ms.author: mathoma

In an Always On availability group, you can configure one or more availability replicas to allow read-only connections when running under the secondary role (that is, when running as a secondary replica). You can also configure each availability replica to allow or exclude read-only connections when running under the primary role (that is, when running as the primary replica).

To facilitate client access to primary or secondary databases of a given availability group, you should define an availability group listener. By default, the availability group listener directs incoming connections to the primary replica. However, you can configure an availability group to support read-only routing, which enables its availability group listener to redirect the connection requests of read-intent applications to a readable secondary replica. For more information, see [Configure Read-Only Routing for an Availability Group (SQL Server)](../../../database-engine/availability-groups/windows/configure-read-only-routing-for-an-availability-group-sql-server.md).

ms.author: mathoma

The [!INCLUDE[ssHADR](../../../includes/sshadr-md.md)] active secondary capabilities include support for taking backups on secondary replicas. Backup operations can put significant strain on I/O and CPU (with backup compression). Offloading backups to a synchronized or synchronizing secondary replica allows you to use the resources on server instance that hosts the primary replica for your tier-1 workloads.

ms.author: mathoma

The [!INCLUDE[ssHADR](../../../includes/sshadr-md.md)] active secondary capabilities include support for read-only access to one or more secondary replicas (*readable secondary replicas*). A readable secondary replica can be in either synchronous-commit availability mode, or asynchronous-commit availability mode. A readable secondary replica allows read-only access to all its secondary databases. However, readable secondary databases are not set to read-only. They are dynamic. A given secondary database changes as changes on the corresponding primary database are applied to the secondary database. For a typical secondary replica, the data, including durable memory optimized tables, in the secondary databases is in near real time. Furthermore, full-text indexes are synchronized with the secondary databases. In many circumstances, data latency between a primary database and the corresponding secondary database is only a few seconds.

ms.author: mathoma

This topic describes how to add a secondary replica to an existing Always On availability group by using [!INCLUDE[ssManStudioFull](../../../includes/ssmanstudiofull-md.md)], [!INCLUDE[tsql](../../../includes/tsql-md.md)], or PowerShell in [!INCLUDE[ssCurrent](../../../includes/sscurrent-md.md)].

ms.author: mathoma

This F1 help topic describes the options of the **Add IP Address** dialog box. This dialog box accessed from the **New Availability Group Listener** dialog box and the **Listener** tab of the **Specify Replicas** page of the [!INCLUDE[ssAoNewAgWiz](../../../includes/ssaonewagwiz-md.md)] or the [!INCLUDE[ssAoAddRepWiz](../../../includes/ssaoaddrepwiz-md.md)] of [!INCLUDE[ssCurrent](../../../includes/sscurrent-md.md)].

ms.author: mathoma

Managing an existing Always On availability group in [!INCLUDE[ssCurrent](../../../includes/sscurrent-md.md)] involves one or more of the following tasks:

- Altering the properties of an existing availability replica, for example to change client connection access (for configuring readable secondary replicas), changing its failover mode, availability mode, or session timeout setting.

ms.author: mathoma

This topic documents interoperability of [!INCLUDE[ssHADR](../../../includes/sshadr-md.md)] with other [!INCLUDE[ssNoVersion](../../../includes/ssnoversion-md.md)] features in [!INCLUDE[ssCurrent](../../../includes/sscurrent-md.md)].

ms.author: mathoma

The [!INCLUDE[ssHADR](../../../includes/sshadr-md.md)] feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring. Introduced in [!INCLUDE[ssSQL11](../../../includes/sssql11-md.md)], [!INCLUDE[ssHADR](../../../includes/sshadr-md.md)] maximizes the availability of a set of user databases for an enterprise. An *availability group* supports a failover environment for a discrete set of user databases, known as *availability databases*, that fail over together. An availability group supports a set of read-write primary databases and one to eight sets of corresponding secondary databases. Optionally, secondary databases can be made available for read-only access and/or some backup operations.

ms.author: mathoma

This topic describes considerations for client connectivity to Always On availability groups, including prerequisites, restrictions, and recommendations for client configurations and settings.