Database Driver for JDB

This chapter provides documentation specific to JDB. It discusses the following:

• Engine initialization (page -2-)
• Connection declaration (page -4-)
• Import conversion (page -4-)
• Formatting for colon-plus processing and binding (page -7-)
• Cursors (page -7-)
• Errors and warnings (page -8-)
• Database transaction processing (page -10-)
• Transaction manager processing (page -13-)
• JDB-specific DBMS commands (page -13-)
• Command directory for Prolifics for JDB (page -18-)

This document is designed as a supplement to information found in the Application Development Guide.

Initializing the Database Engine

Database engine initialization occurs in the source file dbiinit.c. This source file is unique for each database engine and is constructed from the settings in the makevars file. In Prolifics for JDB, this results in the following vendor_list structure in dbiinit.c:

static vendor_t vendor_list[] =
{
	{"jdb", dm_jdbsup, DM_DEFAULT_CASE ,(char *) 0},

	{ (char *) 0, (int (*)()) 0, (int) 0, (char *) 0 }
};

The settings are as follows:

For Prolifics for JDB, the settings can be changed by editing the makevars.jdb file.

Engine Name

You can change the engine name associated with the support routine dm_jdbsup. The application then uses that name in DBMS ENGINE statements and in WITH ENGINE clauses. For example, if you wish to use "tracking" as the engine name, change the following parameter in the makevars.jdb file:

JDB_ENGNAME=tracking

If the application is accessing multiple engines, it makes JDB the default engine by executing:

DBMS ENGINE jdb-engine-name

where jdb-engine-name is the string used in vendor_list. For example,

DBMS ENGINE jdb

or

DBMS ENGINE tracking

Support Routine Name

dm_jdbsup is the name of the support routine for JDB. This name should not be changed.

Case Flag

The case flag, DM_DEFAULT_CASE, determines how Prolifics's database drivers use case when searching for Prolifics variables for holding SELECT results. This setting is used when comparing JDB column names to either a Prolifics variable name or to a column name in a DBMS ALIAS statement.

JDB is case insensitive. Regardless of the case in a SQL statement, JDB creates all database objects-tables, views, columns, etc.-with lower case names. For JDB, the DM_DEFAULT_CASE setting is treated as DM_FORCE_TO_LOWER_CASE. Because JDB uses only lower case, the DM_FORCE_TO_LOWER_CASE setting is the same as DM_PRESERVE_CASE. For either of these flags, Prolifics attempts to match JDB column names to lower case Prolifics variables when processing SELECT results. If your application is using this default, use lower case names when creating Prolifics variables.

If you wish to use upper case variable names, substitute the u option in the makevars file that sets the DM_FORCE_TO_UPPER_CASE flag.

JDB_INIT=u

If you edit makevars.jdb, you must remake your Prolifics executables. For more information on engine initialization, refer to Chapter -#- in the Application Development Guide.

Connecting to the Database Engine

JDB allows your application to use one or more connections. The application can declare any number of named connections with DBMS DECLARE CONNECTION statements; however, you should not have multiple connections to the same database.

The following options are supported for connections to JDB:

Table 1. Database connection options.

database-pathname is a pathname to an existing database.

Additional keywords are available for other database engines. If those keywords are included in your DBMS DECLARE CONNECTION command for JDB, it is treated as an error.

Importing Database Tables

The Import=>Database Objects option in the screen editor creates Prolifics repository entries based on database tables in an JDB database. When the import process is complete, each selected database table has a corresponding repository entry screen.

After the import process is complete, the repository entry screen contains:

• A widget for each column in the table, using the column's characteristics to assign the appropriate widget properties.
• A label for each column based on the column name.
• A table view named for the database table.
• Links that describe the relationship between table views.

Each import session allows you to display and select up to 1000 database tables. Each database table can have up to 255 columns. If your database contains more than 1000 tables, use the filter to control which database tables are displayed.

Table Views

A table view is a group of associated widgets on an application screen. As a general rule, the members of a table view are derived from the same database table. When a database table is first imported to a Prolifics repository, the new repository screen has one table view that is named after the database table. All the widgets corresponding to the database columns are members of that table view.

The import process inserts values in the following table view properties:

• Name - The name of the table view, generally the same as the database table.
• Table - The name of the database table.
• Primary Keys - The columns that are defined as primary keys for the database table.
• Columns - A list of the columns in the database table is displayed when you click on the More button. However, this list is for reference only. It cannot be edited.
• Updatable - A setting that determines if the data in the table can be modified. The default setting for Updatable is Yes.

Links

Links are created from the foreign key definitions entered in the database. The application screen must contain links if you are using the transaction manager and the screen contains more than one table view.

Check the link properties to see if they need to be edited for your application screen. The Parent and Child properties might need to be reversed or the Link Type might need to be changed.

Refer to Chapter -#- in the Application Development Guide for more information on links.

Widgets

A widget is created for each database column. The name of the widget corresponds to the database column name. The Inherit From property is set to @DATABASE indicating that the widget was imported from the database engine. The Justification property is set to Left. Other widget properties are assigned based on the data type.

The following table lists the values for the C Type, Length, and Precision properties assigned to each JDB data type.

Table 2. Importing Database Tables

Other Widget Properties

DT_DATETIME

MM/DD/YY HH:MM

Null Field property

For more information about usage of Prolifics type and C type, refer to Chapter -#- of the Application Development Guide.

Formatting for Colon Plus Processing and Binding

This section contains information about the special data formatting that is performed for the engine. For general information on data formatting, refer to Chapter -#- in the Application Development Guide.

Declaring Cursors

Prolifics uses two cursors for operations performed by DBMS SQL. One cursor is used for SQL SELECT statements and the other for non-SELECT statements. These two cursors might be sufficient for small applications. Larger applications often require more; an application might declare named cursors using DBMS DECLARE CURSOR. For example, master and detail applications often need to declare at least one named cursor: one cursor selects the master rows and additional cursors select detail rows. In short, if an application is processing a SELECT set in increments (i.e., by using DBMS CONTINUE) while it is executing other SELECT statements, two or more cursors are necessary.

Prolifics does not put any limit on the number of cursors an application may declare to an JDB engine. Because each cursor requires memory and JDB resources, however, it is recommended that applications close a cursor when it is no longer needed.

For more information on cursors, refer to Chapter -#- in the Application Development Guide.

Scrolling

Even though JDB does not have native support for non-sequential scrolling in a select set, Prolifics scrolling is available. Before using any of the following commands:

DBMS [ WITH CURSOR cursor-name ] CONTINUE_BOTTOM

DBMS [ WITH CURSOR cursor-name ] CONTINUE_TOP

DBMS [ WITH CURSOR cursor-name ] CONTINUE_UP

the application must set up a continuation file for the cursor. This is done with this command:

DBMS [ WITH CURSOR cursor-name ] STORE FILE [ filename ]

To turn off Prolifics scrolling and close the continuation file, use this command:

DBMS [ WITH CURSOR cursor-name ] STORE

or close the Prolifics cursor with DBMS CLOSE CURSOR.

For more information on scrolling, refer to Chapter -#- in the Application Development Guide.

Error and Status Information

Prolifics uses the global variables described in the following sections to supply error and status information in an application. Note that some global variables can not be used in the current release; however, these variables are reserved for use in other engines and for use in future releases of Prolifics for JDB.

Errors

Prolifics initializes the following global variables for error code information:

JDB returns error codes and messages when it aborts a command. It usually aborts a command because the application used an invalid option or because the user did not have the authority required for an operation. Prolifics writes JDB error codes to the global variable @dmengerrcode and writes JDB messages to @dmengerrmsg.

All JDB errors are Prolifics errors. Therefore, Prolifics always calls the default error handler or the installed error handler when an error occurs.

Using the Default Error Handler

Using an Installed Error Handler

DBMS ONERROR JPL errors
DBMS DECLARE dbi_session CONNECTION FOR ...

proc errors (stmt, engine, flag)
if @dmengerrcode == 0
	msg emsg "JAM error: " @dmretmsg
else
	msg emsg "JAM error: " @dmretmsg " %N" \
	":engine error is " @dmengerrcode " " @dmengerrmsg
return 1

For additional information about engine errors, refer to your JDB documentation. For more information about error processing in Prolifics, refer to Chapter -#- in the Application Development Guide and Chapter -#- in the Programming Guide.

Row Information

Prolifics initializes the following global variables for row information:

As explained on the manual page for @dmrowcount, the value of @dmrowcount after a SQL SELECT is the number of rows fetched to Prolifics variables. This number is less than or equal to the total number of rows in the select set. The value of @dmrowcount after a SQL INSERT, UPDATE, or DELETE is the total number of rows affected by the operation. Note that this variable is reset when another DBMS statement is executed, including DBMS COMMIT.

Using Transactions

A transaction is a unit of work that must be totally completed or not completed at all. JDB has one transaction for each connection. Therefore, in a Prolifics application, a transaction controls all statements executed with a single named connection or the default connection.

The following events commit a transaction on JDB:

• Executing DBMS COMMIT.
• Closing the connection.

The following events roll back a transaction on JDB:

• Executing DBMS ROLLBACK.

Transaction Control on a Single Connection

After an application declares a connection, a transaction automatically starts on that connection.

JDB supports the following transaction commands:

• Commit the transaction on a default or named connection.

	DBMS [ WITH CONNECTION connection ] COMMIT

• Rollback to the beginning of the transaction on a default or named connection.

	DBMS [ WITH CONNECTION connection ] ROLLBACK

Example

# Call the transaction handler and pass it the name 
# of the subroutine containing the transaction commands.

call tran_handle "new_title()"

proc tran_handle (subroutine)
{
# Declare a variable jpl_retcode and 
# set it to call the subroutine.
	vars jpl_retcode
	jpl_retcode = :subroutine

# Check the value of jpl_retcode. If it is 0, all statements 
# in the subroutine executed successfully and the transaction
# was committed. If it is 1, the error handler aborted the
# subroutine. If it is -1, Prolifics aborted the subroutine. 
# Execute a ROLLBACK for all non-zero return codes.

	if jpl_retcode == 0
	{
		msg emsg "Transaction succeeded."
	}
	else
	{
		msg emsg "Aborting transaction."
		DBMS ROLLBACK
	}
}

proc new_title
	DBMS SQL INSERT INTO titles VALUES \
		(:+title_id, :+name, :+genre_code, \
		:+dir_last_name, :+dir_first_name, :+film_minutes, \
		:+rating_code, :+release_date, :+pricecat)
	DBMS SQL INSERT INTO title_dscr VALUES \
		(:+title_id, :+line_no, :+dscr_text)
	DBMS SQL INSERT INTO tapes VALUES \
		(:+title_id, :+copy_num, :+status, :+times_rented)
DBMS COMMIT
return 0

The procedure tran_handle is a generic handler for the application's transactions. The procedure new_title contains the transaction statements. This method reduces the amount of error checking code.

The application executes the transaction by executing

call tran_handle "new_title()"

The procedure tran_handle receives the argument "new_title" and writes it to the variable subroutine. It declares a JPL variable, jpl_retcode. After performing colon processing, :subroutine is replaced with its value, new_title, and JPL calls the procedure. The procedure new_title begins the transaction, performs three inserts, and commits the transaction.

If new_title executes without any errors, it returns 0 to the variable jpl_retcode in the calling procedure tran_handle. JPL then evaluates the if statement, displays a success message, and exits.

If however an error occurs while executing new_title, Prolifics calls the application's error handler. The error handler should display any error messages and return the abort code, 1.

For example, assume the first INSERT in new_title executes successfully but the second INSERT fails. In this case, Prolifics calls the error handler to display an error message. When the error handler returns the abort code 1, Prolifics aborts the procedure new_title (therefore, the third INSERT is not attempted). Prolifics returns 1 to jpl_retcode in the calling procedure tran_handle. JPL evaluates the if statement, displays a message, and executes a rollback. The rollback undoes the insert to the table titles.

Transaction Manager Processing

Transaction Model for JDB

Each database driver contains a standard transaction model for use with the transaction manager. The transaction model is a C program which contains the main processing for each of the transaction manager commands. You can edit this program; however, be aware that the transaction model is subject to change with each release. For JDB, the name of the standard transaction model is tmjdb1.c.

Even though JDB does not enforce referential integrity, the transaction manager checks for duplicate primary key values each time data is inserted or updated. This is performed through processing found in the standard transaction model for JDB. If it finds any duplicate value in the primary key columns, the transaction manager gives an error.

SAVE Commands

If you specify a SAVE command with a table view parameter, it is called a partial command. A partial command is not applied to the entire transaction tree. In the standard transaction models, partial SAVE commands do not commit the database transaction. In order to save those changes, you must do an explicit DBMS COMMIT. Otherwise, those changes could be rolled back if the database engine performs an automatic rollback when the database connection is closed.

JDB-Specific Commands

Prolifics for JDB provides commands for JDB-specific features. This section contains a reference page for each command. If you are using multiple engines or are porting an application to or from another engine, please note that these commands may work differently or may not be supported on some engines.

Using Transactions

COMMIT
Commit a transaction

DBMS [ WITH CONNECTION connection-name ] COMMIT

WITH CONNECTION connection-name

Specify the connection for this command. If the command does not contain a WITH CONNECTION clause, Prolifics issues the commit on the default connection.

Description

After a transaction is terminated, the engine automatically begins a new transaction.

When an application closes its connections with CLOSE_ALL_CONNECTIONS or CLOSE CONNECTION, JDB commits any pending transactions on those connections. However, this procedure is not recommended. Instead, it is strongly recommended that applications use explicit COMMIT and ROLLBACK statements to terminate transactions.

This command is available depending on the setting of various parameters in your environment. Refer to the section on transactions and your documentation for more information.

Example

See Also

ROLLBACK

ROLLBACK
Roll back a transaction

DBMS [ WITH CONNECTION connection-name ] ROLLBACK

WITH CONNECTION connection-name

Specify the connection for this command. If the command does not contain a WITH CONNECTION clause, Prolifics issues the rollback on the default connection.

Description

If a statement in a transaction fails, an application must attempt to reissue the statement successfully or else roll back the transaction. If an application cannot complete a transaction, it should roll back the transaction. If it does not, it might inadvertently commit the partial transaction when it commits a later transaction.

Example

See Also

COMMIT

Command Directory for JDB

The following table lists all commands available in Panther's database driver for JDB. Commands available to all database drivers are described in the Programming Guide.

Table 3. Commands for JDB