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HA option available in SQL Server 2008


1. Failover Clustering


You can use failover clustering to reduce system downtime and provide higher application availability.


2. Database Mirroring


Database mirroring is primarily a software solution to increase database availability by supporting almost instantaneous failover

The mirror database is created by restoring a database backup of the principal database with no recovery. However, you can use it indirectly for reporting by creating a database snapshot on the mirror database. The mirror server continuously brings the mirror database up to date with principal database.

Database mirroring can operate synchronously or asynchronously.

1-1 only

Failover:   Automatic

Failover Duration:  Failover is fast, sometimes < 3 seconds but not more than 10 seconds

Role Change:   Role change is fully automatic

Server Limitation:   Can be applied to only one mirror server

·         high-safety mode

In high-safety mode, a committed transaction is committed on both partners, but at the risk of increased transaction latency.
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.
·         asynchronous operation

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.
 

3. Log Shipping


Log shipping supports an unlimited number of secondary’s for each primary database.

Each secondary database is created by restoring a database backup of the primary database with no recovery, or with standby.

1-M

Before a failover can occur, a secondary database must be brought fully up-to-date by manually applying any unrestored log backups.

Server Limitation:   Can be applied to multiple stand-by servers

Failover:   Manual

Failover Duration:  Can take more than 30 mins

Role Change:   Role change is manual

4. 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
·         Merge

5. 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.
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