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Don Burleson Blog 







Add Streams replication to new server node

Oracle Database Tips by Donald BurlesonJanuary 10, 2013

Question  What is the sequence of steps to add Streams replication to a new server instance?  I have the new instance ready, I just want to replication to also go to this new server node.  What are the steps for adding Streams replication?

Answer.  This DBA has determined the correct sequence of commands to set up oracle replication (and to add new node to existing configuration). Just to mention, I use low level commands from dbms_capture/propagation/apply_adm to get full control, not simplified versions from streams or maintain.

dbms_capture_adm.prepare_table_instantiation not only records Lowest SCN (thing you can see in description of this function in Oracle documentation), but, what is much more important, it writes information about Streams Data Dictionary to Redo Log. This information is a map between identifiers of database objects used in Redo Log and their real names. Because on a remote database database objects with the same name could have different internal identifiers, Streams Data Dictionary have to be somehow populated by capture process from source to apply database before apply process could apply incoming LCRs.

Here is more complete description of Oracle Streams Data Dictionary.

If you don't know about it, you could think that "I had already instantiated this table for replication 2 months ago, so I don't need to do it again". But if you know that instantiation writes mapping information to Redo Log, you know that capture process have to somehow scan it: If you performed a dictionary build after table had been instantiated, and you want to create capture process with first_scn equal to SCN of that build, then you have to reinstantiate your table for replication.

Actually, according to Oracle (unfortunately, scattered) documentation, following conditions have to be met:

1. The database object must have been prepared for instantiation before the new capture process is created.

2. The start SCN for the new capture process must correspond to a time before the database object was prepared for instantiation.

3.The redo logs for the time corresponding to the specified start SCN must be available. Additional redo logs previous to the start SCN might be required as well.

If you will not correctly reinstantiate a table for replication (even if you had instantiated it in the past), apply process will silently ignore corresponding LCRs without generation of any errors (the problem I faced which led to start of this topic).

So the steps to add a new node are following:

1. (Optional) SrcDB: run procedure to speed up start of capture process to be created.

2. (Optional) SrcDB: reinstantiate objects to be replicated if you perform build from step 1.

3. SrcDB -> NewDB: export/import objects to be replicated.

4. NewDB: instantiate objects to be replicated.

5. SrcDB, NewDB: set up capture, propagation and apply processes.

6. SrcDB, NewDB: set instantiation SCNs for objects to which changes will be applied using information about just created capture process on peer DB (I use value of start_scn parameter of the peer capture process).

7. ScrDB, NewDB: start capture, propagation and apply processes.

It is crucial that on source DB build is done before export, because we want also to capture changes that will be made at [export, replication set up] time interval. Also you have to explicitly specify first_scn parameter for capture process. Otherwise implicit call to will be made at capture process creation time and this process won't capture changes from the time interval discussed above.

This sequence is correct if NewDB will receive LCRs only from one DB. If NewDB is supposed to receive LCR's from multiple sources, then it depends on your replication topology. For example, I've managed to include new node to N-way replication without stopping or locking any node (and at the end got identical set of table records of replicated table on all DBs including new one), but steps of course are a bit more complex.
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