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odbc-driver-on-linux-support-for-high-availability-disaster-recovery.md

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title ODBC Driver on Linux and macOS - High Availability and Disaster Recovery | Microsoft Docs
ms.custom
ms.date 04/05/2018
ms.prod sql
ms.prod_service connectivity
ms.reviewer
ms.technology connectivity
ms.topic conceptual
ms.assetid fa656c5b-a935-40bf-bc20-e517ca5cd0ba
author David-Engel
ms.author v-daenge

ODBC Driver on Linux and macOS Support for High Availability and Disaster Recovery

[!INCLUDEDriver_ODBC_Download]

The ODBC drivers for Linux and macOS support [!INCLUDEssHADR]. For more information about [!INCLUDEssHADR], see:

You can specify the availability group listener of a given availability group in the connection string. If an ODBC application on Linux or macOS is connected to a database in an availability group that fails over, the original connection is broken and the application must open a new connection to continue work after the failover.

The ODBC drivers on Linux and macOS iterate sequentially through all IP addresses associated with a DNS hostname if you are not connecting to an availability group listener, and multiple IP addresses are associated with the hostname.

If the DNS server's first returned IP address is not connectable, these iterations can be time consuming. When connecting to an availability group listener, the driver attempts to establish connections to all IP addresses in parallel. If a connection attempt succeeds, the driver discards any pending connection attempts.

Note

Because a connection can fail due to an availability group failover, implement connection retry logic; retry a failed connection until it reconnects. Increasing connection timeout and implementing connection retry logic increases the chance of connecting to an availability group.

Connecting With MultiSubnetFailover

Always specify MultiSubnetFailover=Yes when connecting to a [!INCLUDEssSQL11] availability group listener or [!INCLUDEssSQL11] Failover Cluster Instance. MultiSubnetFailover enables faster failover for all Availability Groups and failover cluster instance in [!INCLUDEssSQL11]. MultiSubnetFailover also significantly reduces failover time for single and multi-subnet AlwaysOn topologies. During a multisubnet failover, the client attempts connections in parallel. During a subnet failover, the driver aggressively retries the TCP connection.

The MultiSubnetFailover connection property indicates that the application is being deployed in an availability group or Failover Cluster Instance. The driver tries to connect to the database on the primary [!INCLUDEssNoVersion] instance by trying to connect to all the IP addresses. When connecting with MultiSubnetFailover=Yes, the client retries TCP connection attempts faster than the operating system's default TCP retransmit intervals. MultiSubnetFailover=Yes enables faster reconnection after failover of either an AlwaysOn Availability Group or an AlwaysOn Failover Cluster Instance. MultiSubnetFailover=Yes applies to both single- and multi-subnet Availability Groups and Failover Cluster Instances.

Use MultiSubnetFailover=Yes when connecting to an availability group listener or Failover Cluster Instance. Otherwise, your application's performance can be negatively affected.

Note the following when connecting to a server in an availability group or Failover Cluster Instance:

  • Specify MultiSubnetFailover=Yes to improve performance when connecting to a single subnet or multi-subnet Availability Group.

  • Specify the availability group listener of the availability group as the server in your connection string.

  • You cannot connect to a [!INCLUDEssNoVersion] instance configured with more than 64 IP addresses.

  • Both [!INCLUDEssNoVersion] Authentication or Kerberos Authentication can be used with MultiSubnetFailover=Yes without affecting the behavior of the application.

  • You can increase the value of loginTimeout to accommodate for failover time and reduce the application's connection retry attempts.

  • Distributed transactions are not supported.

If read-only routing is not in effect, connecting to a secondary replica location in an availability group fails in the following situations:

  1. If the secondary replica location is not configured to accept connections.

  2. If an application uses ApplicationIntent=ReadWrite and the secondary replica location is configured for read-only access.

A connection fails if a primary replica is configured to reject read-only workloads and the connection string contains ApplicationIntent=ReadOnly.

[!INCLUDEspecify-application-intent_read-only-routing]

ODBC Syntax

Two ODBC connection string keywords support [!INCLUDEssHADR]:

  • ApplicationIntent

  • MultiSubnetFailover

For more information about ODBC connection string keywords, see Using Connection String Keywords with SQL Server Native Client.

The equivalent connection attributes are:

  • SQL_COPT_SS_APPLICATION_INTENT

  • SQL_COPT_SS_MULTISUBNET_FAILOVER

For more information about ODBC connection attributes, see SQLSetConnectAttr.

An ODBC application that uses [!INCLUDEssHADR] can use one of two functions to make the connection:

Function Description
SQLConnect Function SQLConnect supports both ApplicationIntent and MultiSubnetFailover via a data source name (DSN) or connection attribute.
SQLDriverConnect Function SQLDriverConnect supports ApplicationIntent and MultiSubnetFailover via DSN, connection string keyword, or connection attribute.

See Also

Connection String Keywords and Data Source Names (DSNs)

Programming Guidelines

Release Notes