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CopyMode attribute

You can tune the number of copies by defining the CopyMode attribute of the BackingMap or ObjectMap objects.

You can tune the number of copies by defining the CopyMode attribute of the BackingMap or ObjectMap objects. The copy mode has the following values:

The COPY_ON_READ_AND_COMMIT value is the default. The COPY_ON_READ value copies on the initial data retrieved, but does not copy at commit time. This mode is safe if the application does not modify a value after committing a transaction. The NO_COPY value does not copy data, which is only safe for read-only data. If the data never changes then you do not need to copy it for isolation reasons.

Be careful when you use the NO_COPY attribute value with maps that can be updated. WebSphere eXtreme Scale uses the copy on first touch to allow the transaction rollback. The application only changed the copy, and as a result, eXtreme Scale discards the copy. If the NO_COPY attribute value is used, and the application modifies the committed value, completing a rollback is not possible. Modifying the committed value leads to problems with indexes, replication, and so on because the indexes and replicas update when the transaction commits. If you modify committed data and then roll back the transaction, which does not actually roll back at all, then the indexes are not updated and replication does not take place. Other threads can see the uncommitted changes immediately, even if they have locks. Use the NO_COPY attribute value for read-only maps or for applications that complete the appropriate copy before modifying the value. If you use the NO_COPY attribute value and call IBM support with a data integrity problem, you are asked to reproduce the problem with the copy mode set to COPY_ON_READ_AND_COMMIT.

The COPY_TO_BYTES value stores values in the map in a serialized form. At read time, eXtreme Scale inflates the value from a serialized form and at commit time it stores the value to a serialized form. With this method, a copy occurs at both read and commit time.

The default copy mode for a map can be configured on the BackingMap object. You can also change the copy mode on maps before you start a transaction by using the ObjectMap.setCopyMode method.

An example of a backing map snippet from an objectgrid.xml file that shows how to set the copy mode for a given backing map follows. This example assumes that you are using cc as the objectgrid/config namespace.

<cc:backingMap name="RuntimeLifespan" copyMode="NO_COPY"/>

See CopyMode best practices for more information.


Parent topic:

Transaction processing overview


Related concepts

Transactions

Map entry locking

Locking strategies

Distribute transactions

Single-partition and cross-data-grid transactions