High Availability (HA) provides a failover solution in the event a server, or database failure. Disaster Recovery (DR) provides a recovery solution in the event of a disaster that causes an entire data center to fail. Developing a HA/DR strategy can be challenging. Microsoft offers several possible HA/DR options, but which you choose will depend on your company and business requirements. We at Scalability Engineers are well equipped with each and HA/DR option.

A partial site failure can be caused by hardware, network, or software failure. Without HA/DR, a partial site failure requires restarting the database management system (DBMS) server that contains the database. The length of time that it takes to restart the database and the server where it is located is unpredictable. It can take several minutes before the database is brought back to a consistent state and made available. With HADR, a standby database can take over in seconds. HA/DR might be your best option if most or all data in your database requires protection or if you perform DDL operations that must be automatically replicated on a standby database.

Our experts are well versed with the following terminologies that are crucial for HA/DR solutions :

• RTO (Recovery Time Objective): The duration of acceptable application downtime, whether from an unplanned outage or scheduled maintenance/upgrades. The primary goal is to restore full service to the point that new transactions can take place.
• RPO (Recovery Point Objective):The ability to accept potential data loss from an outage. It is the time gap or latency between the last committed data transaction before the failure and the most recent data recovered after the failure. The actual data loss can vary depending upon the workload on the system at the time of the failure, the type of failure, and the type of high availability solution used.
• RLO (Recovery Level Objective): This objective defines the granularity with which you must be able to recover data — whether you must be able to recover the whole instance, database or set of databases, or specific tables.
• Quorum: Generally speaking, a quorum is the minimum number of members of an assembly, who must be present before the members can conduct business. The requirement for a quorum protects against unrepresentative action in the name of the body by a disproportionately small number of individuals. In the context of Windows Clustering, the quorum configuration determines the number of failures the cluster can sustain. In the context of SQL database mirroring or availability groups with automatic failover, quorum makes sure that a database is owned by only one partner or replica at a time.

We are well equipped with the following HA/DR techniques:

• ➤ SQL Failover Clustered Instance (FCI)
• ➤ Database Mirroring
• ➤ Transaction Log Shipping
• ➤ Transactional Replication
• ➤ Windows Server Failover Clustering (WSFC)
• ➤ SQL Availability Groups (Always on)