Oracle Distributed Systems

Architecture Components

Oracle has built the replication facility on a variety of triggers, packages, background processes, jobs, and tables, all working in concert to deliver data to multiple sites as if by magic. If you are the DBA for a replicated environment, you must understand the secrets behind this magic. Read on.

The Queues

Queues are the foundation of the replication architecture. DML and DDL changes are entered into these queues, from which they are propagated to remote sites. Table 9.2 summarizes the queues.

Table 9-2. Replication Queues

Queue Name	Relevant Data Dictionary Views	Description
Deferred transaction (a.k.a. deftrans)	Primary: DEFTRAN Other: DEFTRANDEST DEFERRCOUNT DEFERROR	Local transactions that are to be replicated to remote sites are enqueued in to deftrans. A trigger on the replicated table table_name $RT inserts these entries. Note that in Oracle8 this trigger is internalized and is therefore not visible in the data dictionary.
Replication call (a.k.a. defcall)	Primary: DEFCALL Other: DEFCALLDEST	Remote procedure calls are enqueued into the DEFCALL view. In the case of a replicated table, there is one entry for each row that is changed.
Replication catalog (a.k.a. repcatlog)	DBA_REPCAT	DDL modifications to replicated objects as well as administrative tasks such as changing to the propagation mode are tracked in DBA_REPCAT. This view also contains information about errors that may have occurred when performing these tasks.
Job	Primary: DBA_JOBS Other: DEFSCHEDULE DBA_JOBS_RUNNING ...