Day-24.

ADVANCED DB2:

Overview:

This module enables the student to understand the concept of Advanced DB2.

 Outline:

Topics covered,

a.     Data Base Constraints

b.     Journaling 

c.     Commitment Control

d.     Triggers

 Objective:

By the end of this module, the student should be able to:

1. Understanding concept of Data Base Constraints
2. Understanding concept of Journaling 
3. Understanding concept of Commitment Control
4. Understanding concept of Triggers

 Commitment Control

To use commitment control, you do the following:

·   Use the CL commands CRTJRNRCV (Create Journal Receiver) CRTJRN (Create Journal) and STRJRNPF (Journal Physical File) to prepare for using commitment control, and the CL commands STRCCMTCTL (Start Commitment Control) and ENDCMTCTL (End Commitment Control to notify the system when you want to start and end commitment control. See the CL Reference for information on these commands.

·        Specify commitment control on the file-description specifications of the files you want under commitment control.

·        Use the COMIT (Commit) Operation code to apply a group of changes top files under commitment control or use the ROLBK (Roll Back) operation code to eliminate the pending group of changes top files under commitment control.

Specifying Files for Commitment Control

On the file-continuation specifications enter a K in position 53 and the word COMIT in positions 54 through 59. On the file description specifications described the file as having device DISK in positions 40 through 46.

When a program specifies commitment control for a file the specification applies only to the input and output operations made by this program for this file commitment control does not have apply to operations other than input and output operations. It does not apply to files that do not have commitment control specified in the program doing the input output operation.

Commitment Control Operations

The COMIT (Commit) operation tells the system that you have completed a group of changes to the files under commitment control. The ROLBK (Roll Back) operation eliminates the current group of changes to the files under commitment control.

If the system fails it implicitly issues ROLBK operation, You can check the identity of last successfully completed group of changes using the label you specify in factors of the COMIT operation code, and the notify object you specify on the STRCMTCTR command.

At the end of routing step, or when you issue the ENDCMTTCTL command the OS/400 system issues an implicit ROLBK. which eliminates any changes since the last ROLBK or COMIT operation that you issued. To ensure that all your file operations have effect issue a COMIT operation before ending a routing step operation under commitment control.

Commitment Control in the Program Cycle

Commitment Control is intended for full procedural files, where the input and output is under your control. Do not use commitment control with primary and secondary files, where input and output is under the control of the RPG/400 program cycle. The following are some of the reasons for this recommendation.

• You cannot issue a COMIT operation for the last total output in your program.
• It is difficult to program within the cycle for recovery from a locked record condition.
• Level indicators are not reset by the ROLBK operation.
• After A ROLBK operations processing matching records may produce a sequence error.

Example of Using-Commitment Control

The following is an example of the specifications and CL command for a program operating under commitment control. To prepare for using commitment control you issue the following.

CL commands

   CRTJRNRCV JRNRCV (RECEIVER)

The above command creates a journal receiver named RECEIVER

   CRTJRN JRN (JOURNAL) JRNRCV (RECEIVER)

The above command creates a journal named JOURNAL and attaches the journal receiver named RECEIVER.

STRJRNPF FILE (MASTER) JRN (JOURNAL)

The above command directs journal entries for the files MASTER to the JOURNAL. In your program you specify COMIT for the file MASTER.

FMASTER         UF        E          K                                  DISK                KCOMIT

In the calculation specifications, use the COMIT operation to complete a group of operations.

KEY                              CHAINMASTER 50

N50                              UPDATRECORD                       99

          N99                                                                              COMIT

If an operation within a group fails, use the ROLER operation to eliminate the entire group o foperations.

99                                                   ROLBK

Example of Using Commitment Control

To operate your program (named REVISE) under commitment control you issue the commands:

  

   STRCMTCTLLCKLVL(*ALL)

The above command starts commitment control with the higher level of locking

CALL REVISE

      The above command calls your program (named REVISE).

ENDCMTCTL

The above command ends commitment control and causes implicit Roll Back operation.