependent features
- Integrated systems

When to run:
✓ Every commit
✓ Fast targeted check
✓ High confidence/low time
✓ Smart prioritization

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Practical Examples

Example 1: Quick Smoke Test Suite

Workflow: Smoke test (15 minutes)

Purpose: Catch obvious breakage

Test 1: Application Starts
  [Launch application]
  [Check: No errors on startup]
  [Check: Main page loads]
  [Check: All critical elements visible]
  
  Expected: ✅ Pass

Test 2: User Login
  [Navigate to login]
  [Enter valid credentials]
  [Click login]
  [Check: Redirect to dashboard]
  [Check: User profile visible]
  
  Expected: ✅ Pass

Test 3: Core Feature (e.g., Search)
  [Navigate to search]
  [Enter search term]
  [Execute search]
  [Check: Results display]
  [Check: Pagination works]
  
  Expected: ✅ Pass

Test 4: Payment Processing
  [Navigate to checkout]
  [Add test item]
  [Process payment]
  [Check: Success page shown]
  [Check: Order confirmation received]
  
  Expected: ✅ Pass

Test 5: User Logout
  [Click logout]
  [Check: Redirected to login]
  [Check: Session cleared]
  
  Expected: ✅ Pass

Results: ✅ All critical paths working

Example 2: Standard Regression Suite

Workflow: Comprehensive regression test

Purpose: Verify no regressions in important features

Test 1: User Account Management
  [Create account]
  [Edit profile]
  [Change password]
  [Delete account]
  
  Verify: All operations work, data persisted

Test 2: Feature A Workflows
  [Scenario 1: Basic usage]
  [Scenario 2: Edge case]
  [Scenario 3: With constraints]
  
  Verify: All scenarios pass

Test 3: Feature B Integration
  [Feature B with A]
  [Feature B with C]
  [All together]
  
  Verify: Integrations work

Test 4: Data Operations
  [Create data]
  [Read data]
  [Update data]
  [Delete data]
  
  Verify: CRUD operations intact

Test 5: Reports and Exports
  [Generate report]
  [Export to CSV]
  [Export to PDF]
  [Email report]
  
  Verify: All export formats work

Test 6: Permissions and Access
  [Admin access]
  [User access]
  [Guest access]
  [No access (denied)]
  
  Verify: Access control working

Results: ✅ 60 tests passing, comprehensive coverage

Example 3: Risk-Based Regression

Workflow: Smart regression targeting recent changes

Purpose: Efficient regression for specific changes

Recent Changes:
- Modified: User authentication module
- Modified: Payment processing
- Related: Order management (depends on payment)
- Related: Notifications (depends on orders)

Regression Tests (focused):

Payment Module Tests:
  [Test: Payment with valid card]
  [Test: Payment with declined card]
  [Test: Payment timeout handling]
  [Test: Payment with fraud checks]
  
  Verify: No regression in payment processing

Order Management Tests (depends on payment):
  [Test: Create order with payment]
  [Test: Order status updates]
  [Test: Order cancellation]
  [Test: Refund processing]
  
  Verify: Order logic still works with payment changes

Notification Tests (depends on orders):
  [Test: Order confirmation email]
  [Test: Payment notification]
  [Test: Shipment notification]
  
  Verify: Notifications triggered correctly

Related Feature Spot Check:
  [Test: Basic checkout flow]
  [Test: Repeat customer purchase]
  
  Verify: No obvious breakage in related areas

Results: ✅ Risk areas tested, confidence high

Example 4: Nightly Full Regression

Workflow: Complete overnight regression suite

Purpose: Comprehensive validation before release

Execution Time: 2 AM - 5 AM (off-hours)
Scope: 100+ tests covering 90% of features

Test Categories:

Core Functionality (25 tests):
✓ All major features tested
✓ Happy path and edge cases
✓ Basic and advanced usage

Integrations (20 tests):
✓ External service communication
✓ Database operations
✓ File operations
✓ Email/notifications

User Management (15 tests):
✓ Account creation/deletion
✓ Permission levels
✓ Access control
✓ Profile management

Data Operations (15 tests):
✓ CRUD operations
✓ Bulk operations
✓ Data validation
✓ Data migration

Performance (10 tests):
✓ Page load times
✓ Report generation
✓ Bulk processing
✓ Concurrent users

Results Summary:
✓ 103 tests
✓ 98 passed
✓ 2 warnings (slow performance)
✓ 3 skipped (environment specific)

Status: ✅ Ready for release (warnings addressed)

Example 5: Continuous Integration Regression

Workflow: Every-commit regression in CI/CD

Pipeline:

Commit made
  ↓
[Build artifact]
  ↓
[Run unit tests (5 min)]
  ↓
[Run smoke tests (15 min)]
  ↓
[Run integration tests (20 min)]
  ↓
[Run regression suite (45 min)]
  ↓
[Report results]

Results:

✅ All tests passed
   └─ Code merged to develop

❌ Regression test failed
   └─ Block merge
   └─ Alert developer
   └─ Run debug info
   └─ Developer fixes
   └─ Re-run regression

⚠️ Performance degraded
   └─ Flag for review
   └─ Allow merge (warning)
   └─ Create optimization ticket

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Test Suite Organization

By Feature Area

Login Tests
  - Valid credentials
  - Invalid credentials
  - Password reset
  - 2FA verification

Account Tests
  - Create account
  - Edit profile
  - Change password
  - Deactivate account

Search Tests
  - Basic search
  - Advanced filters
  - Search suggestions
  - Sort options

Payment Tests
  - Valid payment
  - Failed payment
  - Refunds
  - Recurring billing

By Risk Level

Critical (must pass):
  - Login
  - Payment
  - Data persistence
  - Critical features

Important (should pass):
  - Most features
  - Common workflows
  - Integrations

Nice-to-have (check):
  - Edge cases
  - Performance
  - UI polish

By Execution Speed

Tier 1 - Smoke (15 min):
  - Critical path only
  - Run on every commit

Tier 2 - Standard (45 min):
  - Important features
  - Run before merge

Tier 3 - Full (2+ hours):
  - Everything
  - Run nightly/pre-release

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Handling Flaky Tests

Detecting Flakiness

Metrics to track:
- Test pass rate over time
- Failure frequency
- Consistency across runs

Rule of thumb:
- 95%+ pass rate = reliable
- 90-95% = investigate
- <90% = fix before using

Fixing Flakiness

Common causes:
1. Timing issues
   → Increase waits
   → Use polling
   → Wait for specific state

2. External dependencies
   → Mock unreliable services
   → Use test environments
   → Add retry logic

3. Data issues
   → Use clean test data
   → Isolate tests
   → Reset state between runs

4. Test order dependencies
   → Run tests in random order
   → Each test independent
   → No shared state

Removing or Quarantining

If test still flaky after fixes:

Option 1: Remove test
- If not testing important behavior
- If behavior tested elsewhere

Option 2: Quarantine test
- Mark as flaky
- Don't block releases
- Monitor separately
- Fix later

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Best Practices

✅ Do

• Automate regression tests - Manual regression doesn't scale

• Run frequently - Daily minimum, ideally per commit

• Prioritize strategically - Smoke suite for speed, full suite nightly

• Keep tests independent - No test order dependencies

• Use test data - Never use production data

• Monitor results - Track pass rates over time

• Fix failing tests quickly - Don't accumulate failures

• Remove outdated tests - Delete tests for removed features

❌ Don't

• Test everything - Select smart subset

• Skip regression - Too easy to miss breaks

• Run infrequently - Daily is minimum

• Ignore flaky tests - Fix or remove them

• Use production data - Test data only

• Run manually - Automate in CI/CD

• Add tests without removing - Keep suite size manageable

• Ignore test failures - Investigate and fix

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Real-World Scenarios

E-Commerce Platform

Every Commit:
✓ Smoke: Login, browse, checkout, payment (15 min)

Before Merge:
✓ Standard: Full E-commerce suite (45 min)
✓ Verify: No regression in selling features

Nightly:
✓ Full: 100+ tests including edge cases (2 hrs)
✓ Performance: Load testing
✓ Integration: Payment gateways, inventory

SaaS Application

Every Commit:
✓ Smoke: Login, dashboard, core feature (15 min)

Per Pull Request:
✓ Risk-based: Test changed modules (30 min)
✓ Integration: Dependent features (20 min)

Nightly:
✓ Full: All 150+ tests (3 hrs)
✓ Compliance: All permissions tests
✓ Data: Backup/recovery scenarios

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Troubleshooting

Issue: Regression tests take too long

Symptoms:

• Suite runs 2+ hours

• Can't run often

• Developers ignore results

Solutions:

1. Split into tiers (smoke/standard/full)

2. Run tiers at different frequencies

3. Optimize execution

4. Remove low-value tests

5. Mock slow operations

Issue: Too many false failures

Symptoms:

• Developers ignore failures

• Tests fail randomly

• Hard to see real issues

Solutions:

1. Fix flaky tests

2. Improve test stability

3. Add waits/polling

4. Mock unreliable services

5. Quarantine problematic tests

Issue: Missing regressions

Symptoms:

• Tests pass but production breaks

• Similar issues keep occurring

• Previous fixes break again

Solutions:

1. Add test for bug when found

2. Increase test coverage

3. Run full suite before release

4. Add regression test for each fix

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Related Topics

• Smoke Testing - Quick health checks

• Integration Testing - Component interactions

• Best Practices - Professional standards

• Performance Optimization - Speed improvements

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Summary

• Regression Testing ensures previous fixes stay fixed

• Use tiered approach - Smoke/Standard/Full at different frequencies

• Automate execution - Integrate with CI/CD

• Keep tests fast - Run often for quick feedback

• Fix flaky tests - Don't ignore failures

• Remove obsolete tests - Keep suite healthy

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Next: Learn about Smoke Testing for quick health checks.