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Oracle Distributed Systems by Charles Dye

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Designing a Replicated System

There are a variety of issues that are unique to sites utilizing Oracle’s advanced replication facilities. Areplicated system succeeds only if the designers are aware of these idiosyncrasies and make the right implementation decisions. Again, the sooner in the process these design choices are made, the better the chances for success.

Transactional Consistency

Oracle’s multi-master replication preserves the order of transactions when it replicates them to participating sites, and it also guarantees the consistency of data at all sites. So, if you have a replicated order entry system that creates an entry in the ORDERS table in one transaction and entries in the ORDER_ITEMS table as a second transaction, Oracle will apply the transactions in the same order at all remote sites.

This is a crucial property of replication; without it, transactions could fail because of violated integrity constraints, and data would not be consistent. Since Oracle respects the order of transactions, row-level transactions do not require any special handling in a replicated environment.

Note

Oracle8 includes a feature called parallel propagation, which is a means to replicate multiple transactions simultaneously. Thus, it is possible, and even likely, that transactions will not be delivered to participating master sites in the same sequence in which they occurred at the originating sites. However, Oracle guarantees that parallel streams of transactions are orthogonal—that is, they ...

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