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Category :
Database
Resources -> Backup and Recovery
DB Version
:
Oracle 9i
OS Details :
Sun Solaris9
What
is an SCN? At a high level, the System Change Number or SCN is a
unique number that the database assigns a transaction when a
transaction is committed. Ever wonder how these SCNs are maintained by
oracle for distributed transactions?
SCNs play an important role
during a distributed transaction. When a distributed transaction is
committed, the highest SCN of all the database instances involved is
assigned to the transaction. This will ensure that that there will
never be a problem with read consistency. For example, if database DB1
has an SCN value of 800 (i.e the next transaction that performs a
commit within the DB1 database will be assigned an SCN value of 800),
and database DB2 has an SCN value of 8000, and if there is a
distributed transaction initiated from database DB1 and the
transaction is completed with a commit on DB1, then the transaction
will be assigned an SCN value of 8000 instead of 800.
The next time you notice
significant increases in the SCN values within a database, chances are
that the database might be part of a distributed transaction.
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