The Main Disadvantage of the waterfall model is that it does not allow for very much Iteration.
for example you are in the testing stage and you realize that something has not been designed correctly.
In theory if following the model as closely as possible and trying to make it iterative this would lead you back to the design stage and then through the implementation until you arrived back at testing again.
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Sanity testing (also known as build verification testing) is used when you want to verify that a new build of the application meets basic functionality requirements before proceeding with further in-depth testing. We try to keep our sanity test set to less than an hour of manual or automated testing to minimize wasted time on builds that don't warrant additional testing. You don't want to test everything but you do want to verify that your basic set of existing functionality still behaves as expected so you can then focus on a specific area of the application.
Smoke testing is used to validate that a fix for a particular issue is working as expected and does not introduce additional regressions into the application. The quick and simple approach is to just verify that the issue being fixed is actually fixed with the change. Obviously, you'll want to do additional regression testing to verify no new issues were introduced but a smoke test is usually intended to quickly verify a fix. Sometimes you will get a patch or single file with the fix to apply it to an existing build of your application rather than a full build of the application, depending on the extent of the change. If this is the case, you'll want to re-verify the fix once it is included in the regular full build of the application.
Generally: A smoke test tells you whether you can test further, and a sanity test can help determine whether or not you should proceed.
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