O'Reilly - Java Message Service

Java Message Service
The logic is fairly simple: if the message is NOT redelivered AND it is NOT already in a
rollback, AND the rollbackOnly flag is true, THEN roll back the transaction. Otherwise
commit the transaction.
The inRollback flag is needed because of the behavior of the Session.rollback( )
method. It causes all of the messages to be redelivered. In our case, that includes the
"SEQUENCE_MARKER" message, which will always be the last message that is redelivered as a
result of the rollback. [4] Without this flag, it would be impossible to tell whether the
messages are being redelivered as a result of the transaction rollback, or for some other
reason.
[4] This illustrates the importance of the JMS provider's role in maintaining proper delivery order,
especially in the redelivery case.
Now you can see the reason for using the delayed rollback. This allows us to isolate the
commit( ), rollback( ), and message redelivery logic in the handling of the
"SEQUENCE_MARKER" message. Dealing with the rollback individually as each message
arrives can quickly become unwieldy.
Finally, let's try this redelivery logic out by simulating a failure. In the Wholesaler
window, type:
Surfboards, 999.99, 499.99
Wetsuits, 299.99, 149.99
Before you type end, simulate an abnormal shut down of the Retailer application by
typing Ctrl-C in the Retailer command window. Then type "end" in the Wholesaler
window.
Now restart the Retailer application. When the retailer app comes back up, it shows that it
is receiving the surfboard and wetsuit messages as redelivered messages and is placing the
order again. The JMS provider knew that the Retailer had failed in the middle of a
transaction. When the Retailer came back up and reconnected, the JMS provider
redelivered the messages to it. The Retailer logic knew that the message was being
redelivered as a result of a failure, as shown in the following section of autoBuy( ). The
logic is fairly simple. If we are at the end marker:
if ( sequence.equalsIgnoreCase("END_SEQUENCE") )
{
AND we are both redelivered AND in a rollback (we aren't in a rollback):
if ( redelivered && inRollback ){ // At the end, start fresh
inRollback = false;
rollbackOnly = false;
session.commit();
}
Else if we are in rollbackOnly mode (we aren't, but we could be and it would still work):
else if ( rollbackOnly ){
inRollback = true;
session.rollback( );
}
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