Goldengate extract from Oracle ADG standby side ALO and ADG mode
Yes, you can extract the data from a Physical Standby database. There are actually two methods available in Oracle GoldenGate.,
1. Archived Log Only Mode – ALO Mode
2. Active Data Guard Mode – ADG Mode
ALO Mode
You can configure Extract to read exclusively from the archived logs. This is known as Archived Log Only
(ALO) mode. In this mode, Extract reads exclusively from archived logs that are stored in a specified location. ALO mode enables Extract to use production logs that are shipped to a secondary database (such as a standby) as the data source. The online logs are not used at all, just extract from Archivelog on standby, so it’s not real-time. Oracle GoldenGate connects to the secondary database to get metadata and other required data as needed. As an alternative, ALO mode is supported on the production system.
Limitations and Requirements for Using ALO Mode
Observe the following limitations and requirements when using Extract in ALO mode.
- Log resets (
RESETLOG) cannot be done on the source database after the standby database is created. - ALO cannot be used on a standby database if the production system is Oracle RAC and the standby database is non-RAC. In addition to both systems being Oracle RAC, the number of nodes on each system must be identical.
- ALO on Oracle RAC requires a dedicated connection to the source server. If that connection is lost, Oracle GoldenGate processing will stop.
- more look here
ADG Mode
since Ogg V12.1.2 Active Data Guard (ADG) – A new processing option (TRANLOGOPTIONS MINEFROMACTIVEDG) is introduced allowing you to configure Classic Extract to mine online redo logs shipped to the Oracle Active Data Guard standby system allowing real-time replication directly from ADG hosts. This mode is useful in load sensitive environments where ADG is already in place or can be implemented. It can also be used as cost effective method to implement high availability using the ADG Broker role planned (switchover) and failover (unplanned) changes.
You can configure classic Extract to access both redo data and metadata in real-time to successfully replicate source database activities using Oracle Active Data Guard. This is known as Active Data Guard (ADG) mode. ADG mode enables Extract to use production logs that are shipped to a standby database as the data source. The online logs are not used as all, but extract from Standby redo log and Archivelog. Oracle GoldenGate connects to the standby database to get metadata and other required data as needed.
Limitations and Requirements for Using ADG Mode
Observe the following limitations and requirements when using Extract in ADG mode.
- Extract in ADG mode will only apply redo data that has been applied to the standby database by the apply process.(I guess it’s to get SCN) If Extract runs ahead of the standby database, it will wait for the standby database to catch up.
- You must explicitly specify ADG mode in your classic Extract parameter file to run extract on the standby database.
- You must specify the database user and password to connect to the ADG system because fetch and other metadata resolution occurs in the database.
- The number of redo threads in the standby logs in the standby database must match the number of nodes from the primary database.
- No new RAC instance can be added to the primary database after classic Extract has been created on the standby database. If you do add new instances, the redo data from the new thread will not be captured by classic Extract.
- Archived logs and standby redo logs accessed from the standby database will be an exact duplicate of the primary database. The size and the contents will match, including redo data, transactional data, and supplemental data. This is guaranteed by a properly configured ADG deployment.
- more look Oracle online doc here
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