Abstract

Computers are complex machines with many degrees of freedom and users have no time to learn how to deal with all of them properly. So, the temptation is to hide some or many or even all of the freedom. Indeed, users are asking for "one-click", "one-line", "no-brain", "easy" solutions. The trouble starts when developers take early decisions based on wrong assumptions about the user's mental model and the mismatch between expectation and observed behavior starts to grow. In the end users cannot understand why the next small feature requires such a huge effort to implement and developers are forced to add to the technical debt. Progress slows down and everyone is frustrated. While that applies to all kind of programming, it only gets worse when it comes to super computers. "No Magic!" means to not hide the complexity but to transport it all the way up into the application and let users configure. We will give examples of options for distributed file locking, options for topologies of distributed runtime environments, and about different ways to display distributed progress. It turns out that users typically start with settings predefined by their administrators and if the observed behavior differs from their expectation they ask for help. Now developers can help them by pointing to some configuration options. Administrators can monitor application behavior and propose or apply modifications to the configuration. Over time the mental models of users and developers converge. The next feature is easier to implement and more likely to match expectations and to make good use of resources. "No Magic!" applies to all levels in the software stack: Framework developers are users of system software and programming languages, and we will give an example of the design of a shared data structure that is easy to use because it does not hide complexity.