Synchronize data from an ApsaraDB RDS for MySQL instance to a ClickHouse cluster

Precautions

• In this scenario, DTS cannot synchronize data definition language (DDL) operations.
• DTS uses the read and write resources of the source and destination databases during initial full data synchronization. This may increase the loads of the database servers. Before you synchronize data, evaluate the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours.
• A maximum of 256 databases can be synchronized to the ClickHouse cluster.
• The names of the databases, tables, and columns to be synchronized must comply with the naming conventions of ClickHouse. For more information, see Limits.

Initial synchronization types

Procedure

1. Log on to the DTS console.In the left-side navigation pane, click Data Synchronization.
2. At the top of the Synchronization Tasks page, select the region where the destination instance resides.
3. Configure the source and destination databases.
   

   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	Select RDS Instance.
   	Instance Region	The source region that you selected on the buy page. You cannot change the value of this parameter.
   	Instance ID	Select the ID of the source RDS instance.
   	Database Account	Enter the database account of the source RDS instance. The account must have the SELECT permission on the required objects, the REPLICATION CLIENT permission, the REPLICATION SLAVE permission, and the SHOW VIEW permission. Note If the database engine of the source RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to configure the database account or database password.
   	Database Password	Enter the password of the source database account.
   	Encryption	Select Non-encrypted or SSL-encrypted. If you want to select SSL-encrypted, you must enable SSL encryption for the RDS instance before you configure the data synchronization task. For more information, see Configure SSL encryption on an ApsaraDB RDS for MySQL instance. Notice The Encryption parameter is available only for regions in the Chinese mainland and the China (Hong Kong) region.
   Destination Instance Details	Instance Type	Select ClickHouse.
   	Instance Region	The destination region that you selected on the buy page. You cannot change the value of this parameter.
   	ClickHouse Cluster ID	Select the ID of the destination ClickHouse cluster.
   	Database Account	Enter the database account of the destination ClickHouse cluster.
   	Database Password	Enter the password of the destination database account.

4. In the lower-right corner of the page, click Set Whitelist and Next.
   In this step, DTS adds the CIDR blocks of DTS servers to the whitelists of the source instance and the destination cluster. This ensures that DTS servers can connect to the source instance and the destination cluster.
5. Select the synchronization policy and the objects to be synchronized.
   

   Parameter	Description
   Initial Synchronization	Select Initial Schema Synchronization, Initial Full Data Synchronization, and Initial Incremental Data Synchronization. For more information, see Initial synchronization types.
   Processing Mode In Existed Target Table	Pre-check and Intercept: checks whether the destination database contains tables that have the same names as tables in the source database. If the source and destination databases do not contain identical table names, the precheck is passed. Otherwise, an error is returned during precheck and the data synchronization task cannot be started. Note You can use the object name mapping feature to rename the tables that are synchronized to the destination database. You can use this feature if the source and destination databases contain identical table names and the tables in the destination database cannot be deleted or renamed. For more information, see Rename an object to be synchronized. Ignore: skips the precheck for identical table names in the source and destination databases. Warning If you select Ignore, data consistency is not guaranteed and your business may be exposed to potential risks. During initial data synchronization, DTS does not synchronize the data records that have the same primary keys as the data records in the destination database. This occurs if the source and destination databases have the same schema. However, DTS synchronizes these data records during incremental data synchronization. If the source and destination databases have different schemas, initial data synchronization may fail. In this case, only some columns are synchronized or the data synchronization task fails.
   Merge Multi Tables	Yes: In online transaction processing (OLTP) scenarios, sharding is implemented to speed up the response to business tables. You can merge multiple source tables that have the same schema into a single destination table. This feature allows you to synchronize data from multiple tables in the source database to a single table in the ClickHouse cluster. Note DTS adds a column named __dts_data_source to the destination table in the ClickHouse cluster. This column is used to record the data source. The data type of this column is varchar. DTS specifies the column values based on the following format: <Data synchronization instance ID>:<Source database name>. <Source table name>. Such column values allow DTS to identify each source table. For example, dts********:dtstestdata.customer1 indicates that the source table is customer1. If you set this parameter to Yes, all of the selected source tables in the task are merged into the destination table. If you do not need to merge specific source tables, you can create a separate data synchronization task for these tables. No is selected by default.
   Synchronization Type	Only the following DML operations are supported: INSERT and UPDATE.
   Select the objects to be synchronized	Select one or more objects from the Available section and click the icon to move the objects to the Selected section. You can select columns, tables, or databases as the objects to be synchronized. After an object is synchronized to the destination cluster, 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 cluster. For more information, see Rename an object to be synchronized. Warning If you set Merge Multi Tables to Yes and select multiple source tables, you must change their names to the same table name in the ClickHouse cluster. Otherwise, data becomes inconsistent. For more information, see Rename an object to be synchronized.
   Specify the retry time for failed connection to the source or destination database	By default, if DTS fails to connect to the source or destination database, DTS retries within the next 12 hours. You can specify the retry time based on your needs. If DTS reconnects to the source and destination databases within the specified time, DTS resumes the data synchronization task. Otherwise, the data synchronization migratetask fails. Note When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time based on your business needs. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.

6. Click Next.
7. Specify the primary key column, sort key, partition key, and sharding key of the tables that you want to synchronize to the ClickHouse cluster.
   

   Note

   • Primary key columns and sort keys can be composite keys. In other words, you can select multiple fields as the primary key columns and sort keys from the drop-down lists. However, you must enter a field or a field expression for the partition key and sharding key. For more information about the primary key columns, sort keys, partition keys, and sharding keys, see Create Table.
   • You can filter tables by table name and set partition keys and sharding keys for a large number of tables at a time. For example, you can enter a field or a field expression in the text box next to Set All Partition Keys To, and then click OK.
8. Wait until the initial synchronization is complete and the data synchronization task is in the Synchronizing state.
   You can view the status of the data synchronization task on the Synchronization Tasks page.