Classical online transaction management (OLTP) is the process of making simultaneous changes in several places “atomically” - that is, all the changes related to a transaction are made or none of the changes are made.
For example, within a single database connection, the DBMS provides some means of demarcating the beginning and end of a transaction. This demarcation ensures that changes to the database are made atomically.  Sometimes, changes must be made atomically across multiple databases. For example, an insurance company might have to change both its claims information and its audit information at the same time, even though the audit information is in a separate database from the claims information. This multiple-database change would ensure that, during a later audit, the company would know which agent took the first report of the loss.  In this case, the existing XA (Transaction Authority) protocol is useful. XA provides a standard mechanism for coordinating changes to multiple databases (called resource managers or RMs) as an atomic unit of work. Basically, the XA protocol asks each RM to vote on whether a commit will be successful. Once an RM has voted “yes,” it must be able to commit the open unit of work without failure. The commit occurs only if all RMs vote “yes.” This process of obtaining a vote, and then performing a commit, is called a “two-phase commit.”