Failover options in C#.NET

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Failover options
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Using the SNAC s failover partner option automates a lot of the hard work in application reconnection logic required for other high-availability options. However, even with database mirroring in place, client connections originating from non-SNAC sources won t be able to take advantage of automatic reconnection, unless the reconnection logic is coded into the application. This is an important high-availability consideration; while the database may fail over immediately and without any data loss, if the clients can t automatically reconnect to the mirror, it can hardly be considered a success, thus devaluing part of the appeal of automatic failover. High-safety mirroring sessions can also use the manual failover method.
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11.5.2 Manual failover
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The manual failover method, available only in high-safety (synchronous) mode, is typically used when preparing for a planned outage such as a hardware or service pack upgrade. We ll see an example of a manual failover later in the chapter, but a summary follows:
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The DBA enacts the manual failover, which swaps the mirroring roles between the principal and mirror. Applications are reconnected to the new principal database once it completes the processing of its redo queue and comes online. Mirroring is suspended and the mirror instance is taken offline for upgrade, during which time the principal database runs exposed that is, failover of any type isn t possible, so if the new principal instance fails, an outage will result. Once the upgrade is complete, the mirror database rejoins the mirroring session and synchronizes outstanding transactions (catches up).
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At this point, the mirroring roles can be reversed to return the instances to their original state, although assuming both servers are configured with the same processing capacity and load, this step shouldn t be required, so the current roles could remain in place. Of course, if there are multiple databases on the server being taken offline for a planned outage, and some of them aren t mirrored, then this approach obviously needs to be reconsidered. The final failover mode is forced service.
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11.5.3 Forced service
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Typically used in disaster-recovery scenarios with high-performance (asynchronous) mirroring, this option brings the mirror database online and makes it available for client connections only if the link between the principal and mirror instances is lost. The critical consideration before enacting this failover mode is the possibility of data loss. If a network connection drops and the principal database continues processing transactions before failover, these transactions won t be available for recovery on the mirror database. As such, forced service is typically only used when service must be resumed as soon as possible and the possibility of data loss is accepted.
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High availability with database mirroring
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In closing our section on failover modes, let s walk through a number of failover scenarios.
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11.5.4 Failure scenarios
To understand how failure is handled based on the mirroring mode and which instance fails, consider table 11.3. Read the notes that follow the table in conjunction with each example.
Table 11.3 Failure Principal Principal Principal Failure scenarios by mirroring topology and transaction safety Mirroring mode Synchronous Synchronous Asynchronous Yes No No Witness Action Automatic failover (see note 1) Manual failover (see note 2) Force service to make mirror available (see note 3) Principal runs exposed and transactions retained (see note 4) Automatic failover not possible
Mirror
Both
Both
Witness
Synchronous
Action Notes:
Assuming the mirror and witness are in contact, automatic failover occurs after the nominated timeout. Mirroring is stopped on the mirror database using ALTER DATABASE <dbname> SET PARTNER OFF. The mirroring database is then recovered using RESTORE DATABASE <dbname> WITH RECOVERY. When the principal server becomes available, mirroring would need to be reestablished, this time in the reverse direction. Service is forced by running this command on the mirror database: ALTER DATABASE <dbname> SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS. For as long as database mirroring exists, transactions generated on the principal must remain in the transaction log, so disk space usage would need to be closely monitored while the mirror instance is unavailable.
Armed with an overview of database mirroring principals and options, let s roll up our sleeves and get into the details of the implementation.
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