Change streams

A log reader reads log records for your data source and forwards them on change streams to the capture service.

A change stream consists of a sequence of log records that contain captured data and sync points. Sync points specify the end of a unit of recovery (UOR).

IMS data sources

A change stream performs these essential change-capture functions:

  • Minimizes concurrent access to the physical logs that store the changes
  • Merges changes from all participating IMS subsystems in the correct order
  • Makes the changes available to the source server for retrieval and processing

When you start replication for the first subscription, the source server deploys a primary stream to transfer changes for all subscriptions on the source server. The bookmark setting for each subscription determines the point in time to begin processing, which continues with the goal of catching up to current processing.

The capture service will initiate a secondary stream when you start replication for a subscription and its restart point is in the past compared to the last change received on the primary change stream. You might notice tape mounts when you start replication or when subscriptions are catching up, indicating that the log reader service is accessing historical logs. If all available secondary change streams are in use, the source server might temporarily suspend change data capture for a subscription.

In situations where the restart position for a subscription is earlier than the oldest record available in the caches, an IMS subscription relies on historical logs and change streams to resume current processing. By design, primary and secondary change streams enable the source server to give priority to subscriptions with the oldest restart position and to subscriptions that are catching up.