The Impact of a Stale Failover Partner Name
The database administrator can change the failover partner at any time. Therefore, a client-supplied failover partner name might be out of date, or stale. For example, consider a failover partner named Partner_B that is replaced by another server instance, Partner_C. Now, if a client supplies Partner_B as the failover partner name, that name is stale. When the client-supplied failover partner name is stale, the behavior of the data access provider equates to the case in which a failover partner name is not supplied by the client.
For example, consider situation in which a client uses one connection string for a series of four connection attempts. In the connection string, the initial partner name is Partner_A, and the failover partner name is Partner_B:
"Server=Partner_A; Failover Partner=Partner_B; Database=AdventureWorks2008R2"
The following table shows four partner configurations and indicates for each whether this connection string works for connecting the client for the first time.
 Note |
|---|
An application can track configuration changes and change its connection string accordingly. This requires extra code but reduces the administrative burden. |
Configuration | Principal server | Mirror server | Behavior when attempting to connect specifying Partner_A and Partner_B |
|---|---|---|---|
Original mirroring configuration. | Partner_A | Partner_B | Partner_A is cached as the initial partner name. The client succeeds in connecting to Partner_A. The client downloads the name of mirror server, Partner_B, and caches it, ignoring the client-supplied failover partner name. |
Partner_A experiences a hardware failure, and failover occurs (disconnecting clients). | Partner_B | none | The Partner_A is still cached as the initial partner name, but the client-supplied failover partner name, Partner_B, permits the client to connect to the current principal server. |
The database administrator stops mirroring (disconnecting clients), replaces Partner_A with Partner_C, and restarts mirroring. | Partner_B | Partner_C | The client attempts to connect to Partner_A and fails; then the client tries Partner_B (the current principal server) and succeeds. The data access provider downloads the name of the current mirror server, Partner_C, and caches it as the current failover partner name. |
Service is manually failed over to Partner_C (disconnecting clients). | Partner_C | Partner_B | Client attempts to connect to Partner_A initially, and then to Partner_B. Both names fail, and eventually the connection request times out and fails. |
