> # Configuring Connection Failover

> Connection failover allows an application to connect to an alternate, or backup, database server if the primary database server is unavailable, for example, because of a hardware failure or traffic overload. Connection failover ensures that the data on which your critical ODBC applications depend is always available.

> You can customize the SequeLink ODBC driver for connection failover by configuring a list of alternate SequeLink servers that are tried if the primary server is not accepting connections. Connection attempts continue until a connection is successfully established or until all the alternate SequeLink servers have been tried the specified number of times.

> For example, suppose you have the environment shown in Figure 1-1 with multiple SequeLink servers: SequeLink Server A, B, and C. SequeLink Server A is designated as the primary database server, SequeLink Server B is the first alternate server, and SequeLink Server C is the second alternate server.

> Figure 1-1. Configuring Connection Failover with the ODBC Client

> First, the application attempts to connect to the primary database server, SequeLink Server A (1). If connection failover is enabled and SequeLink Server A fails to accept the connection, the application attempts to connect to SequeLink Server B (2). If that connection attempt also fails, the application attempts to connect to SequeLink Server C (3).

> In this scenario, it is probable that at least one connection attempt would succeed, but if no connection attempt succeeds, the driver can retry each alternate SequeLink Server (primary and alternate) for a specified number of attempts. You can specify the number of attempts that are made through the connection retry feature. You can also specify the number of seconds of delay, if any, between attempts through the connection delay feature. See "Using Connection Retry" for more information about connection retry.

> A driver fails over to the next alternate SequeLink server only if a successful connection cannot be established with the current alternate server. If the driver successfully establishes communication with a SequeLink Server and the connection request is rejected by the SequeLink Server because, for example, the login information is invalid, then the driver generates an error and does not try to connect to the next SequeLink Server in the list.

> Connection failover provides protection for new connections only and does not preserve states for transactions or queries. For details on configuring connection failover, refer to the SequeLink Administrator's Guide.

> To configure connection failover, you must specify a list of alternate SequeLink Servers that are tried at connection time if the primary server is not accepting connections. To do this, use the AlternateServers connection option.

> Optionally, you can specify the following additional connection failover features:

> - The number of times the SequeLink ODBC driver attempts to connect to the primary and alternate SequeLink servers after the initial unsuccessful connection attempt. By default, the driver does not retry. To set this feature, use the Connection Retry Count (ConnectionRetryCount) connection option. See "Using Connection Retry" for more information.
> - The wait interval, in seconds, between attempts to connect to the primary and alternate SequeLink servers. The default interval is 3 seconds. To set this feature, use the Connection Retry Delay (ConnectionRetryDelay) connection option.
> - Whether the SequeLink ODBC driver will use client load balancing in its attempts to connect to primary and alternate SequeLink servers. If load balancing is enabled, the driver uses a random pattern instead of a sequential pattern in its attempts to connect. The default value is not to use load balancing. To set this feature, use the Load Balancing (LoadBalancing) connection option. See "Using Client Load Balancing" for more information.

> On Windows, you can configure a data source to use connection failover on the Failover tab of the driver's Setup dialog box. Refer to the SequeLink Administrator's Guide for detailed information.

> On UNIX and Linux, you can configure a data source to use connection failover by modifying your system information file (odbc.ini). Refer to the SequeLink Administrator's Guide for detailed information.

> On Linux, UNIX, and Windows, you can use a connection string to direct the driver to use connection failover. Refer to the SequeLink Administrator's Guide for detailed information.