Synchronize data from an ApsaraDB RDS for MySQL instance to a ClickHouse cluster - Data Transmission Service

ApsaraDB for ClickHouse is a columnar database management system (DBMS) designed for online analytical processing (OLAP). Compared with traditional analytical databases, ApsaraDB for ClickHouse delivers faster aggregate queries on large and wide tables. You can use Data Transmission Service (DTS) to synchronize data from a MySQL database (either a self-managed MySQL database or an ApsaraDB RDS for MySQL instance) to an ApsaraDB for ClickHouse cluster. The data synchronization feature enables efficient data transfer and analysis. This topic describes how to synchronize data from an ApsaraDB RDS for MySQL instance to an ApsaraDB for ClickHouse cluster.

Prerequisites

A destination ApsaraDB for ClickHouse cluster of version 20.8 or later is created. For more information, see Create a cluster.

Note

The storage capacity of the ApsaraDB for ClickHouse cluster must be greater than the used storage space of the source ApsaraDB RDS for MySQL instance.

Limits

Type	Description
Limits on source database	The objects to be synchronized cannot be tables without primary keys. If you select tables as the objects to be synchronized and you need to edit the tables (such as renaming tables or columns) in the destination database, 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, an error is returned. In this case, we recommend that you split the tables and configure multiple tasks to synchronize the tables, or configure a task to synchronize the entire database. Binary logs: The binary logging feature is enabled by default. The binlog_row_image parameter must be set to full. Otherwise, error messages are returned during precheck and the data synchronization task cannot be started. For more information about how to set parameters, see Configure parameters for an RDS instance. Important If the source database is a self-managed MySQL database, you must enable binary logging 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 in a dual-primary cluster (the two nodes function as primary and secondary nodes for each other), you must enable the log_slave_updates parameter to ensure that DTS can obtain all binary logs. For more information, see Create an account for a user-created MySQL database and configure binary logging. DTS requires that the local binary logs of the source database be retained for at least 7 days. Otherwise, DTS may fail to obtain the binary logs and the task may fail. In extreme cases, data may be inconsistent or lost. Make sure that you set the retention period of binary logs in accordance with the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not ensure service reliability and performance. If the source is an ApsaraDB RDS for MySQL instance, see Manage local binary logs. During schema synchronization and full data synchronization, do not execute DDL statements to change the schemas of databases or tables. Otherwise, the data synchronization task fails. The data generated by change operation of binary logs, such as data restored from a physical backup or data from a cascade operation, is not recorded or synchronized to the destination database when the data synchronization instance is running. Note If the change data is not recorded and synchronized to the destination database, you can modify the object to be synchronized and remove the databases and tables to which the data belongs on the premise that your business is not affected, and then add the databases and tables to which the data belongs to the object to be synchronized. For more information, see Modify synchronization objects. If the source database is MySQL database 8.0.23 or later, and the data to be synchronized includes invisible columns, the data of the columns cannot be obtained and data loss occurs. Note To make the columns visible, run the `ALTER TABLE <table_name> ALTER COLUMN <column_name> SET VISIBLE;` command. For more information, see Invisible Columns. Tables that do not have primary keys automatically generate invisible primary keys. You need to make the invisible primary keys visible. For more information, see Generated Invisible Primary Keys.
Others	If the DDL statements of the source ApsaraDB RDS for MySQL instance do not comply with the standard MySQL syntax, the synchronization task may fail or data may be lost. DTS does not synchronize RENAME TABLE operations. The time data types in ApsaraDB for ClickHouse have range limits. If the time in the ApsaraDB RDS for MySQL instance is not within the range, the time synchronized to ApsaraDB for ClickHouse is incorrect. For more information about the range limits, see Time information. You cannot select a nullable field as a Partition Key. Otherwise, the synchronization task fails. A maximum of 256 databases can be synchronized to the 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 Naming conventions. During schema synchronization, DTS adds the _sign, _is_deleted, and _version fields to the destination table. If you do not select Schema Synchronization when you configure Synchronization Types, you must manually create tables in the destination database to receive the synchronized data. You must also add the extra fields to the tables. For more information about the requirements for creating tables and the field information, see Table and field information. 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 synchronize a single table or multiple tables (not an entire database), do not use tools such as pt-online-schema-change to perform online DDL operations on the source objects during data synchronization. Otherwise, the synchronization task fails. You can use Data Management (DMS) to perform online DDL operations. For more information, see Change schemas without locking tables. 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. If the source database is an ApsaraDB RDS for MySQL instance for which the EncDB feature is enabled, full data synchronization cannot be performed. Note ApsaraDB RDS for MySQL instances that have the Transparent Data Encryption (TDE) feature enabled support schema synchronization, full data synchronization, and incremental data synchronization. If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified. Note Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.
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 a long time, the synchronization latency may be inaccurate. If the latency of the data 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 synchronize, you can create a heartbeat table. The heartbeat table is updated or receives data every second. DTS executes the CREATE DATABASE IF NOT EXISTS 'test' statement in the source database as scheduled to move forward the binary log file position. If the source database is an ApsaraDB RDS for MySQL instance, take note of the following item: An ApsaraDB RDS for MySQL instance that does not record transaction logs, such as a read-only ApsaraDB RDS for MySQL 5.6 instance, cannot be used as the source database. DTS executes the CREATE DATABASE IF NOT EXISTS 'test' statement in the source database as scheduled 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 statements
DML	INSERT, UPDATE, DELETE
DDL	CREATE TABLE TRUNCATE TABLE ALTER TABLE ADD COLUMN MODIFY COLUMN DROP TABLE, DROP COLUMN

Data type mappings

MySQL and ClickHouse clusters support different data types. The data types cannot be mapped one-to-one. During schema initialization, DTS maps the data types based on the data types that are supported by the destination database. For more information, see Data type mappings for schema initialization.

Permissions required for database accounts

Database	Required permissions	Creation and authorization methods
Source ApsaraDB RDS for MySQL	Read permissions on the objects to synchronize.	Create an account and Modify account permissions.
Destination ApsaraDB for ClickHouse cluster	Version 22.8 and later: Read and write permissions on the destination database. A privileged account meets the requirements. Version 21.8: Read, Write, And Settings Permissions and Allow DDL.	Manage accounts for a Community Edition 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

Use one of the following methods to go to the Data Synchronization page and select the region in which the data synchronization instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Synchronization.
3. In the upper-left corner of the page, select the region in which the data synchronization instance resides.
DMS console
Note
The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data + AI and choose DTS (DTS) > Data Synchronization.
3. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.

Configure the source and destination databases. The following table describes the parameters.

Category	Parameter	Description
N/A	Task Name	The name of the DTS task. DTS automatically generates a task name. 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 Existing Connection	The database that you want to use. You can choose whether to use an existing database based on your business requirements. If you select an existing database, DTS automatically populates the parameters for the database. If you do not select an existing database, you must configure the following database information. Note In the DTS console, register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	Select MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source ApsaraDB RDS for MySQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, the source and destination instances belong to the same Alibaba Cloud account. Select No.
	RDS Instance ID	Select the ID of the source ApsaraDB RDS for MySQL instance.
	Database Account	Enter the database account of the source ApsaraDB RDS for MySQL instance. For information about the permissions that are required for the account, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.
	Encryption	Specifies whether to encrypt the connection to the database. You can select Non-encrypted or SSL-encrypted based on your business requirements. If you want to set this parameter to SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the DTS task. For more information, see Use a cloud certificate to enable SSL encryption.
Destination Database	Select Existing Connection	The database that you want to use. You can choose whether to use an existing database based on your business requirements. If you select an existing database, DTS automatically populates the parameters for the database. If you do not select an existing database, you must configure the following database information. Note In the DTS console, register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	Select ClickHouse.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination ApsaraDB for ClickHouse cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, the source and destination instances belong to the same Alibaba Cloud account. Select No.
	Cluster Type	Select the type of the ApsaraDB for ClickHouse cluster based on your business requirements.
	Cluster ID	Select the ID of the destination ApsaraDB for ClickHouse cluster.
	Database Account	Enter the database account of the destination ApsaraDB for ClickHouse cluster. For information about the permissions that are required for the account, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.

Click Test Connectivity and Proceed in the lower part of the page.
Note
- Make sure that the CIDR blocks of DTS servers can be automatically or manually added to the security settings of the source and destination databases to allow access from DTS servers. For more information, see Add the CIDR blocks of DTS servers.
- If the source or destination database is a self-managed database and its Access Method is not set to Alibaba Cloud Instance, click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the objects to be synchronized.

In the Configure Objects step, configure the objects that you want to synchronize.

Parameter	Description
Synchronization Types	The 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 the selected objects from the source database to the destination cluster. 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 If the source and destination databases contain tables with identical names and the tables in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the tables that are synchronized to the destination database. 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 schema and a data record in the destination database has the same primary key value or unique key value as a data record in the source 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. In this case, only some columns are synchronized, or the data synchronization task fails. Proceed 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 the "Map the name of a single object" section of the Map object names topic. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see the "Map multiple object names at a time" section of the Map object names topic. Note To configure filter conditions for a table, right-click the table in the Selected Objects section. In the dialog box that appears, configure filter conditions. For more information, see Configure filter conditions. If you use the object name mapping feature to rename an object, other objects that depend on the object may fail to be synchronized.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data synchronization tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Time zone of destination database	You can select the time zone for the DateTime data that is written to the ApsaraDB for ClickHouse cluster based on your business requirements.
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 this 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 specify different retry time ranges for multiple data synchronization tasks that have the same source or destination database, the shortest retry time range 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.
Retry Time for Other Issues	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 this 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 Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Migration	During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server. Note This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter.
Enable Throttling for Incremental Data Synchronization	Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Specifies whether to write SQL operations on heartbeat tables to the source database while the DTS instance is running. Valid values: Yes: does not write SQL operations on heartbeat tables. In this case, a latency of the DTS instance may be displayed. No: writes SQL operations on heartbeat tables. In this case, features such as physical backup and cloning of the source database may be affected.
Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, no environment tag is selected.
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.
Monitoring and Alerting	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 enable alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see the "Configure monitoring and alerting when you create a DTS task" section of the Configure monitoring and alerting topic.

Click Next: Configure Database and Table Fields to configure the Type, Primary Key Column, Sort Key, Distribution Key, and Partition Key for the tables to be synchronized in the ApsaraDB for ClickHouse cluster.
Note
- DTS configures a set of data by default. You can select All from the Definition Status drop-down list and modify the data.
- The Primary Key Column and Sort Key can be composite keys. You can select multiple fields from the corresponding drop-down list as the Primary Key Column or Sort Key. You must select one or more columns from the Primary Key Column as the Partition Key. You can select only one field as the Distribution Key. For more information about primary key columns, sort keys, and partition keys, see CREATE TABLE.
- You can leave the Partition Key unspecified. However, you cannot select a nullable field as a partition key. Otherwise, the synchronization task fails.

Save the task settings and run a precheck.
- To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.
- If you do not need to view or have viewed the parameters, click Next: Save Task Settings and Precheck in the lower part of the page.
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 data synchronization task fails the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, rerun the precheck.
- 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 issue. 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.

Purchase an instance.

Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.

On the buy 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 a subscription when you create a data synchronization 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 billed 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 data synchronization instance, you can release the instance to reduce costs.
	Resource Group Settings	The resource group to which the data synchronization instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides instance classes that vary in synchronization speed. You can select an instance class based on your business requirements. For more information, see Instance classes of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, specify the subscription duration and the number of data synchronization 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 Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the dialog box that appears, click OK.
You can view the progress of the task in the task list.

Appendices

Time information

Data type	Minimum value	Maximum value
Date	1970-01-01 00:00:00	2149-06-06 00:00:00
Date32	1925-01-01 00:00:00	2283-11-11 00:00:00
DateTime	1970-01-01 08:00:00	2106-02-07 14:28:15
DateTime64	1925-01-01 08:00:00	2283-11-12 07:59:59

Table and field information

Table information

If you do not use the object name mapping feature, the tables that you create must meet the following requirements.

Important

If the destination table contains the ENGINE parameter, the parameter must be set to ENGINE = ReplicatedReplacingMergeTree(_version, _is_deleted). Otherwise, data inconsistency may occur.

ApsaraDB for ClickHouse Community Edition instance: You must create a local table and a distributed table. The name of the distributed table must be the same as the name of the source table. The name of the local table must be <Name of the distributed table>+_local.
ApsaraDB for ClickHouse Enterprise Edition instance: You must create a table that has the same name as the source table.

Field information

Note

You can use the select * from table_name final where _sign>0; statement to query data in a ClickHouse instance. The where condition filters deleted data, and the final field after the table name filters data with the same sort key.

Version	Name	Data type	Default value	Description
Community Edition earlier than version 23.8Community Edition	_sign	Int8	1	The type of the DML operation. INSERT: The value is 1. UPDATE: The value is 1. DELETE: The value is -1.
	_version	UInt64	1	The timestamp when the data is written to ApsaraDB for ClickHouse.
Enterprise Edition and Community Edition 23.8 and later	_sign	Int8	1	The type of the DML operation. INSERT: The value is 1. UPDATE: The value is 1. DELETE: The value is -1.
	_is_deleted	Int8	0	Indicates whether the record is deleted: Insert: The value is 0. Delete: The value is 1. Update: The value is 0.
	_version	UInt64	1	The timestamp when the data is written to ApsaraDB for ClickHouse.

Prerequisites

Limits

Billing

SQL operations that can be synchronized

Data type mappings

Permissions required for database accounts

Procedure

DTS console

DMS console

Appendices

Time information

Table and field information

Table information

Field information