High availability can be provided in various ways – SQL Server 2005 supports mirroring, fail-over clustering and backup log shipping.

Backup log shipping is the simplest and cheapest high availability system and appears in the workgroup edition of SQL Server 2005 and above. Essentially, it means the shipping of the transaction logs from the database at very regular intervals to another machine, where they are either stored or immediately applied to a copy of the database. Should the original machine fail the alternative can be brought on line relatively rapidly.

Mirroring is more expensive to implement but it does provide a more resilient solution. A synchronised copy of the database is maintained on a separate machine. The mirror can share the load placed on the original database: for example, the original might handle the transactional load and queries and reports can be run against the mirror. Once you have a mirror in place, failover clustering can be implemented to ensure that, should one of the copies fail, the other takes over automatically.

Mirroring (with a two CPU per server limit) and failover clustering (two node limit) are available in the Standard edition of SQL Server 2005. Both are unrestricted in the Enterprise edition.

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