"Modulok" <modu...@gmail.com> wrote
Unit testing functions and methods which rely on passed data is simple
enough. However:
How do I unit test something which relies on external data?
You have to build a test environment.
This needs to be carefully planned to enable every test condition
to be tested.
For example, a function which creates a file only if it doesn't exist
on disk, or a unit test for a function which makes an SQL query?
So you would need a file that exists, one that doesn't(!),
one that exists but is read-only, one that exists but wrong user
ownership, and really a binary file and a text file and an
empty file and a huge file too
For the database it gets tricky, you need to create a database
full of entries with all the possible scenarios you might encounter.
For a large application with multiple joined tables designing such
a test database can take a lot of time - but it is invaluable for testing
and debugging and provided you store a clean copy somewhere
before using it makes regression testing possible.
Once you have the database you then need to write the test driver
code that will provide the right keys for each test - database testing
is one of the biggest challenges in system testing.
And don't forget to test for behaviour with invalid user access on
the database, and for locking and simultaneous updates etc.
And if its on a separate server that you check behaviour when
the server or network is down or running slowly (eg by lowering
server priorioty and running a huge bulk update in the background).
Remember, in testing you are not trying to prove it works but rather
to demonstrate that it doesn't!
--
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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