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<chapter>
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    <copyright>
      <year>2003</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      The contents of this file are subject to the Erlang Public License,
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    <title>Some thoughts about testing</title>
    <prepared>Siri Hansen</prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>why_test_chapter.xml</file>
  </header>
         

  <section>
    <title>Goals</title>

    <p>It's not possible to prove that a program is correct by
    testing. On the contrary, it has been formally proven that it is
    impossible to prove programs in general by testing. Theoretical
    program proofs or plain examination of code may be viable options
    for those that wish to certify that a program is correct. The test
    server, as it is based on testing, cannot be used for
    certification. Its intended use is instead to (cost effectively)
    <em>find bugs</em>. A successful test suite is one that reveals a
    bug. If a test suite results in Ok, then we know very little that
    we didn't know before.</p>

  </section>

  <section>
    <title>What to test?</title>

    <p>
      There are many kinds of test suites. Some concentrate on
      calling every function or command (in the documented way) in 
      a certain interface.
      Some other do the same, but uses all kinds of illegal
      parameters, and verifies that the server stays alive and rejects
      the requests with reasonable error codes. Some test suites
      simulate an application (typically consisting of a few modules of
      an application), some try to do tricky requests in general, some
      test suites even test internal functions with help of special
      load-modules on target.</p>

    <p>Another interesting category of test suites are the ones that
      check that fixed bugs don't reoccur. When a bugfix is introduced,
      a test case that checks for that specific bug should be written
      and submitted to the affected test suite(s).</p>

    <p>Aim for finding bugs. Write whatever test that has the highest
      probability of finding a bug, now or in the future. Concentrate
      more on the critical parts. Bugs in critical subsystems are a lot
      more expensive than others.</p>

    <p>Aim for functionality testing rather than implementation
      details. Implementation details change quite often, and the test
      suites should be long lived. Often implementation details differ
      on different platforms and versions. If implementation details
      have to be tested, try to factor them out into separate test
      cases. Later on these test cases may be rewritten, or just
      skipped.</p>

    <p>Also, aim for testing everything once, no less, no more. It's
      not effective having every test case fail just because one
      function in the interface changed.</p>

  </section>

</chapter>

