It consists of a primary database (the principal)
that forwards database transaction activity to a standby database (the mirror). An
optional component (the witness) monitors the activity between the principal and the
mirror.
Figure 8-1. Simplified view of a mirroring configuration
Note that I specifically said that the principal database forwards ???transaction
activity?????”not the transaction logs themselves.
So far, things seem simple enough. Let??™s start out by examining these three
components.
CHAPTER 8 n DATABASE MIRRORING 196
Principal
The starting point for all mirroring operations is the principal database. It??™s always a user
database, and it always must be in Full Recovery mode (i.e., all transactions must be
logged). System databases cannot be mirrored. (Do you feel a potential problem coming
on?) The principal database is considered to be the primary, functional database; client
applications normally connect to the principal, not to the mirror.
The principal database is always available to users??”with one specific exception,
which I??™ll cover shortly.
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