Knowing how the principal and the mirror communicate is
critical. Why, you may ask? Well, for starters, one method might have your users chasing you through
the cubicles with pitchforks and torches, while the other might have your boss calling security to walk
you out of the building.
Synchronous communication in mirroring is essentially a two-phase commit. The principal doesn??™t
commit its data until it receives confirmation from the mirror that the data in question was committed
successfully on the mirrored database. I??™ve said this before, and I hope that it??™s clear conceptually. It is
in the practice of synchronous, two-phase commit communication where potential problems can occur.
Let??™s say a user is entering data in his application, which is connected to the principal. Every time
the user hits Save, he must wait until both the principal and the mirror complete saving the data. If the
network must undertake a long physical distance or a complicated journey to get the data from the
principal to the mirror, that poor user will sit and wait and wait and wait and . . .
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