[From IT Analysis
For what seems a very long time Oracle has dominated the market for clustered databases with its Real Application Clusters (RAC), but things seem to be changing and, indeed, the market for clustering seems to be polarising.
There are three main reasons for wanting to implement clustering in association with databases. One of these is for performance, because you can spread process loads across multiple nodes; the second is scalability, since you can expect to get reasonable performance across a bigger system; and the third is for high availability or, more particularly, continuous availability (the difference being that the former protects you against unplanned outages whereas the latter means that you can also handle planned outages).
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