Synchronize data between DRDS instances

Limits

• DTS uses read and write resources of the source and destination databases during initial full data synchronization. This may increase the database load. If the database performance is unfavorable, the specification is low, or the data volume is large, database services may become unavailable. For example, DTS occupies a large amount of read and write resources in the following cases: a large number of slow SQL queries are performed on the source database, the tables have no primary keys, or a deadlock occurs in the destination database. Before synchronizing data, you must evaluate the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. For example, you can synchronize data when the CPU usage of the source and destination databases is less than 30%.
• We recommend that you do not scale up or down the DRDS instance, migrate frequently-accessed tables, change shard keys, or perform DDL operations on objects during data synchronization. Otherwise, data synchronization may fail.
• If you need to change the network type of a DRDS instance during data synchronization, you must submit a ticket to modify the network connection information.

SQL operations that can be synchronized

INSERT, UPDATE, and DELETE

Before you begin

Create a database and tables in the destination DRDS instance based on the schemas of the objects in the source DRDS instance. This is because DTS does not support initial schema synchronization between DRDS instances. For more information, see Create a DRDS database and Create a DRDS table.

Procedure

1. Purchase a data synchronization instance. For more information, see Purchase procedure.
   Note On the buy page, set both Source Instance and Target Instance to DRDS and set Synchronization Topology to One-Way Synchronization.
2. Log on to the DTS console.
3. In the left-side navigation pane, click Data Synchronization.
4. At the top of the Synchronization Tasks page, select the region where the destination instance resides.
   
5. Find the data synchronization instance and click Configure Synchronization Channel in the Actions column.
6. Configure the source and destination instances.
   

   Section	Parameter	Description
   N/A	Synchronization Task Name	DTS automatically generates a task name. We recommend that you specify an informative name for easy identification. You do not need to use a unique task name.
   Source Instance Details	Instance Type	The value of this parameter is set to DRDS and cannot be changed.
   	Instance Region	The region of the source instance. The region is the same as the source region that you selected on the buy page. You cannot change the value of this parameter.
   	DRDS Instance ID	Select the ID of the source DRDS instance.
   Destination Instance Details	Instance Type	The value of this parameter is set to DRDS and cannot be changed.
   	Instance Region	The region of the destination instance. The region is the same as the destination region that you selected on the buy page. You cannot change the value of this parameter.
   	DRDS Instance ID	Select the ID of the destination DRDS instance.

7. In the lower-right corner of the page, click Set Whitelist and Next.
   Note In this step, DTS adds the CIDR blocks of DTS servers to the whitelists of the source and destination DRDS instances. This ensures that DTS servers can connect to the DRDS instances.
8. Configure the synchronization policy and objects.
   

   Setting	Description
   Select the processing mode of conflicting tables	Pre-check and Intercept: checks whether the destination tables are empty. If the destination tables are empty, the precheck is passed. If the tables are not empty, an error is returned during the precheck and the data synchronization task cannot be started. Ignore: skips the check for empty destination tables. Warning If you select Ignore, data consistency is not guaranteed and your business may be exposed to potential risks. If the source and destination databases have the same schema, DTS does not synchronize the data records that have the same primary keys as the data records in the destination database. If the source and destination databases have different schemas, initial data synchronization may fail. In this case, only specific columns are synchronized or the data synchronization task fails.
   Select the objects to be synchronized	Select one or more tables from the Available section and click the icon to move the tables to the Selected section. Note You can select only tables as the objects to be synchronized. After an object is synchronized to the destination instance, the name of the object remains unchanged. You can use the object name mapping feature to change the names of the objects that are synchronized to the destination instance. For more information, see Specify the name of an object in the destination instance.

9. Click Next.
10. Specify whether you want to perform initial full data synchronization.
    

    Note During initial full data synchronization, DTS synchronizes the historical data of the required objects from the source database to the destination database. If you do not select Initial Full Data Synchronization, DTS does not synchronize the historical data.
11. In the lower-right corner of the page, click Precheck.
    Note

    • Before you can start the data synchronization task, a precheck is performed. You can start the data synchronization task only after the task passes the precheck.
    • If the task fails to pass the precheck, click the icon next to each failed item to view details. Troubleshoot the issues based on the causes and run the precheck again.
12. Close the Precheck dialog box after the following message is displayed: The precheck is passed.
13. Wait until the initial synchronization is complete and the data synchronization task is in the Synchronizing state.
    On the Synchronization Tasks page, view the status of the data synchronization task.