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<DIV class="panel" style="background-color: #ffffff;border-color: #ECF4D1;border-style: solid;border-width: 1px;"><DIV class="panelHeader" style="border-bottom-width: 1px;border-bottom-style: solid;border-bottom-color: #ECF4D1;background-color: #ECF4D1;"><B>Transaction Management in RDB-Data Access Service (RDB DAS)</B></DIV><DIV class="panelContent" style="background-color: #ffffff;">
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<UL>
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<LI><A href="#RDBDAS-TransactionControl-Overview">Overview</A></LI>
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<LI><A href="#RDBDAS-TransactionControl-DASmanagedtransactions">DAS managed transactions</A></LI>
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<LI><A href="#RDBDAS-TransactionControl-Externallymanagedtransactions">Externally managed transactions</A></LI>
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</UL>
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</DIV></DIV>
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<H3><A name="RDBDAS-TransactionControl-Overview"></A><A name="RDBDAS-TransactionControl-Overview"></A>Overview</H3>
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<P>A typical enterprise application accesses and stores information in one or more databases. Data integrity is <BR>
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the most essential aspect of correct business functioning. Transactions control the concurrent access of data <BR>
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by multiple programs. In the event of a system failure, transactions make sure that after recovery the data will<BR>
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be in a consistent state. This document helps to understand transaction management in RDB DAS.</P>
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<P>One DAS instance is always associated with one database and one Connection. DAS participates in <BR>
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<B>A> DAS managed and B> Externally managed transactions</B>.</P>
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<H3><A name="RDBDAS-TransactionControl-DASmanagedtransactions"></A><A name="RDBDAS-TransactionControl-DASmanagedtransactions"></A>DAS managed transactions</H3>
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<P>By default, the RDB DAS assumes responsibility for managing transaction boundaries by executing commit and <BR>
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rollback operations against the database. A configuration attribute managedtx is assumed with default value <BR>
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TRUE. This signals DAS to control the transaction. The boundaries of these transactions are as follows:</P>
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<P><B>Reading Data</B><BR>
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Queries to the database are always made by calling DAS command's "executeQuery()" method. The associated <BR>
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transaction lives for the duration of this method execution. That is, the transaction can be thought to start <BR>
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when control is transferred to "executeQuery" and the transaction ends just prior to return from this method.</P>
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<P><B>Writing Data</B><BR>
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Writes can be accomplished by two DAS APIs. First is calling the DAS "execute()" command against a command the <BR>
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will perform write operations and, in this case, the transaction boundaries are the same as read queries. That <BR>
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is the transaction life matches the life of the "execute" processing. If the method returns without exception <BR>
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then the application can assume that any changes were successfully committed to the database. If an exception is <BR>
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returned then the changes will have all been rolled back.</P>
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<P>The second API used by clients is the DAS applyChanges() method (see ChangeSummaryProcessing). Again, the <BR>
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boundaries of the transaction match the boundaries of the call to the method. The DAS will create a set of <BR>
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INSERT/UPDATE and DELETE statements to flush the graph changes to the database and all of these statements <BR>
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will be executed as part of a single transaction. If the method returns without exception then the application <BR>
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can assume that all changes were successfully committed to the database. If an exception is returned then the changes <BR>
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will have all been rolled back.</P>
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<P><B>Use</B></P>
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<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>
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Commands in one transaction unit. At the maximum one applyChanges() call having multiple INSERT/UPDATE/DELETE executes <BR>
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under one transaction. Cases where multiple applyChanges() method calls or multiple executeQuery() method calls are <BR>
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required to be part of one transaction, this mode is not useful. In this mode, the database Connection can be internally <BR>
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obtained using DAS Config ConnectionInfo settings or database Connection can also be passed from the client to DAS instance. <BR>
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When DAS obtains database Connection internally, in Config ConnectionInfo, settings can either specify DataSource name or <BR>
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Database Driver related information. Typical examples - </P>
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<UL>
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<LI>When using Tomcat based web app with DAS, Tomcat configured DataSource name is set in the DAS Config.</LI>
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<LI>When using J2SE application client with DAS, Database Driver related information from DAS Config is used to form a database Connection.</LI>
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</UL>
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<P>When multiple SQL statements/DAS commands need to be part of one transaction and also when multiple DAS instances need to participate <BR>
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in one transaction, the other mode "Externally managed transactions" is useful.</P>
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<H3><A name="RDBDAS-TransactionControl-Externallymanagedtransactions"></A><A name="RDBDAS-TransactionControl-Externallymanagedtransactions"></A>Externally managed transactions</H3>
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<P>The RDB DAS provides a straightforward mechanism for working with external transactions. In this mode, DAS is configured to not issue <BR>
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commit/rollback. This behavior is needed to have the DAS participate in a larger transaction since some external party is responsible <BR>
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for these actions. </P>
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<P><B>Use</B></P>
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<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>
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client's transaction if it is available. DAS does not throw exception if external client transaction is not available, but the end result <BR>
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without external transaction is unpredictable. So, when using DAS in this mode, client needs to ensure availability of external transaction <BR>
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encompassing required DAS/s commands. External transaction can be JDBC transaction or JTA transaction. </P>
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<P>When JDBC transaction, it is possible to group multiple Commands from single DAS instance in one transaction. Also nested transactions using <BR>
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multiple DAS instances can be carried out. AS JTA does not support nested transactions, if <B>nested transactions</B> is the requirement, JDBC <BR>
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Transactions should be used. For example:-</P>
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<DIV class="preformatted panel" style="border-width: 1px;"><DIV class="preformattedContent panelContent">
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<PRE>Das1 = DAS.FACTORY.createDAS(jdbcconn1);
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Das2 = DAS.FACTORY.createDAS(jdbcconn2);
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try {
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doSomeDAS1Stuff();
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doSomeDAS2Stuff();
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jdbcconn2.commit();
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doSomeDAS1Stuff();
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jdbcconn1.commit();
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} catch (AnyException ex) {
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jdbcconn1.rollback();
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jdbcconn2.rollback();
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}
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</PRE>
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</DIV></DIV>
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<P>When JTA transaction, multiple units like JMS, DAS1(derbydatabase1), DAS2(derbydatabase2/mysqldatabase2) can be part of single transaction. <BR>
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See example:-</P>
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<DIV class="preformatted panel" style="border-width: 1px;"><DIV class="preformattedContent panelContent">
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<PRE>tx.begin();
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try {
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doSomeJMSStuff();
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doSomeDAS1Stuff();
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doSomeDAS2Stuff();
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tx.commit();
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} catch (AnyException ex) {
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tx.rollback();
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}
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</PRE>
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</DIV></DIV>
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<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>
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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>
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illustrates this setting:</P>
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<DIV class="preformatted panel" style="border-width: 1px;"><DIV class="preformattedContent panelContent">
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<PRE><Config xsi:noNamespaceSchemaLocation="http:///org.apache.tuscany.das.rdb/config.xsd"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
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<ConnectionInfo managedtx="false"/>
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</Config>
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</PRE>
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</DIV></DIV>
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<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>
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instance. There is no meaning to forming Connection internally in DAS using Config info, as such Connections will not be part of external <BR>
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Transaction Control.</P>
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<P><B>Methods Not Used in Externally managed transactions</B><BR>
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DAS does not invoke any method that might interfere with the transaction boundaries set by the Transaction Manager. The list of prohibited <BR>
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methods follows: </P>
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<UL>
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<LI>The commit, setAutoCommit, and rollback methods of java.sql.Connection</LI>
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<LI>The getUserTransaction method of javax.ejb.EJBContext</LI>
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<LI>Any method of javax.transaction.UserTransaction</LI>
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</UL>
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<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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