Unit of work

A unit of work is a recoverable sequence of operations within an application process. A unit of work is sometimes called a logical unit of work.

At any time, an application process has a single unit of work, but the life of an application process can involve many units of work as a result of commit or full rollback operations.

A unit of work is initiated when an application process is initiated. A unit of work is also initiated when the previous unit of work is ended by something other than the end of the application process. A unit of work is ended by a commit operation, a full rollback operation, or the end of an application process. A commit or rollback operation affects only the database changes made within the unit of work it ends. While these changes remain uncommitted, other application processes are unable to perceive them unless they are running with an isolation level of uncommitted read. The changes can still be backed out. Once committed, these database changes are accessible by other application processes and can no longer be backed out by a rollback. Locks acquired by Db2 on behalf of an application process that protects uncommitted data are held at least until the end of a unit of work.

The initiation and termination of a unit of work define points of consistency within an application process. A point of consistency is a claim by the application that the data is consistent. For example, a banking transaction might involve the transfer of funds from one account to another. Such a transaction would require that these funds be subtracted from the first account, and added to the second. Following the subtraction step, the data is inconsistent. Only after the funds have been added to the second account is consistency reestablished. When both steps are complete, the commit operation can be used to end the unit of work, thereby making the changes available to other application processes. The following figure illustrates this concept.