Chapter 5. Bidirectional Q replication on AIX platforms 421
the hot site available for read-only access for reporting purposes to offload
processing from the primary server.
These requirements may be summarized as follows:
򐂰 Maintain a non-dedicated hot site backup. This means that the hot site
backup server has actively updated application tables unrelated to the
financial data application.
򐂰 Replicated tables on the second server must be available for switchover
within 30 minutes.
򐂰 Replicated tables on the second server must be available for read-only
access while the primary server is available.
򐂰 The primary server is to be resynchronized with the secondary after it is
restored to service.
򐂰 Limited resources allowed for replication-only purposes.
5.3 Rationale for the bidirectional solution
Table 2-1 on page 39 lists the evaluation criteria for choosing between
unidirectional, bidirectional, and peer-to-peer replication topologies.
Since Corona’s business requirement is support for a single backup site, the
choice is between a bidirectional and peer-to-peer replication topology.
The bidirectional replication topology is appropriate for Corona for the following
reasons:
򐂰 Less stringent requirements for switchover time (within 30 minutes instead of
an instantaneous requirement, which would tilt in favor of peer-to-peer).
򐂰 Non-dedicated backup server requirement.
򐂰 Absence of conflicts
1
due to the lack of simultaneous updates—only one
server is updated at a given time.
򐂰 Limited computing resources available.
򐂰 As discussed in 2.3, “Choosing a particular Q replication topology” on
page 39, when multiple topologies can address a business requirement, it
would probably be appropriate to choose the topology that is least expensive
and has the minimal impact on existing data. Bidirectional is the lower cost
1
During switchback, there is likely to be some data loss and conflicts due to the fact that all the
changes on the primary server at the time of failure fail to get replicated over to the secondary server.
These conditions are resolved partially by the conflict resolution mechanism during switchback, and
may require manual intervention.

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