Journal Buffers and Journal Files

Defining, Generating, and Punching a DMCL › Journal Buffers and Journal Files

Journal Buffers and Journal Files

Logs Database Activity

Journaling logs database activity on journal files. The following table describes the type of information CA IDMS/DB writes to a journal:

Type of information	Description
Database images	Contain before and after images of modified records and rows
Checkpoints	Describe a transaction event such as a COMMIT or ROLLBACK

Note: For more information about the journal records, see Chapter 19, "Journaling Procedures".

How Do You Use Journal Files?

You use the journal files to recover the database following a system or transaction failure. Typically, journaling occurs for applications that execute under the central version because CA IDMS/DB uses the journals for automatic rollback and warmstart. Journaling is less common for applications that execute in local mode, but may be used for applications that update a large database.

Note: For more information about journaling procedures under the central version and in local mode, see Chapter 19, "Journaling Procedures". Backup and recovery are discussed fully in 21.2, "Backup Procedures".

Journaling Entities

To log information about database activity, CA IDMS/DB requires the following journal entities in a DMCL:

A journal buffer, which allocates space in memory to hold journal pages containing information about database activity. Each DMCL contains only one journal buffer.
Journal files, to which CA IDMS/DB writes the journal pages.

When CA IDMS/DB Writes a Journal Page

CA IDMS/DB writes a journal page to the active journal file when one of the following conditions exist:

The page in the journal buffer is full
An update transaction terminates. A transaction terminates when the application program issues a COMMIT, COMMIT WORK, ROLLBACK, ROLLBACK WORK or FINISH statement or similar task-level statement, or when the application program aborts.
The journal page contains before images for records or rows on a database page which must be written to the database.

Types of Journal Files

CA IDMS/DB supports the following types of journal files:

Type	Medium
Disk journal file	Disk
Archive journal file	Sequential tape or disk file (1)
Tape journal file	Sequential tape or disk file (1)

Note: (1) To be used for manual recovery, journal files on disk must be copied to tape.

Files You Choose Depend on the Runtime Environment

The type of journal files you define to a DMCL depends on whether the DMCL will be used under the central version or to journal updates made by a local mode application. A typical journaling configuration appears below:

Type of configuration	Description
Under the central version	Define: 2 or more disk journals 1 or more archive journals
In local mode	Define 1 tape journal

Type of configuration

Description

Under the central version

Define:

2 or more disk journals
1 or more archive journals

In local mode

Define 1 tape journal

A DMCL defined with disk and archive journals can be used in local mode provided journaling is not necessary. Only a DMCL defined with a tape journal file can be used to journal in local mode.

Multiple Archive Files

You can define more than one archive journal. When CA IDMS/DB offloads a disk journal, it writes journal images to each archive file, thereby reducing the risk of unreadable archive journal files.