A flush is issued from source to establish reference points, typically to help in maintenance activities. It may also be issued to test replication. The flush command is issued from source sp_ctrl and requires source SID as a parameter in case of a non-RAC source. This puts a marker in the data stream and stops Post when the marker reaches Post queue. When dealing with a RAC source, there is not one source instance that Capture deals with. The question arises as to which instance Shareplex deals with when a flush is issued in a RAC environment.
General information.
When a flush is issued in a RAC environment, it is issued with the alias SID provided as the parameter to the flush. When the command is issued, Shareplex will look at the current instance involved in transaction and will insert the flush marker in the queues via that instance. The Post will eventually stop due to flush, regardless of which instance on source was active when the flush was issued, and regardless of whether the target is a RAC or a non-RAC. This serves the purpose of establishing a reference point, whether it be for maintenance or for simulating transactions without involving actual tables in replication. It is immaterial as to which of the RAC thread (or instance) was used for generating the flush marker that traversed through Shareplex queue and stopped Post.
The following shows the difference in the output of "show capture detail" in RAC and non-RAC environment:
RAC source:
sp_ctrl > show capture detail
Host: hostname
Operations
Source Status Captured Since
---------- --------------- ---------- ------------------
o.alias_SID Running 203 23-Feb-07 05:30:11
Thread Instance Host Redo Log Log Offset
------ ------------ ------------ -------- ----------
1 instance1 hostname 95 88747536
2 instance2 hostname 49 4042768
non-RAC source:
sp_ctrl > show capture detail
Host: hostname
Operations
Source Status Captured Since
---------- --------------- ---------- ------------------
o.instance Running 2085257 10-Mar-08 17:04:13
Oracle current redo log : 461
Capture current redo log : 461
Capture log offset : 42260772
