Yes No No The principal continues to operate but generates an
error that it cannot connect to the mirror.
No Yes No Manual failover must be initiated to move to the
mirror.
Yes Yes Yes, but No failover occurs. An error is noted, and until the
down witness is restored or replaced, all failover is manual.
CHAPTER 8 n DATABASE MIRRORING 200
Understanding the Details
OK, you have a principal database, a mirror database, and an optional witness. What are
the other components to database mirroring that make it so complex? So far, you know
where the original data resides (in the principal), where the standby data resides (in the
mirror), and the component that facilitates failover (the witness). Now it??™s time to explore
how these pieces connect and how they communicate. This is where things get complex.
Connections
Communications for database mirroring occur over TCP/IP connections. A port number
is an important facet of such connections??”you likely know that. However, with database
mirroring, you need to be concerned about an additional facet: the endpoint.
Ports
Database mirroring connects by means of TCP ports bound to a specific IP address.
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