LUW

[kesavadas_thekkillath]

Can anybody briefly explain what is an LUW

[Former Member]

Hi

The R/3 system is multi user system and many users access the same information at the same time, which is mainly DATA. Consider the case where one user is modifying a record, and second user is trying to delete the same record. If the second user is successful in deleting the record then the first user will face problem for modifying the record that is already deleted. The avoid such situation, R/3 system has provided Logical Unit of Work, which is defined as a locking mechanism to protect transaction integrity. Of course, there are other measures, which ensures data integrity like check table i.e. foreign key relationship. Within SAP system there are three types of transaction and may be distinguished as:

• Database transaction known as LUW. It can be defined as a period in which operation requested must be performed as a unit, i.e. all or nothing operation. At the end of LUW, either of the database changes are committed or rolled back.

• Update transaction or SAP LUW. One SAP LUW can have several databases LUW. So a set of a database is either committed or rolled back. The special ABAP/4 command COMMIT WORK, marks the end of a SAP LUW.

• ABAP/4 transaction. Is made up of a set of related task combined under one transaction code. ABAP/4 transactions are for programming environment, in which ABAP/4 transaction functions like one complete object containing screens, menus and transaction codes.

R/3 system has provided in built locking mechanism, which defines the Logical Unit of Work. Also user can set his own locking mechanism. The LUW starts when a lock entry in the system table is created, and it ends when the lock is released.

To provide the user the facility to communicate with the table in order to modify or delete or insert data, R/3 has provided tool called SCREEN PAINTER. This tool allows you to design screen, process screen through program and update the database table. SAP has provided one and only one way to update the database table, i.e. transaction. Though you can update database table by using open SQL statement through program, SAP usually doesn’t recommend this kind of updating. Many standard transactions are available to update standard table but if the need arises, the developer should be able to develop new transaction, which allows the updating of database tables. This can be achieved by using various components of screen painter.

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Regards

Anji

[Former Member]

Hi,

Logical Unit of Work (LUW in DATABASE) please check out the link below it might help you

http://help.sap.com/saphelp_nw2004s/helpdata/en/41/7af4bca79e11d1950f0000e82de14a/content.htm

SAP LUW please check out the link below it might help you

http://help.sap.com/saphelp_nw04/helpdata/en/41/7af4bfa79e11d1950f0000e82de14a/content.htm

R/3 LUW please check out the link below it might help you

http://help.sap.com/saphelp_nw04/helpdata/en/41/7af4c5a79e11d1950f0000e82de14a/content.htm

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[Former Member]

SAP LUW (logical unit of work )

http://help.sap.com/saphelp_nw04/helpdata/en/41/7af4bfa79e11d1950f0000e82de14a/content.htm

[Former Member]

Transactions and Logical Units of Work Locate the document in its SAP Library structure

In everyday language, a transaction is a sequence of actions that logically belong together in a business sense and that either procure or process data. It covers a self-contained procedure, for example, generating a list of customers, creating a flight booking, or sending reminders to customers. From the user’s viewpoint, it forms a logical unit.

The completeness and correctness of data must be assured within this unit. In the middle of a transaction, the data will usually be inconsistent. For example, when you transfer an amount in financial accounting, this must first be deducted from one account before being credited to another. In between the two postings, the data is inconsistent, since the amount that you are posting does not exist in either account. It is essential for application programmers to know that their data is consistent at the end of the transaction. If an error occurs, it must be possible to undo the changes made within a logical process.

In the R/3 System, there are three terms frequently used in this context:

Database Logical Unit of Work (LUW)

A database LUW is the mechanism used by the database to ensure that its data is always consistent.

SAP LUW

An SAP LUW is a logical unit consisting of dialog steps, whose changes are written to the database in a single database LUW.

SAP Transaction

An SAP transaction is an application program that you start using a transaction code. It may contain one or more SAP LUW.

Database Logical Unit of Work (LUW) Locate the document in its SAP Library structure

From the point of view of database programming, a database LUW is an inseparable sequence of database operations that ends with a database commit. The database LUW is either fully executed by the database system or not at all. Once a database LUW has been successfully executed, the database will be in a consistent state. If an error occurs within a database LUW, all of the database changes since the beginning of the database LUW are reversed. This leaves the database in the state it was in before the transaction started.

This graphic is explained in the accompanying text

The database changes that occur within a database LUW are not actually written to the database until after the database commit. Until this happens, you can use a database rollback to reverse the changes. In the SAP System, database commits and rollbacks can be triggered either implicitly or using explicit commands.

Implicit Database Commits

A work process can only execute a single database LUW. The consequence of this is that a work process must always end a database LUW when it finishes its work for a user or an external call. Work processes trigger an implicit database commit in the following situations:

· When a dialog step is completed

Control changes from the work process back to the SAP GUI.

· When a function module is called in another work process (RFC).

Control passes to the other work process.

· When the called function module (RFC) in the other work process ends.

Control returns to the calling work process.

· When a WAIT statement interrupts the work process.

Control passes to another work process.

· Error dialogs (information, warning, or error messages) in dialog steps.

Control passes from the work process to the SAP GUI.

Explicit Database Commits

There are two ways to trigger an explicit database commit in your application programs:

· Call the function module DB_COMMIT

The sole task of this function module is to start a database commit.

· Use the ABAP statement COMMIT WORK

This statement starts a database commit, but also performs other tasks (refer to the keyword documentation for COMMIT WORK).

Implicit Database Rollbacks

The following cases lead to an implicit database rollback:

· Runtime error in an application program

This occurs whenever an application program has to terminate because of an unforeseen situation (for example, trying to divide by zero).

· Termination message

Termination messages are generated using the ABAP statement MESSAGE with the message type A or X. In certain cases (updates), they are also generated with message types I, W, and E. These messages end the current application program.

Explicit Database Rollbacks

You can trigger a database rollback explicitly using the ABAP statement ROLLBACK WORK. This statement starts a database rollback, but also performs other tasks (refer to the keyword documentation for ROLLBACK WORK).

From the above, we can draw up the following list of points at which database LUWs begin and end.

A Database LUW Begins

· Each time a dialog step starts (when the dialog step is sent to the work process).

· Whenever the previous database LUW ends in a database commit.

· Whenever the previous database LUW ends in a database rollback.

A Database LUW Ends

· Each time a database commit occurs. This writes all of the changes to the database.

· Each time a database rollback occurs. This reverses all of the changes made during the LUW.

Database LUWs and Database Locks

As well as the database changes made within it, a database LUW also consists of database locks. The database system uses locks to ensure that two or more users cannot change the same data simultaneously, since this could lead to inconsistent data being written to the database. A database lock can only be active for the duration of a database LUW. They are automatically released when the database LUW ends. In order to program SAP LUWs, we need a lock mechanism within the R/3 System that allows us to create locks with a longer lifetime (refer to The SAP Locking Concept).

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