How to setup a synchronously Mirrored Database
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To guarantee maximum resilience, it is possible to deploy two separate database servers. At this, only one database server is connected to the DLS (single node or cluster), while the data are mirrored on the other server. In case the main server ("Principal") fails, the second server ("Mirror") assumes its function. For database mirroring, two models are supported:
- Synchronous Mirroring: A third server ("Witness") monitors the state of the main server. If the Principal should fail, the Witness will switch over to the Mirror, which will hereby become the Principal. Moreover, the Witness checks whether all transactions have been completed on the Mirror, too. Thus it is ensured that no data loss will occur. However, the performance will be slightly lower than with asynchronous mirroring.
- Asynchronous Mirroring: There is no Witness, and the switchover to the mirror server is done by "Forced Service", i. e., by the DLS.