Databases Reference
In-Depth Information
include support for duplicate live servers so that operations can be split between
the servers, improving performance.
Why not use server mirroring all the time? After all, it protects against a wider
range of potential problems. There are two potential problems with server mirror-
ing. The first is that not all DBMSs support a backup server or failover option. The
other is that these solutions are relatively expensive, requiring duplicate hardware
and, in many configurations, multiple software licenses (one for each server).
If the solutions are so expensive, what's the justification for using them?
Backup servers, especially configurations that provide for automatic failover, give
you a way of designing a database system with a guarantee of near-zero down
time. Imagine, for a moment, a database system that supports a stock broker-
age. Clients need the ability to post trades and have them recorded and executed
immediately any time, day or night. Any outage, no matter how brief, means
delayed or lost trades. Not only does this mean lost revenue, it would probably
mean losing any customers that were affected. If word gets out, it could means
losing even more customers, and possibly your business.
11.3.5 Using Data Backups
The surest way to recover from a failure is to have reliable backups. Data back-
ups let you recover from the most catastrophic failures, even the loss of the entire
server. Backup features vary by DBMS, but in every case, regular backups are a
critical requirement, not an option.
Backup design must be part of your overall database design solution. You
should be ready to implement your backup plan as you implement the database.
You can judge how complete your backup plan is by whether or not it answers
four key questions: “what,” “when,” “where,” and “how.” “Who” would also be
a question, except that the “who” ultimately falls on the database administrator,
who either runs the backups, or makes sure that they are run on a regular basis.
Answering the “What” of Backups
What do you back up? That's the easiest question. You back up everything at
least once. You need to have a backup copy of all of your data, including lookup
and reference data that doesn't change, to facilitate recovery in case of data loss.
The more critical the data is to your business, the more important that it be
backed up, and all data is important. If not, then why is it taking up space on
your server? Of course, more volatile data needs to be backed up more fre-
quently, but make sure you have a copy of everything.
Everything
literally means everything, including database metadata. The need
to back up production data is easy to see, but that's not the only data in your
database. If your DBMS includes system databases that store information about
databases and their contents, you need to back these up also. The same is true
of the data dictionary, if maintained as a separate component in your DBMS.
