Always On HA Overview

1.    Introduction

High Availability of an application or service provided to the end user and reducing the downtime of the outages occurred if any (Disaster Recovery Management) plays a major role in the lifecycle of any application. System outages are either anticipated (planned) or result of failures (unplanned) which may lead to data loss directly/indirectly impacts the end user requirements. In these situations, the primary objective of the software engineers/developers is to bring back the system online as soon as possible with minimal the data loss. AlwaysOn is a feature in the SQL server 2012 which provides flexible and cost efficient high availability and disaster recovery solution. AlwaysOn provides automatic recovery from failures avoiding the downtime which increases the high availability of critical applications online thereby reducing the data loss. AlwaysOn can be configured at both database level and instance level.

1.1.       Availability Groups (AG) & AG Listener

Availability group is a collection of databases in which AlwaysOn is configured at database level. Availability group enhances capabilities of database mirroring and ensures the availability of databases. Client application connects to the Availability group’s databases through a virtual network name called Availability group listener (AG Listener).Client need not required to find the active physical instance of the SQL server instead Client connects to the AG Listener which determines the available primary SQL server. A SQL server instance can have multiple availability groups.

1.2.       Availability Replicas and Roles

Availability Replicas are the user databases residing in the Availability Group (AG). Each database in the availability group can have a maximum of four availability replicas but only one replica in the AG can act as primary replica which is enabled for read-write operations and remaining replicas act as secondary read only replicas. The secondary replicas are considered as the backup.

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