migrate PolarDB for MySQL to ApsaraDB for ClickHouse - Data Transmission Service

Data Transmission Service (DTS) supports data migration from a PolarDB for MySQL cluster to an ApsaraDB for ClickHouse cluster. This lets you stream and centrally analyze your business data.

Prerequisites

You must create a destination ApsaraDB for ClickHouse cluster that runs version 20.8 or later. For more information, see Create a cluster.

Note

The storage space of the ApsaraDB for ClickHouse cluster must be larger than the used storage space of the source PolarDB for MySQL cluster.

Usage notes

Type	Description
Limits on the source database	The server that hosts the source database must have sufficient outbound bandwidth. Otherwise, the data migration speed is affected. If you migrate data at the table level and need to perform editing operations such as object name mapping, a single data migration task can migrate a maximum of 1,000 tables. If the number of tables exceeds this limit, a request error is reported after you submit the task. In this case, split the tables to be migrated and configure multiple tasks to migrate them in batches. Alternatively, configure a task to migrate the entire database. If you need to migrate incremental data: The binary logging feature must be enabled and the loose_polar_log_bin parameter must be set to on. Otherwise, error messages are returned during the precheck and the data migration task fails to be started. For more information, see Enable binary logging and Modify parameters. Note If you enable the binary logging feature for a PolarDB for MySQL cluster, you are charged for the storage space that is occupied by binary logs. The binary logs of a PolarDB for MySQL cluster must be stored for at least three days. We recommend that you store the binary logs 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 configure the retention period of binary logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed. Note For more information about how to set retention period of the binary logs of a PolarDB for MySQL cluster, see Modify the retention period. During schema migration and full data migration, do not perform DDL operations to change the schemas of databases or tables. Otherwise, the data migration task fails. Note During full data migration, DTS queries the source database. This creates a metadata lock, which may block DDL operations on the source database. If you perform only full data migration, do not write new data to the source database. Otherwise, the source and destination data will be inconsistent. To maintain real-time data consistency, select schema migration, full data migration, and incremental data migration.
Other limits	Read-only nodes of the source PolarDB for MySQL cluster cannot be migrated. DTS does not migrate Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster. The migration of INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, and FOREIGN KEY (FK) objects is not supported. Primary/standby switchover is not supported for the database instance during full data migration. If a switchover occurs, reconfigure the migration task promptly. If online DDL change operations in temporary table mode, including but not limited to multi-table merge scenarios, are performed on the source database, data may be lost in the destination database or the migration instance may fail. When you use DMS or the gh-ost tool to perform online DDL changes at the source, DTS migrates only the original DDL statement to the destination. In this scenario, DTS does not need to migrate a large amount of temporary table data, but this may cause table locks at the destination. Note Online DDL changes made using tools such as pt-online-schema-change at the source are not supported. If such changes exist at the source, data may be lost at the destination or the migration instance may fail. If the DDL statements of the source PolarDB for MySQL cluster do not follow standard MySQL syntax, the migration task may fail or data may be lost. The RENAME TABLE operation cannot be migrated. The time-type data in ApsaraDB for ClickHouse has a specific range. If the time data in PolarDB for MySQL is outside this range, the time migrated to ApsaraDB for ClickHouse will be incorrect. For more information about the range, see Time information. The Partition Key cannot be a nullable field. Otherwise, the migration task will fail. Note Partition keys support only BIGINT, INT, TIMESTAMP, DATETIME, and DATE data types. The number of databases to be migrated must not exceed the limit of ApsaraDB for ClickHouse, which is 256. The names of the databases, tables, and columns to be migrated must comply with the naming conventions of ApsaraDB for ClickHouse. For more information about the conventions, see Object naming conventions. During schema migration, DTS adds the _sign, _is_deleted, and _version fields to the destination tables. If you do not select Schema Migration when you configure Migration Types, you must manually create the tables to receive data in the destination and add the extra fields to the tables. For more information about the requirements for creating tables and the field information, see Tables and fields. Before you migrate data, evaluate the performance of the source and destination databases. We recommend that you migrate data during off-peak hours. Otherwise, initial full data synchronization will consume read and write resources of the source and destination databases, which may increase the database load. If you migrate one or more tables instead of an entire database, do not use tools such as pt-online-schema-change to perform online DDL changes on the migration objects in the source database. Otherwise, the migration will fail. You can use Data Management (DMS) to perform online DDL changes. For more information, see Change schemas without locking tables. During DTS migration, do not write data from sources other than DTS to the destination database. Otherwise, data inconsistency between the source and destination databases will occur. If an instance fails, DTS helpdesk will try to recover the instance within 8 hours. During the recovery process, operations such as restarting the instance and adjusting parameters may be performed. Note When parameters are adjusted, only the parameters of the DTS instance are modified. The parameters of the database are not modified. The parameters that may be modified include but are not limited to those described in Modify instance parameters.

Billing

Migration type	Link Setup Fee	Data transfer cost
Schema migration and full data migration	Free of charge.	This example is free of charge.
Incremental data migration	Charged. For more information, see Billing overview.	This example is free of charge.

SQL operations that can be incrementally migrated

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

Data type mappings

A one-to-one mapping is not possible because PolarDB for MySQL and ApsaraDB for ClickHouse support different data types. During initial schema synchronization, DTS maps the data types based on the types supported by the destination database. For more information, see Data type mappings between heterogeneous databases.

Permissions required for database accounts

Database	Required permissions	How to create and grant permissions
Source PolarDB for MySQL cluster	Read permissions on the migration objects.	Create and manage a database account and Manage the password of a database account.
Destination ApsaraDB for ClickHouse cluster	For version 22.8 or later, you need read and write permissions on the destination database. A privileged account meets these requirements. Version 21.8: Read, Write, and Set Permissions and Enable DDL.	Account management for Community-compatible Edition clusters

Procedure

Use one of the following methods to go to the Data Migration page and select the region in which the data migration instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Migration.
3. In the upper-left corner of the page, select the region in which the data migration instance resides.
DMS console
Note
The actual operation 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 > DTS (DTS) > Data Migration.
3. From the drop-down list to the right of Data Migration 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	Configuration	Description
None	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	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select PolarDB for MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB for MySQL cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	This example shows migration within the same Alibaba Cloud account. Select No.
	PolarDB Cluster ID	Select the ID of the source PolarDB for MySQL cluster.
	Database Account	Enter the database account for the source PolarDB for MySQL cluster. For more information about the required permissions, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.
	Encryption	Select a method as needed. For more information about the Secure Sockets Layer (SSL) encryption feature, see Set SSL encryption.
Destination Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	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	This example shows migration within the same Alibaba Cloud account. Select No.
	Cluster Type	Select the type of the ApsaraDB for ClickHouse cluster as needed.
	Cluster ID	Select the ID of the destination ApsaraDB for ClickHouse cluster.
	Database Account	Enter the database account for the destination ApsaraDB for ClickHouse cluster. For more information about the required permissions, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.

In the lower part of the page, click Test Connectivity and Proceed.
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 DTS server IP addresses to a whitelist.

Configure the objects to be migrated.

On the Configure Objects page, configure the objects that you want to migrate.

Configuration	Description
Migration Types	To perform only full data migration, select Schema Migration and Full Data Migration. To ensure service continuity during data migration, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If you do not select Schema Migration, make sure a database and a table are created in the destination database to receive data and the object name mapping feature is enabled in Selected Objects. If you do not select Incremental Data Migration, we recommend that you do not write data to the source database during data migration. This ensures data consistency between the source and destination databases.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains tables that use 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 migration 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 migrated 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 the following potential risks: If the source and destination databases have the same schema, and a data record has the same primary key as an existing data record in the destination database, the following scenarios may occur: During full data migration, DTS does not migrate the data record to the destination database. The existing data record in the destination database is retained. During incremental data migration, DTS migrates 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, only specific columns are migrated or the data migration 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 make sure that the capitalization of object names is consistent with that of 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. Click the icon to add the objects to the Selected Objects section. Note You can select objects to migrate at the database or table level.
Selected Objects	To rename an object that you want to migrate 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 set filter conditions for data, right-click the table to be migrated in the Selected Objects box and set the conditions in the dialog box that opens. For more information, see Set filter conditions. If you use the object name mapping feature, other objects that depend on the renamed object may fail to migrate.

Click Next: Advanced Settings to configure advanced settings.

Configuration	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the data migration task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data migration tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Time zone of destination database	The time zone of the date and time data type such as DateTime. You can select a time zone during the data synchronization to the destination ClickHouse cluster.
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 migration task is started, DTS immediately retries a connection within the retry time range. Valid values: 10 to 1,440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS is reconnected to the source and destination databases within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails. Note If you specify different retry time ranges for multiple data migration tasks that share the same source or destination database, the value that is specified later 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 the earliest opportunity after the source database and destination instance are released.
Retry Time for Other Issues	The retry time range for other issues. For example, if DDL or DML operations fail to be performed after the data migration task is started, DTS immediately retries the operations within the retry 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 retry time range, DTS resumes the data migration task. Otherwise, the data migration 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	Specifies whether to 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 enable throttling for full data migration based on your business requirements. To configure throttling, you must 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. This reduces the loads of the destination database server. Note You can configure this parameter only if you select Full Data Migration for the Migration Types parameter.
Enable Throttling for Incremental Data Migration	Specifies whether to enable throttling for incremental data migration. To configure throttling, you must configure the RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) parameters. This reduces the loads of the destination database server. Note You can configure this parameter only if you select Incremental Data Migration for the Migration Types parameter.
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	You can select an environment tag to identify the instance as needed. This example does not require a selection.
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 migration task. If the task fails or the migration latency exceeds the specified threshold, the alert contacts receive notifications. Valid values: No: does not configure 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 and then configure the Type, Primary Key Column, Sort Key, Distribution Key, and Partition Key parameters for the table that is migrated.
- DTS provides a set of default configurations. You can modify these configurations by setting Definition Status to All.
- The Primary Key Column and Sort Key can be composite keys. This means you can select multiple fields from the drop-down lists to serve as the Primary Key Column or Sort Key. You must also select one or more columns of the Primary Key Column to serve as the Partition Key. The Distribution Key can only be a single field. For more information about primary keys, sort keys, and partition keys, see CREATE TABLE.
  Note
  You can leave the Partition Key blank, but you cannot select a nullable field for it, or the migration task will fail.
  Partition keys support only the BIGINT, INT, TIMESTAMP, DATETIME, and DATE data types. For more information about their calculation logic, see Calculation logic for partition keys.

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 migration task, DTS performs a precheck. You can start the data migration 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 analyze the causes based on the check results, troubleshoot the issues. Then, 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 the 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 the instance.

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

On the Purchase Instance page, configure the Instance Class parameter for the data migration instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Resource Group	The resource group to which the data migration instance belongs. Default value: default resource group. For more information, see What is Resource Management?
New Instance Class	Instance Class	DTS provides instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Instance classes of data migration instances.

Read and agree to Data Transmission Service (Pay-as-you-go) Service Terms by selecting the check box.
Click Buy and Start. In the message that appears, click OK.
You can view the progress of the task on the Data Migration page.
Note
- If a data migration task cannot be used to migrate incremental data, the task automatically stops. The Completed is displayed in the Status section.
- If a data migration task can be used to migrate incremental data, the task does not automatically stop. The incremental data migration task never stops or completes. The Running is displayed in the Status section.

Appendix

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

Tables and fields

Tables

If you do not use the object name mapping feature, make sure that the tables you create meet the following requirements:

Important

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

If the destination database is an ApsaraDB for ClickHouse Community-compatible Edition cluster, you must create a local table and a distributed table. The name of the distributed table must be the same as that of the source table. The name of the local table is in the <Distributed table name>+_local format.
If the destination database is an ApsaraDB for ClickHouse Enterprise Edition cluster, you must create a table with the same name as the source table.

Fields