Comparing Items

This section describes how to compare synchronization items to check if there are any differences between source and destination databases. DRS allows you to compare the source and destination databases at the object-level or data-level. You can determine the proper time for service cutover to minimize the service downtime.

Object-level comparison: compares databases, indexes, tables, views, and table sorting rules.
Data-level comparison: compares rows and values of tables.
- Full synchronization tasks do not support data-level comparisons.
- If you modify data in the destination database, the data comparison results may be inaccurate.
- If DDL operations were performed on the source database, you need to compare the objects again to ensure the accuracy of the comparison results.
- In the many-to-one row comparison scenario, the number of rows in the table in the source database is compared with that in the aggregation table mapped to the destination database.
- If the source is a PostgreSQL database, the index and constraint names will be changed during table mapping. As a result, the index and constraint names are inconsistent.
- To prevent resources from being occupied for a long time, DRS limits the row comparison duration. If the row comparison duration exceeds the threshold, the row comparison task stops automatically. If the source database is a relational database, the row comparison duration is 60 minutes. If the source database is a non-relational database, the row comparison duration is 30 minutes.

Prerequisites

You have logged in to the DRS console.

Procedure

On the Data Synchronization Management page, click the target synchronization task name in the Task Name/ID column.
Click the Synchronization Comparison tab.
Compare synchronization items.
- On the Object-Level Comparison tab, check whether the comparison results of the source and destination databases are consistent. Locate a comparison item you want to view and click View Details in the Operation column.
- On the Data-Level Comparison tab, click Create Comparison Task. In the displayed dialog box, specify Comparison Type, Comparison Time, and Object. Then, click OK.
  - Comparison Type: compares rows and values.
    - Row comparison: checks whether the source table has the same number of rows as the destination table.
      After a task enters the incremental comparison phase, you can create a row comparison task.
    - Value comparison: checks whether the source table has the same data as the destination table.
      After a task enters the incremental synchronization phase, you can create a value comparison task. After the full synchronization is complete, data in the source database cannot be changed. Otherwise, the comparison result will be inconsistent.
      The value comparison function only applies to tables with single-column primary key or unique index. You can use row comparison for tables that do not support value comparison. Therefore, you can compare data by row or value based on scenarios.
  - Comparison Policy: DRS supports one-to-one and many-to-one comparison policies.
    - One-to-one: compares the number of rows in a table in the source database with that in the table mapped to the destination database.
    - Many-to-one: compares the number of rows in a table in the source database with that in the aggregate table mapped to the destination database.
      
      If you select Row Comparison for Comparison Type, the Comparison Policy option becomes available.
  - Comparison Time: You can select Start upon task creation or Start at a specified time. There is a slight difference in time between the source and destination databases during synchronization. Data inconsistency may occur. You are advised to compare migration items during off-peak hours for more accurate results.
  - Object: You can select objects to be compared based on the scenarios.
After the comparison creation task is submitted, the Data-Level Comparison tab is displayed. Click to refresh the list and view the comparison result of the specified comparison type.

Currently, the value comparison function only applies to tables with one column with a primary key or unique index. You can use the row comparison function for tables that do not support value comparison. Therefore, you can compare data by row or value based on scenarios.

If you want to view the row or value comparison details, click View Results.

If you want to download the row comparison or value comparison result, locate a specified comparison type and click Export Report in the Operation column.

You can also view comparison details of canceled comparison tasks.