Using incremental aggregation, we apply captured changes in the source data (CDC part) to aggregate calculations in a session. If the source changes incrementally and we can capture the changes, then we can configure the session to process those changes. This allows the Integration Service to update the target incrementally, rather than forcing it to delete previous loads data, process the entire source data and recalculate the same data each time you run the session.
When the session runs with incremental aggregation enabled for the first time say 1st week of Jan, we will use the entire source. This allows the Integration Service to read and store the necessary aggregate data information. On 2nd week of Jan, when we run the session again, we will filter out the CDC records from the source i.e the records loaded after the initial load. The Integration Service then processes these new data and updates the target accordingly.
Use incremental aggregation when the changes do not significantly change the target.If processing the incrementally changed source alters more than half the existing target, the session may not benefit from using incremental aggregation. In this case, drop the table and recreate the target with entire source data and recalculate the same aggregation formula .
INCREMENTAL AGGREGATION, may be helpful in cases when we need to load data in monthly facts in a weekly basis.
Let us see a sample mapping to implement incremental aggregation:
Look at the Source Qualifier query to fetch the CDC part using a BATCH_LOAD_CONTROL table that saves the last successful load date for the particular mapping.
Look at the ports tab of Expression transformation.
Look at the ports tab of Aggregator Transformation.
Now the most important session properties configuration to implement incremental Aggregation
If we want to reinitialize the aggregate cache suppose during first week of every month we will configure the same session in a new workflow at workflow level with the Reinitialize aggregate cache property checked in.
Example with Data
Now have a look at the source table data:
After the first Load on 1st week of Jan 2010, the data in the target is as follows:
Now during the 2nd week load it will process only the incremental data in the source i.e those records having load date greater than the last session run date. After the 2nd weeks load after incremental aggregation of the incremental source data with the aggregate cache file data will update the target table with the following dataset:
|1111||6008||201001||450||The cache file updated after aggretation|
|2222||6009||201001||500||The cache file updated after aggretation|
|3333||5003||201001||300||The cache file remains the same as before|
|4444||1234||201001||350||New group row inserted in cache file|
|5555||6157||201001||500||New group row inserted in cache file|
Understanding Incremental Aggregation Process
The first time we run an incremental aggregation session, the Integration Service processes the entire source. At the end of the session, the Integration Service stores aggregate data for that session run in two files, the index file and the data file. The Integration Service creates the files in the cache directory specified in the Aggregator transformation properties.
Each subsequent time we run the session with incremental aggregation, we use the incremental source changes in the session. For each input record, the Integration Service checks historical information in the index file for a corresponding group. If it finds a corresponding group, the Integration Service performs the aggregate operation incrementally, using the aggregate data for that group, and saves the incremental change. If it does not find a corresponding group, the Integration Service creates a new group and saves the record data.
When writing to the target, the Integration Service applies the changes to the existing target. It saves modified aggregate data in the index and data files to be used as historical data the next time you run the session.
Each subsequent time we run a session with incremental aggregation, the Integration Service creates a backup of the incremental aggregation files. The cache directory for the Aggregator transformation must contain enough disk space for two sets of the files.
The Integration Service creates new aggregate data, instead of using historical data, when we configure the session to reinitialize the aggregate cache, Delete cache files etc.
When the Integration Service rebuilds incremental aggregation files, the data in the previous files is lost.
Note: To protect the incremental aggregation files from file corruption or disk failure, periodically back up the files.