ALLBASE/SQL databases are contained in a DBEnvironment.
A DBEnvironment is one or more databases that share
the same system catalog and the same logging and recovery.
Databases within a DBEnvironment can be related to each
other through ISQL and in applications if security allows it.
Databases that are contained in separate DBEnvironments cannot
be linked to each other.
An interactive user or application program
can only access one DBEnvironment at
a time. (However, the same application can
maintain several DBEnvironment connections and access different
DBEnvironments in sequence. Refer to the section
"Using Multiple Connections and Transactions with Timeouts"
in the ALLBASE/SQL Reference Manual chapter "Using ALLBASE/SQL.")
As shown in Figure 2-10 “DBEnvironment used by Integrated Peripherals, Inc.”,
the DBEnvironment used by Integrated
Peripherals, Inc., has three databases.
The PurchDB database
has been designed in the present chapter; the other
databases are included to illustrate the use of
multiple databases in a single DBEnvironment. All three databases
are placed in a single DBEnvironment because the data, although
used separately in most applications, is sometimes used together in
reports and global applications.
The number of databases that you can put in a DBEnvironment is
limited only by the amount of disk space available to you.
There are some reasons why you might create separate DBEnvironments
for databases.
You may have several users who need to create and drop objects
frequently. Consider putting their databases into different
DBEnvironments, since the frequent creation and dropping of objects
by several users in the same DBEnvironment requires heavy use of
the system catalog and can cause deadlocks.
You may have databases with different users functioning as DBA.
DBA authority is granted at the DBEnvironment level. Databases with
different DBAs should be in different DBEnvironments.
Recovery time increases as the size of the DBEnvironment increases.
Databases are logically separated by the owner name of the tables,
indexes, and views. They can also be physically separated for
independent manipulation through selective allocation of space
using DBEFiles and DBEFileSets. The allocation of space or
physical design of the database is covered in the next chapter.
The DBEnvironment designed for Integrated Peripherals, Inc. is the
basis for the sample DBEnvironment PartsDBE, which is included
with the ALLBASE/SQL product. Most of the examples in the rest
of this book refer to tables or other objects in PartsDBE.
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