Usually, you only see this error when automatic failover is deliberately not
being used.
While you can configure a standby database used in log shipping as readable, the
mirror database is inaccessible. You cannot read it, back it up, and so on. You can use
the mirror database for reporting purposes by means of database snapshots, but that
method of viewing the mirror database is limited. I??™ll cover that method in more detail
in the next chapter.
CHAPTER 8 n DATABASE MIRRORING 199
Witness
While the principal and the mirror are both databases, the witness is not??”it??™s a separate
SQL Server instance. The witness is an optional component of a mirroring environment,
but it??™s required if you automate failover. People often ask, ???Do I need to have a third
server for every mirroring session???? No. One instance can act as the witness to any number
of mirroring sessions. SQL Server Express can act as a witness, so really the witness
can be any server available.
The role of the witness is to do nothing but help decide if a failover needs to occur. It
continually talks to both the principal and the mirror, and if either is unavailable, it facilitates
automatic failover.
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