Combine unit testing with mutation testing. By automatically mutating application code, the quality of the test suite itself can be evaluated: for example, a m

Combine unit testing with mutation testing. By automatically mutating application code, the quality of the test suite itself can be evaluated: for example, a method is mutated to return null--and tests still pass! That could indicate a real bug or nothing. End-to-end (e2e) is ideal to really exercise everything, but they can take a long time to finish. The feedback cycle is an eternity compared to unit tests. One way to improve e2e is to have an agent on the app server and an agent on the test server, building a probabilistic model of what e2e tests to actually run; then it's a real time-saver to only execute those tests. At least one company offers this. In the end, it's preferable to have both unit tests for microservices and e2e tests that run against one or more browsers.