SQL/MX Queuing and Publish/Subscribe Services

Major Queuing Features
HP NonStop SQL/MX Queuing and Publish/Subscribe Services—523734-002
2-8
AFTER LAST ROW
If a third application enqueues an invoice, either the first or second application can
dequeue the new invoice:
CONTRACTNBR AMOUNT PRIORITY
----------- ----------- -----------
500 55000 5
Note that if the second application did not use skip conflict access, it would not be able
to access this new invoice, because it would be waiting on a locked row and therefore
would eventually time out:
SELECT * FROM
(DELETE FROM STREAM(invoices)) AS invoices;
*** ERROR[8551] Error 73 was returned by the Distribution
Service on \TESTSYS.$DB.PUBS.INVOICES.
--- 0 row(s) selected.
For an example of using skip conflict access for the cursor specification in an
application, see Skip Conflict Access on page 3-5.
AFTER LAST ROW
When a statement that selects from a stream begins executing, it returns rows that
already exist in the table as well as rows that are inserted or updated in the table after
the statement begins executing. For some applications, it is necessary to select only
newly published rows, not existing rows, from the stream.
When you use the AFTER LAST ROW clause with a stream table specification, the
statement does not read rows that existed in the table when the statement began
executing, but only returns a row if it was inserted or updated after the statement
began executing.
This behavior can be useful if the subscriber is not interested in the rows that were
published while it was not subscribing. For example, the subscriber could be involved
in real-time fraud detection, monitoring scans of items by a cashier so that the
application can take immediate action. In such a case, items scanned in the past would
be of no interest.
Alternatively, the application might be managed in a way that prevents publishing while
the subscribers are inactive. The operator could first shut down the publishing
processes and then shut down the subscribers after they have timed out (see Stream
Timeout Attribute on page 2-3). To restart the application, the operator would always
start the subscribers before the publishers.
Using Scenario 1. The Quotes Channel (page B-2), if you want the application to see
only quotes published after the SELECT statement began executing, you would
include the AFTER LAST ROW clause in its syntax:
SELECT * FROM STREAM(quotes) AFTER LAST ROW;