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The ins and outs of journaling -- Part I

Ron Turull discusses what journaling is, how it works and the four basic journal categories.

Ron Turull

After all these years, journaling is still a seldom-used and rarely understood feature of OS/400-i5/OS and its...

integrated database, DB2 UDB for iSeries (formally DB2/400). Many sites simply don't know how to set it up properly to perform its chief function of providing another level of data protection. Here, and in the next few installments, we'll discuss what journaling is, how it works, and how to set it up to give you the greatest level of data protection.

What journaling is and how it works
Journaling, generally speaking, is a process of recording activity. In accounting, it is the process of recording financial activity -- monies in and monies out. On the iSeries, journaling refers to the process of recording object activity.

More Information
  • Capturing the record change

  • Remote journaling between systems

  • Journaling resources

Journaling on the iSeries typically involves the recording of the activity related to files, namely, physical files. When a file is being journaled, activity such as file-opens, file-closes and data updates are recorded. For example, when a program writes out a new record or updates an existing one, the system makes an entry in the associated journal. The entry contains such information as the job name and program name that made the change(s) to the file, as well as a copy of the record that was changed. (Note: Journaling can be configured to record a copy of the record before it is changed and/or to record a copy of the record after it is changed.)

How to set up journaling in three easy steps

  1. Create one or more journal receivers. Use the Create Journal Receiver (CRTJRNRCV) command to create a journal receiver. Think of the journa as a notebook binder and the journal receiver as the pages (i.e., the notebook paper where information is written). The journal receiver is where the journal entries are actually recorded. The journal "connects" the receiver to the file. It's a good habit to name the journal receiver the same as the journal, plus a numeric suffix such as 0 or 1. (See the next step on what to name the journal.) Also, you should put journal receivers in the same library as the file. (This is very important in order to simplify recovery procedures, which we will discuss in Part III.) For maximum protection, consider storing the journal receiver in a different ASP than the file so that their storage will not be on the same disk unit.
  2. Create a journal. Use the Create Journal (CRTJRN) command to create a journal and specify the receiver created in step 1. Although you can journal multiple files to the same journal (and, in some cases, that is actually preferable), you will generally want to have a journal "serving" a single file. A good practice is to name the journal the same as the file and put it in the same library as the file, but store it in the same ASP as the journal receiver.
  3. Start journaling the file. This is done by using the Start Journal Physical File (STRJRNPF) command. This is how you associate a file to a journal. Once the association is made, the system will record in the journal receiver a copy of any record added, updated or deleted from the file. Other activity, such as when the file is opened and closed, can also be recorded in the journal receiver if you choose by selecting the appropriate options on the STRJRNPF command.

For more information on the journal commands mentioned above and other journal-related commands, please see the subsequent section "Quick reference to journaling commands."

Four basic journal entry categories
The most common journal entries fall into four basic categories. Each category is represented by a one-character code (shown in parenthesis in the following list). Within each category are a number of different journal entry types. Each journal entry type is represented by a two-character entry code (also shown in parenthesis within the category descriptions below).

  1. Journal and journal receiver operations (J). These include such things as references to the previous receiver (PR) or the next receiver (NR) in a chain. Also, at IPL-time, an entry is made (e.g., an IN entry for IPL after normal end) marking a critical chronological boundary in the file activity.
  2. File operations (F). This category includes file opens (OP) and file closes (CL).
  3. Record operations (R). Record updates (UP), deletes (DL), and new records written (PT and PX) all fall into this category.
  4. Commitment control (C). Anything related to commitment control falls into this category. Some examples are begin commitment control (BC), start a commit cycle (SC), commit operation (CM) and rollback operation (RB).

There are other categories and entry types in addition to those listed above. However, those are what you will come across most often. For a complete listing, see the documentation or online help.

Quick reference to journaling commands

APYJRNCHG. Uses the journal entries to apply changes that have occurred since a database file was saved or some other specified time.

CHGJRN. Use this command to change the attributes of a journal or to attach new journal receivers to a journal.

CMPJRNIMG. This command compares and lists the difference between the before-image and after-image of a record, or between the current after-image of a record and the previous after-image of the record.

CRTJRN. Use this command to create a journal.

CRTJRNRCV. Use this command to create a journal receiver.

DLTJRN. Use this command to delete a journal.

DLTJRNRCV. Use this command to delete a journal receiver.

DSPJRN. This command displays or prints the journal entries that are in the journal receivers associated with the specified journal. This command has outfile support so you can list the journal entries to a database output file for further processing or analysis.

DSPJRNRCVA. Use this command to display the attributes of a journal receiver.

ENDJRNPF. This command ends journaling for the specified physical file.

RCVJRNE. This command allows a specified user program to continuously receive journal entries one at a time as they are written to the journal. The behavior is similar to an exit program.

RMVJRNCHG. Use this command to remove changes that have occurred to a database file from a specified point in time to some previous point in time (allowed only if before-images were recorded during the time).

RTVJRNE. Use this command to retrieve a journal entry and place it in CL program variables.

SNDJRNE. Use this command to write user-defined entries to a journal (i.e., journal receiver).

STRJRNPF. Use this command to start journaling for the physical file.

WRKJRN. This command displays a menu from which you can perform many journal-related functions, such as system-assisted recovery of journaled files.

WRKJRNA. This command displays the attributes of a journal and the associated receivers.

About the author: Ron Turull is editor of Inside Version 5. He has more than 20 years' experience programming for and managing AS/400-iSeries systems.

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