In this new post on thePHP.cc blog today Sebastian Bergmann relates the unfortunate disintegration of the Mars Climate Orbiter (back in 1999) back to a lesson on software testing and errors.

One of the most important tasks in software testing is to find the smallest scope in which a test case can be implemented. The smaller the scope in which a test is run, the faster it can be executed and the more precise its result. Unit Tests exercise one unit of code in isolation from all collaborators. Integration Tests verify the interaction of two or more collaborators in isolation from the rest of the system. Edge-to-Edge Tests run the software as end-to-end as possible in a single process (and without using a web browser or a web server). End-to-End Tests, or System Tests, look at the whole system and in the case of a web application send a HTTP request from a web browser to a web server running the software to inspect the HTTP response that is sent back.

He talks some about the difference between unit tests and acceptance tests and how "easy and seductive" functional tests can be over unit testing. He points out how fragile (and sometimes slow) this can be though, and how their failure only shows a problem and not where it is.

The promise of being able to develop both the business model as well as the software that implements it in an agile fashion should be reason enough for enterprises to invest in a modern, highly decoupled software architecture. And when the members of the software development team communicate well, both among themselves and with the other stakeholders, then there is not much that can really impede the success of the project.