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<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2003</year><year>2009</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<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>
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