How source data changing synchronize to destination after exp from source?

Question

Stream creation, how the source data changing synchronize to the destination after exp data from source?
Time of Stream creation:
a. exp/imp
exp erpdev/erpdev owner=erpdevobject_consistent=y file=erpdev.dmp grants=y rows=y indexes=y statistics=none
imp erpdev/erpdev file=erpdev.dmp ignore=ycommit=y log=import.log streams_instantiation=y
b. confirming source and dest database SCN
BEGIN
DBMS_APPLY_ADM.SET_SCHEMA_INSTANTIATION_SCN(
SOURCE_SCHEMA_NAME=> 'erpdev',
source_database_name=> 'ERPDB',
instantiation_scn=> &iscn ,
recursive =>TRUE);
END;
c. 'START_APPLY' of dest database
EXEC DBMS_APPLY_ADM.START_APPLY('STRMADMIN_APPLY');
Question:
1.
Stream start to work at "c. START_APPLY of dest database", how the source data changing synchronize to the destination after exp data from source?
2.
What was "b. confirming source and dest database SCN" for？

Answer 1

You missed a step : the prepare table and its role. The prepare table for capture returns an SCN which will tells Streams at which SCN it must start capture any DML. At this stage the Apply may be configured or not, but if the source tables is modified, an LCR will be created when you start the capture and this LCR will be sent to the apply site when the apply site is finally up.
During the process of transfering the content of the Source site to target site there is always room for a desync to take place if the source site is active. Two cases exists and must be examined:
- Prepare table first, then export source --> any DML after the prepared SCN and before the start of the export will be sent as LCR while it is also into the export->> duplicate DML
- Export first then prepare table --> A DML occured AFTER the start of the export but before the SCN returned by the prepared SCN: this DML will be missing in the epxort (and at target site) and not yet captured neither since it occured before the prepare table.
So what is the best? The answer the first. You resolve the issue with the apply SCN which has into the export, for each table the SCN at the start of the export. In this scenario, you have prepared first, export next source, import in target and set the scn of each table i(at apply site) using the one found nto the export (hence the parameter STREAMS_INSTANTIATION ).
Now even all SCN captured on source after the prepared table but before the start of the export, will effectively be sent to apply site but will be IGNORED by the apply process has it would be a SCN aging before the start of the export. The SCN of the tables into the export file will become the SCN of the apply process. any SCN lower will be ignored by the apply process.
A last word in respect of a DML that started AFTER the start of the export and BEFORE the end of the export on source site (during the export). Since it is after the prepare table, it is captured and since it is after the start of the export, it is not into the export (we assume the export is consistent - which is not always true). So you need this LCR as the mutation it describes occured after the start of the export and is not in the export: everything is ok.
Having said that, still the best way to setup a target site is still to avoid any transction on source site during the setup.
I hope you understand this well for it is the way it works.