Monday, 16 July 2012

Incremental Aggregation

When we enable the session option-> Incremental Aggregation the Integration Service performs incremental aggregation, it passes source data through the mapping and uses historical cache data to perform aggregation calculations incrementally.
When using incremental aggregation, you apply captured changes in the source to aggregate calculations in a session. If the source changes incrementally and you can capture changes, you can configure the session to process those changes. This allows the Integration Service to update the target incrementally, rather than forcing it to process the entire source and recalculate the same data each time you run the session.
For example, you might have a session using a source that receives new data every day. You can capture those incremental changes because you have added a filter condition to the mapping that removes pre-existing data from the flow of data. You then enable incremental aggregation.
When the session runs with incremental aggregation enabled for the first time on March 1, you use the entire source. This allows the Integration Service to read and store the necessary aggregate data. On March 2, when you run the session again, you filter out all the records except those time-stamped March 2. The Integration Service then processes the new data and updates the target accordingly.Consider using incremental aggregation in the following circumstances:
  • You can capture new source data. Use incremental aggregation when you can capture new source data each time you run the session. Use a Stored Procedure or Filter transformation to process new data.
  • Incremental changes do not significantly change the target. 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 complete source data.
Note: Do not use incremental aggregation if the mapping contains percentile or median functions. The Integration Service uses system memory to process these functions in addition to the cache memory you configure in the session properties. As a result, the Integration Service does not store incremental aggregation values for percentile and median functions in disk caches.
Integration Service Processing for Incremental Aggregation
(i)The first time you run an incremental aggregation session, the Integration Service processes the entire source. At the end of the session, the Integration Service stores aggregate data from 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.
(ii)Each subsequent time you run the session with incremental aggregation, you 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.
(iii)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.
(iv) If the source changes significantly and you want the Integration Service to continue saving aggregate data for future incremental changes, configure the Integration Service to overwrite existing aggregate data with new aggregate data.
Each subsequent time you 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.
(v)When you partition a session that uses incremental aggregation, the Integration Service creates one set of cache files for each partition.
The Integration Service creates new aggregate data, instead of using historical data, when you perform one of the following tasks:
  • Save a new version of the mapping.
  • Configure the session to reinitialize the aggregate cache.
  • Move the aggregate files without correcting the configured path or directory for the files in the session properties.
  • Change the configured path or directory for the aggregate files without moving the files to the new location.
  • Delete cache files.
  • Decrease the number of partitions.
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.
Preparing for Incremental Aggregation:
When you use incremental aggregation, you need to configure both mapping and session properties:
  • Implement mapping logic or filter to remove pre-existing data.
  • Configure the session for incremental aggregation and verify that the file directory has enough disk space for the aggregate files.
Configuring the Mapping
Before enabling incremental aggregation, you must capture changes in source data. You can use a Filter or Stored Procedure transformation in the mapping to remove pre-existing source data during a session.
Configuring the Session
Use the following guidelines when you configure the session for incremental aggregation:
(i) Verify the location where you want to store the aggregate files.
  • The index and data files grow in proportion to the source data. Be sure the cache directory has enough disk space to store historical data for the session.
  • When you run multiple sessions with incremental aggregation, decide where you want the files stored. Then, enter the appropriate directory for the process variable, $PMCacheDir, in the Workflow Manager. You can enter session-specific directories for the index and data files. However, by using the process variable for all sessions using incremental aggregation, you can easily change the cache directory when necessary by changing $PMCacheDir.
  • Changing the cache directory without moving the files causes the Integration Service to reinitialize the aggregate cache and gather new aggregate data.
  • In a grid, Integration Services rebuild incremental aggregation files they cannot find. When an Integration Service rebuilds incremental aggregation files, it loses aggregate history.
(ii) Verify the incremental aggregation settings in the session properties.
  • You can configure the session for incremental aggregation in the Performance settings on the Properties tab.
  • You can also configure the session to reinitialize the aggregate cache. If you choose to reinitialize the cache, the Workflow Manager displays a warning indicating the Integration Service overwrites the existing cache and a reminder to clear this option after running the session.

1 comment:

  1. Good post! Thanks for sharing this information I appreciate it. God bless!

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