Could any one help me with this question:
If I execute JDBC batchUpdate, which updates several tables and is not wrapped into any transactions, will it lock any tables or rows?
My code executes a bunch of UPDATE statements and all of them look as follows
String sql = "UPDATE contacts SET ref_counter = ? where uid = ?";
jdbcTemplate.batchUpdate(sql, new CustomBatchPreparedStatementSetter(elements));
Any link to documentation will be appreciated (I haven't managed to find any...)
Thanks in advance!
Locking (if any) is implementation dependent, so not defined by JDBC itself.
Related
Before I explain my problem I would like to say that I know the basics of JDBC but not really used to it.
I am using an updatable result set to hold data from 2 different tables, as in the following sample code:
searchQry = "SELECT ct.CustomerName, ct.Email, ct.PhoneNo, ot.ItemName
FROM CUSTOMER_TABLE ct JOIN ORDER_Table ot
ON ct.OrderID = ot.OrderID";
prestmt = dbcon.prepareStatement(searchQry, ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);
uprs = prestmt.executeQuery();
uprs.updateLong("ut.PhoneNo", 7240987456L);
uprs.updateString("otItemName", "GTA5");
uprs.updateRow();
I would like to know if I will update the database from somewhere else (not using the same result set object) while the result set, upsr, connected to the database, whether uprs will get updated with it or it will throw an error or it will go with the old data itself. Sorry if it a newbie question but I can't really test that on my DB without knowing the outcomes and safe measures.
Please, suggest me if there is any better way to update the underlining db along with the data in the ResultSet without having any transaction issues when changing from different places.
Using:
Oracle Database for JDBC connection.
I have to write a sql update trigger statement for Apache Derby. I am usually working with Sql Server and T-SQL. But now I have to use Derby. Unfortunately I am very new to Derby and I couldn't find a proper solution in the Derby manual.
My Problem is that I have to check for a condition in the update trigger and based on the result of this condition I would do either an UPDATE or an INSERT, so in T-SQL I would use an IF-ELSE-condition. Can somebody tell me what the equivalent is in Derby or an alternative way? I already considered the WHEN-clause, but this seems the wrong direction.
I have following code till now:
CREATE TRIGGER UPDATE_EVENTS
AFTER UPDATE
ON ACCIDENTS
REFERENCING OLD AS oldRow NEW AS newRow
FOR EACH ROW MODE DB2SQL
-- In the following, I would usually use an IF-ELSE Statement,
-- but I can't use this in Derby. So I tried the optional WHEN Statement,
-- but there I could not have an else "path", right?
-- This should be the If-Case
WHEN((SELECT COUNT(*) FROM VIEW_EVENTS WHERE ID_DATE = newRow.ID_DATE) > 0)
UPDATE VIEW_EVENTS
SET DETAILS = newRow.DETAILS,
PARTICIPANTS = newRow.PARTICIPANTS
WHERE ID_DATE = newRow.ID_DATE
-- And this should be the else case
WHEN((SELECT COUNT(*) FROM VIEW_EVENTS WHERE ID_DATE = newRow.ID_DATE) <= 0)
INSERT INTO VIEW_EVENTS
( ID_KEY,
ID_DATE,
DETAILS,
PARTICIPANTS
)
VALUES
( newRow.ID_KEY,
newRow.ID_DATE,
newRow.DETAILS,
newRow.PARTICIPANTS
);
This Statement is just a mini example to show you my problem. I hope you can help me :).
Best regards,
Yalcin
Do not tag indiscriminately. Your question has nothing to do with sql server.
But it seems that your goal is not directly achievable - as has been discussed (did you search?) here. Derby does not support multi-statement triggers. It seems that you need to use multiple triggers.
I stumbled upon a problem with locking row in Oracle DB. The purpose of the lock is to prevent more than one transaction reading data from the DB because this data influences the generation of new data and is changed in terms of a transaction.
In order to make the lock, I've put the #Lock annotation over SpringData find method which retrieves data that participates in the transaction.
#Lock(LockModeType.PESSIMISTIC_WRITE)
User findUserById(#Param("id") String operatorId);
After this code is implemented I get log message
org.hibernate.loader.Loader - HHH000444: Encountered request for locking however dialect reports that database prefers locking be done in a separate select (follow-on locking); results will be locked after initial query executes
Besides, it has no effect and causes
org.springframework.dao.DataIntegrityViolationException: could not execute batch; SQL [insert into ...]
The issue can be solved when rewriting the lock using entity manager
entityManager.lock(userByIdWithLockOnReadWrite, LockModeType.PESSIMISTIC_WRITE);
or
entityManager.unwrap(Session.class).lock(userByIdWithLockOnReadWrite, LockMode.PESSIMISTIC_WRITE);
The issue doesn't appear on MariaDB (MySQL).
Maybe there are some special rules of using the annotation?
You said that:
The purpose of the lock is to prevent more than one transaction
reading data from the DB because this data influences the generation
of new data and is changed in terms of a transaction.
Oracle uses MVCC (Multiversion Concurrency Control) so Readers don't block Writers and Writers don't block Readers. Even if you acquire a row-level lock with Oracle, and you modify that row without committing, other transactions can still read the last committed value.
Related to this log message:
org.hibernate.loader.Loader - HHH000444: Encountered request for locking however dialect reports that database prefers locking be done in a separate select (follow-on locking); results will be locked after initial query executes
The follow-on locking mechanism is due to Oracle not being able to apply the lock when doing Oracle 11g pagination, using DISTINCT or UNION ALL.
If you're using Oracle 12i, then you can update the Hibernate dialect to Oracle12cDialect and pagination and locking will work fine since Oracle 12 uses the SQL standard pagination and it no longer requires a derived table query.
This does not happen in MariaDB or any other database. It's just an Oracle pre-12 limitation.
If you are using Hibernate 5.2.1, we added a new hint HINT_FOLLOW_ON_LOCKING which disables this mechanism.
So, your Spring Data query becomes:
#QueryHints(value = { #QueryHint(name = "hibernate.query.followOnLocking", value = "false")}, forCounting = false)
#Lock(LockModeType.PESSIMISTIC_WRITE)
User findUserById(#Param("id") String operatorId);
you can also apply it manually:
User user = entityManager.createQuery(
"select u from User u where id = :id", User.class)
.setParameter("id", id);
.unwrap( Query.class )
.setLockOptions(
new LockOptions( LockMode.PESSIMISTIC_WRITE )
.setFollowOnLocking( false ) )
.getSingleResult();
Is there any option to make a Transaction(TxB) to wait for some time (without throwing Lock Acquisition Exception) for another Transaction(TxA) to release the Lock.
We use java datastax cassandra driver 2.1.2. Cassandra version we use is 2.0.9.
We have statement which we build with QueryBuilder and we are setting consistency level to statement on TWO explicitly.
Select selectStatement = QueryBuilder.select().from(ARTICLES);
selectStatement.where(eq(ORGANIZATION_ID, organizationId));
selectStatement.setConsistencyLevel(ConsistencyLevel.TWO);
final ResultSet rs = session.execute(selectStatement);
//call to all() will be removed since it is enough to iterate over result set
//and then you get pagination for free instead of loading everything in
//memory
List<Row> rows = rs.all();
for (final Row row : rows) {
//do something with Row, convert to POJO
}
We get exception like this:
com.datastax.driver.core.exceptions.ReadTimeoutException: Cassandra timeout during read query at consistency ALL (3 responses were required but only 2 replica responded)
com.datastax.driver.core.exceptions.ReadTimeoutException.copy (ReadTimeoutException.java:69)
com.datastax.driver.core.DefaultResultSetFuture.extractCauseFromExecutionException (DefaultResultSetFuture.java:259)
com.datastax.driver.core.ArrayBackedResultSet$MultiPage.prepareNextRow (ArrayBackedResultSet.java:279)
com.datastax.driver.core.ArrayBackedResultSet$MultiPage.isExhausted (ArrayBackedResultSet.java:239)
com.datastax.driver.core.ArrayBackedResultSet$1.hasNext (ArrayBackedResultSet.java:122)
com.datastax.driver.core.ArrayBackedResultSet.all (ArrayBackedResultSet.java:111)
I know that calling all() on ResultSet makes it load all articles for organization in memory and work with it and creates load on cassandra. This will be removed as noted in comments. This can cause read timeout but I am still puzzled why in exception message there is ALL.
Question is why exception is telling that consistency level ALL is used when we set it to TWO for original statement. Is all() internally doing something with query and using CL ALL by default?
Your problem is almost certainly https://issues.apache.org/jira/browse/CASSANDRA-7947 . You are seeing an error message from failing to perform read repair. It is unrelated to your original consistency level. This is fixed in 2.1.3+.
I work at a gaming cybercafe, and we've got a system here (smartlaunch) which keeps track of game licenses. I've written a program which interfaces with this system (actually, with it's backend MySQL database). The program is meant to be run on a client PC and (1) query the database to select an unused license from the pool available, then (2) mark this license as in use by the client PC.
The problem is, I've got a concurrency bug. The program is meant to be launched simultaneously on multiple machines, and when this happens, some machines often try and acquire the same license. I think that this is because steps (1) and (2) are not synchronised, i.e. one program determines that license #5 is available and selects it, but before it can mark #5 as in use another copy of the program on another PC tries to grab that same license.
I've tried to solve this problem by using transactions and table locking, but it doesn't seem to make any difference - Am I doing this right? Here follows the code in question:
public LicenseKey Acquire() throws SmartLaunchException, SQLException {
Connection conn = SmartLaunchDB.getConnection();
int PCID = SmartLaunchDB.getCurrentPCID();
conn.createStatement().execute("LOCK TABLE `licensekeys` WRITE");
String sql = "SELECT * FROM `licensekeys` WHERE `InUseByPC` = 0 AND LicenseSetupID = ? ORDER BY `ID` DESC LIMIT 1";
PreparedStatement statement = conn.prepareStatement(sql);
statement.setInt(1, this.id);
ResultSet results = statement.executeQuery();
if (results.next()) {
int licenseID = results.getInt("ID");
sql = "UPDATE `licensekeys` SET `InUseByPC` = ? WHERE `ID` = ?";
statement = conn.prepareStatement(sql);
statement.setInt(1, PCID);
statement.setInt(2, licenseID);
statement.executeUpdate();
statement.close();
conn.commit();
conn.createStatement().execute("UNLOCK TABLES");
return new LicenseKey(results.getInt("ID"), this, results.getString("LicenseKey"), results.getInt("LicenseKeyType"));
} else {
throw new SmartLaunchException("All licenses of type " + this.name + "are in use");
}
}
You must do two things:
Wrap your code in a transaction (to avoid autocommit releasing locks immediately)
Use SELECT ... FOR UPDATE and mysql will give you the lock you need (released on commit)
SELECT ... FOR UPDATE is better than LOCK TABLE as it can possibly get by with row-level locking, instead of automatically locking the whole table
According to the online manual, the correct syntax for locking is:
LOCK TABLES ...
and you have
LOCK TABLE ...
but you don't have any error checking. Hence you're probably failing to get the lock and it's silently ignoring that.
FWIW, I'd put your cleanup code (UNLOCK TABLES, conn.commit(), etc) in a finally block to ensure that you always clean up properly in the event of an exception.
As it is, you appear to be potentially leaking database connection handles, and never releasing the lock if there's no free license.
I would like to suggest just doing an update statement and checking how many rows where updated. i will write it out in psudo code.
int uniqueId = SmartLaunchDB.getCurrentPCID();;
int updatedRows = execute('UPDATE `licensekeys` SET `InUseByPC` = uniqueId WHERE `InUseByPC` NOT null LIMIT1')
if (updatedRows == 1)
SUCCESS
else
FAIL
If it succeeds you can then get the licence key/ID by doing a select.
As is so often the case, OP is an idiot. The code I posted was actually working, but I've just discovered a duplicate row in the database - I guess someone entered the same license twice by mistake. This led me to believe that a concurrency bug I had fixed (by introducing table locks) was still unfixed.
Thanks for the general advice, I've introduced better exception handling to this method.