How to synchronize data from an ApsaraDB RDS for MySQL instance to an ApsaraDB for ClickHouse cluster - ApsaraDB for ClickHouse

ApsaraDB for ClickHouse is a column-oriented database management system (DBMS) for online analytical processing (OLAP) of queries. ApsaraDB for ClickHouse provides aggregate queries on wide tables with a speed that is one order of magnitude faster than other analytical databases. You can use Data Transmission Service (DTS) to synchronize data from a MySQL database, such as a self-managed MySQL database or an ApsaraDB RDS for MySQL instance, to an ApsaraDB for ClickHouse cluster. This way, you can transfer and analyze data with ease. This topic describes how to synchronize data from an ApsaraDB RDS for MySQL instance to an ApsaraDB for ClickHouse cluster.

Prerequisites

Data Transmission Service (DTS) is authorized to access Alibaba Cloud resources. For more information, see Authorize DTS to access Alibaba Cloud resources.

An ApsaraDB for ClickHouse cluster of V20.8 or later is created. For more information, see Create an ApsaraDB for ClickHouse cluster.
Note
The available storage space of the destination ApsaraDB for ClickHouse cluster is larger than the total size of the data in the source ApsaraDB RDS for MySQL instance.
A database is created in the destination ApsaraDB for ClickHouse cluster to receive data. For more information, see Create a database.
Important
We recommend that you specify the same name for the database in the destination ApsaraDB for ClickHouse cluster as the database that stores the data to be synchronized in the source ApsaraDB RDS for MySQL instance. Otherwise, you must use the name mapping feature to map the name of the source database to the name of the destination database in the Selected Objects section during the Configure Objects and Advanced Settings step. For more information, see Map object names.

Limits

Category	Description
Limits on the source database	If you select tables as the objects to be synchronized and you need to edit the tables in the destination database, such as renaming tables or columns, up to 1,000 tables can be synchronized in a single data synchronization task. If you run a task to synchronize more than 1,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to synchronize the tables in batches or configure a task to synchronize the entire database. The following requirements for binary logs must be met: By default, the binary logging feature is enabled for an ApsaraDB RDS for MySQL instance. Make sure that the binlog_row_image parameter is set to full. Otherwise, error messages are returned during precheck and the data synchronization task cannot be started. For more information, see Modify instance parameters. Important If the source database is a self-managed MySQL database, you must enable the binary logging feature and set the binlog_format parameter to row and the binlog_row_image parameter to full. If the source database is a self-managed MySQL database deployed in a dual-primary cluster, you must set the log_slave_updates parameter to ON. This ensures that DTS can obtain all binary logs. For more information, see Create an account for a self-managed MySQL database and configure binary logging. The binary logs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the binary logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. Make sure that you set the retention period of binary logs based on the preceding requirements. Otherwise, the service level agreement (SLA) of DTS does not guarantee service reliability or performance. For more information about how to manage the binary log files of an ApsaraDB RDS for MySQL instance, see Manage binary log files.
Other limits	If the DDL statements that are executed on the source ApsaraDB RDS for MySQL instance do not comply with the standard MySQL syntax, a synchronization task may fail or data may be lost. The RENAME TABLE operation cannot be synchronized. The values of the DATETIME data stored in ApsaraDB for ClickHouse range from 1970-01-01 00:00:00 to 2106-02-07 06:28:15. If the timestamp of the data to be synchronized from the source database does not fall into this time range, the timestamp of the synchronized data in the destination database is not correct. All selected columns of the Partition Key parameter must be non-empty fields. Otherwise, a synchronization task fails. Up to 256 databases can be synchronized to an ApsaraDB for ClickHouse cluster. The names of the databases, tables, and columns to be synchronized must comply with the naming conventions of ApsaraDB for ClickHouse. For more information, see the Object naming conventions section of the "Limits" topic. If the destination cluster is an ApsaraDB for ClickHouse Community Edition cluster, DTS adds the _sign and _version fields to the destination table. If the destination cluster is an ApsaraDB for ClickHouse Enterprise Edition cluster, DTS does not add fields to the destination table. Note If you perform incremental data synchronization, DTS records the timestamp that is generated when data is written to the destination ApsaraDB for ClickHouse Community Edition cluster in the _version field, and the types of DML operations in the _sign field. A value of 1 indicates an INSERT or UPDATE operation and a value of -1 indicates a DELETE operation. 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. During initial full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers. If you select one or more tables instead of an entire database as the objects to be synchronized, do not use tools such as pt-online-schema-change to perform DDL operations on the tables during data synchronization. Otherwise, data may fail to be synchronized. You can use Data Management (DMS) to perform online DDL operations. For more information, see Perform lock-free DDL operations. During data synchronization, we recommend that you use only DTS to write data to the destination database. This prevents data inconsistency between the source and destination databases.
Special cases	If the source database is a self-managed MySQL database, take note of the following items: If you perform a primary/secondary switchover on the source database when the data synchronization task is running, the task fails. DTS calculates synchronization latency based on the timestamp of the latest synchronized data in the destination database and the current timestamp in the source database. If no DML operation is performed on the source database for an extended period of time, the synchronization latency may be inaccurate. If the latency of the synchronization task is too high, you can perform a DML operation on the source database to update the latency. Note If you select an entire database as the object to be synchronized, you can create a heartbeat table. The heartbeat table is updated or receives data every second. DTS regularly executes the CREATE DATABASE IF NOT EXISTS `test` statement in the source database to move forward the binary log file position.

Billing

Synchronization type	Task configuration fee
Schema synchronization and full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

SQL operations that can be synchronized

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	CREATE TABLE TRUNCATE TABLE ALTER TABLE ADD COLUMN MODIFY COLUMN DROP TABLE and DROP COLUMN

Data type mappings

The data types supported by ApsaraDB RDS for MySQL instances and ApsaraDB for ClickHouse clusters do not have one-to-one correspondence. During initial schema synchronization, DTS establishes mappings between source fields and destination fields based on the data types supported by the destination database. For more information, see Data type mappings for schema synchronization.

Permissions required for database accounts

Database type	Required permission	References
Source ApsaraDB RDS for MySQL instance	Read permissions on the objects to be synchronized.	Create an account on an ApsaraDB RDS for MySQL instance and Modify the permissions of a standard account on an ApsaraDB RDS for MySQL instance.
Destination ApsaraDB for ClickHouse cluster	An ApsaraDB for ClickHouse cluster of V22.8: the read and write permissions on the destination database. In this case, you can use a privileged account. An ApsaraDB for ClickHouse cluster of V21.8: the read, write, and set permissions and the permissions to perform DDL operations.	Manage database accounts of an ApsaraDB for ClickHouse cluster.

Note

If the source database account that you use is not created and authorized by using the ApsaraDB RDS for MySQL console, make sure that the account has the REPLICATION CLIENT, REPLICATION SLAVE, SHOW VIEW, and SELECT permissions.

Procedure

Go to the Data Synchronization Tasks page.
1. Log on to the DMS console.
2. In the top navigation bar, click DTS.
3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
Note
- The actual operation may vary depending on the mode and layout of DMS. For more information, see Simple mode and Customize the layout and style of the DMS console.
- You can also go to the Data Synchronization Tasks page of the new DTS console.
From the drop-down list to the right of Data Synchronization Tasks, select the region in which you want to create the data synchronization task.
Note
If you use the new DTS console, select the region in which you want to create the data synchronization task in the upper-left corner of the page.

On the Data Synchronization page, click Create Task. On the Create Task wizard page, configure the source and destination databases.

Section	Parameter	Description
N/A	Task Name	The name of the task. DTS automatically assigns a name to the task. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select an existing DMS database instance. (Optional. If you have not registered a DMS database instance, ignore this option and configure database settings in the section below.)	The database instance that you want to use. You can choose whether to select an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
	Database Type	The type of the source database. Select MySQL.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source ApsaraDB RDS for MySQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
	RDS Instance ID	The ID of the source ApsaraDB RDS for MySQL instance.
	Database Account	The database account of the source ApsaraDB RDS for MySQL instance. For information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
	Database Password	The password of the database account.
	Encryption	Specifies whether to encrypt the connection to the database. Select Non-encrypted or SSL-encrypted based on your business requirements. If you select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the data synchronization task. For more information, see Configure the SSL encryption feature.
Destination Database	Select an existing DMS database instance. (Optional. If you have not registered a DMS database instance, ignore this option and configure database settings in the section below.)	The database instance that you want to use. You can choose whether to select an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
	Database Type	The type of the destination database. Select ClickHouse.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination ApsaraDB for ClickHouse cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
	Cluster Type	The type of the destination ApsaraDB for ClickHouse cluster.
	Cluster ID	The ID of the destination ApsaraDB for ClickHouse cluster.
	Database Account	The database account of the destination ApsaraDB for ClickHouse cluster. For information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
	Database Password	The password of the database account.

In the lower part of the page, click Test Connectivity and Proceed.
If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you must ensure that the ECS instance can access the database. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the whitelist of the database to allow DTS to access the database. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.
Warning
If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database or instance, or to the ECS security group rules, security risks may arise. Therefore, before you use DTS to synchronize data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

Configure the objects to be synchronized and the advanced settings.

Parameter	Description
Synchronization Types	By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of selected objects from the source instance to the destination instance. The historical data is the basis for subsequent incremental synchronization.
Processing Mode of Conflicting Tables	Precheck and Report Errors: 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 tables that have identical table names, the precheck is passed. Otherwise, an error is returned during the 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 Map object names. Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases. Warning If you select Ignore Errors and Proceed, data inconsistency may occur, and your business may be exposed to potential risks. If the source and destination databases have the same schemas, and a data record has the same primary key value as an existing data record in the destination database: During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained. During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten. If the source and destination databases have different schemas, data may fail to be initialized, only some columns are synchronized, or the data synchronization task fails. Operate with caution.
Capitalization of Object Names in Destination Instance	The capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to ensure that the capitalization of object names is consistent with that in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
Source Objects	Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section. Note You can select databases, tables, or columns as the objects to be synchronized.
Selected Objects	To rename an object that you want to synchronize to the destination instance, right-click the object in the Selected Objects section. For more information, see Map the name of a single object. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time. Note To specify filter conditions to synchronize data that meets the specified conditions, right-click the table to be synchronized in the Selected Objects section. In the dialog box that appears, enter one or more SQL conditions. For more information, see Set filter conditions. If you use the object name mapping feature to map the names of objects in the source database to those in the destination database or collection, the objects that depend on the renamed objects may fail to be synchronized.

Click Next: Advanced Settings to configure advanced settings.

Parameter or setting	Description
Select the dedicated cluster used to schedule the task	By default, DTS schedules tasks to shared clusters. You do not need to configure this parameter. You can purchase a dedicated cluster of specified specifications to run DTS synchronization tasks. For more information, see What is a DTS dedicated cluster.
Set Alerts	Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts will receive notifications. Valid values: No: does not configure alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert contacts. For more information, see Configure monitoring and alerting for a new DTS task.
Retry Time for Failed Connections	The retry time range for failed connections. If the source or destination database fails to be connected after the data synchronization task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Note If you set different retry time ranges for multiple DTS tasks that have the same source or destination database, the shortest retry time range that is set takes precedence. When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
The wait time before a retry when other issues occur in the source and destination databases.	The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set the parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Important The value of the The wait time before a retry when other issues occur in the source and destination databases parameter must be smaller than the value of the Retry Time for Failed Connection parameter.
Enable Throttling for Full Data Migration	During full data migration, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and BPS of Full Data Migration parameters for full data synchronization tasks to reduce the pressure on the destination database. Note This parameter can be configured only if Full Data Synchronization is selected as Synchronization Types.
Enable Throttling for Incremental Data Synchronization	You can configure the RPS of Incremental Data Migration and BPS of Incremental Data Migration parameters for incremental data synchronization tasks to reduce the pressure on the destination database.
Environment Tag	You can add environment tags to database instances based on your business requirements. In this example, no environment tag is added.
Configure ETL	Specifies whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: configures the ETL feature. You can enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: does not configure the ETL feature.

Click Next: Configure Database and Table Fields, and then configure the Type, Primary Key Column, Sort Key, Distribution Key, and Partition Key parameters of the destination table in the ApsaraDB for ClickHouse cluster.
Note
- By default, the parameters are configured by DTS. To modify the parameters, set the Definition Status parameter to All.
- You can select multiple columns for the Primary Key Column or Sort Key parameter from the corresponding drop-down list. One or more columns specified for the Primary Key Column parameter can be selected for the Partition Key parameter. You can select only one column for the Distribution Key parameter. For more information, see Create Table.
- The Partition Key parameter is optional. However, if you configure this parameter, all selected columns of the Partition Key parameter must be non-empty fields. Otherwise, the synchronization task fails.
Click Next: Save Task Settings and Precheck in the lower part of the page.
You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameter settings of the API operation that is called to configure the instance.
Note
- Before you can start the data synchronization task, DTS performs a precheck. You can start the data synchronization task only after the task passes the precheck.
- If the task fails to pass the precheck, click View Details next to each failed item. After you troubleshoot the issues based on the causes, run a precheck again.
- If an alert is triggered for an item during the precheck:
  If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.
  If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur, and your business may be exposed to potential risks.
Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.

On the Purchase Instance page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay for the instance when you create an instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. Pay-as-you-go: A pay-as-you-go instance is charged on an hourly basis. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go instance, you can release the pay-as-you-go instance to reduce costs.
	Resource Group	The resource group on which the instance is run. Default value: default resource group. For more information, see What is Resource Management?.
	Instance Class	DTS provides various synchronization specifications that provide different performance. The synchronization speed varies based on the synchronization specifications that you select. You can select a synchronization specification based on your business scenario. For more information, see Specifications of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, set the subscription duration and the number of instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years. Note This parameter is available only if you select the Subscription billing method.

Read and select the check box to agree to the Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.