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  PUBLIC "-//OASIS//DTD DITA Task//EN" "task.dtd">
<task id="GUID-5F641F7A-AEAA-5C3E-9179-C3707F10E6DC" xml:lang="en"><title>Starting
and Finishing Transactions: Tutorial</title><shortdesc>This topic shows you how to start, finish and roll back a transaction
with the Comms Database. </shortdesc><prolog><metadata><keywords/></metadata></prolog><taskbody>


<prereq id="GUID-17B4987D-BD5C-5CD8-8E51-251431BD4E70"><p>Before you start,
you must understand: </p> <ul>
<li id="GUID-CE226134-308A-5B1C-BE3F-8AD1976CA1F0"><p>the general concept
of the Comms Database </p> </li>
<li id="GUID-5616BB6A-1FAE-56C3-B996-6AA7A1D41A75"><p>the specific concept
of Comms Database Transactions </p> </li>
<li id="GUID-F546E315-A6F2-5612-9DB2-66C6037E0FC3"><p>how to write and build
application code to run on Symbian platform </p> </li>
</ul> </prereq>
<context id="GUID-47D01F8A-228E-5D1B-8764-13AC73E99950"><p>A transaction is
a sequence of read and write operations. In a transaction all operations must
be successful to make sure of the integrity of the data. </p> <p>A tool or
application must mark the start of a transaction. The tool or application
must also mark the end of a transaction. </p> <p>A transaction ends normally
or the transaction is rolled back. A transaction is rolled back when one of
the read or write operations fails. Other events can cause the tool or application
to abandon the transaction. Other events depend on the purpose of the tool
or application. </p> <p>Create a session with the Comms Database before you
start a transaction. </p> <p>There are many ways to implement transactions.
The procedure below is one method. The method shows the principle of transactions.
The method you use depends on the organisation of your code. </p> </context>
<steps id="GUID-012CB103-90DC-54F1-A4B4-512498EBC563">
<step id="GUID-1E102596-F9BE-57BD-84A4-4C51BCD8A340"><cmd>Make sure that you
have created a session before you start your first transaction. </cmd>
</step>
<step id="GUID-02951E12-339B-52D2-B42F-4977DC543090"><cmd>Start the transaction
before you start the operations that access the database. Commit the transaction
if all operations are successful. Rollback the transaction if any operation
fails. </cmd>

<info>The code uses the functions: <xref href="GUID-1CDD0B97-8B00-3373-9908-512C9BC1CF51.dita"><apiname>CMDBSession::OpenTransactionL()</apiname></xref>, <xref href="GUID-1CDD0B97-8B00-3373-9908-512C9BC1CF51.dita"><apiname>CMDBSession::CommitTransactionL()</apiname></xref> and <xref href="GUID-1CDD0B97-8B00-3373-9908-512C9BC1CF51.dita"><apiname>CMDBSession::RollbackTransactionL()</apiname></xref>. </info>
<stepxmp><codeblock id="GUID-FAD16FB7-AC98-5579-AF89-848633BA7D10" xml:space="preserve">...

// This code fragment assumes that a session with the Comms Database has been created.
// iDb is a pointer to a CMDBSession object

TInt rc;

TRAP(rc,PerformSequenceOfOperationsL());
if (!rc)
    {
    // Sequence of read and write operations has failed.
    iDb-&gt;RollbackTransactionL();
    ...
    }
...</codeblock> </stepxmp>
<stepxmp><codeblock id="GUID-B7D81AF6-6D77-5CE9-A91A-E8093BA2802E" xml:space="preserve">...
void PerformSequenceOfOperationsL()
    {
    // Start a transaction
    iDb-&gt;OpenTransactionL();
    ...
    // Sequence of read and write operations
    ...
    // Commit the changes to the Comms Database. This assumes that
    // all operations have completed successfully, and no other event has
    // caused the sequence of operations to be abandoned.
    iDb-&gt;CommitTransactionL()
    ...
    }
...</codeblock> </stepxmp>
</step>
</steps>
</taskbody><related-links>
<link href="GUID-1AFDDD6F-CB99-587D-A0B5-D3F5B27F7135.dita"><linktext>Comms Database
                concepts</linktext></link>
</related-links></task>