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Major Queuing Features

HP NonStop SQL/MX Queuing and Publish/Subscribe Services—523734-002

2-7

Skip Conflict Access

The skip conflict access method is important in implementing an efficient transactional

queuing feature. It prevents concurrent transactions from blocking each other by

waiting for the release of locks on rows currently being inserted or updated while other

rows are available that are not currently locked by another transaction.

Transactional queuing implies that entries are read and deleted with a single operation

as part of a transaction. Entries are removed from the queue after the associated

transaction commits. Newly inserted or updated entries become available after the

associated transaction commits. If necessary, you can execute several operations as

part of the same transaction—for example, dequeueing a request and completing the

work of that request as part of the same transaction.

To prevent concurrent transactions—for example, multiple applications dequeueing

requests—from interfering with each other, applications use skip conflict access.

From Scenario 2. The Invoices Queue (page B-5), this SELECT statement uses the

skip conflict access mode:

SELECT * FROM

(DELETE FROM STREAM(invoices)

FOR SKIP CONFLICT ACCESS) AS invoices;

CONTRACTNBR AMOUNT PRIORITY ARCHIVE

----------- ----------- ----------- -------

100 10500 1 Y

200 20390 2 Y

300 30800 3 Y

Another application issues the same SELECT statement:

SELECT * FROM

(DELETE FROM STREAM(invoices)

FOR SKIP CONFLICT ACCESS) AS invoices;

This second application cannot access the rows locked by the first application.

However, the skip conflict access prevents the second application from timing out as a

result of waiting for rows locked by the first application. If the second statement were

not using SKIP CONFLICT ACCESS, the second application would be waiting on the

lock held on the first row by the first application, and the first application would be

waiting for more rows.