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 columnar database built for online analytical processing (OLAP). It provides fast aggregate analysis and queries on large wide tables. Its speed is an order of magnitude faster than other analytic 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 helps you centralize data for analysis. This topic uses an RDS MySQL instance as an example to demonstrate how to synchronize its data 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.

Limitations

Type	Description
Source database limitations	Tables without a primary key cannot be synchronized. If you synchronize data at the table level and need to edit objects, such as mapping table or column names, a single data synchronization task supports a maximum of 1,000 tables. If you exceed this limit, an error is reported after you submit the task. In this case, split the tables into multiple tasks or configure a task to synchronize the entire database. Binary logs: ApsaraDB RDS for MySQL enables binary logging by default. Ensure that the binlog_row_image parameter is set to full. Otherwise, the precheck fails and the synchronization task cannot start. For instructions, see Configure instance parameters. Important If your source instance is a self-managed MySQL database, enable binary logging and set binlog_format to row and binlog_row_image to full. If your self-managed MySQL database is a dual-primary cluster (where both nodes act as primary and secondary), enable the log_slave_updates parameter so DTS can capture all binary log events. For instructions, see Create an account and configure binary logging for a self-managed MySQL database. The local binary logs for an ApsaraDB RDS for MySQL instance must be retained for at least three days (seven days is recommended). For a self-managed MySQL database, retain local binary logs for at least seven days. Otherwise, DTS may fail to retrieve binary logs, causing the task to fail. In extreme cases, this may cause data inconsistency or data loss. Issues caused by binary log retention periods shorter than DTS requires are not covered under the DTS SLA. Note To configure the retention period for local binary logs on an ApsaraDB RDS for MySQL instance, see Automatically delete local logs. Do not run DDL operations that change database or table schemas during schema synchronization or full synchronization. Otherwise, the synchronization task fails. Note During full synchronization, DTS queries the source database. This creates metadata locks that may block DDL operations on the source database. Data generated by changes that do not write to binary logs—such as data restored from physical backups or created by cascade operations—is not synchronized to the destination database. Note If this occurs, remove the affected database or table from the synchronization objects. Then add it back. You can do this only if your business allows it. For more information, see Modify synchronization objects. If your source database is MySQL 8.0.23 or later and contains invisible hidden columns, DTS may not read those columns. This may cause data loss. Note Run the `ALTER TABLE <table_name> ALTER COLUMN <column_name> SET VISIBLE;` command to make the hidden column visible. For more information, see Invisible Columns.
Other limitations	If the DDL statements of the source RDS MySQL instance do not follow standard MySQL syntax, the synchronization task may fail or data may be lost. If your source database uses online DDL operations in temporary table mode—including but not limited to multi-table merge scenarios—or adds function-based indexes to unique key columns, data loss or task failure may occur in the destination database. When you use DMS or the gh-ost tool to perform online DDL operations on the source, DTS synchronizes only the original DDL statements to the destination. In this scenario, DTS does not need to synchronize a large amount of temporary table data, but this may cause tables to be locked at the destination. Note DTS does not support synchronizing online DDL changes made with tools like pt-online-schema-change. If such changes exist at the source, data may be lost at the destination, or the synchronization instance may fail. Synchronization of INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, and FK is supported. The RENAME TABLE operation is not supported. If a primary key or unique key conflict occurs while the task is running: If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database: During full data synchronization, DTS retains the destination record and skips the source record. During incremental synchronization, DTS overwrites the destination record with the source record. If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution. Time-type data in ApsaraDB for ClickHouse has range limitations. If the time data in RDS MySQL is outside this range, the time synchronized to ApsaraDB for ClickHouse will be incorrect. For information about the range limitations, see Time information. The Partition Key cannot be a nullable field. Otherwise, the synchronization task will fail. Note Only fields of the BIGINT, INT, TIMESTAMP, DATETIME, and DATE types are supported as partition keys. The number of databases to be synchronized does not exceed the limit of ApsaraDB for ClickHouse, which is 256. The names of the databases, tables, and columns to be synchronized comply with the naming conventions of ApsaraDB for ClickHouse. For more information about the conventions, see Object naming conventions. During schema synchronization, DTS adds the _sign, _is_deleted, and _version fields to the destination tables. If you do not select Schema Synchronization when you configure Synchronization Types, you must manually create the tables for receiving data at the destination and add the extra fields to the tables. For information about table creation requirements and field details, see Table and field information. Before synchronizing data, evaluate the performance of the source and destination databases. We recommend that you perform data synchronization during off-peak hours. Otherwise, initial full data synchronization will consume read and write resources of the source and destination databases, which may increase database loads. If you synchronize one or more tables instead of the entire database, do not use tools like pt-online-schema-change to perform online DDL operations on the synchronization objects in the source database. Otherwise, the synchronization will fail. You can use Data Management (DMS) to perform online DDL operations. For more information, see Change schemas without locking tables. During DTS synchronization, do not allow any data other than that from DTS to be written to the destination database. Otherwise, data inconsistency between the source and destination databases will occur. If your ApsaraDB RDS for MySQL instance has Always-Encrypted enabled, full data synchronization is not supported. Note ApsaraDB RDS for MySQL instances with Transparent Data Encryption (TDE) enabled support schema synchronization, full data synchronization, and incremental data synchronization. If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters. Note Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.
Other notes	For a self-managed MySQL source database: If a primary/secondary switchover occurs in the source database during synchronization, the task fails. DTS calculates latency by comparing the timestamp of the last synchronized record with the current time. If no DML operations run for a long time in the source database, latency reporting may become inaccurate. If latency appears too high, run a DML operation in the source database to update the latency. Note If you select a full database for synchronization, create a heartbeat table. Update or write to this table every second. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command in the source database to advance the binary log offset. If your source database is Amazon Aurora MySQL or another clustered MySQL instance, ensure the domain name or IP address used in the task configuration—and its DNS resolution—always points to a read/write (RW) node. Otherwise, synchronization may fail. For an ApsaraDB RDS for MySQL source database: Read-only instances—such as ApsaraDB RDS for MySQL 5.6 read-only instances—that do not record transaction logs cannot serve as source databases. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command in the source database to advance the binary log offset.

Billing

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

Supported SQL operations

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

Data type mappings

Because MySQL and ApsaraDB for ClickHouse clusters support different data types, a one-to-one mapping is not possible. When DTS performs initial schema synchronization, it maps data types based on the types supported by the destination database. For more information, see Data type mappings for initial schema synchronization.

Database account permissions

Database	Required permissions	Creation and authorization method
Source RDS MySQL	Read permissions on the objects to be synchronized.	Create an account and Modify account permissions.
Destination ApsaraDB for ClickHouse cluster	Version 22.8 or later: Read and write permissions on the destination database. A privileged account meets the requirements. Version 21.8: Read/Write And Settings and Allow DDL.	Community-compatible Edition Account Management

Note

If the source database account you are using was not created and authorized through the RDS MySQL console, make sure the account has the REPLICATION CLIENT, REPLICATION SLAVE, SHOW VIEW, and SELECT permissions.

Procedure

Go to the data synchronization task list page in the destination region. You can do this in one of two ways.
DTS console
1. Log on to the DTS console.
2. In the navigation pane on the left, click Data Synchronization.
3. In the upper-left corner of the page, select the region where the synchronization instance is located.
DMS console
Note
The actual steps may vary depending on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top menu bar, choose Data + AI > DTS (DTS) > Data Synchronization.
3. To the right of Data Synchronization Tasks, select the region of the synchronization instance.
Click Create Task to open the task configuration page.

Configure the source and destination databases.

Category	Configuration	Description
None	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source RDS MySQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	This example shows synchronization within the same Alibaba Cloud account. Select No.
	RDS Instance ID	Select the ID of the source RDS MySQL instance.
	Database Account	Enter the database account of the source RDS MySQL instance. For information about permission requirements, see Database account permissions.
	Database Password	Enter the password for the specified database account.
	Encryption	Select Non-encrypted or SSL-encrypted as needed. If you set this to SSL-encrypted, you must enable SSL encryption for the RDS for MySQL instance beforehand. For more information, see Use a cloud certificate to quickly enable SSL link encryption.
Destination Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	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 synchronization 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 of the destination ApsaraDB for ClickHouse cluster. For information about permission requirements, see Database account permissions.
	Database Password	Enter the password for the specified database account.

After completing the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
- Ensure that you add the CIDR blocks of the DTS servers (either automatically or manually) to the security settings of both the source and destination databases to allow access. For more information, see Add the IP address whitelist of DTS servers.
- If the source or destination is a self-managed database (i.e., the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the task objects.

On the Configure Objects page, specify the objects to synchronize.

Configuration	Description
Synchronization Types	DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks for tables with the same names in the destination database. If any tables with the same names are found, an error is reported during the precheck and the data synchronization task does not start. Otherwise, the precheck is successful. Note If you cannot delete or rename the table with the same name in the destination database, you can map it to a different name in the destination. For more information, see Database Table Column Name Mapping. Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example: If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database: During full data synchronization, DTS retains the destination record and skips the source record. During incremental synchronization, DTS overwrites the destination record with the source record. If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.
Capitalization of Object Names in Destination Instance	Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.
Source Objects	In the Source Objects box, click the objects, and then click to move them to the Selected Objects box. Note You can select objects to synchronize at the database or table level.
Selected Objects	To rename a single object in the destination instance, right-click the object in the Selected Objects box. For more information, see Map a single object name. To rename multiple objects in bulk, click Batch Edit in the upper-right corner of the Selected Objects box. For more information, see Map multiple object names in bulk. Note To filter data, you can right-click the table to synchronize in the Selected Objects box and set the filter conditions in the dialog box. For more information, see Set filter conditions. If you use the object name mapping feature, other objects that depend on the mapped object may fail to be synchronized.

Click Next: Advanced Settings.

Configuration	Description
Dedicated Cluster for Task Scheduling	By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.
Time zone of destination database	You can select the time zone for DateTime data that is written to the ApsaraDB for ClickHouse cluster.
Retry Time for Failed Connections	If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails. Note If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances. DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.
Retry Time for Other Issues	If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.
Enable Throttling for Full Data Synchronization	During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s). Note This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
Enable Throttling for Incremental Data Synchronization	You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).
Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Choose whether DTS writes heartbeat SQL information to the source database while the instance is running. Yes: Does not write heartbeat SQL information to the source database. The DTS instance may display latency. No: Writes heartbeat SQL information to the source database. This may interfere with source database operations like physical backups and cloning.
Environment Tag	You can select an environment tag to identify the instance as needed. No selection is needed for this example.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert notifications. For more information, see Configure monitoring and alerting during task configuration.

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 synchronize to ClickHouse.
- DTS provides a default configuration. You can set Definition Status to All to modify the configuration.
- The Primary Key Column and Sort Key can be composite keys. You can select multiple fields from the drop-down lists to define the Primary Key Column or Sort Key. You must also select one or more columns from the Primary Key Column as the Partition Key. Only one field can be selected as the Distribution Key. For more information about primary key columns, sort keys, and partition keys, see CREATE TABLE.
  Note
  The Partition Key is optional, but it cannot be a nullable field, or the synchronization task will fail.
  Only fields of the BIGINT, INT, TIMESTAMP, DATETIME, and DATE types are supported as partition keys. For information about the calculation logic, see Calculation logic for partition keys.

Save the task and perform a precheck.
- To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.
- If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.
- If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.
- If the precheck generates warnings:
  For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.
  For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.

Purchase the instance.

When the Success Rate reaches 100%, click Next: Purchase Instance.

On the Purchase page, select the billing method and link specifications for the data synchronization instance. For more information, see the following table.

Category	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay upfront for a specific duration. This is cost-effective for long-term, continuous tasks. Pay-as-you-go: You are billed hourly for actual usage. This is ideal for short-term or test tasks, as you can release the instance at any time to save costs.
	Resource Group Settings	The resource group to which the instance belongs. The default is default resource group. For more information, see What is Resource Management?.
	Instance Class	DTS offers synchronization specifications at different performance levels that affect the synchronization rate. Select a specification based on your business requirements. For more information, see Data synchronization link specifications.
	Subscription Duration	In subscription mode, select the duration and quantity of the instance. Monthly options range from 1 to 9 months. Yearly options include 1, 2, 3, or 5 years. Note This option appears only when the billing method is Subscription.

Read and select the checkbox for Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start, and then click OK in the OK dialog box.
You can monitor the task progress on the data synchronization page.

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

Table and field information

Table information

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

Important

If the destination table includes an ENGINE, it must be ENGINE = ReplicatedReplacingMergeTree(_version, _is_deleted). Otherwise, data inconsistency may occur.

ClickHouse Community Edition instance: You need to create one local table and one distributed table. The name of the distributed table must be the same as the source table name. The name of the local table must be <distributed_table_name>_local.
ClickHouse Enterprise Edition instance: You need to create a table with the same name as the source table.

Field information