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

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title High availability and disaster recovery on Linux and macOS
description Learn about how the Microsoft ODBC Driver for Linux and macOS supports Always On availability groups and failover clusters.
author David-Engel
ms.author davidengel
ms.date 05/06/2020
ms.service sql
ms.subservice connectivity
ms.custom linux-related-content
ms.topic conceptual

High availability and disaster recovery on Linux and macOS

[!INCLUDEDriver_ODBC_Download]

The ODBC drivers for Linux and macOS support Always On availability groups. For more information about Always On availability groups, see:

You can specify the availability group listener of a particular 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. 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 aren't connecting to an availability group listener. If the DNS server's first returned IP address isn't connectable, these iterations can be time consuming.

When you're 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, you should implement connection retry logic. Retry a failed connection until it reconnects. Increasing the connection timeout and implementing connection retry logic increases the chance of connecting to an availability group.

Connect with MultiSubnetFailover

Always specify MultiSubnetFailover=Yes when you're connecting to a [!INCLUDEssSQL11] availability group listener or [!INCLUDEssSQL11] failover cluster instance. MultiSubnetFailover enables faster failover for all availability groups and failover cluster instances in [!INCLUDEssSQL11].

This connection property also significantly reduces failover time for single and multi-subnet Always On topologies. During a multi-subnet 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 you connect 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 Always On availability group, or an Always On failover cluster instance. MultiSubnetFailover=Yes applies to both single- and multi-subnet availability groups and failover cluster instances.

Use MultiSubnetFailover=Yes when you're connecting to an availability group listener or failover cluster instance. Otherwise, your application's performance can be negatively affected.

Recommendations

When you're connecting to a server in an availability group or failover cluster instance:

  • Specify MultiSubnetFailover=Yes to improve performance when you're 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 can't 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 aren't supported.

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

  • If the secondary replica location isn't configured to accept connections.

  • 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 Always On availability groups:

  • 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 Always On availability groups 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