In a multi-system z/OS environment with shared DASD, changes can be written to a database or VSAM data set on any z/OS system. To use PowerExchange CDC in this type of environment, changes must be captured from all z/OS systems. Also, the changes that are captured from multiple z/OS systems for the same database or data set must be merged to reserve the chronological context of the change.

For example, the online CICS system runs on one z/OS system but an overnight batch job, which updates the same VSAM data sets, runs on a different z/OS system. In this example, the VSAM data sets are being changed on multiple z/OS systems but in a serial fashion, either through CICS or batch processing. It is also possible to change the same database or data set at the same time, or nearly the same time, on multiple z/OS systems. For example, changes are made to an IMS database from multiple z/OS systems at the same time.

Post-Log Merge is a PowerExchange Logger configuration option that allows the change data that has been captured and logged to multiple Loggers on multiple z/OS systems to be merged and extracted as if the data had been captured on a single system.

The multi-Logger merge process is performed by the Post-Log Merge job or task. This job or task extracts logged data from each of the member PowerExchange Loggers and merges this data in the proper chronological order for use by the PowerExchange extraction process. This process results in a single merged change stream, which is provided to the extraction process.