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Getting Started with HP IMAGE/SQL: HP 3000 MPE/ iX Computer Systems > Chapter 2 Basic Concepts
What Is a DBEnvironment? |
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In IMAGE/SQL, you create one or more databases in a structure called a DBEnvironment. The structure of a DBEnvironment is shown in Figure 2-2. The following objects are the most important parts of the DBEnvironment:
Objects are structures created and stored in an IMAGE/SQL DBEnvironment. IMAGE/SQL connects the DBEnvironment objects with the TurboIMAGE/XL database structures. This is discussed in greater detail in Chapter 5, "IMAGE/SQL Tasks." To better understand the DBEnvironment and its objects, imagine that it is like a library that stores books, periodicals, or other information. Like a library, the DBEnvironment is a physical location for information, so you need to set aside physical space for it. Also like a library, the DBEnvironment uses a logical method for storing and retrieving information. When you create a DBEnvironment, IMAGE/SQL creates a physical file known as the DBEnvironment Configuration File or DBECon file. This file contains basic information that is used every time the DBEnvironment is opened. The DBECon file has the same name as the DBEnvironment itself. If the DBEnvironment is like a library, the DBECon file is like a building directory that points to the other DBEnvironment components. You must allocate some physical storage space by creating DBEFiles--operating system files that hold DBEnvironment data. Like the individual bookcases in a library, they have a specific capacity. DBEFiles have both physical names (operating system names) and logical names by which the files are known internally to the IMAGE/SQL system catalog, to be explained shortly. DBEFiles are grouped together in logical groupings known as DBEFileSets. These are something like the different subject categories in a library. You create a DBEFileSet with a logical name (this corresponds to the subject category name). Then you add DBEFiles to the DBEFileSet as needed, as you would add bookcases to the category to add more space. For example, a section may be labeled "Computer Periodicals" and hold many bookcases with different magazines in them. DBEFileSets do not have a specific capacity; you can always add more DBEFiles to create more space in them. The data in a DBEnvironment is stored in databases, which are groups of tables having the same owner. The database in a DBEnvironment is like division ownership of certain books or periodicals in a library. In a university, some of the periodicals may belong to a computer science library, others may belong to a medical library, though these may be housed inside the same building and use the same card catalog. In such a subdivision of the library, certain books and periodicals could only be checked out by people belonging to the appropriate division or having special permission. When you create an object in the DBEnvironment, you are its owner by default, and therefore it belongs to your database. The most important objects you create are tables. A table is like a periodical stored in a library, and the rows in the table are like individual articles in the periodical. Tables have only logical names; a table name is like the title of the periodical. When you create a table, you only need to define its name and characteristics and associate it with a DBEFileSet. This is like adding a new periodical to a subject category in the library. To enable readers to find issues of ComputerWorld, the librarian puts an entry in a card catalog that shows where the issues are stored. In the DBEnvironment, SQLCore inserts an entry in the system catalog for each database object you create. The system catalog is created at the time you create a DBEnvironment. The system catalog is physically located in a DBEFile known as DBEFILE0. This file contains extra space for the additional entries that are made by SQLCore each time you create a new object. Like a good librarian, IMAGE/SQL keeps the system catalog up-to-date for you, without your being aware of it. The system catalog is a set of tables for internal use by SQLCore. These tables are associated with a DBEFileSet known as SYSTEM. SQLCore uses the system catalog to look up database objects much as you would use the card catalog in a library to look up the bookcase and shelf number for a book or periodical. IMAGE/SQL provides logging of all transactions that take place in the DBEnvironment. Like the library's record of items checked out and returned, the log file is a record of the rows in database tables that are added, deleted, or changed. The log makes it possible to keep data consistent when multiple users are accessing the system, and it makes recovery possible in the event of a system failure. Information about the system log is stored in the DBECon file. Because logging is essentially automatic, it will not be discussed any further in this manual. For more information, refer to the chapter "Backup and Recovery" in the ALLBASE/SQL Database Administration Guide. |
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![[Structure of a DBEnvironment]](img/elements.gif)