I've implemented an Active/Passive failover cluster for SQL Server 2000. How is Active/Active mode different from Active/Passive mode? And is it possible to perform Active/Active clustering for this version of the database system?

Yes, you can do Active/Active clustering with SQL Server 2000. In SQL Server failover clustering, the OS and SQL Server work together to provide availability in case of an application failure, hardware failure, or OS error. Failover clustering provides hardware redundancy through a configuration in which vital, shared resources are automatically transferred from a failing computer to an equally configured server. Failover clustering is available only with the Enterprise Edition of SQL Server 2000. (For more information about SQL Server failover clustering, see SQL Server Books Online (BOL) and the following October 2003 articles: Michael Hotek's "High Availability Solutions," and Brian Knight's "Clustering SQL Server.")

In SQL Server, Active/Active clustering simply means having two separate instances running in the cluster—one (or more) per machine. So, Active/Active clustering refers to a two-node cluster, with each node running SQL Server instances but each with a different set of databases. This setup is conceptually the same as having two Active/Passive nodes, except that one node must be a named instance.

Note that only one server at a time can open a SQL Server database. If you want to implement a system in which you can update data in two places at once, you need to read about SQL Server's various data-replication options in BOL or by searching on the keyword replication on the SQL Server Magazine home page (http://www.sqlmag.com).