While High Availability mode does require a witness, there
should be no reason why an organization can??™t run in High Availability if it want to. The
witness could be an older server running SQL Server Express Edition, so no additional
SQL Server licensing fees would be required.
However, remember that High Availability mode communicates synchronously??”
that is, it runs in two-phase commit mode. That means slow insert, update, or delete
operations. High-volume data-entry systems may experience pauses while users are
CHAPTER 8 n DATABASE MIRRORING 209
entering data. It??™s best not to use High Availability mode with applications that have
data-bound controls, in which updates are automatically transmitted for every field
that is updated on a form. It??™s best to use High Availability mode when data is captured
from the form and then inserted or updated in one action. Granted, users will
still see a delay after hitting Save, but at least they won??™t feel the delay throughout the
entire data-entry process.
Failover
In the event that a principal fails in High Availability mode, mirroring will automatically
transfer users to the mirror node, as long as they??™re using SNAC.
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