Frequently Asked Questions: Lock Concept

The following section contains some common questions and answers regarding the lock concept and is intended to act as a quick reference guide. Most aspects are described in greater detail under Functions of the SAP Lock Concept.

The questions are sorted by topic areas.

·        Architecture

·        Programming

·        Problem analysis

·        High availability

·        Performance

Architecture

 Question

Where is the lock table stored?                                        

 Answer

In the main memory, shared memory, of the Enqueue Server. All work processes in the enqueue server have access to it. External application servers execute their lock operations in the enqueue process on the enqueue server. Communication in this case takes place via the relevant dispatchers and the message server. 

 Question

Can locks exist directly after startup?              

 Answer

Yes, the saved locks, which were inherited by the update task, are reloaded to the lock table during startup (see first question).             

 Question

Why is an enqueue work process required in a central system? Don't all work processes have the same access to the shared memory and thus to the lock table?     

 Answer

Although the enqueue process is not used in a central system, it does not do any harm. Since almost all customers install an application server sooner or later, problems will inevitably arise if the enqueue process is missing. For this reason, the enqueue diagnosis function will output an error if an enqueue process has not been configured.                           

 Question

Are the locks in the lock table also set at the database level? If not, database functions could be used to process objects locked in the SAP system.                                        

 Answer

Locks are not set on the database. The lock table is stored in the main memory of the enqueue server. See Relationship Between SAP Locks and Database Locks.                               

 Question

Is a lock table built if an enqueue work process is not started on the enqueue server in the instance profile?           

 Answer

Yes, because the work processes on the enqueue server use the lock table directly, and not via the enqueue process. The latter is only responsible for lock requests from external application servers.  

Programming

 Question

What does the _SCOPE parameter mean? How does the lock owner concept work?            

 Answer

See Lock Owners and _SCOPE-Parameter.

 Question

How can I find out who is currently holding the ungranted lock? In other words, how can check the program after an ENQUEUE to determine which use is currently holding the lock so that I can let him or her know?                                  

 Answer

When the ENQUEUE_... function module is returned, the name of the lock owner is listed in SY-MSGV1.

Question

Can I use special characters in my lock argument (especially the ‘at’ sign (@))?

 Answer

The ‘at’ symbol is used as a wildcard in SAP locks (enqueues). In other words, it can stand for any other character during collision checks. For example, the parameter value 12345@ locks the quantities 123450 to 123459, 12345a to 12345z, and 12345A to 12345Z, and all other values with any special character in the 6th character position.               

This is described in detail in the section Lock Collisions.

In order to prevent the wildcard mechanism from being activated in SAP locks when it is not required, you need to ensure when enqueue function modules are called that key value parameters do not contain any wildcard characters.                                     

If key values that you want to use to lock individual entities do contain wildcard characters, you have to replace the wildcards with different characters before the enqueue is called.          

Problem Analysis

 Question

What should I do first if a problem arises?                          

 Answer

Use the diagnosis functions:

sm12  Extras ® Diagnosis and then

sm12 Extras ® Diagnosis in update

If a problem is reported, back up the trace files dev_w*, dev_disp, dev_eq* and check the Syslog.           

 Question

The following message is displayed in the diagnosis details in SM12:

Lock management operation mode                                  

Internal lock management in same process                         

What does this message mean and what are the other options?         

 Answer

"Internal lock management in same work process" in the diagnosis function means that you are logged onto the enqueue server and your work process can access the lock table straight away. You do not have to delegate enqueue requests to an enqueue process on a remote enqueue server. If you are logged onto an application server that is not an enqueue server, the diagnosis function will provide you with the name of the enqueue server.     

Each SAP System has exactly one application server that functions as an enqueue server. This enqueue server maintains the lock table, which is located in a shared memory segment. All of the work processes on the enqueue server can access the lock table. All work processes on other application servers delegate their enqueue requests to a special enqueue work process on the enqueue server.

                                              

This procedure is configured automatically. The parameter line "rdisp/enqname =<application server name>" in the default profile DEFAULT.PFL indicates which application server is currently acting as the enqueue server. When an application server detects that its name matches the name of the enqueue server, it creates the lock table and all of its work processes process enqueue requests inline. If an application server detects that its name does not match the name of the enqueue server, it sends all enqueue requests to the enqueue server.

Work processes of the type "enqueue" guarantee that incoming requests are processed immediately. One enqueue process is usually sufficient. In very large SAP Systems with many application servers, a second process can be beneficial. However, it is not expedient to define more than two enqueue processes. If the transaction SM50 -> [CPU] shows that only the first enqueue process is being used, the bottleneck is due to something else. 

 Question

The Syslog often contains messages such as "Enqueue: total wait time during locking: 2500 seconds". How should I analyze this problem? Or is the entry not critical? (There are no records of terminations or timeouts.)                                                  

 Answer

The message is output for information purposes only but may indicate parallel processing errors with ABAP programs. The specified wait time is the time that has elapsed since startup due to the use of the WAIT parameter when the enqueue function module was called.                                                 

The WAIT parameter enables a lock attempt to be repeated a number of times, for example, so that the update task does not have to be cancelled when a lock is set temporarily by other programs. The work process remains busy between the lock attempts.  

High availability

 Question

What happens to locks when the enqueue server is restarted?                                                             

 Answer

If they have not been saved to disk in the backup file, they will be lost. The locks that are inherited by the update task when COMMIT WORK is executed after CALL FUNCTION .. IN UPDATE TASK are saved to disk. The locks are saved to disk when the update request becomes valid, that is, with the COMMIT WORK. Each time the enqueue server is restarted, the lock entries saved on the disk are reloaded to the lock table.

A lock is saved to disk at the point at which the backup flag is set.     

If the enqueue replication service is used as part of a high availability solution, locks will not be lost if the enqueue server fails or is restarted. See next question.

 Question

The enqueue server is a single-point-of-failure in the SAP System. Can I guarantee high availability for the Enqueue Server?                   

 Answer

To guarantee this you must use the standalone Enqueue Server with the Replication Server. This is described in the documentation Standalone Enqueue Server. 

SAP note 524816 contains the prerequisites that must be fulfilled for using the standalone Enqueue Servers with the Replication Server.

Performance

 Question

How fast are lock operations?                                

 Answer

In work processes on the enqueue server, a few 100 microseconds. In work processes of external application servers you have to include network communications and process changes. Depending on CPU and network load this amounts to a few milliseconds.                                           

 Question

With a single-process system as an enqueue server, we have reached X  SD Benchmark users. Can this number be increased by using a multiprocessor system (message server on the same machine as the enqueue server)? Can we assume that scaling is linear (number of CPUs * X SD users)? How many processes are advisable if message servers, dispatchers, one dialog, and two enqueue processes are to run on the system?                  

 Answer

A significant increase in the enqueue server throughput can be expected by using several processors. The CPU load on the enqueue server is distributed relatively evenly between message servers, dispatchers, and enqueue work processes, which means that up to 3 processors can be occupied simultaneously. The dispatchers and the message server represent the bottleneck with the enqueue.

Linear scaling can be expected for up to three processors, even if lock requests are so frequent that message server, dispatchers, and work processes are occupied simultaneously. Due to asynchronous system processes (for example, syncer), using more processors can further enhance throughput.