[ http://jira.jboss.com/jira/browse/JBMQ-2?page=comments#action_12310800 ]
Adrian Brock commented on JBMQ-2:
---------------------------------
The fundamental problem is databases that try to retrieve the entire
result set into memory regardless of the size of the query.
This can cause out of memory problems before the message cache gets chance
to "soften" the messages.
I don't see how this can be done in an easily portable way,
e.g. here is the MySQL mechanism to avoid the problem which is very
inefficient:
"By default, ResultSets are completely retrieved and stored in memory. In most
cases this is the most efficient way to operate, and due to the design of the
MySQL network protocol is easier to implement. If you are working with
ResultSets that have a large number of rows or large values, and can not
allocate heap space in your JVM for the memory required, you can tell the
driver to 'stream' the results back one row at-a-time.
To enable this functionality, you need to create a Statement instance in the
following manner:
stmt = conn.createStatement(java.sql.ResultSet.TYPE_FORWARD_ONLY,
java.sql.ResultSet.CONCUR_READ_ONLY);
stmt.setFetchSize(Integer.MIN_VALUE);
The combination of a forward-only, read-only result set, with a fetch size of
Integer.MIN_VALUE serves as a signal to the driver to "stream" result sets
row-by-row. After this any result sets created with the statement will be
retrieved row-by-row.
There are some caveats with this approach. You will have to read all of the
rows in the result set (or close it) before you can issue any other queries on
the connection, or an exception will be thrown. Also, any tables referenced by
the query that created the streaming result will be locked until all of the
results have been read or the connection closed."
> Need better control over the pm memory usage during destination recovery
> ------------------------------------------------------------------------
>
> Key: JBMQ-2
> URL: http://jira.jboss.com/jira/browse/JBMQ-2
> Project: JBoss MQ
> Type: Patch
> Versions: JBossAS-3.2.5
> Reporter: Scott M Stark
> Assignee: Adrian Brock
>
>
> When starting up jboss with a destination with either a large number or very
> large persistent messages, the jdbc pm may run out of memory if the select *
> from the db results in the jdbc driver pulling in all of the records. We need
> to either write the pm with this expectation or allow for some configuration
> to avoid this.
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