For simplicity, in the following sections we??™ll examine
replication using a simple two-site example??”a primary and a secondary. Oracle can,
however, perform N-way or multimaster replication involving more than two sites
with all sites replicating to all others.
Data Replication??”Synchronous and Asynchronous
Whenever youhave a data replication scenario, youalways have a primary site, from
which the replication originates, and a secondary site, which is the recipient of the
data replication. (In a multimaster scenario, you can have more than one master site,
and a single machine can be a master for one replication plan and a secondary site
for another.) When youdesign your replication plan, youmu st consider the degree
to which data at the secondary site can differ for a period of time from the data at the
primary site. This difference is referred to as data divergence. When youimplement
replication, Oracle generates triggers on all specified tables. These triggers are fired
as part of the primary site transactions. The triggers either update the secondary
site??™s data as part of the same transaction (synchronous replication) or place an entry
in a deferred transaction queue that will be used later to update the secondary site
(asynchronous replication).
Pages:
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626