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<LI><A href="#RDBDAS-TransactionControl-Overview">Overview</A></LI>
<LI><A href="#RDBDAS-TransactionControl-DASmanagedtransactions">DAS managed transactions</A></LI>
<LI><A href="#RDBDAS-TransactionControl-Externallymanagedtransactions">Externally managed transactions</A></LI>
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<H3><A name="RDBDAS-TransactionControl-Overview"></A><A name="RDBDAS-TransactionControl-Overview"></A>Overview</H3>
<P>A typical enterprise application accesses and stores information in one or more databases. Data integrity is <BR>
the most essential aspect of correct business functioning. Transactions control the concurrent access of data <BR>
by multiple programs. In the event of a system failure, transactions make sure that after recovery the data will<BR>
be in a consistent state. This document helps to understand transaction management in RDB DAS.</P>
<P>One DAS instance is always associated with one database and one Connection. DAS participates in <BR>
<B>A> DAS managed and B> Externally managed transactions</B>.</P>
<H3><A name="RDBDAS-TransactionControl-DASmanagedtransactions"></A><A name="RDBDAS-TransactionControl-DASmanagedtransactions"></A>DAS managed transactions</H3>
<P>By default, the RDB DAS assumes responsibility for managing transaction boundaries by executing commit and <BR>
rollback operations against the database. A configuration attribute managedtx is assumed with default value <BR>
TRUE. This signals DAS to control the transaction. The boundaries of these transactions are as follows:</P>
<P><B>Reading Data</B><BR>
Queries to the database are always made by calling DAS command's "executeQuery()" method. The associated <BR>
transaction lives for the duration of this method execution. That is, the transaction can be thought to start <BR>
when control is transferred to "executeQuery" and the transaction ends just prior to return from this method.</P>
<P><B>Writing Data</B><BR>
Writes can be accomplished by two DAS APIs. First is calling the DAS "execute()" command against a command the <BR>
will perform write operations and, in this case, the transaction boundaries are the same as read queries. That <BR>
is the transaction life matches the life of the "execute" processing. If the method returns without exception <BR>
then the application can assume that any changes were successfully committed to the database. If an exception is <BR>
returned then the changes will have all been rolled back.</P>
<P>The second API used by clients is the DAS applyChanges() method (see ChangeSummaryProcessing). Again, the <BR>
boundaries of the transaction match the boundaries of the call to the method. The DAS will create a set of <BR>
INSERT/UPDATE and DELETE statements to flush the graph changes to the database and all of these statements <BR>
will be executed as part of a single transaction. If the method returns without exception then the application <BR>
can assume that all changes were successfully committed to the database. If an exception is returned then the changes <BR>
will have all been rolled back.</P>
<P><B>Use</B></P>
<P>This mode is useful for simple data operations, where there is no need of having group of SQL statements or group of DAS <BR>
Commands in one transaction unit. At the maximum one applyChanges() call having multiple INSERT/UPDATE/DELETE executes <BR>
under one transaction. Cases where multiple applyChanges() method calls or multiple executeQuery() method calls are <BR>
required to be part of one transaction, this mode is not useful. In this mode, the database Connection can be internally <BR>
obtained using DAS Config ConnectionInfo settings or database Connection can also be passed from the client to DAS instance. <BR>
When DAS obtains database Connection internally, in Config ConnectionInfo, settings can either specify DataSource name or <BR>
Database Driver related information. Typical examples - </P>
<UL>
<LI>When using Tomcat based web app with DAS, Tomcat configured DataSource name is set in the DAS Config.</LI>
<LI>When using J2SE application client with DAS, Database Driver related information from DAS Config is used to form a database Connection.</LI>
</UL>
<P>When multiple SQL statements/DAS commands need to be part of one transaction and also when multiple DAS instances need to participate <BR>
in one transaction, the other mode "Externally managed transactions" is useful.</P>
<H3><A name="RDBDAS-TransactionControl-Externallymanagedtransactions"></A><A name="RDBDAS-TransactionControl-Externallymanagedtransactions"></A>Externally managed transactions</H3>
<P>The RDB DAS provides a straightforward mechanism for working with external transactions. In this mode, DAS is configured to not issue <BR>
commit/rollback. This behavior is needed to have the DAS participate in a larger transaction since some external party is responsible <BR>
for these actions. </P>
<P><B>Use</B></P>
<P>The scope of transaction support is a blend of <B>Supports</B> and <B>Mandatory</B> transaction attributes in J2EE scenario. i.e. DAS will use the <BR>
client's transaction if it is available. DAS does not throw exception if external client transaction is not available, but the end result <BR>
without external transaction is unpredictable. So, when using DAS in this mode, client needs to ensure availability of external transaction <BR>
encompassing required DAS/s commands. External transaction can be JDBC transaction or JTA transaction. </P>
<P>When JDBC transaction, it is possible to group multiple Commands from single DAS instance in one transaction. Also nested transactions using <BR>
multiple DAS instances can be carried out. AS JTA does not support nested transactions, if <B>nested transactions</B> is the requirement, JDBC <BR>
Transactions should be used. For example:-</P>
<DIV class="preformatted panel" style="border-width: 1px;"><DIV class="preformattedContent panelContent">
<PRE>Das1 = DAS.FACTORY.createDAS(jdbcconn1);
Das2 = DAS.FACTORY.createDAS(jdbcconn2);
try {
doSomeDAS1Stuff();
doSomeDAS2Stuff();
jdbcconn2.commit();
doSomeDAS1Stuff();
jdbcconn1.commit();
} catch (AnyException ex) {
jdbcconn1.rollback();
jdbcconn2.rollback();
}
</PRE>
</DIV></DIV>
<P>When JTA transaction, multiple units like JMS, DAS1(derbydatabase1), DAS2(derbydatabase2/mysqldatabase2) can be part of single transaction. <BR>
See example:-</P>
<DIV class="preformatted panel" style="border-width: 1px;"><DIV class="preformattedContent panelContent">
<PRE>tx.begin();
try {
doSomeJMSStuff();
doSomeDAS1Stuff();
doSomeDAS2Stuff();
tx.commit();
} catch (AnyException ex) {
tx.rollback();
}
</PRE>
</DIV></DIV>
<P>In both the examples above, the DAS1 and DAS2 have been configured to not manage transactions since this is handled by the calling program.<BR>
The DAS is set to behave this way by a piece of configuration (typically via a configuration xml file). The following config file snippet <BR>
illustrates this setting:</P>
<DIV class="preformatted panel" style="border-width: 1px;"><DIV class="preformattedContent panelContent">
<PRE><Config xsi:noNamespaceSchemaLocation="http:///org.apache.tuscany.das.rdb/config.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<ConnectionInfo managedtx="false"/>
</Config>
</PRE>
</DIV></DIV>
<P><B>Note</B> When using DAS with "Externally managed transactions", it is required by the client to pass appropriate Connection object to DAS <BR>
instance. There is no meaning to forming Connection internally in DAS using Config info, as such Connections will not be part of external <BR>
Transaction Control.</P>
<P><B>Methods Not Used in Externally managed transactions</B><BR>
DAS does not invoke any method that might interfere with the transaction boundaries set by the Transaction Manager. The list of prohibited <BR>
methods follows: </P>
<UL>
<LI>The commit, setAutoCommit, and rollback methods of java.sql.Connection</LI>
<LI>The getUserTransaction method of javax.ejb.EJBContext</LI>
<LI>Any method of javax.transaction.UserTransaction</LI>
</UL>
<P><B>Note</B> DAS never allows AutoCommit(TRUE) for the Connection it is using and throws Exception if this mode is detected.</P>
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