Data analysis with observers

Since Asap simulations tend to be very large, it is often advantageous to analyze the simulation while it is running. This is particularly the case for parallel simulations, where memory constraints may make it difficult to load the simulation later. In such cases, observers can be very useful.

An observer is an object which is informed about changes to another object, and then “observes” it. In its usual form, two objects are observed, the “subject” and the “observer”. The subject informs the observer when it has been modified, the observer then analyzes data it reads form the subject.

In ASE/Asap, this is altered slightly, and three objects are involved. The Atoms object (the subject), a Dynamics object, and an observer. The Dynamics object modifies the Atoms, and notifies the observer that the atoms have been modified. The observer then analyzes data it gets form the atoms. This is done because a Dynamics sometimes modifies the Atoms object more than once during a “time step”, and the Atoms cannot know this. The resulting pattern is illustrated in the figure below.

The observer pattern as used in ASE/Asap. When the dynamics informs the observer that it has modified the atoms, the observer can do whatever analysis of the atoms it was written to do.