Directed Acyclic Graphs

What makes version control systems different from backup applications is, among others, the ability to represent more than linear history. This is necessary, both to support the simultaneous parallel development by different developers (each developer in his or her own clone of repository), and to allow independent parallel lines of development—branches. For example, one might want to keep the ongoing development and work on bug fixes for the stable version isolated; this is possible by using individual branches for the separate lines of development. Version control system (VCS) thus needs to be able to model such a (non-linear) way of development and to have some structure to represent multiple revisions.

Fig 1. A generic example of the Directed Acyclic Graph (DAG). The same graph is represented on both sides: in free-form on the left, left-to-right order on the right.

The structure that Git uses (on the abstract level) to represent the possible non-linear history...