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WorkingWithOptimisticConcurrencyControl
The RDB DAS provides a straightforward mechanism for support of OptimisticConcurrencyControl. To enable OCC a client must designate an integer version column in the configuration (usually via a config xml file). The following example is an example of a cofig file that identifies a version column:<Config
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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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<Command
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name="select
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book
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1"
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SQL="select
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*
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from
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BOOK
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where
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BOOK_ID
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=
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1"
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kind="Select"/>
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<Table
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tableName="BOOK">
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<Column
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columnName="BOOK_ID"
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primaryKey="true"/>
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<Column
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columnName="OCC"
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collision="true"
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managed="true"/>
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</Table>
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</Config>
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This
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line
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in
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the
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config
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file
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_<Column
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columnName="OCC"
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collision="true"
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managed="true"/>_designates
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the
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column
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named
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"OCC"
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as
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the
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version
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column
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and
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also
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sets
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"managed"
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to
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true.
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The
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"managed"
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attribute
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has
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to
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do
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with
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how
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the
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version
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column
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is
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changed.
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In
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order
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for
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OCC
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to
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work,
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some
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agent
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must
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change
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the
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value
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of
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the
...
version
...
column
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each
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time
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the
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row
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is
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modified.
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A
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new
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value
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in
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the
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version
...
column
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is
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what
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tells
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the
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DAS
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that
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the
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row
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has
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been
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modified.
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If
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a
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version
...
column
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is
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"managed"
...
then
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the
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DAS
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is
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responsible
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for
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incrementing
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the
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value
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of
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the
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column
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each
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time
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some
...
column
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in
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the
...
row
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is
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changed.
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The
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other
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option
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is
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"managed=
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false"
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and
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in
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this
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case,
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the
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DAS
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will
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assume
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some
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other
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agent
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is
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responsible
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for
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updating
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the
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version
...
column.
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This
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other
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agent
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is
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likely
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to
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be
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the
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client
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application
...
itself
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or
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the
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database
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via
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a
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trigger
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or
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some
...
other
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mechanism.
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When
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applying
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changes
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made
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to
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a
...
graph
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(see
...
...
),
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the
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DAS
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will
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throw
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an
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exception
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if
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any
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table
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row
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has
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been
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modified
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since
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the
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row
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was
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read.
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For
...
example,
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if
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a
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DAS
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client
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reads
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a
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row
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representing
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a
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specific
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Customer
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and
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then
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tries
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to
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update
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that
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Customer,
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the
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DAS
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will
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throw
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an
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exception
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if
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the
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Customer
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has
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already
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been
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modified
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by
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some
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other
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agent.
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The
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following
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example
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illustrates
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applying
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changes
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and
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checking
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for
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a
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collision
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failure:
| Code Block |
|---|
DAS\* das = DASImpl::getFactory().createDAS(Config("ManagedBooksConfig.xml"), getConnection()); // Read a book instance CommandPtr select = das->getCommand("select book 1"); DataObjectPtr root = select->executeQuery(); DataObjectPtr book = root->getDataObject("BOOK1"); book->setInt("QUANTITY", 2); // Try to apply changes and catch potential update collision try { { das.applyChanges(root); } catch (DASOptimisticConcurrencyControlException&) { { recover(); } |