Row Images

Enterprise Replication evaluates the initial and final images of a row and any changes that occur between the two row images to determine which rows to replicate. Each row image contains the data in the row and the action that is performed on that row.

A row might change more than once in a particular transaction. For example, a transaction might insert and then update a row before committing. Enterprise Replication evaluates the net effect (final state) of a transaction based on the row buffers in the log. Enterprise Replication then determines what must be replicated, based on the net effect, the initial state of the row, and whether the replicate definition (in particular, the WHERE clause) applies to the initial and final state. Enterprise Replication evaluates the row-image type (INSERT, UPDATE, DELETE), the results of evaluating the replicate WHERE clause for both the initial and final image, and whether the replication key changes as a result of the transaction.

The following table shows the logic that determines which rows are candidates for replication. The source and destination tables are assumed to be initially synchronized (identical before replication begins). If the tables were not synchronized, anomalous behavior might result.

Table 1. Enterprise Replication Evaluation Logic
Initial Image	Replicate Evaluates	Final Image	Replicate Evaluates	Replication-Key Changed?	Send to Destination Database Server	Comments
INSERT	T or F	DELETE	T or F	Yes or no	Nothing	Net change of transaction results in no replication
INSERT	T or F	UPDATE	T	Yes or no	INSERT with final row image	Inserts final data of transaction
INSERT	T or F	UPDATE	F	Yes or no	Nothing	Final evaluation determines no replication
UPDATE	T	DELETE	T or F	Yes or no	DELETE with initial row image	Net result of transaction is delete
UPDATE	F	DELETE	T or F	Yes or no	Nothing	Net change of transaction results in no replication
UPDATE	T	UPDATE	T	Yes	DELETE with initial row image and INSERT with final row image	Ensures old replication key does not replicate
UPDATE	T	UPDATE	T	No	UPDATE with final row image	Simple update
UPDATE	T	UPDATE	F	Yes or no	DELETE with initial row image	Row no longer matches replicate definition
UPDATE	F	UPDATE	T	Yes or no	INSERT with final row image	Row now matches replicate definition
UPDATE	F	UPDATE	F	Yes or no	Nothing	No match exists, and therefore, no replication

The following rules apply to the information in the table:

The initial image is the before image of the transaction in the logical log.
The replicate evaluates to T (true) or F (false).
The final image is the image of the transaction that is replicated.

After Enterprise Replication identifies transactions that qualify for replication, Enterprise Replication transfers the transaction data to a queue.