Copy-on-write

Copy-on-write (sometimes referred to as "COW") is an optimization strategy used in computer programming. The fundamental idea is that if multiple callers ask for resources which are initially indistinguishable, they can all be given pointers to the same resource. This state of affairs can be maintained until a caller tries to modify its "copy" of the resource, at which point a separate (private) copy is made for that caller to prevent its changes from becoming visible to everyone else. All of this happens transparently to the callers. The primary advantage is that if no caller ever makes any modifications, no private copy need ever be created.