In-Memory Caching and Persistence v6.2

The data changes are fetched and stored in memory buffers to optimize the data replication process. This avoids the overhead associated with repeatedly fetching the same set of changes from the database server when there are multiple target databases.

This approach is sufficient as long as all of the target databases are accessible during a replication event and the data fits within the available cache.

However, if one or more of the target databases is unavailable due to network connectivity problems, server down time, etc. the in-memory data changes must be persisted for later retrieval when the target databases becomes available for synchronization with the source database.

The xDB Replication Server architecture utilizes Java object serializatio to persist the in-memory state of the data. Object serialization is the conversion of object data and other relevant information to a sequence of bytes that can then be stored in a file.

The following are examples that can result in the eviction of in-memory data to persistent storage:

• Before the next replication event occurs, the in-memory cache is filled with the data changes and needs to be evicted to accommodate a new set of changes.
• In the replication system, there are multiple target databases. During a synchronization event, all of the changes available in the cache are applied successfully to some of the target databases. However one or more of the other target databases cannot be accessed. All of the applied changes held in memory must be persisted and retained so that these changes can be reloaded and applied when the inaccessible databases becomes available.

The cache size corresponds to the heap size configured for the publication server by the -Xmxnnnm setting of the JAVA_HEAP_SIZE parameter in the xDB Startup Configuration file. See xDB Replication Configuration File for information on the xDB Startup Configuration file.

The persistence I/O overhead can be minimized by increasing the heap size value and defining a more frequent synchronization interval such as for every few seconds. See Creating a Schedule for information on setting a replication schedule.

The data changes are persisted in a local file on the host running the publication server. The file is stored in the directory XDB_HOME/xdata.

Each time persistence occurs, a new file is created. After the files have been processed, they are periodically removed from disk.