Initializing an HADR system in multiple standby mode is similar to single standby mode. The main difference is that you must enable multiple standby mode by setting the hadr_target_list database configuration parameter on all the databases in your setup.
This task covers how to initialize HADR in multiple standby mode. If you want to convert a single standby setup to a multiple standby setup, see Enabling multiple standby mode on a preexisting HADR setup.
Multiple standby mode requires the hadr_target_list configuration parameter to be set on all participating databases. This parameter lists the standbys in the scenario when the database becomes a primary. It is required even on a standby. Mutual inclusion is required (that is, if A has B in its target list, B must have A in its target list). This ensures that after a takeover from any standby, the new primary can always keep the old primary as its standby. The first standby that you specify in the target list is designated as the principal HADR standby database. Additional standbys are auxiliary HADR standby databases. The target list need not always include all participants. As well, there is no requirement for symmetry or reciprocity if there is more than one standby; even if you designate that database A has database B as its principal standby, database B does not have to designate A as its principal standby. Each standby specified in the target list of database A, must also have database A in its target list. Working out the target list for each database is an important step.
As a special case, multiple standby mode can be configured with only one standby. For example, you can configure two databases as primary and standby in multiple standby mode. The behavior is not same as single standby setup because multiple standby behavior such as automated configuration will be in effect and because standby targets can be added or removed dynamically.
To initialize HADR in multiple standby mode:
The standby databases start in local catchup state, in which locally available log files are read and replayed. After all local logs have been replayed, the databases enter remote catchup pending state. After the primary starts, the standbys enter remote catchup state, in which log pages are received from the primary and replayed. After all of the log files that are on the disk of the primary database have been replayed on the standbys, what happens depends on the type of what happens next depends on the type of synchronization mode. A principal standby in SUPERASYNC and any auxiliary standby will stay in remote catchup mode. A principal standby with a SYNC, NEARSYNC, or ASYNC mode will enter peer mode.