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Ideally, each class should be tested in isolation. However, some higher-level classes have complex dependencies on lower-level classes that can make unit testing hard (e.g. we don't want to have to set up an running RAMCloud cluster to run unit tests, but some classes will invoke RPCs to other servers). In situations like this, write mock classes that can be used instead of lower-level classes to simplify writing unit tests. We have already written many such classes. For example, if a class invokes system calls, there is a MockSyscall class that can be used to avoid actually making system calls.
Write tests that don't break easily when
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unrelated code changes
It's easy to write tests that are extremely sensitive to the overall structure of the system, such that the tests will break if almost any code changes. When this happens, it becomes hard to make code changes, because we have to update a lot of tests afterwards. Try to avoid such tests. Ideally, a test is sensitive to the behavior of a particular piece of code (the code it is validating), but not sensitive to other code in the system. One way to create more robust tests is to setup a test by invoking API methods of a class, which are less likely to change, rather than by directly modifying fields of structures, which are more likely to change.
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