How to implement locking in ABAP

[Former Member]

Hello everyone,

I am doing dialog programming and I have a screen that creates some template. When the user opens up the screen it defualts to next available template number to be created (for example if template No 1 exists then it will default to template no 2 so template 2 will be created). But if multiple users open up the screen then it will show template 2 for both of them and I want to avoid this, i.e. I like to implement some locking mechanism so one user opens up the screen that template is locked and the other cannot create the same template. Please share any ideas and suggestions, how I can imeplement this!

Thanks.

Mithun

[Former Member]

Hello everyone,

I am doing dialog programming and I have a screen that creates some template. When the user opens up the screen it defualts to next available template number to be created (for example if template No 1 exists then it will default to template no 2 so template 2 will be created). But if multiple users open up the screen then it will show template 2 for both of them and I want to avoid this, i.e. I like to implement some locking mechanism so one user opens up the screen that template is locked and the other cannot create the same template. Please share any ideas and suggestions, how I can imeplement this!

Thanks.

Mithun

[Former Member]

In the data dictionary, you can create a lock object.

But you also have to decide what you want to do if the second user encounters a lock. Do you give an error; do you go on to the next template?

Rob

[Former Member]

Thanks Rob. Could you point me to some example or documentation on "create lock object". I don't have to give an error. I have to go on to next template and just give them warning.

Thanks.

Mithun

[Former Member]

Please see the SAP help:

[Lock Objects|http://help.sap.com/saphelp_47x200/helpdata/EN/a2/3547360f2ea61fe10000009b38f839/frameset.htm]

Rob

[RaymondGiuseppi]

Look at this documentation [The SAP Lock Concept (BC-CST-EQ)|http://help.sap.com/printdocu/core/Print46c/en/data/pdf/BCCSTEQ/BCCSTEQ_PT.pdf]

Regards

[Former Member]

You can achieve this by the following methods

1. Lock objects.

[http://help.sap.com/saphelp_nw70/helpdata/en/cf/21eea5446011d189700000e8322d00/content.htm|http://help.sap.com/saphelp_nw70/helpdata/en/cf/21eea5446011d189700000e8322d00/content.htm]

2. Using FM ENQUEUE_ESNDX

You can also do one thing.... call FM ENQUEUE_ESINDX using RELID as 'ZZ' and pass the

template number generated say T2. When the first user calls this FM, he creates a lock on the

template number T2. When the second user calls the FM using T2, the FM returns SY-SUBRC

as not equal to 0. In your program you can use your own logic to generate the next available

template number and create a lock on the same. Remember to call the DEQUEUE_ESINDX

when the lock can be removed.

3. Try using number ranges for the template...and create the same in NRIV table.

Hope this helps. Reward points for all helpful answers.

Thanks,

Balaji

[Former Member]

Hai Mithun Dha

Lock Objects

The R/3 System synchronizes simultaneous access of several users to the same data records with a lock mechanism. When interactive transactions are programmed, locks are set and released by calling function modules (see Function Modules for Lock Requests). These function modules are automatically generated from the definition of lock objects in the ABAP Dictionary.

Structure of a Lock Object

The tables in which data records should be locked with a lock request are defined in a lock object together with their key fields. When tables are selected, one table (the primary table) is first selected. Further tables (secondary tables) can also be added using foreign key relationships (see also Conditions for Foreign Keys).

Lock Arguments

The lock argument of a table in the lock object consists of the key fields of the table.

The lock argument fields of a lock object are used as input parameters in the function modules for setting and removing locks generated from the lock object definition. When these function modules are called, the table rows to be locked or unlocked are specified by defining certain values in these fields. These values can also be generic. The lock argument fields therefore define which subset of the table rows should be locked.

The simplest case of a lock object consists of exactly one table and the lock argument of the table is the primary key of this table. Several tables can also be included in a lock object. A lock request therefore can lock an entire logical object, and not only a record of a table. Such a logical object can be for example a document comprising an entry in a header table and N entries in a position table.

Locks can also be set from programs in other systems with the corresponding interfaces if the lock object was defined with RFC authorization.

A lock mode can be assigned for each table in the lock object. This mode defines how other users can access a locked record of the table.

Table SFLIGHT in the flight model contains all the scheduled flights of a carrier. Field SEATSMAX contains the number of seats available. Field SEATSOCC contains the number of seats already booked. If a booking is made for a customer (by a travel agency or sales desk), you must check whether there are enough seats available. The number of seats booked is incremented when the booking is made.

This mechanism must ensure that two sales desks do not make the same booking at the same time and that the flight is not overbooked.

This can be done by creating lock object ESFLIGHT. Only the table SFLIGHT must be included in this lock object. The flight can then be locked (with the function modules generated from the lock object) when booking. If another sales desk also wants to book seats for this flight, the lock will prevent the flight from being overbooked.

Activating a lock object in the ABAP Dictionary automatically creates function modules for setting (ENQUEUE_<lock object name>) and releasing (DEQUEUE_<lock object name>) locks.

The generated function modules are automatically assigned to function groups. You should not change these function modules and their assignment to function groups since the function modules are generated again each time the lock object is activated.

Never transport the function groups, which contain the automatically generated function modules. The generated function modules of a lock object could reside in a different function group in the target system. Always transport the lock objects. When a lock object is activated in the target system, the function modules are generated again and correctly assigned to function groups.

Parameters of the Function Modules

Field Names of the Lock Object

The keys to be locked must be passed here.

A further parameter X_<field> that defines the lock behavior when the initial value is passed exists for every lock field <field>. If the initial value is assigned to <field> and X_<field>, then a generic lock is initialized with respect to <field>. If <field> is assigned the initial value and X_<field> is defined as X, the lock is set with exactly the initial value of <field>.

Parameters for Passing Locks to the Update Program

A lock is generally removed at the end of the transaction or when the corresponding DEQUEUE function module is called. However, this is not the case if the transaction has called update routines. In this case a parameter must check that the lock has been removed.

Parameter _SCOPE controls how the lock or lock release is passed to the update program (see The Owner Concept for Locks). You have the following options:

_SCOPE = 1: Locks and lock releases are not passed to the update program. The lock is removed when the transaction is ended.

_SCOPE = 2: The lock or lock release is passed to the update program. The update program is responsible for removing the lock. The interactive program with which the lock was requested no longer has an influence on the lock behavior. This is the standard setting for the ENQUEUE function module.

_SCOPE = 3: The lock or lock release is also passed to the update program. The lock must be removed in both the interactive program and in the update program. This is the standard setting for the DEQUEUE function module.

Parameters for Lock Mode

A parameter MODE_<TAB> exists for each base table TAB of the lock object. The lock mode for this base table can be set dynamically with this parameter. Valid values for this parameter are S (shared), E (exclusive) and X (exclusive but not cumulative).

The lock mode specified when the lock object for the table is created is the default value for this parameter. This default value can however be overridden as required when the function module is called.

If a lock set with a lock mode is to be removed by calling the DEQUEUE function module, this call must have the same value for the parameter MODE_<TAB>.

Controlling Lock Transmission

Parameter _COLLECT controls whether the lock request or lock release should be performed directly or whether it should first be written to the local lock container. This parameter can have the following values:

Initial value: The lock request or lock release is sent directly to the lock server.

X : The lock request or lock release is placed in the local lock container. The lock requests and lock releases collected in this lock container can then be sent to the lock server at a later time as a group by calling the function module FLUSH_ENQUEUE.

Behavior for Lock Conflicts (ENQUEUE only)

The ENQUEUE function module also has the parameter _WAIT. This parameter determines the lock behavior when there is a lock conflict.

You have the following options:

Initial value: If a lock attempt fails because there is a competing lock, the exception FOREIGN_LOCK is triggered.

X : If a lock attempt fails because there is a competing lock, the lock attempt is repeated after waiting for a certain time. The exception FOREIGN_LOCK is triggered only if a certain time limit has elapsed since the first lock attempt. The waiting time and the time limit are defined by profile parameters.

Controlling Deletion of the Lock Entry (DEQUEUE only)

The DEQUEUE function module also has the parameter _SYNCHRON.

If X is passed, the DEQUEUE function waits until the entry has been removed from the lock table. Otherwise it is deleted asynchronously, that is, if the lock table of the system is read directly after the lock is removed, the entry in the lock table may still exist.

Exceptions of the ENQUEUE Function Module

FOREIGN_LOCK: A competing lock already exists. You can find out the name of the user holding the lock by looking at system variable SY-MSGV1.

SYSTEM_FAILURE: This exception is triggered when the lock server reports that a problem occurred while setting the lock. In this case, the lock could not be set.

If the exceptions are not processed by the calling program itself, appropriate messages are issued for all exceptions.

Reference Fields for RFC-Enabled Lock Objects

The type of an RFC-enabled function module must be completely defined. The parameters of the generated function module therefore have the following reference fields for RFC-enabled lock objects:

Parameters

Reference fields

X_<field name>

DDENQ_LIKE-XPARFLAG

_WAIT

DDENQ_LIKE-WAITFLAG

_SCOPE

DDENQ_LIKE-SCOPE

_SYNCHRON

DDENQ_LIKE-SYNCHRON

All the tables that can be included in a lock object must be linked with foreign keys. There are a number of restrictions to the valid relationships.

The foreign key relationships of the tables of the lock object must form a tree. The tables are the nodes of the tree. The links of the tree mean is the check table of.

The foreign key fields must be key fields of the foreign key table.

The foreign key relationships defined between the base tables of the lock objects may not have any field that is checked against more than one other field. A field therefore may not occur twice as foreign key field in a relationship and may not be checked against two different fields in two different foreign key relationships.

You must keep one restriction in mind for multi-structured foreign keys. If a field is assigned to a field that is outside the check table, the table containing this field must be in a sub-tree that contains the check table of this foreign key relationship as a root.

These conditions are always satisfied if the key of the foreign key table is an extension of the key of the check table.

Conditions 2, 3 and 4 are meaningless if the particular foreign key field was excluded from the assignment to the key fields of the check table by marking it as generic or setting it to a constant. This is also true for multi-structured foreign keys if the foreign key field refers to a table that is not contained in the lock object.

Regards.

Eshwar.

[Former Member]

You can create a lock on a object of SAP thorugh transaction SE11 and enter any meaningful name start with EZ Example EZTEST_LOCK.

Use: you can see in almost all transaction when you are open an object in Change mode SAP could not allow to any other user to open the same object in change mode.

Example: in HR when we are enter a personal number in master data maintainance screen SAP can't allow to any other user to use same personal number for changes.

Technicaly:

When you create a lock object System automatically creat two function module.

1. ENQUEUE_<Lockobject name>. to insert the object in a queue.

2. DEQUEUE_<Lockobject name>. To remove the object is being queued through above FM.