CDC Guide for Linux, UNIX, and Windows

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Db2 CDC Considerations

Consider the following CDC capabilities and restrictions when planning Db2 CDC processing.

To extract change data on a Db2 client machine that is remote from the Db2 server where the change data is captured, both machines must have the same architecture. Otherwise, change data capture processing might fail with the error message PWX-20628.

For Db2 9.7 and later sources, PowerExchange can capture change data from tables that use Db2 row compression. These tables were created or altered with the COMPRESS YES option.

If the source tables are compressed, ensure that you have a compression dictionary that is compatible with the compressed Db2 log records from which PowerExchange reads change data for the tables. Otherwise, Db2 cannot decompress the log records for PowerExchange read requests. Usually, the compatible compression dictionary is available because Db2 maintains the current compression dictionary and a backup of the previous compression dictionary on disk.

If you run the Db2 REORG TABLE utility or the Db2 LOAD utility with the REPLACE or RESUME NO option against compressed source tables, Informatica recommends that you specify the KEEPDICTIONARY option for the utility. The KEEPDICTIONARY option forces Db2 to retain the current compression dictionary if it exists. If you use the RESETDICTIONARY option, Db2 rebuilds compression dictionary. In this case, the previous compression dictionary that matches the Db2 log records might not be available any longer.

PowerExchange cannot capture change data for the following Db2 datatypes:

DECFLOAT, LOB, and XML datatypes. You can create a capture registration for a table that includes columns with DECFLOAT, LOB, and XML datatypes. However, the registration does not include these columns, and PowerExchange does not capture change data for them. PowerExchange does capture change data for the other columns in the registered table that have supported datatypes.

User-defined datatypes. Tables that include columns with user-defined datatypes cannot be registered for change data capture. PowerExchange cannot capture change data for these tables.

To add or drop partitions in a partitioned database and then redistribute table data across the updated partition group, or to reconfigure a database partition group, you must use a special procedure. Otherwise, PowerExchange might not be able to resume change data capture properly.

If you alter a column datatype to or from FOR BIT DATA, PowerExchange does not detect the datatype change. PowerExchange continues to use the datatype that is specified in the existing capture registration.

If you alter a source table to change the DEFAULT value of a Db2 column of CDC interest, PowerExchange does not detect this DDL change during capture processing. As a result, the correct DEFAULT value is not available when PowerExchange performs the following operations:

Delivers the pre-existing short rows for a table to which columns were added.

Delivers rows for source tables that use the VALUE COMPRESSION option and that include a column with the COMPRESS SYSTEM DEFAULT option and a default value.

In a partitioned database, if an UPDATE to a table row changes the partition key and that change causes the row to move to another partition, PowerExchange processes the UPDATE as two operations: a DELETE and an INSERT. However, based on the Db2 log information, PowerExchange cannot predictably determine the order in which to perform the DELETE and INSERT operations. If the INSERT is processed first, both the original row and the updated row appear on the target until the DELETE is processed.

The maximum length of a row from which PowerExchange can capture change data is 128,000 bytes.

PowerExchange uses multithreaded processing for change data capture. By default, PowerExchange uses up to nine threads. To configure the number of threads, specify the THREADING parameter in the UDB CAPI CONNECTION statement. If you have a Db2 partitioned database, you can use a maximum of one thread for each database partition node plus two additional threads for the CAPI and merge processing.

PowerExchange does not support Db2 10.5 sources on zLinux.

Planning for Db2 CDC

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