Oracle deadlock - how to use "synchronised" keyword in a transaction?

Hi,
I use WL6.1 SP4, Oracle 8.1.6, with some Java objects which execute a
lot
of SQL queries (mixed update, insert and select) using plain JDBC
calls,
and Weblogic connection pools. These objects are called by servlets.
I experienced recently deadlocks when two users call the object at the
same
time (See error below).
I execute the queries using "synchronized" keyword in the following
way:
synchronized (this)
conConnection.setAutoCommit(false);
executeTransaction(myStatement);
conConnection.commit();
executeTransaction is overriden in sub-classes and is the method which
executes
all the queries.
It calls methods in other objects. These methods are not declared as
synchronized.
1) Should they?
2) Should I use the keyword "synchronized" in another way?
3) This part of code is also called when I do only "select"
statements. I guess
it should only be synchronized when we do "update" and "insert" which
could lead
to a deadlock?
4) Do you have any idea why this deadlock occurs as I use the
"synchronized"
keyword, and one thread should wait until the other one has finished?
Thanks for any idea,
Stéphanie
----------------- error:
<ExecuteThread: '4' for queue: 'default'> <> <> <000000> <SQL request
sent to database: UPDATE PARTICIPANT par SET par.PARTICIPANTLASTRANK =
4 WHERE par.IDPARTICIPANT = 8983566>
<ExecuteThread: '11' for queue: 'default'> <> <> <000000> <SQL request
sent to database: UPDATE PARTICIPANT par SET par.PARTICIPANTLASTRANK =
6 WHERE par.IDPARTICIPANT = 8983570>
ORA-00060: deadlock detected while waiting for resource
     at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:134)
     at oracle.jdbc.ttc7.TTIoer.processError(TTIoer.java:289)
     at oracle.jdbc.ttc7.Oall7.receive(Oall7.java:573)
     at oracle.jdbc.ttc7.TTC7Protocol.doOall7(TTC7Protocol.java:1891)
     at oracle.jdbc.ttc7.TTC7Protocol.parseExecuteFetch(TTC7Protocol.java:1093)
     at oracle.jdbc.driver.OracleStatement.executeNonQuery(OracleStatement.java:2047)
     at oracle.jdbc.driver.OracleStatement.doExecuteOther(OracleStatement.java:1940)
     at oracle.jdbc.driver.OracleStatement.doExecuteWithTimeout(OracleStatement.java:2709)
     at oracle.jdbc.driver.OracleStatement.executeUpdate(OracleStatement.java:796)
     at weblogic.jdbc.pool.Statement.executeUpdate(Statement.java:872)
     at weblogic.jdbc.rmi.internal.StatementImpl.executeUpdate(StatementImpl.java:89)
     at weblogic.jdbc.rmi.SerialStatement.executeUpdate(SerialStatement.java:100)
     at bfinance.framework.EDBBLBean.executeSQL(EDBBLBean.java:299)

Hi Stepanie,
I'd try to group update statement together. Usually it helps.
Regards,
Slava Imeshev
"Stephanie" <[email protected]> wrote in message
news:[email protected]...
Thanks for your answer.
In the case you describe, is there a way to ensure that tx-2 waits for
tx-1
to be finished before beginning?
My transaction which causes the problem is the following (simplified):
UPDATE tableA SET islast=0 WHERE externalid=myid;
for (int i=0; i< aVector.size(); i++) {
INSERT INTO tableA (id, islast, ranking, externalid) (SELECT
SEQ_tableA.nextval, 1, 0, myid);
UPDATE tableA SET ranking = /*calculated ranking */
WHERE externalid=myid AND islast=1;
UPDATE tableB ....
commit;
tx-1 and tx-2 execute this transaction at the same time. tx-1 begins
The deadlock appears when tx-2 executes the second UPDATE tableA
query.
I don't see how I can avoid to execute these two update queries, so if
I can find another way to prevent deadlock, it would be great!
Stéphanie
Joseph Weinstein <[email protected]_this> wrote in message
news:<[email protected]_this>...
Stephanie wrote:
Hi,
I use WL6.1 SP4, Oracle 8.1.6, with some Java objects which execute a
lot
of SQL queries (mixed update, insert and select) using plain JDBC
calls,
and Weblogic connection pools. These objects are called by servlets.
I experienced recently deadlocks when two users call the object at the
same
time (See error below).Hi. The error you are getting isn't necessarily from a lack ofsynchronization
of your java objects. It has to do with the order in which you accessDBMS
data. You are getting ordinary DBMS deadlocks, which are caused when
two DBMS connections each have a lock the other wants, in order toproceed.
The DBMS will quickly discover this and will kill one transaction inorder to
let the other one proceed:
time 0: tx-1 and tx-2 have started.....
time 1: tx-1: update tableA set val = 1 where key = 'A'
time 2: tx-2: update tableB set val = 2 where key = 'B'
time 3: tx-1: update tableB set val = 1 where key = 'B' (waitsbecause tx-2 has the row
locked)
time 4: tx-2: update tableA set val = 2 where key = 'A' (waitsbecause tx-1 has the row
locked)
This is a deadlock. The solution is to organize your application code sothat every
transaction accesses the data in the same order, eg: update tableAfirst, then update tableB.
This will prevent deadlocks.
Joe Weinstein at BEA
I execute the queries using "synchronized" keyword in the following
way:
synchronized (this)
conConnection.setAutoCommit(false);
executeTransaction(myStatement);
conConnection.commit();
executeTransaction is overriden in sub-classes and is the method which
executes
all the queries.
It calls methods in other objects. These methods are not declared as
synchronized.
1) Should they?
2) Should I use the keyword "synchronized" in another way?
3) This part of code is also called when I do only "select"
statements. I guess
it should only be synchronized when we do "update" and "insert" which
could lead
to a deadlock?
4) Do you have any idea why this deadlock occurs as I use the
"synchronized"
keyword, and one thread should wait until the other one has finished?
Thanks for any idea,
Stéphanie
----------------- error:
<ExecuteThread: '4' for queue: 'default'> <> <> <000000> <SQL request
sent to database: UPDATE PARTICIPANT par SET par.PARTICIPANTLASTRANK =
4 WHERE par.IDPARTICIPANT = 8983566>
<ExecuteThread: '11' for queue: 'default'> <> <> <000000> <SQL request
sent to database: UPDATE PARTICIPANT par SET par.PARTICIPANTLASTRANK =
6 WHERE par.IDPARTICIPANT = 8983570>
ORA-00060: deadlock detected while waiting for resource
at
oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:134)
at oracle.jdbc.ttc7.TTIoer.processError(TTIoer.java:289)
at oracle.jdbc.ttc7.Oall7.receive(Oall7.java:573)
atoracle.jdbc.ttc7.TTC7Protocol.doOall7(TTC7Protocol.java:1891)
atoracle.jdbc.ttc7.TTC7Protocol.parseExecuteFetch(TTC7Protocol.java:1093)
atoracle.jdbc.driver.OracleStatement.executeNonQuery(OracleStatement.java:2047
atoracle.jdbc.driver.OracleStatement.doExecuteOther(OracleStatement.java:1940)
atoracle.jdbc.driver.OracleStatement.doExecuteWithTimeout(OracleStatement.java
:2709)
atoracle.jdbc.driver.OracleStatement.executeUpdate(OracleStatement.java:796)
atweblogic.jdbc.pool.Statement.executeUpdate(Statement.java:872)
atweblogic.jdbc.rmi.internal.StatementImpl.executeUpdate(StatementImpl.java:89
atweblogic.jdbc.rmi.SerialStatement.executeUpdate(SerialStatement.java:100)
at bfinance.framework.EDBBLBean.executeSQL(EDBBLBean.java:299)

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