Data mirroring, the ability to generate an up-to-the-second mirror image of an operational database, will become part of Microsoft SQL Server 2005 as Microsoft released Service Pack 1 for its database Wednesday.

Data mirroring is an option with the Oracle database and IBM's DB2 database. SQL Server's Service Pack 1, as previously announced, pushes the Microsoft system onto the same high-availability playing field as its competitors.

SQL Server 2005 already includes such continuous-operation features as the ability to take database snapshots, which allows the reconstruction of the database at a previous point in time. Some database users, for example, record a snapshot every hour. Its predecessor, SQL Server 2000, had the ability to "fail over" from one server to another in a cluster, avoiding downtime due to a hardware failure.

But the data mirroring feature comes closer to the "continuous availability" goal set by Microsoft Senior VP Paul Flessner earlier this month. Data mirroring "is a milestone in support of that vision," says Carol Dullmeyer, Microsoft senior product manager.

With mirroring, two servers are linked together, each creating the same log of events from a stream of transactions. Mirroring software also listens for the "heartbeat" of the primary server. If it detects an outage, within a few seconds the backup copy of the data will become the primary database server, picking up where the other left off, Dullmeyer explains.

As a downed server or failed network segment comes back on line, the mirroring system reasserts itself, with what was formerly the primary server working as the mirror image server.

The feature moves SQL Server beyond "warm failover" capabilities, where an outage might result in a few minutes of downtime, to the status of a "hot failover" system, where an outage remains nearly imperceptible to users. Backup and recovery techniques, where the state of the database is rebuilt from a database server log, is more time-consuming and considered a cold start, or restart.

Microsoft is also pushing the Management Studio features of SQL Server in its Enterprise Standard and Workgroup editions down into its free Express version in the Service Pack 1 release, says Dullmeyer.

Microsoft's closer ties with SAP were also evident this week as the competition heats up between SAP and Oracle. The Reporting Services in SQL Server can work better with SAP's NetWeaver Business Intelligence, the data storehouse for SAP applications. Microsoft will also supply SAP NetWeaver-ready versions of its Data Provider, a business intelligence tool, and Query Designer, used for querying the SAP Business Warehouse.

"It's important to have reporting facilities for the SAP Business Warehouse," Dullmeyer notes. Microsoft released SQL Server 2005 last November. Service Pack 1 represents its first upgrade of the release.