Link related to Datebase table - useful

http://www.sapabapiq.com/2012/04/data-dictionarybuffering-indexes-in-sap.html

Table buffering in SAP - important

Pls refer the belo link:
http://www.sap-perf.ca/sap-table-buffering/

 Which Tables should be Buffered? 
Only transparent tables and pooled tables can be buffered. Cluster tables cannot be buffered.
Character data types must be assigned to all key fields of buffered tables, that is the data type of the field must be mapped to one of the ABAP types C, N, D or T (see
Mapping to ABAP Data Types).The following two points speak against table buffering:

• The data read by the application must always be up-to-date. Delays caused by the synchronization mechanism (see

Synchronization of Local Buffers) cannot be accepted.

The table data is frequently modified. In this case the cost of synchronization could be greater than the gain in performance resulting from buffering. Roughly speaking, it does not make sense to buffer a table if more than one percent of the accesses to the table are modifying accesses.

The table containing currency exchange rates is updated only once a day, but it is read frequently. Buffering is recommended in this case.Typical candidates for buffering include customizing and system tables. In certain cases master data with customizing character can also be buffered.
The contents of buffered tables are not always up-to-date in a distributed system. You can bypass the buffer and read the data directly from the database table with the ABAP command "SELECT SINGLE ... BYPASSING BUFFER". If a buffered table is accessed frequently with this command, you should consider whether it is really necessary for the table to be buffered or whether it is essential to have the current state of the database.
You must define whether and how a table is buffered in its technical settings (see
Technical Settings).



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http://help.sap.com/saphelp_nw04/helpdata/en/cf/21f244446011d189700000e8322d00/content.htm


 Buffering Database Tables 
Buffering a table improves the performance when accessing the data records contained in the table.
The table buffers reside locally on each application server in the system. The data of buffered tables can thus be accessed directly from the buffer of the application server. This avoids the time-consuming process of accessing the database.

Buffering is particularly important in client/server environments, as it takes considerably longer to access a table with the network than it does to access a table that is buffered locally. Depending on the network load, this factor can lie between 10 and 100.
The difference in performance is somewhat less marked in central systems (systems with only one application server) than in local ones (systems with several application servers). However, even in central systems, a reduction in process changes and increased sophistication of the buffering over that provided by the database system have a noticeable effect on performance.How are the Buffers Filled?If a program accesses the data of a buffered table, the database interface determines whether this data is in the buffer of the application server. If this is the case, the data is read directly from the buffer. If the data is not in the buffer of the application server, it is read from the database and loaded into the buffer. The next access to this data can then use the buffer.
The buffering type determines which records are loaded into the buffer during an access.
How are the Local Buffers Synchronized?A buffered table is generally read on all application servers and held in the buffer there. If a program changes the data contained in the table on an application server, this is noted in the log table by the database interface. The buffers still have the old status on all the other application servers, so that the programs might read obsolete data.
A synchronization mechanism runs at a fixed time interval, usually every 1-2 minutes. The log table is read and the buffer contents that were changed by other servers are invalidated. In the next access, the data of invalidated tables is read directly from the database and updated in the buffer.DisplacementIf more space is required in the buffer due to new data, the data that has not been accessed for the longest time is displaced. The data is displaced asynchronously at certain times that are defined dynamically by the buffer accesses. The data is only displaced if at this time the free space in the buffer is less than a given value or if the access quality is not good enough.Resetting the Table BuffersYou can reset the table buffers on the corresponding application servers by entering $TAB in the command field. All the data in the buffer is invalidated.
Only use this command if inconsistencies occurred in the buffer! It can take several hours to fill the buffers in large systems. Performance is considerably reduced during this time.See also:Local Buffer SynchronizationWhich Tables Should be Buffered?
How are Table Buffers Implemented Technically?
Which Accesses Proceed Directly to the Database?How can you Analyze the Buffer Quality?

Work Processes in SAP

http://help.sap.com/saphelp_wp/helpdata/en/fc/eb2e7d358411d1829f0000e829fbfe/content.htm


Work Processes 
Work processes execute the individual dialog steps in R/3 applications. The next two sections describe firstly the structure of a work process, and secondly the different types of work process in the R/3 System.Structure of a Work ProcessWork processes execute the dialog steps of application programs. They are components of an application server. The following diagram shows the components of a work process:

Each work process contains two software processors and a database interface.Screen ProcessorIn R/3 application programming, there is a difference between user interaction and processing logic. From a programming point of view, user interaction is controlled by screens. As well as the actual input mask, a screen also consists of flow logic. The screen flow logic controls a large part of the user interaction. The R/3 Basis system contains a special language for programming screen flow logic. The screen processor executes the screen flow logic. Via the dispatcher, it takes over the responsibility for communication between the work process and the SAPgui, calls modules in the flow logic, and ensures that the field contents are transferred from the screen to the flow logic.ABAP-ProzessorThe actual processing logic of an application program is written in ABAP - SAP’s own programing language. The ABAP processor executes the processing logic of the application program, and communicates with the database interface. The screen processor tells the ABAP processor which module of the screen flow logic should be processed next. The following screen illustrates the interaction between the screen and the ABAP processors when an application program is running.
Database InterfaceThe database interface provides the following services:

• Establishing and terminating connections between the work process and the database.
• Access to database tables
• Access to R/3 Repository objects (ABAP programs, screens and so on)
• Access to catalog information (ABAP Dictionary)
• Controlling transactions (commit and rollback handling)
• Table buffer administration on the application server.

The following diagram shows the individual components of the database interface:

The diagram shows that there are two different ways of accessing databases: Open SQL and Native SQL.
Open SQL statements are a subset of Standard SQL that is fully integrated in ABAP. They allow you to access data irrespective of the database system that the R/3 installation is using. Open SQL consists of the Data Manipulation Language (DML) part of Standard SQL; in other words, it allows you to read (SELECT) and change (INSERT, UPDATE, DELETE) data. The tasks of the Data Definition Language (DDL) and Data Control Language (DCL) parts of Standard SQL are performed in the R/3 System by the ABAP Dictionary and the authorization system. These provide a unified range of functions, irrespective of database, and also contain functions beyond those offered by the various database systems.
Open SQL also goes beyond Standard SQL to provide statements that, in conjunction with other ABAP constructions, can simplify or speed up database access. It also allows you to buffer certain tables on the application server, saving excessive database access. In this case, the database interface is responsible for comparing the buffer with the database. Buffers are partly stored in the working memory of the current work process, and partly in the shared memory for all work processes on an application server. Where an R/3 System is distributed across more than one application server, the data in the various buffers is synchronized at set intervals by the buffer management. When buffering the database, you must remember that data in the buffer is not always up to date. For this reason, you should only use the buffer for data which does not often change.
Native SQL is only loosely integrated into ABAP, and allows access to all of the functions contained in the programming interface of the respective database system. Unlike Open SQL statements, Native SQL statements are not checked and converted, but instead are sent directly to the database system. Programs that use Native SQL are specific to the database system for which they were written. R/3 applications contain as little Native SQL as possible. In fact, it is only used in a few Basis components (for example, to create or change table definitions in the ABAP Dictionary).
The database-dependent layer in the diagram serves to hide the differences between database systems from the rest of the database interface. You choose the appropriate layer when you install the Basis system. Thanks to the standardization of SQL, the differences in the syntax of statements are very slight. However, the semantics and behavior of the statements have not been fully standardized, and the differences in these areas can be greater. When you use Native SQL, the function of the database-dependent layer is minimal.Types of Work ProcessAlthough all work processes contain the components described above, they can still be divided into different types. The type of a work process determines the kind of task for which it is responsible in the application server. It does not specify a particular set of technical attributes. The individual tasks are distributed to the work processes by the dispatcher.
Before you start your R/3 System, you determine how many work processes it will have, and what their types will be. The dispatcher starts the work processes and only assigns them tasks that correspond to their type. This means that you can distribute work process types to optimize the use of the resources on your application servers.
The following diagram shows again the structure of an application server, but this time, includes the various possible work process types:



The various work processes are described briefly below. Other parts of this documentation describe the individual components of the application server and the R/3 System in more detail. Dialog Work ProcessDialog work processes deal with requests from an active user to execute dialog steps.Update Work ProcessUpdate work processes execute database update requests. Update requests are part of an SAP LUW that bundle the database operations resulting from the dialog in a database LUW for processing in the background.Background Work ProcessBackground work processes process programs that can be executed without user interaction (background jobs).Enqueue Work ProcessThe enqueue work process administers a lock table in the shared memory area. The lock table contains the logical database locks for the R/3 System and is an important part of the SAP LUW concept. In an R/3 System, you may only have one lock table. You may therefore also only have one application server with enqueue work processes.Spool Work ProcessThe spool work process passes sequential datasets to a printer or to optical archiving. Each application server may contain only one spool work process.
The services offered by an application server are determined by the types of its work processes. One application server may, of course, have more than one function. For example, it may be both a dialog server and the enqueue server, if it has several dialog work processes and an enqueue work process.
You can use the system administration functions to switch a work process between dialog and background modes while the system is still running. This allows you, for example, to switch an R/3 System between day and night operation, where you have more dialog than background work processes during the day, and the other way around during the night.

Lock concept in sap

The R/3 Lock Concept 
Das R/3 SperrkonzeptReasons for Setting LocksSuppose a travel agent want to book a flight. The customer wants to fly to a particular city with a certain airline on a certain day. The booking must only be possible if there are still free places on the flight. To avoid the possibility of overbooking, the database entry corresponding to the flight must be locked against access from other transactions. This ensures that one user can find out the number of free places, make the booking, and change the number of free places without the data being changed in the meantime by another transaction. Lock Mechanisms in the Database SystemThe database system automatically sets database locks when it receives change statements (INSERT, UPDATE, MODIFY, DELETE) from a program. Database locks are physical locks on the database entries affected by these statements. You can only set a lock for an existing database entry, since the lock mechanism uses a lock flag in the entry. These flags are automatically deleted in each database commit. This means that database locks can never be set for longer than a single database LUW; in other words, a single dialog step in an R/3 application program.
Physical locks in the database system are therefore insufficient for the requirements of an R/3 transaction. Locks in the R/3 System must remain set for the duration of a whole SAP LUW, that is, over several dialog steps. They must also be capable of being handled by different work processes and even different application servers. Consequently, each lock must apply on all servers in that R/3 System. SAP LocksTo complement the SAP LUW concept, in which bundled database changes are made in a single database LUW, the R/3 System also contains a lock mechanism, fully independent of database locks, that allows you to set a lock that spans several dialog steps. These locks are known as SAP locks.
The SAP lock concept is based on lock objects. Lock objects allow you to set an SAP lock for an entire application object. An application object consists of one or more entries in a database table, or entries from more than one database table that are linked using foreign key relationships.
Before you can set an SAP lock in an ABAP program, you must first create a lock object in the ABAP Dictionary. A lock object definition contains the database tables and their key fields on the basis of which you want to set a lock. When you create a lock object, the system automatically generates two function modules with the names ENQUEUE_<lock object name> and DEQUEUE_<lock object name> . You can then set and release SAP locks in your ABAP program by calling these function modules in a CALL FUNCTION statement.
See also: Example Transaction: SAP Locking.These function modules are executed in a special enqueue work process. Within an R/3 System, enqueue work processes run on a single application server. This server maintains a central lock table for the entire R/3 System in its shared memory.

The enqueue function module sets an SAP lock by writing entries in the central lock table. If the lock cannot be set because the application object (or a part of it) is already locked, this is reflected in the return code sy-subrc. The following diagram shows the components of the R/3 System that are involved in setting a lock.
Unlike the database, which sets physical locks, the SAP lock mechanism sets logical locks. This means that

• A locked database entry is not physically locked in the database table.

The lock entry is merely entered as a lock argument in the central R/3 lock table. The lock argument is made up of the primary key field values for the tables in the lock object. These are import parameters of the enqueue function module. The lock is independent of database LUWs. It is released either implicitly when the database update or the SAP transaction ends, or explicitly, using the corresponding dequeue function module. You can use a special parameter in the update function module to set the exact point at which the lock is released during the database update.

• A locked entry does not necessarily have to exist in a database table.

You can, for example, set a lock as a precaution for a database entry that is not written to the database until the update at the end of the SAP LUW.

• The effectiveness of the locks depends on cooperative application programming.

Since there are no physical locks in the database tables themselves, all programs that use the same application objects must look in the central table themselves for any locks. There is no mechanism that automatically prevents a program from ignoring the locks in the lock table.Lock TypesThere are two types of lock in the R/3 System:

• Shared lock

Shared locks (or read locks) allow you to prevent data from being changed while you are reading it. They prevent other programs from setting an exclusive lock (write lock) to change the object. It does not, however, prevent other programs from setting further read locks.

• Exclusive lock

Exclusive locks (or write locks) allow you to prevent data from being changed while you are changing it yourself. An exclusive lock, as its name suggests, locks an application object for exclusive use by the program that sets it. No other program can then set either a shared lock or an exclusive lock for the same application object.Lock DurationWhen you set a lock, you should bear in mind that if it remains set for a long time, the availability of the object to other transactions is reduced. Whether or not this is acceptable depends on the nature of the task your program is performing.
Remember in particular that setting too many shared locks without good reason can have a considerable effect on programs that work with database tables. If several programs running concurrently all set a shared lock for the same application object in the system, it can make it almost impossible to set an exclusive lock, since the program that needs to set that lock will be unable to find any time when there are no locks at all set for that object. Conversely, a single exclusive lock prevents all other programs from reading the locked object.
At the end of an SAP LUW, you should release all locks. This either happens automatically during the database update, or explicitly, when you call the corresponding dequeue function module. Locks that are not linked to a database update are released at the end of the SAP transaction.

Update Function module

It is basically used to bundle distributed updates within different programs spots, to one place (in FM).

Such FM would store all the UPDATE/INSERT/DELETE statements which otherwise you would write in some program place. Now when system reaches CALL FUNCTION 'XXX' IN UDPDATE TASK it doesn't go inside. Instead in registeres this XXX FM in VBLOG table (you can see update tasks in SM13) to be executed later. Now when in program it reaches COMMIT WORK statement, it looks into that table and calls each registered functions.
The aim is to either COMMIT all the changes at once, or ROLLBACK them all. This means that if inside one of any FM these statements are encountered system writes changes to DB permanently. Next it clears VBLOG table (so no FM are registered for change anymore) and continues the program.


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Creating Update Function Modules 
To create a function module, you first need to start the Function Builder. Choose Tools ® ABAP Workbench, Function Builder. For more information about creating function modules, refer to the ABAP Workbench Tools documentation.To be able to call a function module in an update work process, you must flag it in the Function Builder. When you create the function module, set the Process Type attribute to one of the following values:

• Update with immediate start

Set this option for high priority ("V1") functions that run in a shared (SAP LUW). These functions can be restarted by the update task in case of errors.

• Update w. imm. start, no restart

Set this option for high priority ("V1") functions that run in a shared (SAP LUW). These functions may not be restarted by the update task.

• Update with delayed start

Set this option for low priority ("V2") functions that run in their own update transactions. These functions can be restarted by the update task in case of errors.
To display the attributes screen in the Function Builder, choose Goto → Administration.
Defining the InterfaceFunction modules that run in the update task have a limited interface:

• Result parameters or exceptions are not allowed since update-task function modules cannot report on their results.

• You must specify input parameters and tables with reference fields or reference structures defined in the ABAP Dictionary.

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CALL FUNCTION update_function IN UPDATE TASK
                             [EXPORTING p1 = a1 p2 = a2 ...]
                             [TABLES t1 = itab1 t2 = itab2 ...].

This statement registers the update function module specified in update_function. update_function must be a character-type field, which during execution of the statement contains the name of an update function module in uppercase letters.

An update function module is a function module, for which in the Function Builder the property update module is marked. The registration of an update function module is an essential part of the update task.

The function module is not executed immediately, but is scheduled for execution in a special work process (update work process). For this purpose, the name of the function module including the passed actual parameters is stored as a log record in the database table VBLOG. If the statement is executed during the update task, the addition IN UPDATE TASK is ignored.

If the statement
SET UPDATE TASK LOCAL is executed before registration of an update function module in the current SAP LUW, registration takes place in the ABAP memory rather than on the database, and for the current work process.

The actual execution is triggered by the statement COMMIT WORK. The formal parameters of the function module receive the values of the actual parameters from table VBLOG. A function module that is registered several times will also be excuted several times with the respective parameter values.
If a COMMIT WORK statement is not executed after registration of a function module during execution of the current program, the function module is not executed and is deleted from table VBLOG at the end of the program.

Addition 1

... EXPORTING p1 = a1 p2 = a2 ...

Addition 2

... TABLES t1 = itab1 t2 = itab2 ...

Effect

The additions EXPORTING and TABLES have the same syntax and meaning as in the parameter_list of the general function module call, except that for EXPORTING, no Referenzvariablen or data objects that contain reference variables can be output as actual parameters.

Notes

• The additions IMPORTING, CHANGING and EXCEPTIONS of the general function module call may be specified, but they are ignored during the execution. The additions for a dynamic parameter transfer are not allowed.
  
  
• While an update function module is processed in an update work process, you are not allowed to execute the statements SUBMIT, CALL DIALOG, CALL SCREEN, CALL TRANSACTION, COMMIT WORK, ROLLBACK WORK and all other statements that provoke a database commit.
  
  
• If during the update an error occurs, the update work process executes a database rollback, returns the log record with a note into table VBLOG and informs the user whose program has created the log record by SAPMail. After removing the error cause, the returned log records can be updated again.

LUW and SAP LUW

Transactions and Logical Units of Work 

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.

There are three relevant concepts 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 ABAP application program that you start using a transaction code. It may contain one or more SAP LUWs.

The following sections of this documentation explain these three terms in more detail.

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Database Logical Unit of Work (LUW) 

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.

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 R/3 System, database commits and rollbacks can be triggered either implicitly or using explicit commands.

Implicit Database Commits in the R/3 System

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 in the R/3 System

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 in the R/3 System

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 in the R/3 System

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 R/3 Locking Concept).

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Logical Unit of Work (LUW )
A Logical Unit of Work (LUW or database transaction) is an inseparable sequence of database operations which must be executed either in its entirely or not at all. For the database system, it thus constitutes a unit.

LUWs help to guarantee database integrity. When an LUW has been successfully concluded, the database is once again in a correct state. If, however, an error occurs within an LUW, all database changes made since the beginning of the LUW are canceled and the database is then in the same state as before the LUW started.

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 LUWs.