Oracle7 Server Distributed Systems Volume I: Distributed Data

Coordination of System Change Numbers

Each committed transaction has an associated system change number (SCN) to uniquely identify the changes made by the SQL statements within that transaction. In a distributed system, the SCNs of communicating nodes are coordinated when:

A connection occurs using the path described by one or more database links.

A distributed SQL statement executes (the execute phase completes).

A distributed transaction commits.

Among other benefits, the coordination of SCNs among the nodes of a distributed system allows global distributed read-consistency at both the statement and transaction level. If necessary, global distributed time-based recovery can also be completed.

During the prepare phase, Oracle7 determines the highest SCN at all nodes involved in the transaction. The transaction then commits with the high SCN at the commit point site. The commit SCN is then sent to all prepared nodes with the commit decision.