What is the difference between failover clustering and high availability?

Is high availability part of how SQL Server Failover Clustering works? Or these two competing technologies?

From what I've read, it seems like HA is one of about three different ways you can "cluster" SQL Servers for failover. The other one would be "Database Mirroring". Dont know about the third one.

Please correct me If I'm wrong.


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asked Dec 18, 2009 at 04:45 PM in Default

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2 answers: sort voted first

High availability simply refers to the fact that the server system is in some way tolerant to failure.

That can be helped, in general, through hardware redundancy. For example, if a machine has redundant power supplies, then if one fails the machine will keep running. You can go a long way using hardware redundancy.

On top of that, you then have application redundancy, which usually refers to the ability for an application running on multiple hardware installations to respond to clients in a consistent manner from any of those hardware installations. That way, if the hardware does totally fail, or the O/S dies on a particular machine, another machine can carry on.

SQL Server caters for application redundancy in three four ways:

  • Clustering
  • Mirroring
  • Replication
  • Log Shipping (thanks Jack!)

Hope that clears it up a bit, but I'm sure some other people will have some good things to say too!

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answered Dec 18, 2009 at 04:52 PM

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Matt Whitfield ♦♦
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I'd add log shipping to the the HA solution although with replication and log shipping the failover is manually while with Clustering and Mirroring (depending on the way it is setup) is supposed to be automatic.

Dec 18, 2009 at 06:43 PM Jack Corbett

Fair point - I've always really thought of log shipping as 'budget replication' but I guess it's worth mentioning in it's own right - will edit the post ;)

Dec 19, 2009 at 09:29 AM Matt Whitfield ♦♦
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Matt covers it well. I would like to break out the replication solution into three different solutions.

  1. SQL Server replication - SQL Server manages changes and replicates them to a second server
  2. Disk level block replication - a third party tool monitors disk and replicates (DoubleTake)
  3. Disk appliance replication - SAN replication

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answered Dec 22, 2009 at 08:06 AM

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Mark Allison
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asked: Dec 18, 2009 at 04:45 PM

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