How to implement Pessimistic Locking?

Hello,
I am using JDev 10g with integrated mapping workbench. There are option to set up optimistic locking, but I would like to implement pessimistic locking. How do I approach this?
Thanks,
Dmitry.

Dmitry,
Pessimistic locking is not configured on the descriptor it is executed at the API level on either a session or query. If you are using the UnitOfWork directly then I would recommend looking in the docs at
UnitOfWork.refreshAndLockObject(Object)
UnitOfWork.refreshAndLockObject(Object, short lockMode)
If you wish to configure it on a query you can look at:
ObjectLevelReadQuery.setLockMode(short lockMode)
ObjectLevelReadQuery.acquireLocks()
ObjectLevelReadQuery.acquireLocksWithoutWaiting()
In the case of a query it should only be executed against a UnitOfWork where the database TX can be started to maintain the database locks.
Doug

Similar Messages

How to Implement shared lock

Hi friends,
For a table I have created views. I want to create shared lock for that table.
How to achieve that.
can we create shared lock for views?
Thanks.

Hi Pagidala,
Go to Se11-> Select Lock object radiobutton->give some name starting with E for example EZDEMO_LOCK->click on Create ->give short description->clcik on Tables Tab->give the table name for which you want to implement shared lock example ZTABLE ->lock mode you select as Read Lock.After this save check and activate your lock object.
We have selected READ LOCK since it protects read access to an object. The read lock allows other transactions read access but not write access to the locked area of the table.
Check the below link for the complete information on Lock Concept.
http://help.sap.com/saphelp_nw04/helpdata/en/41/7af4c5a79e11d1950f0000e82de14a/content.htm
Cheers!!
VEnk@
Edited by: Venkat Reddy on Oct 29, 2008 11:54 AM

How to implement optemistic locking in pl/sql for oracle database

i have search the net to find example of how i can do the optimistic locking in pl/sql, all the resources i found were old , for example "Optimistic Locking with Concurrence in Oracle.
As " by Graham Thornton Snr. Database Architect / Oracle DBA in 2001, but at the end he said that the approach will not work fine if the update being made is a relative update, this apprach includes:-
1.add a timestamp column to an exsiting table.
2.add a preinsert trigger on the table to set the timestamp.
3.add a preupdate trigger to comapre the old time stamp with the new one.
So where i can find updated resources about this issue.
Edited by: 812643 on 17-Nov-2010 12:39

totally plagiarized from expert oracle database architecture 9i, 10g, 11g by Tom Kyte pg201
one popular implementation of optimistic locking is to keep the old and new values in the application and upon updating the data use and update like this
update table
set column1 =: new_column1, column2 = : new_column2, ...
where primary_key = :primary_key
and decode( column1, :old_column1, 1 ) = 1
and decode( column2, :old_column2, 1 ) = 1
another implementation
optimistic locking using a version column
add a single column to each database table you wish to protect from lost updates. this column is generally either a number or date/timestamp column
It is typically maintened via a row trigger on the table, which is responsible for incrementing the number column or updating the date/timestamp column
every time a row is modified.
another one on page 204
optimistic locking usin a checksum
similiar to the version column implementation but it uses the base data itself to compute a virtual version column.
the ones suggested where owa_opt_lock.checksum, dbms_obfuscation_toolkit.md5, dbms_crypto.hash, ora_hashEdited by: pollywog on Nov 17, 2010 3:48 PM
might be a good book for you to look into getting it has a whole chapter on locking and latching.
Edited by: pollywog on Nov 17, 2010 3:54 PM

How to implement the pessimistic locking using toplink with sybase

we want to allocate the unique primary key to each row when many user try to insert the records concurrently..So what we are trying to do is we calculate the maximum of Primary Key and incremented it by 1. Now we want to Apply the locking concept on the so that unique key will be allocated to each newly inserted row
Can you please tell me
1. how we can genrate unique primary key in toplink using sybase?
2.how to implement the pessimistic or optimistic locking ?which one will be preferable?

Hi brother
I think that this link can help you
http://download-east.oracle.com/docs/cd/A97688_16/toplink.903/b10064/database.htm#1007986
Good luck

How to implement the shared lock in a table view

Hi,
How to implement the shared lock in a table view.for multiple users to edit on this same table.
Thanks in advance
Swathi

Hi,
Please refer this link to find solution to your querry.
Hope it helps.
https://www.sdn.sap.com/irj/sdn/go/portal/prtroot/docs/library/uuid/c322c690-0201-0010-fb86-811c52b0acc2.
Regards,
Rahul

How to implement locking in ABAP

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

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.

Pessimistic Locking (relating to COBOL)

Hi,
Apologies for the SA I've written below - just wanted to explain the context around my problem.
I'm working on an application which replaces the Micro Focus COBOL file handler & allows data in COBOL VSAM files to be migrated to Oracle in such a way that the COBOL source code doesn't have to change.
The last problem we're having is in replicating COBOL's pessimistic locking paradigm in that a COBOL READ with LOCK statement will take a lock on the row that's read, and won't release the lock until an explicit UNLOCK statement is issued.
Each IO command from COBOL ends up calling an Oracle procedure, so what I'm after is ideas on how to create a procedure which when called will select one record & create a row lock, and hold that lock until another stored procedure call (to the unlock) procedure is made.
Using Select ... for update will work provided we're in a database transaction, however that causes other headaches because currently we're sharing one database connection across all files that are opened by a running COBOL program, & the files need to be locked & unlocked independently from each other.
If that's the only way to achieve the lock I can work with that by changing our application to create a connection for a file when it needs to lock a record, but does anyone have any out of the box ideas?
thanks, Darren.

This is not an easy problem to solve due to a very big difference between file (ISAM) based locking and RDBMS transactions.
A lock in a RBMS is not just a "mere row lock" - it is part of a transaction. And a lock cannot span a transaction. To emulate file-based locking in any RDBMS will be problematic as a result.
It can be done in Oracle. Heck, there is little one cannot do in Oracle. However, some designs (like this file-based record locking) simply does not scale.
And precisely for that reason, Oracle does not have a Lock Manager. There is no special monolithic structure (memory or table) that contains a list of locks. Any new lock, any release of an existing lock, will require access to that structure - and serialised access. This means it will become a huge bottleneck the more locks there are and the more processes wants locks. And this is the reason why there are lock escalation problems in SQL-Server, why locks are "expensive". Why the Ingres RDBMS manual goes into great lengths of how to "tune" the database for this, to the effect of having to decide whether to boost OLTP processing or read-only queries - as you cannot run both as effectively as you want.
In order for you to emulate the file-lock approach, this is exactly what you will need to simulate in Oracle. A monolithic lock manager.
Not that difficult to design and implement in Oracle. Pessimistic locking in Oracle is essentially "turned off" and you manage application row locking via your own PL/SQL lock manager in Oracle. Using autonomous transactions extensively.
But this will not scale. Sure, it can handle maybe 200 users. Maybe more. But as the userbase increases, so the system will slow down - as you have no choice but to enforce serialisation in your PL/SQL lock manager to handling locking.
There is a price to pay for not wanting to change the Cobol code to correctly use Oracle's concurrency model.
Personally, I would argue long and very hard that this price is unacceptable and that none of the ROI in an "expensive" RDBMS is realised. Any RDBMS makes a very poor ISAM file system. So why bother?

Pessimistic Locking mechanism in SAP Gateway.

Hi All,
           I am new to SAP Gateway. I know there are two types of locking mechanism ( optimistic and pessimistic) . Optimistic we do by etag and if-match.
Is there any idea of how to do by pessimistic approach.
For example we are reading a contract and then updating it. So, in get_entity we have to put the lock while reading and we have to unlock while updating. But how does it works. Whether the lock automatically get released after the get_entity method.
Kindly guide me in proceeding further.

Hi Sundar,
Here is my opinion :
          Gateway is Completely Stateless till the current version which is SP8. So Pessimistic Locking would not be available out of the box by Gateway. You need to implement your custom logic to achieve this behaviour. However simulating a Pessimistic Lock in a complete stateless architecture will lead to many complexities which often might lead to inconsistency in the Application. As mentioned by Krishna, you can store lock information in some table in backend , but it might get messy very easily. I would not recommend to do so. However if you are really keen to do Pessimistic Locking kind of mechanism, what about the following approach -----
1. In the OData service, let us have an extra Entity called LOCK which has properties such as USER, RESOURCE_ID(such as contract ID),DATE,TIME. USER and RESOURCE_ID is the key in this entity. This data is stored in a table in database.
2. When a particular USER opens the CONTRACT screen in EDIT mode, the UI fires a POST(Create) call for Entity LOCK with the USER and RESOURCE_ID(contract ID in this case). In the backend database table if no entry exists with that USER ID and RESOURCE_ID, then a new entry is created and the USER is allowed to EDIT the contract. If an entry already exist in table then it means some other user is already in Edit mode , and the current user is refused to access the resource in edit mode.
3. When the USER saves the CONTRACT you need to fire a DELETE request for that Particular LOCK entity. Thus is resource is dequeued.
However this is like a vanilla flow. Handling can be complicated if user closes the browser, disconnects the network connection, opens parallel session etc etc.
          Rather a clean approach would be an Optimistic lock. Fiori Applications are using this mechanism to handle concurrency control. You may refer to my reply on Locking mechanism in SAP Gateway. for a little more information on Optimistic Locking.
I agree that even though Optimistic locking works pretty well in REST based applications as it allows operations to be free from overhead of long transaction , there can be situations as mentioned below, where an Optimistic lock might not be preferred -
1. Where the chances of concurrency collision is very high (Example: Online Reservation of Tickets)
2. If modifying the resource by the user requires a lot of typing (However JS libraries such as Garlic.js, sisyphus.js or HTML5 Local storage can help us here )
Now it is upto you to decide upon the approach ..

How can i explicitly lock a row of a Table

Hi
How can we implement optimistic locking , i am using Oracle as Database for my Application .
Assume that i am working on a single row of a Table .
Now please tell me how can i explicitly lock this row , so that other simultanoues users insert or update operations are not affected to this particular row .
I am not using any ORM related technology
Please help .

raviprivate wrote:
Hi
How can we implement optimistic locking , i am using Oracle as Database for my Application .
Assume that i am working on a single row of a Table .
Now please tell me how can i explicitly lock this row , so that other simultanoues users insert or update operations are not affected to this particular row .Either you want optimistic locking which does not use (database) locks at all, or you want pessimistic locking which does use database locks.
In optimistic locking everyone is allowed to read and to update without taking out a lock. Usually the optimistic lock is realised by including either a 'version' or 'last_modified' column which with every update is either increased (version) or set to the current_timestamp (last_modified). The update statement must be issued with a condition including that column (eg UPDATE ... WHERE id=<id> AND version=<version at time of read>). If the column was already changed by another user or process zero rows will have been updated signalling to the program that another user already changed the row and the user must redo his changes, or a more complex method of change comparison can be done.

Need help in Pessimistic locking

I am getting the Maiximum of primaryKey field using reportquery ..now i want to lock the object of the report query. i am doing as following
ExpressionBuilder exp=new ExpressionBuilder();
ReportQuery query=new ReportQuery();
query.setReferenceClass(Abc.class);
query.addAttribute("memberId",exp.get("memberId").maximum());
query.acquireLocksWithoutWaiting();
Vector results = (Vector)uow.executeQuery(query);
System.out.println("results---->"+results);
now i increment the value of primary key as follows
ReportQueryResult result1=(ReportQueryResult)results.get(0);
System.out.println("hello"+result1.getByIndex(0));
Integer ctr=(Integer)result1.getByIndex(0);
System.out.println("ctr:-"+ctr);
int count=ctr.intValue();
count=count+1;
System.out.println("incrementedValue:-"+count)
here i insert the new record to the database ;;
the whole code work fine but when i trying to implement the pessimistic locking by
     query.acquireLocksWithoutWaiting();
it will gives the following error..Please help me ..thanks in Advance
ServerSession(28523022)--Connection(27817788)--SELECT MAX(MemberId) FROM Abc FOR UPDATE OF * NOWAIT
Local Exception Stack:
Exception [TOPLINK-4002] (OracleAS TopLink - 10g (9.0.4.5) (Build 040930)): oracle.toplink.exceptions.DatabaseException
Exception Description: com.sybase.jdbc3.jdbc.SybSQLException: Incorrect syntax near '*'.
Internal Exception: com.sybase.jdbc3.jdbc.SybSQLException: Incorrect syntax near '*'.
Error Code: 102
     at oracle.toplink.exceptions.DatabaseException.sqlException(DatabaseException.java:227)
     at oracle.toplink.internal.databaseaccess.DatabaseAccessor.executeCall(DatabaseAccessor.java:698)
     at oracle.toplink.threetier.ServerSession.executeCall(ServerSession.java:506)
     at oracle.toplink.internal.queryframework.CallQueryMechanism.executeCall(CallQueryMechanism.java:131)
     at oracle.toplink.internal.queryframework.CallQueryMechanism.executeCall(CallQueryMechanism.java:115)
     at oracle.toplink.internal.queryframework.CallQueryMechanism.executeSelectCall(CallQueryMechanism.java:194)
     at oracle.toplink.internal.queryframework.CallQueryMechanism.selectAllRows(CallQueryMechanism.java:565)
     at oracle.toplink.internal.queryframework.ExpressionQueryMechanism.selectAllRowsFromTable(ExpressionQueryMechanism.java:733)
     at oracle.toplink.internal.queryframework.ExpressionQueryMechanism.selectAllReportQueryRows(ExpressionQueryMechanism.java:694)
     at oracle.toplink.queryframework.ReportQuery.execute(ReportQuery.java:442)
     at oracle.toplink.queryframework.DatabaseQuery.execute(DatabaseQuery.java:493)
     at oracle.toplink.queryframework.ReadQuery.execute(ReadQuery.java:125)
     at oracle.toplink.publicinterface.Session.internalExecuteQuery(Session.java:1958)
     at oracle.toplink.threetier.ServerSession.internalExecuteQuery(ServerSession.java:629)
     at oracle.toplink.threetier.ClientSession.internalExecuteQuery(ClientSession.java:392)
     at oracle.toplink.publicinterface.Session.executeQuery(Session.java:1086)
     at oracle.toplink.publicinterface.UnitOfWork.internalExecuteQuery(UnitOfWork.java:2189)
     at oracle.toplink.publicinterface.Session.executeQuery(Session.java:1086)
     at oracle.toplink.publicinterface.Session.executeQuery(Session.java:1038)
     at com.pearson.Test.Test.insert(Test.java:664)
     at com.pearson.Test.Test.main(Test.java:78)
Caused by: com.sybase.jdbc3.jdbc.SybSQLException: Incorrect syntax near '*'.
     at com.sybase.jdbc3.tds.Tds.a(Unknown Source)
     at com.sybase.jdbc3.tds.Tds.nextResult(Unknown Source)
     at com.sybase.jdbc3.jdbc.ResultGetter.nextResult(Unknown Source)
     at com.sybase.jdbc3.jdbc.SybStatement.nextResult(Unknown Source)
     at com.sybase.jdbc3.jdbc.SybStatement.nextResult(Unknown Source)
     at com.sybase.jdbc3.jdbc.SybStatement.queryLoop(Unknown Source)
     at com.sybase.jdbc3.jdbc.SybStatement.executeQuery(Unknown Source)
     at com.sybase.jdbc3.jdbc.SybPreparedStatement.executeQuery(Unknown Source)
     at oracle.toplink.internal.databaseaccess.DatabaseAccessor.executeSelect(DatabaseAccessor.java:802)
     at oracle.toplink.internal.databaseaccess.DatabaseAccessor.executeCall(DatabaseAccessor.java:651)
     ... 19 more

I will explain you what I am trying to do:- we want to allocate the primary key to each new inserted row. suppose there are more than one user who are trying to insert the record in the database on the click of save button..how we can handle this situation by allocating them unique Id to each row..how we can genrate unique primary key to all the users who are inserting the new rows.
Please help me how can i genrate unique primary key if more than one user inserting the data in the database. As you said use sequencing in toplink how can i use this?our database is sybase is it support Pessimistic locking or optimistic locking. need urgent help
thanks in advance

Pessimistic locking Vs Optimitic locking In ERP

Hi all
" Pessimistic locking Vs Optimistic locking To advice MY ERP "
Im SASI , Now I have a problem for using locking system in update problem
I am using Oracle 10g and Java 5.0(Netbeans)
My PL to advice for pessimistic locking system ,
Now I want to what is the difference between & How to help my ERP
I am reading lot of information they are all confuse to me.
1) Select .. for update nowait --> this statement will lock that record upto the user commit,
in my problem I am wait for the same record that fellow when will release I'm updated.
2) Do the modified (I have column upto 25 ) how to change to another transaction in some other table
ex orderdetails -order cancel
stock detail -stock maintain
orderdetails for update ........................... long time at the same time waiting for stock detail updatation .
ALL the guys How to handle the above top and I want solution above problem
very urgent .....
Thank
SASIKUMARA

Pessimistic Locking:
You lock any and all rows before you update them. In Oracle that is done with a SELECT FOR UPDATE.
Problem:
Stateless clients (like web clients/app tier connections) cannot use pessimistic locking as this locking is specific to a single Oracle session. These stateless clients can use/re-use/create any Oracle session. Thus should they attempt to lock rows in one session, that state information will be missing when they attempt to update the rows from another session.
Work-around - Optmistic Locking:
To work around this problem with stateless clients, optimistic locking is used. The client does not lock the rows it wants to update. It optimistically assume that the rows will be unchanged at the time it wants to update them.
When it does update the rows, it needs to check the existing copy of the row in the database, with the old copy of the row it retrieved from the database. If these two copies matches, then it can proceed to write the changed copy of the row.
This is usually done via an UPDATE SQL like this:
UPDATE emp
SET name = :newname, surname = :newsurname, ...
WHERE emp_id = :emp_id
AND name = :oldname
AND surname = :oldsurname
AND ... (for all other columns) As you can see, should any column in the row be changed from the last time that the client viewed the row, the UPDATE will fail to update the changed row.
Not implementing optimistic locking for stateless clients leads to issues like lost updates. Two clients update the same row - the 2nd client overwrites the changes of the 1st client and thus the 1st client's changes becomes lost.

JPA Pessimistic Locking

Hi,
Can anybody tell me why the following ain't working?
I'm trying to implement a J2SE client that will update objects using pessimistic locking. I've created the simplest testcase for some testing, but I can't seem to get it right, I must be overlooking something I guess...
TestCase:
//...imports and package declaration
public class ToplinkLockingTestCase extends TestCase {
private EntityManager entityManager;
     @Override
     protected void setUp() throws Exception {
entityManager = Persistence.createEntityManagerFactory("default").createEntityManager();
     public void testUpdateWithPessimisticLock() throws Exception {
          final long sendingId = 991L;
          entityManager.getTransaction().begin();
          Query query = entityManager.createQuery("SELECT sending FROM ProductiveSending sending WHERE sending.id = :id");
          query.setParameter("id", sendingId);
          query.setHint(TopLinkQueryHints.PESSIMISTIC_LOCK, PessimisticLock.Lock);
          Sending sending = (Sending) query.getSingleResult();
          sending.setAcceptedAmount(sending.getAcceptedAmount() + 10);
          LOGGER.info("Sending --> " + sending);
          entityManager.getTransaction().commit();
The problem with the example above, is that the invocation of the setter, setAcceptedAmount() deadlocks. The process just stops, I guess it's waiting upon release of a lock. However, since I'm on the same client, I would expect that the lock has been taken by the same client that is trying to invoke the update, is it not?
What I can see from the output logging is that the SELECT ... FOR UPDATE is using another Connection(ID) than the UPDATE statement. How is that possible?
[TopLink Finer]: 2007.11.19 02:10:35.146--ServerSession(25877218)--Thread(Thread[main,5,main])--client acquired
[TopLink Finest]: 2007.11.19 02:10:46.422--UnitOfWork(18686163)--Thread(Thread[main,5,main])--Execute query ReportQuery(be.cm.apps.tpb.business.domain.sending.ProductiveSending)
[TopLink Fine]: 2007.11.19 02:10:46.452--ServerSession(25877218)--Connection(20536747)--Thread(Thread[main,5,main])--SELECT Zending_Id, Facturatie_Mode_Cd, Aanvaard_Bdr, Verworpen_Bdr, File_Naam, Ontvangst_Dt, Geannuleerd_Ind, Facturatie_Type_Cd, Verworpen_Blokk_Error_Ind, Totaal_Lijnen, Verworpen_Drempel_Ind, Medium_Ontbreekt_Ind, last_positie, Totaal_Bdr, Laatste_Wijz_Id, Verwerking_Tijd, RowVersion, Verwerking_Dt, Ucp_Ziekenfonds_Nr, Process_Instance_Id, Antw_Verstuurd_Dt, Verworpen_Lijnen, Derde_Instelling_Ref , Facturatie_Jaar, Zending_Nr, Facturatie_Maand, Versie_Nr, Derde_Nummer, Verworpen_Uniciteit_Ind FROM SREFTPB_ZENDING WHERE ((Zending_Id = ?) AND (Facturatie_Mode_Cd = ?)) FOR UPDATE
     bind => [991, P]
[TopLink Finest]: 2007.11.19 02:10:46.762--UnitOfWork(18686163)--Thread(Thread[main,5,main])--Register the existing object Sending[id=991, sendingNumber=492, thirdNumber=71014688, invoicingMonth=8, invoicingYear=2007, fileName=10061122.000, versionNumber=1]
[TopLink Finest]: 2007.11.19 02:10:46.772--UnitOfWork(18686163)--Thread(Thread[main,5,main])--Revert the object's attributes Sending[id=991, sendingNumber=492, thirdNumber=71014688, invoicingMonth=8, invoicingYear=2007, fileName=10061122.000, versionNumber=1]
[TopLink Finest]: 2007.11.19 02:10:46.772--UnitOfWork(18686163)--Thread(Thread[main,5,main])--track pessimistic locked object Sending[id=991, sendingNumber=492, thirdNumber=71014688, invoicingMonth=8, invoicingYear=2007, fileName=10061122.000, versionNumber=1] with UnitOfWork 18.686.163
JUnitConfigurationHelper - The LogManager 4.3.1 is properly initialized and logs to the junit console
20071119:14:11:03:336|DEBUG|be.cm.apps.tpb.business.domain.ToplinkLockingTestCase|INFO|Sending --> Sending[id=991, sendingNumber=492, thirdNumber=71014688, invoicingMonth=8, invoicingYear=2007, fileName=10061122.000, versionNumber=1]
[TopLink Finer]: 2007.11.19 02:11:11.238--UnitOfWork(18686163)--Thread(Thread[main,5,main])--begin unit of work commit
[TopLink Finer]: 2007.11.19 02:11:11.248--ClientSession(17385679)--Connection(5501096)--Thread(Thread[main,5,main])--begin transaction
[TopLink Finest]: 2007.11.19 02:11:11.248--UnitOfWork(18686163)--Thread(Thread[main,5,main])--Execute query UpdateObjectQuery(Sending[id=991, sendingNumber=492, thirdNumber=71014688, invoicingMonth=8, invoicingYear=2007, fileName=10061122.000, versionNumber=1])
[TopLink Finest]: 2007.11.19 02:11:11.248--UnitOfWork(18686163)--Thread(Thread[main,5,main])--Assign return row DatabaseRecord(
     SREFTPB_ZENDING.RowVersion => 12)
[TopLink Fine]: 2007.11.19 02:11:11.258--ClientSession(17385679)--Connection(5501096)--Thread(Thread[main,5,main])--UPDATE SREFTPB_ZENDING SET Aanvaard_Bdr = ?, RowVersion = ? WHERE ((Zending_Id = ?) AND (RowVersion = ?))
     bind => [40.0, 12, 991, 11]

OK so in the end I decided to switch to Eclipse Link and used the query hints feature to add exclusive locks to my objects (rows), if another client tries for the same lock then an exception is thrown after a timeout occurs. If the default timeout is lowered in MySQL then only a short delay is incurred by doing this.
In summary something like:
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innodb_lock_wait_timeout=0
it should be noted that on MySQL will only work with InnoDB tables.

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I work on behalf of HP
The advice and opinions given here are my own and not those of HP

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Steve, thank you so much!!!
Your example helped me a lot. I am able to run the process, but I still have three doubts:
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