This section introduces several SQL Server high-availability solutions that improve the availability of servers or databases. A high-availability solution masks the effects of a hardware or software failure and maintains the availability of applications so that the perceived downtime for users is minimized.

SQL Server provides several options for creating high availability for a server or database. High-availability options include the following:

• Failover clustering
  Failover clustering provides high-availability support for an entire instance of SQL Server. A failover cluster is a combination of one or more nodes, or servers, with two or more shared disks. Applications are each installed into a Microsoft Cluster Service (MSCS) cluster group, known as a resource group. At any time, each resource group is owned by only one node in the cluster. The application service has a virtual name that is independent of the node names, and is referred to as the failover cluster instance name. An application can connect to the failover cluster instance by referencing the failover cluster instance name. The application does not have to know which node hosts the failover cluster instance.
  A SQL Server failover cluster instance appears on the network as a single computer, but has functionality that provides failover from one node to another if the current node becomes unavailable. For example, during a non-disk hardware failure, operating system failure, or planned operating system upgrade, you can configure an instance of SQL Server on one node of a failover cluster to fail over to any other node in the disk group.
  A failover cluster does not protect against disk failure. You can use failover clustering to reduce system downtime and provide higher application availability. Failover clustering is supported in SQL Server Enterprise and SQL Server Developer, and, with some restrictions, in SQL Server Standard. For more information about failover clustering, see Getting Started with SQL Server 2008 Failover Clustering and Installing a SQL Server 2008 Failover Cluster.
  
• Database mirroring
  Database mirroring is primarily a software solution to increase database availability by supporting almost instantaneous failover. Database mirroring can be used to maintain a single standby database, or mirror database, for a corresponding production database that is referred to as the principal database.
  The mirror database is created by restoring a database backup of the principal database with no recovery. This makes the mirror database is inaccessible to clients. However, you can use it indirectly for reporting by creating a database snapshot on the mirror database. The database snapshot provides clients with read-only access to the data in the database as it existed when the snapshot was created.
  Each database mirroring configuration involves a principal server that contains the principal database, and a mirror server that contains the mirror database. The mirror server continuously brings the mirror database up to date with the principal database.
  Database mirroring runs with either synchronous operation in high-safety mode, or asynchronous operation in high-performance mode. In high-performance mode, the transactions commit without waiting for the mirror server to write the log to disk, which maximizes performance. In high-safety mode, a committed transaction is committed on both partners, but at the risk of increased transaction latency.
  In its simplest configuration, database mirroring involves only the principal and mirror servers. In this configuration, if the principal server is lost, the mirror server can be used as a warm standby server, with possible data loss. High-safety mode supports an alternative configuration, high-safety mode with automatic failover. This configuration involves a third server instance, known as a witness, which enables the mirror server to act as a hot standby server. Failover from the principal database to the mirror database typically takes several seconds.
  Database mirroring partners and witnesses are supported by SQL Server 2005 Standard SP1 and later versions, and by SQL Server 2005 Enterprise Edition SP1 and later versions. But the partners must use the same edition, and asynchronous database mirroring (high-performance mode) is supported only by SQL Server 2005 Enterprise Edition SP1 and later versions. Witnesses are also supported by SQL Server 2005 Workgroup SP1 and later versions and by SQL Server 2005 Express Edition SP1 and later versions. For more information about database mirroring, see Database Mirroring.
  
• Log shipping
  Like database mirroring, log shipping operates at the database level. You can use log shipping to maintain one or more warm standby databases for a corresponding production database that is referred to as the primary database. Standby databases are also referred to as secondary databases. Each secondary database is created by restoring a database backup of the primary database with no recovery, or with standby. Restoring with standby lets you use the resulting secondary database for limited reporting.
  A log shipping configuration includes a single primary server that contains the primary database, one or more secondary servers that each have a secondary database, and a monitor server. Each secondary server updates its secondary database at set intervals from log backups of the primary database. Log shipping involves a user-modifiable delay between when the primary server creates a log backup of the primary database and when the secondary server restores the log backup. Before a failover can occur, a secondary database must be brought fully up-to-date by manually applying any unrestored log backups.
  Log shipping provides the flexibility of supporting multiple standby databases. If you require multiple standby databases, you can use log shipping alone or as a supplement to database mirroring. When these solutions are used together, the current principal database of the database mirroring configuration is also the current primary database of the log shipping configuration.
  Log shipping is supported in the SQL Server Enterprise, Standard, and Workgroup editions. For more information about log shipping, see Log Shipping Overview and Log Shipping Administration.
  
• Replication
  Replication uses a publish-subscribe model. This lets a primary server, referred to as the Publisher, distribute data to one or more secondary servers, or Subscribers. Replication enables real-time availability and scalability across these servers. It supports filtering to provide a subset of data at Subscribers, and also allows for partitioned updates. Subscribers are online and available for reporting or other functions, without query recovery. SQL Server offers three types of replication: snapshot, transactional, and merge. Transactional replication provides the lowest latency and is usually used for high availability. For more information, see Improving Scalability and Availability.
  Replication is supported in all editions of SQL Server. Replication publishing is not available with SQL Server Express or SQL Server Compact 3.5.
  

  Important:
  A well designed and implemented backup and restore strategy is important to any high-availability solution. For more information, see Backing Up and Restoring Databases in SQL Server, and Backing Up and Restoring Replicated Databases.

• Scalable shared databases
  The scalable shared database feature lets you scale out a read-only database built exclusively for reporting. The reporting database must reside on a set of dedicated, read-only volumes whose primary purpose is hosting the database. By using commodity hardware for servers and volumes, you can scale out a reporting database that provides the same view of the reporting data on multiple reporting servers. This feature also allows a smooth update path for the reporting database. For more information, see Scalable Shared Databases Overview.
  

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