SQL Server Native Client Support for High Availability, Disaster Recovery

Article
08/31/2016

This topic discusses SQL Server Native Client support (added in SQL Server 2012) for Always On Availability Groups. For more information about Always On Availability Groups, see Availability Group Listeners, Client Connectivity, and Application Failover (SQL Server), Creation and Configuration of Availability Groups (SQL Server), Failover Clustering and AlwaysOn Availability Groups (SQL Server), and Active Secondaries: Readable Secondary Replicas (AlwaysOn Availability Groups).

You can specify the availability group listener of a given availability group in the connection string. If a SQL Server Native Client application 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.

If you are not connecting to an availability group listener, and if multiple IP addresses are associated with a hostname, SQL Server Native Client will iterate sequentially through all IP addresses associated with DNS entry. This can be time consuming if the first IP address returned by DNS server is not bound to any network interface card (NIC). When connecting to an availability group listener, SQL Server Native Client attempts to establish connections to all IP addresses in parallel and if a connection attempt succeeds, the driver will discard any pending connection attempts.

Note

Increasing connection timeout and implementing connection retry logic will increase the probability that an application will connect to an availability group. Also, because a connection can fail because of an availability group failover, you should implement connection retry logic, retrying a failed connection until it reconnects.

Connecting With MultiSubnetFailover

Always specify MultiSubnetFailover=Yes when connecting to a SQL Server 2012 availability group listener or SQL Server 2012 Failover Cluster Instance. MultiSubnetFailover enables faster failover for all Availability Groups and failover cluster instance in SQL Server 2012 and will significantly reduce failover time for single and multi-subnet AlwaysOn topologies. During a multi-subnet failover, the client will attempt connections in parallel. During a subnet failover, SQL Server Native Client will aggressively retry the TCP connection.

The MultiSubnetFailover connection property indicates that the application is being deployed in an availability group or Failover Cluster Instance, and that SQL Server Native Client will try to connect to the database on the primary SQL Server instance by trying to connect to all the IP addresses. When MultiSubnetFailover=Yes is specified for a connection, the client retries TCP connection attempts faster than the operating system's default TCP retransmit intervals. This enables faster reconnection after failover of either an AlwaysOn Availability Group or an AlwaysOn Failover Cluster Instance, and is applicable to both single- and multi-subnet Availability Groups and Failover Cluster Instances.

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

Specifying MultiSubnetFailover=Yes when connecting to something other than an availability group listener or Failover Cluster Instance may result in a negative performance impact, and is not supported.

Use the following guidelines to connect to a server in an availability group or Failover Cluster Instance:

Use the MultiSubnetFailover connection property when connecting to a single subnet or multi-subnet; it will improve performance for both.
To connect to an availability group, specify the availability group listener of the availability group as the server in your connection string.
Connecting to a SQL Server instance configured with more than 64 IP addresses will cause a connection failure.
Behavior of an application that uses the MultiSubnetFailover connection property is not affected based on the type of authentication: SQL Server Authentication, Kerberos Authentication, or Windows Authentication.
You can increase the value of loginTimeout to accommodate for failover time and reduce application 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 will fail in the following situations:

If the secondary replica location is not configured to accept connections.
If an application uses ApplicationIntent=ReadWrite (discussed below) and the secondary replica location is configured for read-only access.

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

Upgrading to Use Multi-Subnet Clusters from Database Mirroring

A connection error will occur if the MultiSubnetFailover and Failover_Partner connection keywords are present in the connection string. An error will also occur if MultiSubnetFailover is used and the SQL Server returns a failover partner response indicating it is part of a database mirroring pair.

If you upgrade a SQL Server Native Client application that currently uses database mirroring to a multi-subnet scenario, you should remove the Failover_Partner connection property and replace it with MultiSubnetFailover set to Yes and replace the server name in the connection string with an availability group listener. If a connection string uses Failover_Partner and MultiSubnetFailover=Yes, the driver will generate an error. However, if a connection string uses Failover_Partner and MultiSubnetFailover=No (or ApplicationIntent=ReadWrite), the application will use database mirroring.

The driver will return an error if database mirroring is used on the primary database in the availability group, and if MultiSubnetFailover=Yes is used in the connection string that connects to a primary database instead of to an availability group listener.

Specifying Application Intent

When ApplicationIntent=ReadOnly, the client requests a read workload when connecting to an AlwaysOn enabled database. The server will enforce the intent at connection time and during a USE database statement but only to an Always On enabled database.

The ApplicationIntent keyword does not work with legacy, read-only databases.

A database can allow or disallow read workloads on the targeted AlwaysOn database. (This is done with the ALLOW_CONNECTIONS clause of the PRIMARY_ROLE and SECONDARY_ROLETransact-SQL statements.)

The ApplicationIntent keyword is used to enable read-only routing.

Read-Only Routing

Read-only routing is a feature that can ensure the availability of a read only replica of a database. To enable read-only routing:

You must connect to an Always On Availability Group availability group listener.
The ApplicationIntent connection string keyword must be set to ReadOnly.
The Availability Group must be configured by the database administrator to enable read-only routing.

It is possible that multiple connections using read-only routing will not all connect to the same read-only replica. Changes in database synchronization or changes in the server's routing configuration can result in client connections to different read-only replicas. To ensure that all read-only requests connect to the same read-only replica, do not pass an availability group listener to the Server connection string keyword. Instead, specify the name of the read-only instance.

Read-only routing may take longer than connecting to the primary because read only routing first connects to the primary and then looks for the best available readable secondary. Because of this, you should increase your login timeout.

ODBC

Two ODBC connection string keywords were added to support Always On Availability Groups in SQL Server Native Client:

ApplicationIntent
MultiSubnetFailover

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

The equivalent connection properties are:

SQL_COPT_SS_APPLICATION_INTENT
SQL_COPT_SS_MULTISUBNET_FAILOVER

For more information about ODBC connection properties in SQL Server Native Client, see SQLSetConnectAttr.

The functionality of the ApplicationIntent and MultiSubnetFailover keywords will be exposed in the ODBC Data Source Administrator for DSNs that use the SQL Server Native Client driver, beginning in SQL Server 2012.

A SQL Server Native Client ODBC application can use one of three functions to make the connection:

Function	Description
SQLBrowseConnect	The list of servers returned by `SQLBrowseConnect` will not include VNNs. You will only see a list of servers without any indication if the server is a standalone server, or a primary or secondary server in a Windows Server Failover Clustering (WSFC) cluster that contains two or more SQL Server instances that have been enabled for Always On Availability Groups. If you connect to a server and get a failure, it may be because you have connected to a server, and the `ApplicationIntent` setting is not compatible with the server configuration. Because `SQLBrowseConnect` does not recognize servers in a Windows Server Failover Clustering (WSFC) cluster that contains two or more SQL Server instances that have been enabled for Always On Availability Groups, `SQLBrowseConnect` ignores the `MultiSubnetFailover` connection string keyword.
SQLConnect	`SQLConnect` supports both `ApplicationIntent` and `MultiSubnetFailover` via a data source name (DSN) or connection properties.
SQLDriverConnect	`SQLDriverConnect` supports `ApplicationIntent` and `MultiSubnetFailover` via connection string keywords, connection properties, or DSN.

OLE DB

OLE DB in the SQL Server Native Client does not support the MultiSubnetFailover keyword.

OLE DB in the SQL Server Native Client will support application intent. Application intent will behave the same for OLE DB applications as ODBC applications (see above).

One OLE DB connection string keyword added to support Always On Availability Groups in SQL Server Native Client:

Application Intent

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

The equivalent connection properties are:

SSPROP_INIT_APPLICATIONINTENT
DBPROP_INIT_PROVIDERSTRING

A SQL Server Native Client OLE DB application can use one of the methods to specify application intent:

IDBInitialize::Initialize
IDBInitialize::Initialize uses the previously configured set of properties to initialize the data source and create the data source object. Specify application intent as a provider property or as part of the extended properties string.

IDataInitialize::GetDataSource
IDataInitialize::GetDataSource takes an input connection string that can contain the Application Intent keyword.

IDBProperties::GetProperties
IDBProperties::GetProperties retrieves the value of the property that is currently set on the data source. You can retrieve the Application Intent value through the DBPROP_INIT_PROVIDERSTRING property and SSPROP_INIT_APPLICATIONINTENT property.

IDBProperties::SetProperties
To set the ApplicationIntent property value, call IDBProperties::SetProperties passing in the SSPROP_INIT_APPLICATIONINTENT property with value "ReadWrite" or "ReadOnly" or DBPROP_INIT_PROVIDERSTRING property with value containing "ApplicationIntent=ReadOnly" or "ApplicationIntent=ReadWrite".

You can specify application intent in the Application Intent Properties field of the All tab in the Data Link Properties dialog box.

When implicit connections are established, the implicit connection will use the application intent setting of the parent connection. Similarly, multiple sessions created from the same data source will inherit the data source's application intent setting.

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