High availability of an application is a function of all components such as hardware, backup/recovery, operating system, vendor agreements, sufficient training, extensive quality assurance/testing, rigorous stands and procedures and some overall risk-mitigating strategies, such as spreading…
Backup, Training, Quality Assurance , Standard / Procedures, Server Instance isolation
1. Hardware
- Redundant power supplies ( and UPSs)
- Redundant fan systems
- Fault-tolerant disk, such as RAID ( 1 through 10 ), preferably “hot swappable”
- ECC memory
- Redundant Ethernet connections
2. Backup
3. Operating System
- Upgrades to your OS
- Antivirus software
- firewalls for external-facing systems
Vendor Agreements
- software licenses
- software support agreements
- hardware service agreements
Different Levels of HA:
- Microsoft Cluster Services ( non-SQL Server based )
- SQL clustering
- Data replication ( including peer-to-peer configurations )
- Log shipping
- Database Mirroring
Microsoft Cluster Services ( MSCS )
- MSCS is the advance windows operating system configuration that defines and manages between two and eight servers as “nodes” in a cluster.
- These nodes are aware of each other and can be setup to take over cluster-aware applications from any node that fails ( failed server ).
- This sluster configuration also share and controls one or more disk subsystems as part of its high-availability capability.
- MSCS is available only with MS Windows Enterprises Edition Advanced Server and Data Center OS.
- MSCS can be set up in an active/passive or active/active mode.If active or passive server fails other can take over share disk and cluster-aware applications in instaneously.
SQL Clustering ( Chapter 17 )
- Creating a SQL Server instance that is clustered is done by actually creting a virtual SQL server instance that is known to the application.
- two physical SQL server instances that share one set of databases.
- In an active/passive configuration, only one SQL server instance is active at a time
- If active server fails, the passive server simply takes over instantaneously.
- This is possible because MSCS also controls shared disk where the databases are.