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Oracle Tips by Burleson |
Oracle Streams
Chapter 4 -
Capture
and Propagate Configuration
Understanding the SCN values
Most of the Streams-related dynamic performance
views show the SCN number. This helps the DBA understand the
progress of the Streams process. This also helps in troubleshooting
and monitoring. For example, the dba_capture view records the
following SCN values which helps track the Capture process progress.
-
captured_scn: The captured_scn
is the one that corresponds to the most recent change scanned in the
redo log by a Capture process. This value keeps incrementing to a
higher number as the database transactional activity
goes on. As long as the Capture process is running, this value
should change gradually.
-
applied_scn: The applied_scn for a Capture process is the SCN of the most recent
event dequeued by the relevant Apply processes. It indicates that
all events with SCNs lower than this one have been dequeued by all
Apply processes that apply changes captured by the Capture process.
The applied_scn for a Capture process is equivalent to the
low-watermark SCN for such an Apply process.
-
first_scn: The first_scn is the lowest SCN in the redo log from which a Capture
process can capture changes. If a first_scn is specified
during creation of the Capture process, the database must be able to
access redo log information from the specified SCN and those with
higher values.
-
start_scn: The start_scn is the SCN from which a Capture process begins to
capture changes.
-
required_checkpoint_scn:
This value indicates the lowest checkpoint SCN for which the Capture
process requires redo information. At very regular intervals, the
Capture process records a checkpoint where it records its current
state persistently in the data dictionary of the database running
the Capture process. The redo log file that contains the required
checkpoint SCN, and all subsequent redo log files must be available
to the Capture process. If a Capture process is stopped and
restarted, it starts scanning the redo log from the SCN that
corresponds to its required_checkpoint_scn.
The above text is
an excerpt from:
Oracle Streams
High Speed Replication and Data
Sharing
ISBN 0-9745993-5-2
by Madhu Tumma
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