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GenerateDeltaTable configuration

  ≥ 2022.R3 

In order to track row-level changes between versions of a Delta table in an optimized way, Change Data Feed on Azure Databricks option has been included in Sidra through the GenerateDeltaTable column in the Entity table.

In the next table, the actions of Change Data Feed are depicted:

Changes Databricks - Change Data Feed
Fields TABLE_COLUMNS + _change_type + _commit_version + commit_timestamp
Insertions Adds a new row with the values of the source table and with _change_type = insert
Updates Adds two rows, one with the previous data and change_type= update_preimage; and another one with the updated data with _change_type = update_postimage. Both are with the same _commit version and _commit_timestamp
Deletions Adds a new row with the data and with _change_type= delete

Configuration

Considerations

The metadata created after a first ingestion is needed to be able to modify the Entity property GenerateDeltaTable. When the metadata pipeline is executed, and the metadata created, the metadata pipeline must be executed again to update the configuration for the tables. For executing the pipeline, two ways are ava:

  • Directly executing the metadata pipeline.
  • Executing the Orchestrator if the schema evolution is activated by the Sidra Web -note that in this last case the question Do you want the metadata to be refreshed automatically with each data extraction? should be marked as true.

Configuration steps

For the configuration, follow the next steps:

  1. Set the Entity property GenerateDeltaTable to true.
  2. Update the LastUpdated date of the Entity to be greater than the LastDeployed date.
  3. Execute the orchestrator for the DIP or the FileIngestionDatabricks ADF pipeline for the Provider.
  4. After the execution, the Databricks table history should have changed. Execute in Databricks this to visualize it:
%sql DESCRIBE HISTORY [Provider].[table]

Change-Data-Feed-1

By updating the data in the source (by modifying, creating, or removing rows), the tracking table in Databricks should update. To check it, execute:

%sql SELECT * FROM table_changes('[provider].[table]', '2022-04-21 05:45:46')
# Timestamp is indicated to review the specific changes

Change-Data-Feed-2

In the example above, the delta table view appears with 3 inserts, one update and one delete.

Databricks creates a folder (_change_data) in the same directory of the table to store the changes done in the table. These changes are stored in parquet format.

Scenarios with other parameters of Entity table

The displayed tables are the ones generated by Databricks after setting the delta.enableChangeDataFeed to true in the Databricks table (as shown above in the first image).

ConsolidationMode as Snapshot

In this consolidation mode option, since we are appending all the data for the Asset, the Change Data Feed will identify everything as an insert. Deletions will be identified the same way, since we set a deletion with a new row with all values to NULL except for the Id and the SidraIsDeleted values.

Change-Data-Feed-3

ConsolidationMode as Merge

For this case:

  • Inserts are identified with the operation in one line.
  • Updates are identified with the operation in two lines.
  • Deletions in data source are represented as an insert with all values to NULL except for the Asset id and the SidraIsDeleted as true.

Change-Data-Feed-4


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Last update: 2022-11-17
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