> # Configuring Connection Failover

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

> You can customize the ADO Client 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 the following illustration with multiple SequeLink servers: SequeLink Server A, B, and C. SequeLink Server A is designated as the primary SequeLink server, SequeLink Server B is the first alternate server, and SequeLink Server C is the second alternate server.

> First, the application attempts to connect to the primary SequeLink 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.

> 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 Alternate Servers connection property. Connection attempts continue until a connection is successfully established or until all the database servers in the list have been tried once (the default).

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

> - The number of times the ADO Client 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 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 connection option.
> - Whether the ADO data Client uses 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 connection option. See "Using Client Load Balancing" for more information.

> ### Connection Failover Properties

> Table 3-3 summarizes the connection properties that control how connection failover works with the ADO Client. See Table 3-1 for details about configuring each property.

> Table 3-3. Summary: Connection Failover Properties for the ADO Data Provider
> Property | Characteristic
> --- | ---
> Alternate Servers | A list of alternate SequeLink servers to which the data provider will attempt to connect if the primary SequeLink server is unavailable. A port number and an IP address or server name identifying each server are required. If the primary SequeLink server is an LDAP server, each alternate server must be an LDAP server.
> Connection Retry Count | Number of times the data provider retries the primary database server, and if specified, alternate servers until a successful connection is established. The default is 5.
> Connection Retry Delay | Wait interval, in seconds, between connection retry attempts when the ConnectionRetryCount property is set to a positive integer. The default is 1.
> Host | The TCP/IP address or server name of primary SequeLink server. LDAP: If LDAP is enabled, this identifies the TCP/IP address of the LDAP server. This can also be a list of LDAP servers separated by a blank space (for example, "ld1.foo.com ld2.foo.com ld3.foo.com"). If the first LDAP server in the list does not respond, the ADO Client will try to connect to the next LDAP server in the list.
> Load Balancing | Sets whether the ADO Client uses client load balancing in its attempts to connect to the list of database servers (primary and alternate). If client load balancing is enabled, the ADO Client uses a random pattern instead of a sequential pattern in its attempts to connect. The default is 0 (client load balancing is disabled).
> Port | Port listening for connections on the primary SequeLink server. If LDAP is enabled, this identifies the TCP/IP port on which the LDAP server is listening. If you do not specify a port, the default port for LDAP (389) will be used.

> See "Configuring Connection Failover" and "Using Client Load Balancing" for overviews of connection failover and client load balancing.