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copied!<blockquote> <p>Code coverage will ensure I'm <em>hitting</em> every functional piece of code</p> </blockquote> <h3>"Hit" <em>does not</em> mean "Testing"</h3> <p>The problem with only doing web-testing is that it only ensures that you <em>hit</em> the code, and that it <em>appears</em> to be correct at a high-level.</p> <p>Just because you loaded the page, and it didn't crash, doesn't mean that it actually works correctly. Here are some things I've encountered where 'web tests' covered 100% of the code, yet completely missed some very serious bugs which unit testing would not have.</p> <ol> <li>The page loaded correctly from a cache, but the actual database was broken</li> <li>The page loaded every item from the database, but only displayed the first one - it appeared to be fine even though it failed completely in production because it took too long</li> <li>The page displayed a valid-looking number, which was actually wrong, but it wasn't picked up because 1000000 is easy to mistake for 100000</li> <li>The page displayed a valid number by coincidence - 10x50 is the same as 25x20, but one is WRONG</li> <li>The page was supposed to add a log entry to the database, but that's not visible to the user so it wasn't seen.</li> <li>Authentication was bypassed to make the web-tests actually work, so we missed a glaring bug in the authentication code.</li> </ol> <p>It is easy to come up with hundreds more examples of things like this.</p> <p>You need both unit tests to make sure that your code actually does what it is supposed to do at a low level, and then functional/integration (which you're calling web) tests on top of those, to prove that it actually works when all those small unit-tested-pieces are chained together.</p>

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