Testing Modern Applications is a practical engineering guide to building a test suite that catches real regressions in a system that has distributed services, asynchronous communication, third-party APIs, and cloud infrastructure. It starts from a single observation: the testing practices that worked for a monolith do not scale to modern applications, and the teams that treat testing as a discipline rather than a checkbox ship software their users can rely on.The book walks through the full discipline — why modern testing is different, the testing pyramid and its critics, unit testing fundamentals, test doubles and dependency injection, integration testing with testcontainers, contract testing and consumer-driven contracts, end-to-end testing with Playwright, property-based testing with Hypothesis, mutation testing and test quality, test data and fixture management, testing asynchronous and eventual systems, performance and load and chaos testing, testing in continuous delivery, and the future of software testing.It covers the failure modes that quietly wreck test suites: a suite that passes on every commit and lets a production bug through, a flaky test that trains the team to retry rather than investigate, a mock that drifts from the real service, an integration test that fails under parallel execution because of shared state, a contract test that is not enforced in CI, a browser test that breaks because a CSS class changed, a property test that generates inputs the system was never designed to handle, a mutation score that is high because the tests assert on unimportant details, a load test that runs against a smaller environment, a chaos experiment that takes down production. Each is presented with the failure, the countermeasure, and the operational tradeoff.
