# Graphs

The FIB is essentially a collection of related graphs. Terminology from graph theory is often used in the sections that follow. From Wikipedia:

... a graph is a representation of a set of objects where some pairs of objects are connected by links. The interconnected objects are represented by mathematical abstractions called vertices (also called nodes or points), and the links that connect some pairs of vertices are called edges (also called arcs or lines) ... edges may be directed or undirected.

In a directed graph the edges can only be traversed in one direction - from child to parent. The names are chosen to represent the many to one relationship. A child has one parent, but a parent many children. In undirected graphs the edge traversal can be in either direction, but in FIB the parent child nomenclature remains to represent the many to one relationship. Children of the same parent are termed siblings. When the traversal is from child to parent it is considered to be a forward traversal, or walk, and from parent to the many children a back walk. Forward walks are cheap since they start from the many and move toward the few. Back walks are expensive as the start from the few and visit the many.

The many to one relationship between child and parent means that the lifetime of a parent object must extend to the lifetime of its children. If the control plane removes a parent object before its children, then the parent must remain, in an incomplete state, until the children are themselves removed. Likewise if a child is created before its parent, the parent is created in an incomplete state. These incomplete objects are needed to maintain the graph dependencies. Without them when the parent is added finding the affected children would require a search through many databases for those children. To extend the lifetime of parents all children thereof hold a lock on the parent. This is a simple reference count. Children then follow the add-or-lock/unlock semantics for finding a parent, as opposed to a malloc/free.