It supports promoting slaves to masters, but, like MySQL,
does not provide any mechanism to detect that nodes have failed.
An upgrade to Slony, Slony-II, is in the very early stages of development now. It
plans to provide multimaster synchronous replication for PostgreSQL based on the
Spread group-communication framework.
Multimaster replication: PGCluster
PGCluster (http://pgcluster.projects.postgresql.org/) is a product that offers multimaster
replication and clustering for PostgreSQL. It provides both load balancing and high
availability for a database cluster. The software handles failover, and yields a readily
available solution if three or more physical servers are used.
124 | Chapter 4: Database
PGCluster??™s replication style is synchronous; updates are propagated to all servers
before the update transaction succeeds. Thus, it should only be used in environments
where all master servers are at the same location and are always connected.
Asynchronous replication, in which changes are propagated to other servers some
time after the transaction commits, is generally considered a hard problem. Asynchronous
replication is also application-specific, as the proper way to handle conflicts
between two committed transactions depends on the application??™s needs.
Oracle
Oracle??™s clustering product is Oracle Real Application Clusters (RAC). In contrast to
the shared-nothing clustering solutions available for other DBMSs, RAC is a sharedeverything
clustering product.
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