Synchronize data from MySQL to SelectDB - Data Transmission Service

Prerequisites

You have created a target ApsaraDB for SelectDB instance with storage space exceeding the storage used by the source ApsaraDB RDS for MySQL instance. For more information, see Create an instance.

Considerations

Type	Description
Source database limitations	Requirements for synchronization objects: If all tables to be synchronized have a primary key or unique constraint: Ensure that the columns that form the key or constraint contain unique values. If the synchronization objects include tables that have neither a primary key nor a unique constraint: When you configure the data synchronization instance, we recommend that you select Schema Synchronization for Synchronization Types. Then, in the Configurations for Databases, Tables, and Columns step, set the Engine for the table to duplicate. Otherwise, the instance may fail or data loss may occur. Note During schema synchronization, DTS adds columns to the destination tables. For more information, see Additional columns. If you synchronize at the table level and need to edit mappings (such as column name mapping), each synchronization task supports up to 1,000 tables. If you exceed this limit, the task fails with an error. To fix this, split the tables across multiple tasks or configure a full-database synchronization task. 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	You can synchronize data only to tables that use the Unique or Duplicate engine in Alibaba Cloud SelectDB instances. Unique engine If a destination table uses the Unique engine, ensure that all unique keys of the destination table exist in the source table and are included in the synchronization objects. Otherwise, data inconsistency may occur. Duplicate engine If a destination table uses the Duplicate engine, any of the following conditions may cause duplicate data in the destination database. You can manually deduplicate the data based on the additional columns (_is_deleted, _version, and _record_id): The data synchronization instance was retried. The data synchronization instance was restarted. Two or more DML operations were performed on the same row after the data synchronization instance started. Note When the destination table uses the Duplicate engine, DTS converts UPDATE or DELETE statements to INSERT statements. DTS does not synchronize INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, or foreign key (FK) objects. When you configure parameters in the Selected Objects section, you can only set the `bucket_count` (number of buckets) parameter. Note The value of the `bucket_count` parameter must be a positive integer. The default value is auto. During data synchronization, do not create a cluster in the destination Alibaba Cloud SelectDB instance. Otherwise, the task fails. You can restart the data synchronization instance to resume the task. If the name of a synchronized database or table does not start with a letter, rename it by using the object name mapping feature. If the name of a synchronized object (such as a database, table, or column) contains Chinese characters, rename it by using the object name mapping feature, for example, to an English name. Otherwise, the task may fail. DTS does not support DDL operations that modify multiple columns at once or consecutive DDL operations on the same table. During data synchronization, do not add BE nodes to the Alibaba Cloud SelectDB database. Otherwise, the task fails. You can restart the data synchronization instance to resume the task. In a many-to-one table synchronization scenario, where data from multiple source tables is synchronized to a single destination table, ensure that the schemas of all source tables are identical. Otherwise, data inconsistency or task failure may occur. In MySQL, M in `VARCHAR(M)` represents the number of characters. In Alibaba Cloud SelectDB, N in `VARCHAR(N)` represents the number of bytes. If you do not use the schema synchronization feature provided by DTS, we recommend setting the length of a VARCHAR field in Alibaba Cloud SelectDB to four times the length of the corresponding VARCHAR field in MySQL. 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. Before you synchronize data, evaluate the performance of the source and destination databases. Perform data synchronization during off-peak hours. Initial full data synchronization consumes read and write resources of both the source and destination databases, which may increase the database load. Initial full data synchronization runs concurrent INSERT operations, which causes fragmentation in the tables of the destination database. As a result, the tablespace of the destination database is larger than that of the source database after the initial full data synchronization completes. During data synchronization, do not use tools such as pt-online-schema-change to perform online DDL changes on the synchronization objects in the source database. Otherwise, the task fails. During data synchronization, if sources other than DTS write data to the destination database, data inconsistency may 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. During incremental synchronization, DTS uses a batch synchronization strategy to reduce the load on the destination. By default, DTS writes to a single synchronization object at most once every 5 seconds. Therefore, DTS synchronization tasks may experience regular synchronization latency—typically within 10 seconds. To reduce this regular synchronization latency, modify the DTS instance parameter `selectdb.reservoir.timeout.milliseconds` in the console to adjust the batch interval. The allowed range is [1000, 10000] milliseconds. Note When you adjust the batch interval, a smaller value increases the write frequency of DTS. This may increase the load and write response time (RT) on the destination, which in turn increases DTS synchronization latency. Adjust the value based on the load on the destination. 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.
Special cases	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.

SQL for incremental synchronization

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	ADD COLUMN MODIFY COLUMN CHANGE COLUMN DROP COLUMN and DROP TABLE TRUNCATE TABLE RENAME TABLE Important The RENAME TABLE operation may cause data inconsistency. For example, if the synchronization object is only a specific table and you rename the table in the source instance during synchronization, the data of the table is not synchronized to the destination database. To prevent this issue, select the entire database to which the table belongs as the synchronization object when you configure the data synchronization task. Make sure that the databases to which the table belongs before and after the RENAME TABLE operation are both included in the synchronization object.

Database account permissions

Database	Required permissions	Actions
Source ApsaraDB RDS for MySQL	Read and write permissions on the objects to be synchronized	Create an account and Modify account permissions
Destination ApsaraDB for SelectDB instance	Cluster access permission (Usage_priv) and read and write permissions on the database (Select_priv, Load_priv, Alter_priv, Create_priv, and Drop_priv)	Manage cluster permissions and Manage basic permissions

Note

If you created the source database account outside the ApsaraDB RDS for MySQL console, ensure that it 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 DMS console layout and style.
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 navigate to the task configuration page.

Configure the source and destination databases.

Section	Parameter	Description
N/A	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 for the source ApsaraDB RDS for MySQL instance.
	Replicate Data Across Alibaba Cloud Accounts	In this example, data is synchronized within 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 required permissions, see Permissions required for database accounts.
	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 ApsaraDB for SelectDB.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region for the destination ApsaraDB for SelectDB instance.
	Replicate Data Across Alibaba Cloud Accounts	In this example, data is synchronized within the same Alibaba Cloud account. Select No.
	Instance ID	Select the ID of the destination ApsaraDB for SelectDB instance.
	Database Account	Enter the database account of the destination ApsaraDB for SelectDB instance. For information about the required permissions, see Permissions required for database accounts.
	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.

Parameter	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. Important Data types are converted when data is synchronized from MySQL to ApsaraDB for SelectDB. If you do not select Schema Synchronization, you must create tables that use the Unique or Duplicate model with the appropriate schema in the destination ApsaraDB for SelectDB instance in advance. For more information, see Data type mappings, Additional columns, and Data models.
Processing Mode of Conflicting Tables	Precheck and Report Errors: DTS checks for tables with the same name in the destination database. If a conflict exists, DTS reports an error during the precheck and does not start the task. Note If it is not practical to delete or rename the conflicting table, you can map the table to a different name. For more information, see Map object names. Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database. Warning If you select Ignore Errors and Proceed, data inconsistency may occur. For example: If the table schemas are consistent, destination records with the same primary or unique key as source records are overwritten. If the table schemas are inconsistent, the task may fail or only partially synchronize data. Proceed 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 at the database or table level.
Selected Objects	To change the name of a synchronization object in the destination instance, right-click the object in the Selected Objects box. For more information, see Map object names. If you select Schema Synchronization for Synchronization Types, select tables as the synchronization objects, and need to set the number of buckets (the `bucket_count` parameter), right-click the table in the Selected Objects box. In the Parameter Settings section, set Enable Parameter Settings to Yes, specify a Value based on your requirements, and then click OK. Note To select the incremental SQL operations to be synchronized at the database or table level, right-click the object in the Selected Objects box and select the desired SQL operations in the dialog box that appears. To filter data by using a WHERE clause, right-click the table in the Selected Objects box and set the filter conditions in the dialog box that appears. For more information, see Configure filter conditions. If you use the object name mapping feature, other objects that depend on the renamed object may fail to be synchronized.

Click Next: Advanced Settings.

Parameter	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?.
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).
Environment Tag	You can select an environment tag to identify the instance based on your requirements. This example does not require a tag.
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.
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.

Optional: After completing the preceding configurations, click Next: Configure Database and Table Fields to set the Primary Key Column, Distribution Key, and Engine for the destination tables.
Note
- This step is available only if you select Schema Synchronization for Synchronization Types when you configure the synchronization objects. You can set Definition Status to All to modify the settings.
- You can select multiple columns to form a composite Primary Key Column. You must also select one or more columns from the Primary Key Column as the Distribution Key.
- For a table without a primary key or a unique constraint, you must select duplicate for the Engine. Otherwise, the task may fail or data loss may occur.

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.

Data type mapping

Category	MySQL type	SelectDB type
NUMERIC	TINYINT	TINYINT
	TINYINT UNSIGNED	SMALLINT
	SMALLINT	SMALLINT
	SMALLINT UNSIGNED	INT
	MEDIUMINT	INT
	MEDIUMINT UNSIGNED	BIGINT
	INT	INT
	INT UNSIGNED	BIGINT
	BIGINT	BIGINT
	BIGINT UNSIGNED	LARGEINT
	BIT(M)	INT
	DECIMAL	DECIMAL Note ZEROFILL is not supported.
	NUMERIC	Decimal
	FLOAT	FLOAT
	DOUBLE	DOUBLE
	BOOL BOOLEAN	BOOLEAN
DATE AND TIME	DATE	DATEV2
	DATETIME[(fsp)]	DATETIMEV2
	TIMESTAMP[(fsp)]	DATETIMEV2
	TIME[(fsp)]	VARCHAR
	YEAR[(4)]	INT
STRING	CHAR VARCHAR	VARCHAR Important To prevent data loss, CHAR and VARCHAR(n) data is converted to VARCHAR(4*n) when synchronized to the destination ApsaraDB for SelectDB instance. If no data length is specified, the default is VARCHAR(65533). If the data length exceeds 65533, the data is converted to STRING.
	BINARY VARBINARY	STRING
	TINYTEXT TEXT MEDIUMTEXT LONGTEXT	STRING
	TINYBLOB BLOB MEDIUMBLOB LONGBLOB	STRING
	ENUM	STRING
	SET	STRING
	JSON	STRING

Additional columns

Note

This table lists the additional columns that DTS automatically adds or that you must manually add to the destination duplicate model table.

Parameter	Type	Default	Description
_is_deleted	Int	0	Indicates whether the row is deleted. Insert: The value is 0. Update: The value is 0. Delete: The value is 1.
_version	Bigint	0	For data from a full synchronization, the value is 0. For data from an incremental synchronization, the value is the commit timestamp (in seconds) of the change from the source database binlog.
_record_id	Bigint	0	For data from a full synchronization, the value is 0. For data from an incremental synchronization, the value is the record ID from the incremental log. This ID uniquely identifies the log entry. Note The ID is unique and increases monotonically.

Prerequisites

Considerations

Unique engine

Duplicate engine