Introduction to Data Management

Managing Records With ENSCRIBE

4-4 15873 Tandem Computers Incorporated

Access Coordination Several processes can have access to one file at the same time. To coordinate

simultaneous access, each process must indicate, when opening a file, how it intends

to use that file. First, the process indicates the access mode that specifies the kinds of

operations that the process will perform: read/write, read-only, or write-only access.

Then, the process indicates the exclusion mode that determines how much access

other competing processes will be allowed: shared access, exclusive access, or

protected access. The access and exclusion modes operate on a file from the time the

file is opened until it is closed. The access and exclusion modes are managed by the

GUARDIAN 90 operating system.

In addition to the access modes, your program can explicitly invoke locking

mechanisms that prevent concurrent access to data for short periods of time. These

locks can be applied to an entire file or to individual records within the file.

Nowait and Waited I/O ENSCRIBE permits your application process to execute concurrently with its file

operations by means of nowait I/O. With nowait I/O, when your process calls a

procedure to read or write data, the process continues to execute, running

concurrently with the file operation. Your process receives notification of I/O

completion through a separate procedure call.

If your process does not request nowait I/O, waited I/O remains in effect. In this

case, when a file operation is requested by a system procedure call, the application

process is suspended and waits until the file operation completes.

Support for Database

Development

The Data Definition Language (DDL) helps you design and develop relational

databases that contain ENSCRIBE records. DDL lets you define both record structures

and data structures. These definitions are kept in dictionaries that are accessible to

programming languages and that drive the ENABLE and ENFORM software

described in this section, and the PATHMAKER software described in Section 5.

Support for Application

Generation

Based on ENSCRIBE records, the ENABLE program generator can automatically

generate simple interactive applications through which end users can add, retrieve,

update, and delete records in your database. Once these applications are produced,

they can be modified and customized as needed. They can also be called as modules

from within larger applications.

In addition, more complex, powerful, and flexible applications based on ENSCRIBE

records can be generated with the PATHMAKER online application code generator or