You seem to imply that I think integration tests aren’t valuable. But I’m not saying that. What I’m saying is that it’s easier to produce unit tests that cover sections of code and test their contract across edg-cases easier than it’s to do the same with integration tests.
An example: Let’s say you’re writing a DNS resolver.
You need to parse all of the various RDATA portions of records. It’s far easier to write an exhaustive suite of unit tests that verify the code is parsing those data elements properly and throw in many tests for edge-cases that might mean insecure software if done incorrectly.
But it’s absolutely invaluable that we also have integration tests that verify an entire Message parses, and even moves through the entire stack from parsing to response. But why construct an entire Message when all you need to test is the RDATA? It’s easier and less complex test code to localize the RDATA test, and then to do a smaller set of Message based tests that verify the message moves through all the stacks properly.
Then popping all the way up the stack, you actually need to generate network traffic and send real DNS queries and verify that those work. That means multiple processes in your end-to-end test. Trying to produce a full set of tests that would validate individual edges cases in all RDATAs would be an extreme amount of redundant code, and also would fail due to many other reasons not related to an RDATA change, thus making it more time consuming to track down a failure.
Each area of tests is absolutely important and all are necessary for different reasons.