When replication is enabled for a LOB column, Replication Agent makes an entry in the prefixBLOB_COLUMNS_ table to support replication for that column.
When Replication Agent processes a transaction that affects a LOB column, the LOB data is not stored in the transaction log because of its possible size. Instead, the Replication Agent Log Reader component reads the LOB data directly from the primary database at the time it processes the transaction.
Because of the way Replication Agent processes the LOB column data when replicating transactions, it is possible to compromise transaction integrity. For example, if two transactions change the data in a LOB column and the Log Reader does not process the first transaction until after the second transaction has been committed, when the LOB data is read from the primary database, the value of that data is the result of the second transaction. In this event, the value of the LOB data in the first transaction is never sent to the replicate database. After the second transaction is processed by the Log Reader, the primary and replicate databases are synchronized again, but for a period of time between processing the first and second transactions, the replicate database contains data that does not match the originating transaction.
This problem occurs only when a LOB column is changed more than once by a sequence of transactions. The period of time over which the problem exists could be significant if the replication system throughput is slow or if a replication system component fails. As soon as the last transaction that changes the LOB column is processed at the replicate site, the problem is corrected.
