[anon946]

Atomicity can be important at any level. For example, assume that adding a friend involves two edges in a graph, one edge in each direction. Now assume that there is some other piece of code that does some analysis or processing of friend relationships. This piece of code might rely on there always being two edges, and crash or give erroneous results if not.

Thus, the adding of the two edges must be atomic (when observed from the rest of the system).

(The example is a bit contrived, but hopefully gets the idea across.)