What is End-to-End Testing and When Should It Be Used Instead of Integration Testing?

​In modern development, E2E testing is essentially the "Evolution of System Testing" adapted for Agile and CI/CD environments. Here is a practical breakdown to address your confusion:
​1. What is the main purpose?
​The goal is to verify the User Experience (UX) rather than individual functions. It ensures that the "Happy Path" (e.g., Search → Add to Cart → Checkout) works seamlessly across the entire stack, including the frontend, backend, database, and 3rd-party integrations.
​2. E2E vs. Integration Testing
​Integration Testing: Validates that two or more modules (e.g., API and DB) talk to each other correctly. It’s fast and runs in a "headless" or internal environment.
​E2E Testing: Validates the system from the user's perspective (usually via a browser). It’s a "black-box" test that covers the flow from "End to End."
​3. When to use it (and when to avoid it)
​Use it for: Critical business flows where failure means a direct loss of revenue (e.g., Login, Payment). These act as a safety net before deployment to prevent regressions.
​Avoid it for: Testing every single edge case or validation logic. E2E tests are "expensive" in terms of execution time and maintenance. Use Unit/Integration tests for granular logic.
​4. The Challenges (The "Dark Side" of E2E)
​Flakiness: E2E tests often fail due to network lag or minor UI changes rather than actual bugs.
​Execution Time: Running a full suite can take hours, which slows down your CI/CD pipeline.
​Missing Non-functional Requirements: While E2E focuses on the "flow," it might overlook security, performance, or stress testing—areas that a traditional "System Test" would typically cover.
​Summary
​Think of E2E testing as a high-level safeguard for user stories. It’s the modern replacement for traditional System Testing in Agile, but it should be used selectively (following the Test Pyramid) to keep your CI/CD pipeline fast and reliable.