Advanced Queuing - Oracle

Propagation Issues
Optimizing Propagation
In setting the number of JOB_QUEUE_PROCESSES, DBAs should be aware that this
number is determined by the number of queues from which the messages have to
be propagated and the number of destinations (rather than queues) to which
messages have to be propagated.
A scheduling algorithm handles propagation. The algorithm optimizes available job
queue processes and minimizes the time it takes for a message to show up at a
destination after it has been enqueued into the source queue, thereby providing
near-OLTP behavior. The algorithm can handle an unlimited number of schedules
and various types of failures. While propagation tries to make the optimal use of the
available job queue processes, the number of job queue processes to be started also
depends on the existence of nonpropagation-related jobs such as replication jobs.
Hence, it is important to use the following guidelines to get the best results from the
scheduling algorithm.
The scheduling algorithm uses the job queue processes as follows (for this
discussion, an active schedule is one that has a valid current window):
If the number of active schedules is less than half the number of job queue
processes, the number of job queue processes acquired corresponds to the
number of active schedules.
If the number of active schedules is more than half the number of job queue
processes, after acquiring half the number of job queue processes, multiple
active schedules are assigned to an acquired job queue process.
If the system is overloaded (all schedules are busy propagating), depending on
availability, additional job queue processes will be acquired up to one less than
the total number of job queue processes.
If none of the active schedules handled by a process has messages to be
propagated, then that job queue process will be released.
The algorithm performs automatic load balancing by transferring schedules
from a heavily loaded process to a lightly load process such that no process is
excessively loaded.
Handling Failures in Propagation
The scheduling algorithm also has robust support for handling failures. It may not
be able to propagate messages from a queue due to various types of failures. Some
of the common reasons include failure of the database link, non-availability of the
remote database, non-existence of the remote queue, remote queue not started and
security violation while trying to enqueue messages into the remote queue. Under
4-12 Oracle9i Application Developer’s Guide - Advanced Queuing