Resource recovery functions

Resource recovery based on the two-phase commit protocol has two functions:

Commit: During the commit process, all changes to both local and distributed resources are made permanently.
Backout: During the backout process, all pending changes to both local and distributed resources are not made.

The set of changes that are to be made or not made as a unit are called a unit of recovery (UR). A UR represents an application program's changes to resources since the last commit or backout or, for the first UR, since the beginning of the application. Each UR is associated with a context, which consists of the UR, or more than one UR, with the associated application programs, resource managers, and protected resources.

A context, which is sometimes called a work context, represents a work request. The life of a context consists typically of a series of URs. Figure 1 shows the relation of the context, application program, and URs.

Figure 1. Context as a Series of URs