[
http://issues.apache.org/jira/browse/DERBY-421?page=comments#action_12320544 ]
Mamta A. Satoor commented on DERBY-421:
---------------------------------------
Kathey, thanks for your explanation of prepareIsolationLevel in
GenericLanguageConnectionContext.java.
As for your question, following set of steps hopefully will help understand why
the flag needs to be reset.
Say, the user application has following code.
XAConnection xac = xaDataSource.getXAConnection();
At this point, xac will be EmbedXAConnection object with real database
connection EmbedConnection in it.
The user application can then request a connection handle (BrokeredConnection)
from EmbedXAConnection. Derby will set the state of this connection handle to
that of the real database connection.
Connection con = xac.getConnection();
Let's say the local transaction then sets the state using JDBC api
con.setTransactionIsolation(Connection.TRANSACTION_READ_UNCOMMITTED);
This call will change the isolation level of the real database connection to
the new value and the state of the connection handle also gets set correctly
inside BrokeredConnection.setTransactionIsolation method.
But if the local transaction sets the state using SQL
BrokeredStatement.executeUpdate("set current isolation = RR");
real database connection's isolation level gets set to new value but the state
information in BrokeredConnection doesn't.
Now if the user application says
XAResource.start(xid, XAResource.TMNOFLAGS);
Before starting a global transaction, EmbedXAConnection's start method checks
if there is already a real database connection and a handle to it. If yes, it
calls connection handle's setState(true). (The current Derby code without my
changes) sets the state of the real database connection object to the handle's
state. But handle's state is out of sync at this point because of earlier SQL
set current isolation. And hence the state setting onto real connection will be
incorrect. (this is the cause of Derby-421)
To fix this, I have a call to connection handle's (BrokeredConnection's)
getIsolationUptoDate() method prior to setting the real connection's state to
connection handle's state. If the isolation was changed via JDBC/SQL, the
connection handle's state will be brought to the correct value, and the flag
used to keep track of isolation level change will be set to false.
Similar thing needs to happen when the global transaction is ended because
isolation level could have been set using SQL inside the global transaction as
shown below.
BrokeredStatement.executeUpdate("set current isolation = UR");
Again, this will update the real database connection's isolation to UR, but
BrokeredConnection's state does not get changed.
To correct the state information of BrokeredConnection, when the global
transation is ended as follows
XAResource.end(xid, XAResource.TMSUCCESS);
EmbedXAConnection's end method checks if the BrokeredConnection is not null. If
not null, it brings the isolation level upto date by calling
BrokeredConnection.getIsolationUptoDate and then resets the flag.
If say, the flag is not reset after fixing the BrokeredConnection's state
information during the global transaction start and end times, then once
isolation level is changed in a global transaction, local transaction will
always take the isolation level of last global transaction that just ended.
This is incorrect. Local transaction should take the isolation level of last
global transaction only if the isolation level was changed in that transaction.
Following is the incorrect behavior with no flag resetting:
Start a local transaction.
isolation level default CS
Start global transaction 1
isolation level default CS
Change isolation level in global transaction 1 to UR(the flag gets set
to true at this point)
Exit global transaction 1. Isolation level is local transaction
isolation level UR
Change isolation level in local transaction to RS
Start global transaction 2
isolation level RS
End global transaction 2 and rejoin global transaction 1
isolation level UR
Exit global transaction 1. The isolation level in local transaction at this
point should have been RS but it got set to UR which is the last global
transaction's isolation level. This happened because the flag was not reset to
false at the end of first global transaction when the isolation was set using
JDBC/SQL.
Kathey, I hope this is not too much information and helps answer your question.
If I have missed a scenario in which we could run into problem, let me know.
Also, Dan, I am making couple changes to the code and putting some more
comments and will send a new review package soon(after running the tests) with
explanation as to why we need so many methods.
> starting an XA transaction resets the isolation level set with SET CURRENT
> ISOLATION
> ------------------------------------------------------------------------------------
>
> Key: DERBY-421
> URL: http://issues.apache.org/jira/browse/DERBY-421
> Project: Derby
> Type: Sub-task
> Components: JDBC
> Reporter: Kathey Marsden
> Assignee: Mamta A. Satoor
> Attachments: derby421XAIsolation082205.txt
>
> When an XA Transaction is started the isolation level set with SET CURRENT
> ISOLATION gets reset to CS.
> Embedded setTransactionIsolation does not have this problem but this problem
> is the root cause of DERBY-414 because client implements
> setTransactionIsolation by sending SET CURRENT ISOLATION
> $ java TestSetCurrentIsolation
> Database product: Apache Derby
> Database version: 10.2.0.0 alpha
> Driver name: Apache Derby Embedded JDBC Driver
> Driver version: 10.2.0.0 alpha
> SET CURRENT ISOLATION = UR
> CURRENT ISOLATION: UR
> getTransactionIsolation:TRANSACTION_READ_UNCOMMITTED:1
> Isolation level after xa start
> CURRENT ISOLATION: CS
> getTransactionIsolation:TRANSACTION_READ_COMMITTED:2
> $
> import java.sql.*;
> import javax.sql.*;
> import javax.transaction.xa.*;
> public class TestSetCurrentIsolation
> {
> public static void main(String[] args) throws Throwable
> {
> try
> {
> final org.apache.derby.jdbc.EmbeddedXADataSource ds =
> new org.apache.derby.jdbc.EmbeddedXADataSource();
> ds.setDatabaseName("C:\\drivers\\derby\\databases\\SCHEDDB");
> ds.setUser("dbuser1");
> ds.setPassword("******");
> XAConnection xaConn = ds.getXAConnection();
> Connection conn = xaConn.getConnection();
> conn.setAutoCommit(true);
> System.out.println("Database product: " +
> conn.getMetaData().getDatabaseProductName());
> System.out.println("Database version: " +
> conn.getMetaData().getDatabaseProductVersion());
> System.out.println("Driver name: " +
> conn.getMetaData().getDriverName());
> System.out.println("Driver version: " +
> conn.getMetaData().getDriverVersion());
> Statement stmt = conn.createStatement();
> System.out.println("SET CURRENT ISOLATION = UR");
> stmt.executeUpdate("SET CURRENT ISOLATION = UR");
> showIsolationLevel(conn);
> conn.setAutoCommit(false);
> XAResource xaRes = xaConn.getXAResource();
> Xid xid = new TestXid(1,(byte) 32, (byte) 32);
> xaRes.start(xid, XAResource.TMNOFLAGS);
> System.out.println("Isolation level after xa start");
> showIsolationLevel(conn);
>
> xaRes.end(xid, XAResource.TMSUCCESS);
> xaRes.rollback(xid);
> conn.close();
> xaConn.close();
> }
> catch (SQLException sqlX)
> {
> System.out.println("Error on thread 1.");
> do sqlX.printStackTrace();
> while ((sqlX = sqlX.getNextException()) != null);
> }
> catch (Throwable th)
> {
> System.out.println("Error on thread 1.");
> do th.printStackTrace();
> while ((th = th.getCause()) != null);
> }
> }
> /**
> * @param conn
> * @throws SQLException
> */
> private static void showIsolationLevel(Connection conn) throws
> SQLException {
> PreparedStatement ps = conn.prepareStatement("VALUES CURRENT
> ISOLATION");
> ResultSet rs = ps.executeQuery();
> //ResultSet rs = conn.createStatement().executeQuery("VALUES
> CURRENT ISOLATION");
> rs.next();
> System.out.println("CURRENT ISOLATION: " + rs.getString(1));
> System.out.println("getTransactionIsolation:" +
>
> getIsoLevelName(conn.getTransactionIsolation()));
>
> }
>
> public static String getIsoLevelName(int level)
> {
> switch (level) {
> case java.sql.Connection.TRANSACTION_REPEATABLE_READ:
> return "TRANSACTION_REAPEATABLE_READ:" + level;
>
> case java.sql.Connection.TRANSACTION_READ_COMMITTED:
> return "TRANSACTION_READ_COMMITTED:" + level;
> case java.sql.Connection.TRANSACTION_SERIALIZABLE:
> return "TRANSACTION_SERIALIZABLE:" + level;
> case java.sql.Connection.TRANSACTION_READ_UNCOMMITTED:
> return "TRANSACTION_READ_UNCOMMITTED:" + level;
> }
> return "UNEXPECTED_ISO_LEVEL";
> }
> }
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