Continuous
Nocontinuous
Causes the LogMiner process to attach to the database and begin
extracting records in "near-real" time. When the Continuous
qualifier is specified, the RMU Unload After_Journal command
extracts records from the online after-image journal files of the
database until it is stopped via an external source (for example,
Ctrl/y, STOP/ID, $FORCEX, or database shutdown).
A database must be explicitly enabled for the Continuous LogMiner
feature. To enable the Continuous LogMiner feature, use the RMU
Set Logminer command with the Enable and Continuous qualifiers;
to disable use of the Continuous LogMiner feature, use the RMU
Set Logminer command with the Enable and Nocontinuous qualifiers.
The output from the Continuous LogMiner process is a continuous
stream of information. The intended use of the Continuous
LogMiner feature is to write the changes into an OpenVMS
mailbox or pipe, or to call a user-supplied callback routine.
Writing output to a disk file is completely functional with the
Continuous LogMiner feature, however, no built-in functionality
exists to prevent the files from growing indefinitely.
It is important that the callback routine or processing of
the mailbox be very responsive. If the user-supplied callback
routine blocks, or if the mailbox is not being read fast enough
and fills, the RMU Unload After_Journal command will stall. The
Continuous LogMiner process prevents backing up the after-image
journal that it is currently extracting along with all subsequent
journals. If the Continuous LogMiner process is blocked from
executing for long enough, it is possible that all available
journals will fill and will not be backed up.
When a database is enabled for the Continuous LogMiner feature,
an AIJ "High Water" lock (AIJHWM) is utilized to help coordinate
and maintain the current .aij end-of-file location. The lock
value block for the AIJHWM lock contains the location of the
highest written .aij block. The RMU Unload After_Journal command
with the Continuous qualifier polls the AIJHWM lock to determine
if data has been written to the .aij file and to find the highest
written block. If a database is not enabled for the Continuous
LogMiner feature, there is no change in locking behavior; the
AIJHWM lock is not maintained and thus the Continuous qualifier
of the RMU Unload After_Journal command is not allowed.
In order to maintain the .aij end-of-file location lock,
processes that write to the after-image journal file must use
the lock to serialize writing to the journal. When the Continuous
LogMiner feature is not enabled, processes instead coordinate
allocating space in the after-image journal file and can write
to the file without holding a lock. The Continuous LogMiner
process requires that the AIJHWM lock be held during the .aij
I/O operation. In some cases, this can reduce overall throughput
to the .aij file as it serves to reduce multiple over-lapped I/O
write operations by multiple processes.
The Save_Metadata and Restore_Metadata qualifiers are
incompatible with the Continuous qualifier.