Is HA a subset of clustering?
In my opinion, the answer to your question is no.
High Availability (HA) is a design goal; are your systems sufficiently available for them to achieve the goals for which you purchased them? If so, then they are highly available, otherwise they are not. And every application in every enterprise has its own availability requirements. Ebay.com's e-commerce systems are more critical to its operation (and its bottom line) than its development servers, for instance.
Technologies like clustering, mirroring, replication and tape backups are just that, technologies. They are the tools that will enable you to increase your systems' availability to appropriate levels but they themselves are not HA.
I hope this helps. Feel free to follow up if you have additional questions.
Evan L. Marcus
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This was first published in July 2002