June 03, 2009


Microsoft SQL Server 2005 offers four solutions to mitigate the risk of hardware failure. In order of data-availability and minimized data-loss, your choices are log shipping, replication, and mirroring or clustering.

Log shipping :
In log shipping, log backups from a primary database server are ‘shipped’ to a secondary database server at an alterable, pre-determined interval. As the logs arrive at the secondary server, they are ‘applied’ to the database. Before a failover can occur, all unrestored logs must first be manually applied to the secondary server.
The advantage of log shipping is that the same logs can be applied to multiple standby servers for multiple purposes, and is an integral component of any disaster recovery plan. The disadvantage is that failover cannot be automated, that downtime can be lengthy, and that there is a distinct possibility of data-loss depending on the shipping interval.
Log shipping is supported in all versions of SQL Server 2005, excepting ‘Express’ and ‘Compact’ editions.
Replication :
Replication is an automated distribution of data to one or more receiving nodes. Microsoft refers to this relationship as a publish-subscribe model—a Publisher (the primary database) pushes its information to a secondary database server called a Subscriber.
Subscriber nodes are available for reporting, data warehousing, and backup purposes, and should be considered for any application where the strain of these tasks should not be performed by the primary online node.
For online applications that are very lookup-intensive (e.g., searches), connections can be sent to a farm of Subscriber nodes. In that same scenario, database writes (e.g., ecommerce purchases) can be sent to the Publisher for processing. This application architecture ensures proper scalability in the database layer, and is a very effective load-balancing mechanism.
SQL Server 2005 supports three types of replication: snapshot, transactional, and merge. Snapshot replication is a good choice when data changes infrequently or the dataset is small. Merge replication is appropriate when multiple environments work independently and on separate sets of data, which later need to be re-integrated (merged). Transactional replication is the best choice when data changes frequently and needs to be pushed to subscribers in near-real-time.
Replication is supported by all editions of SQL Server 2005, but only ‘publishing’ is not available in the ‘Express’ or ‘Compact’ editions.

Mirroring :
Database mirroring is new in SQL Server 2005. Mirroring is a form of database clustering to keep an exact real-time copy of the database on a second server. This setup requires three servers: a Principal, a Mirror, and a Witness. The Witness brokers the relationship between the Principal and the Mirror, and initiates a seamless automated failover when necessary.
The advantage of mirroring is that it is a cost-effective solution when only one or very few databases need to be maintained, and the entire dataset fits onto the servers’ disk drives. The drawback is that the Mirror cannot be accessed for reporting or backups (only snapshots). Additionally, Mirroring is not a server but a database clustering technology. This means that the Mirror server must have all the proper logins, SQL Agent jobs, SQL Server Integration Services, and other supporting components and configurations. Lastly, Mirroring is not appropriate for inter-dependent databases, as only one database might failover, breaking the dependencies.

Failover Clustering:
Failover clustering is as much a function of the underlying operating system as it is SQL Server. In failover clustering for SQL Server 2005, the advantages and caveats flip-flop when compared to database mirroring. Failover clustering has higher hardware and resource requirements—such as shared external storage (a SAN)—but can accommodate a much higher volume of data and databases.

Windows Server 2003, with Microsoft Cluster Service (MSCS) supports Active/Passive and Active/Active cluster topologies. The “Standard” edition of SQL Server 2005 supports only two nodes in the cluster, whereas “Enterprise” supports up to eight active nodes. An Active/Passive cluster only supports one instance of a database. Should the Active node fail, the Passive node will take over the workload. Up until then, the Passive node has been sitting around idly with nothing to do, except for receiving transaction updates from the Active server. However, in Active/Passive clustering the financial benefit is that the Passive node does not require a separate database license, yet can be used for backup and reporting purposes of the idle database.
Active/Active clustering supports multiple database instances, meaning that individual nodes in the cluster can be online and actively collecting and manipulating data, and also failover to another node in the cluster. In an Active/Active cluster all nodes need to be licensed.

· Replication would be primarily suited for online-offline data synchronization scenarios (laptop , mobile devices, remote servers).
· Log shipping could be used to have a failover server with manual switching.
· Database Mirroring is an automatic failover technique
· Failover Clustering is an advanced type of database mirroring.

Failover clustering is an availability technology that provides redundancy at the hardware level and is built on top of Windows Clustering technology, i.e. it is not specific to SQL Server.
For example, the processor blows up on Server A. Fortunately Server A is part of a SQL Server Cluster and so Server B takes over the job of providing the SQL Server Service, within a matter of seconds. All of this occurs automatically and is transparent to the database users and or application being served by the cluster.
The main difference between Database Mirroring and clustering is that SQL Clustering provides redundancy at the instance level whereas database mirroring provides redundancy at the database level.
Log shipping is considered more of a redundancy technology.
For example, it can be used to provide a full copy of your primary environment, typically used as a warm standby that can be manually brought online. This can be used to provide additional redundancy to your backup strategy. Log shipping can also be used to offload reporting from a primary server by creating a read only copy of the production database at an alternative location/server.
Replication is quite a diverse technology and can be used to cater for a number of different scenarios, the choice of which will determine the specific type of replication that is implemented.
For example, merge replication can be used to support distributed processing by spreading the workload of an application across several servers, i.e. distributed processing architectures.
Merge replication often requires an application that is relatively aware of its environment. Techniques such as conflict resolution also have to be taken into consideration in order to ensure data consistency across the entire integrated environment.
Transactional Replication can be used in a similar fashion to log shipping however you can limit the specific objects that are replicated to the subscriber. This can be useful if only a subset of tables is required for reporting purposes.

A failover cluster is a combination of one or more physical disks in a Microsoft Cluster Service (MSCS) cluster group, known as a resource group, that are participating nodes of the cluster. The resource group is configured as a failover clustered instance that hosts an instance of SQL Server. A SQL Server failover clustered instance appears on the network as if it were a single computer, but has functionality that provides failover from one node to another if one node becomes unavailable.
Failover clusters provide high-availability support for an entire Microsoft SQL Server instance, in contrast to database mirroring, which provides high-availability support for a single database. Database mirroring works between failover clusters and, also, between a failover cluster and a nonclustered host.

Typically, when mirroring is used with clustering, the principal server and mirror server both reside on clusters, with the principal server running on the failover clustered instance of one cluster and the mirror server running on the failover clustered instance of a different cluster. You can establish a mirroring session in which one partner resides on the failover clustered instance of a cluster and the other partner resides on a separate, unclustered computer, however.
If a cluster failover makes a principal server temporarily unavailable, client connections are disconnected from the database. After the cluster failover completes, clients can reconnect to the principal server on the same cluster, or on a different cluster or an unclustered computer, depending on the operating mode.
When deciding how to configure database mirroring in a clustered environment, the operating mode you use for mirroring is significant.
High-Safety mode Session with Automatic Failover
If you intend to mirror a database in high-safety mode with automatic failover, a two-cluster configuration is recommended for the partners. This configuration provides maximum availability. The witness can reside either on a third cluster or on an unclustered computer.
If the node running the current principal server fails, automatic failover of the database begins within a few seconds, while the cluster is still failing over to another node. The database mirroring session fails over to the mirror server on the other cluster or unclustered computer, and the former mirror server becomes the principal server. The new principal server rolls forward its copy of the database as quickly as possible and brings it online as the principal database. After the cluster failover completes, which typically takes several minutes, the failover clustered instance that was formerly the principal server becomes the mirror server
The following illustration shows an automatic failover between clusters in a mirroring session running in high-safety mode with a witness (which supports automatic failover).
The three server instances in the mirroring session reside on three distinct clusters: Cluster_A, Cluster_B, and Cluster_C. On each cluster, a default instance of SQL Server is running as a SQL Server failover clustered instance. When the mirroring session starts, the failover clustered instance on Cluster_A is the principal server, the failover clustered instance on Cluster_B is the mirror server, and the failover clustered instance on Cluster_C is the witness in the mirroring session. Eventually, the active node on Cluster_A fails, which causes the principal server to become unavailable.
Before the cluster has time to fail over, the loss of the principal server is detected by the mirror server, with the help of the witness. The mirror server rolls forward its database and brings it online as the new principal database as quickly as possible. When Cluster_A finishes failing over, the former principal server is now the mirror server, and it synchronizes its database with the current principal database on Cluster_B.
High-Safety Mode Session Without Automatic Failover
If you are mirroring a database in high-safety mode without automatic failover, another node in the cluster will act as the principal server if the node running the current principal server fails. Note that while the cluster is unavailable, the database is unavailable.
High-Performance Mode Session
If you intend to mirror a database in high-performance mode, consider placing the principal server on the failover clustered instance of a cluster and placing the mirror server on an unclustered server in a remote location. If the cluster fails over to a different node, the failover clustered instance will continue as the principal server in the mirroring session. If the entire cluster has problems, you can force service onto the mirror server


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Sripal Vindyala said...

Thank you very much. Nice explanation.