Enable capture - Connect_CDC - connect_cdc_mimix_share - 6.x

Prior to capturing changes and replicating data, you must enable change capture for the request. Enabling capture for the request affects all the tables in the request. You can also enable/disable capture for an individual table.

To enable capture:

1. Start the Oracle Change Selector.

   This includes turning on supplemental logging for each table. Connect CDC assigns a unique name for the supplemental log group created. Table supplemental logging adds additional column information to the redo logs using a log group.

   Oracle provides two types of log groups, Conditional and Unconditional. The Change Selector uses both Conditional and Unconditional log groups depending on the number of columns selected for replication. If there are more than 33 columns selected for replication, an Unconditional log group is created.

   • Unconditional log groups–The before images of specified columns are logged any time the table is updated, regardless of whether the update affected any of the specified columns. This is sometimes referred to as an ALWAYS log group.

   • Conditional log groups–The before images of specified columns are logged only if at least one of the columns in the log group is updated.

2. Create a lightweight trigger on tables that have one or more Large Object (LOB) columns being replicated to capture changes on the LOB columns only. A trigger is used to avoid writing LOB data to the redo logs and having to copy the data and parse the LOB values.

• If an update statement does not affect any LOB columns, the trigger exits.

• If there are updates to LOB columns, a row that contains the tranid, row_id, column numbers and request_id is written into an internal table. The row is used to retrieve the LOB value when it is needed.