Synchronize MySQL data to SelectDB - Data Transmission Service

ApsaraDB for SelectDB provides sub-second responses for massive data queries, high-concurrency point queries of up to 10,000 queries per second, and high-throughput complex analytics. Data Transmission Service (DTS) lets you synchronize data from a MySQL database, such as a self-managed MySQL database or an RDS for MySQL instance, to an ApsaraDB for SelectDB instance to meet your massive data analytics needs. This topic uses an RDS for MySQL instance as an example to describe the procedure.

Prerequisites

Create a destination ApsaraDB for SelectDB instance. The storage space of the destination instance must be larger than the storage space used by the source RDS for MySQL instance. For more information, see Create an instance.

Precautions

Type	Description
Source database limits	Requirements for synchronization objects: All tables to be synchronized have a primary key or a UNIQUE constraint: Make sure that the table fields are unique. Otherwise, duplicate data may appear in the destination database. The synchronization objects include tables that have neither a primary key nor a UNIQUE constraint: When you configure the instance, select Schema Synchronization for Synchronization Types. On the Configurations for Databases, Tables, and Columns step, set Engine to duplicate for the tables. Otherwise, the instance may fail or data may be lost. Note DTS adds fields to the destination table during schema synchronization. For more information, see Additional column information. If you synchronize data at the table level and need to edit the objects, such as mapping table or column names, you can synchronize a maximum of 1,000 tables in a single task. If you exceed this limit, an error is reported after you submit the task. In this case, split the tables into multiple synchronization tasks or configure a task to synchronize the entire database. Binary logs: Binary logging is enabled for RDS for MySQL instances by default. You must set the binlog_row_image parameter to full. Otherwise, a precheck error is reported, and the data synchronization task cannot start. For more information about how to set instance parameters, see Set instance parameters. Important If the source instance 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 self-managed MySQL database is a primary/primary cluster where the two databases are the primary and secondary of each other, you must enable the log_slave_updates parameter. This ensures that DTS can obtain all binary logs. For more information, see Create a database account for a self-managed MySQL database and configure binary logging. Retain the binary logs of the RDS for MySQL instance for at least 3 days. We recommend a retention period of 7 days. For a self-managed MySQL database, retain the binary logs for at least 7 days. If the retention period is shorter, the DTS task may fail because DTS cannot obtain the binary logs. In extreme cases, data inconsistency or data loss may occur. Issues that are caused by a binary log retention period shorter than the required period are not covered by the DTS Service-Level Agreement (SLA). Note For more information about how to set the Retention Period of binary logs for an RDS for MySQL instance, see Automatically delete local logs. During initial schema synchronization and initial full data synchronization, do not perform Data Definition Language (DDL) operations that change the schema of databases or tables. Otherwise, the data synchronization task fails. Note During initial full data synchronization, DTS queries the source database. This creates metadata locks, which may block DDL operations on the source database. During the runtime of the synchronization instance, data changes from operations that are not recorded in binary logs, such as data restored from a physical backup or data generated by cascade operations, are not synchronized to the destination database. Note If this occurs, you can remove the database or table that contains the data from the synchronization objects and then add it back. This is allowed if your business permits. For more information, see Modify synchronization objects. If the source database is a MySQL database of version 8.0.23 or later and the data to be synchronized contains invisible columns, data loss may occur because the data in these columns cannot be obtained. Note You can run the `ALTER TABLE <table_name> ALTER COLUMN <column_name> SET VISIBLE;` command to make the invisible columns visible. For more information, see Invisible Columns.
Other limits	Currently, you can only synchronize data to tables that use the Unique or Duplicate engine in the SelectDB instance. The destination table uses the Unique engine If the destination table uses the Unique engine, make sure that all unique keys in the destination table also exist in the source table and are included in the synchronization objects. Otherwise, data inconsistency may occur. The destination table uses the Duplicate engine If the destination table uses the Duplicate engine, duplicate data may appear in the destination database in the following cases. You can remove duplicates based on the additional columns (_is_deleted, _version, and _record_id): The synchronization instance has been retried. The synchronization instance has been restarted. After the synchronization instance starts, two or more DML operations are performed on the same data record. Note When the destination table uses the Duplicate engine, DTS converts UPDATE or DELETE statements into INSERT statements. Synchronization of INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, and FK is not supported. When you configure parameters in the Selected Objects box, you can only set the `bucket_count` parameter. Note The value of `bucket_count` must be a positive integer. The default value is auto. During data synchronization, do not create a new cluster in the destination SelectDB instance. Otherwise, the task fails. You can try to restart the synchronization instance to resume the failed task. SelectDB instances only support database and table names that start with a letter. If the name of a database or table to be synchronized does not start with a letter, you must use the mapping feature to change the name. If the name of a synchronization object (database, table, or column) contains Chinese characters, you must use the mapping feature to change the name, for example, to an English name. Otherwise, the task may fail. DDL operations that modify multiple columns at once and consecutive DDL operations on the same table are not supported. During data synchronization, do not add backend (BE) nodes to the SelectDB database. Otherwise, the task fails. You can try to restart the synchronization instance to resume the failed task. In a multi-table merge scenario, where data from multiple source tables is synchronized to a single destination table, make sure that the source tables have the same schema. Otherwise, data inconsistency or task failure may occur. In MySQL, M in `VARCHAR(M)` indicates the character length. In SelectDB, N in `VARCHAR(N)` indicates the byte length. If you do not use the schema synchronization feature provided by DTS, we recommend that you set the length of the VARCHAR field in SelectDB to four times the length of the VARCHAR field in MySQL. When you perform online DDL operations on the source database using DMS or the gh-ost tool, DTS only synchronizes the original DDL to the destination. In this scenario, DTS does not need to synchronize a large amount of temporary table data, but this may cause locked tables in the destination. Note Online DDL changes made on the source using tools such as pt-online-schema-change are not supported. If such changes occur 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. We also recommend that you perform data synchronization during off-peak hours. Otherwise, initial full data synchronization consumes read and write resources of the source and destination databases, which may increase the database load. Initial full data synchronization concurrently executes INSERT operations. This causes fragmentation in the tables of the destination database. Therefore, after the initial full synchronization is complete, the tablespace of the destination instance is larger than that of the source instance. During data synchronization, do not use tools such as pt-online-schema-change to perform online DDL changes on the synchronization objects of the source database. Otherwise, the task fails. During DTS synchronization, if data other than from DTS is written to the destination database, data inconsistency may occur between the source and destination databases. If the always-encrypted (EncDB) feature is enabled for the RDS for MySQL instance, full data synchronization is not supported. Note For RDS for MySQL instances with Transparent Data Encryption (TDE) enabled, schema synchronization, full data synchronization, and incremental data synchronization are supported. During incremental synchronization, DTS uses a batch synchronization policy to reduce the load on the destination. By default, DTS writes data to a single synchronization object at most once every 5 seconds. Therefore, a normal synchronization latency of less than 10 seconds may occur. To reduce this normal latency, modify the `selectdb.reservoir.timeout.milliseconds` parameter in the console to adjust the batching time. The allowed range is [1000, 10000] milliseconds. Note When you adjust the batching time, a shorter time increases the write frequency of DTS. This may increase the load and write response time (RT) of the destination, which in turn increases the DTS synchronization latency. Adjust the time based on the load of the destination. 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 or adjusting its parameters may be performed. Note When parameters are adjusted, only the parameters of the DTS instance are modified. The parameters in the database are not modified. The parameters that may be modified include but are not limited to those described in Modify instance parameters.
Special cases	When the source database is a self-managed MySQL database: If a primary/secondary switchover occurs on the source database during synchronization, the sync task fails. The latency of DTS is calculated by comparing the timestamp of the last synchronized data record in the destination database with the current timestamp. If no DML operations are performed on the source database for a long time, the displayed latency may be inaccurate. If the displayed latency is too high, you can perform a DML operation on the source database to update the latency information. Note If you choose to synchronize the entire database, you can also create a heartbeat table. The heartbeat table is updated or written to every second. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command on the source database to advance the binary log offset. If the source database is an Amazon Aurora MySQL instance or another cluster-mode MySQL instance, make sure that the domain name or IP address configured for the task and its resolved result always point to the read/write (RW) node address. Otherwise, the sync task may not run as expected. When the source database is an RDS for MySQL instance: RDS for MySQL instances that do not record transaction logs, such as read-only instances of RDS for MySQL 5.6, are not supported as a source. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command on the source database to advance the binary log offset.

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.

Supported SQL for incremental synchronization

Operation type	SQL statement
DML	INSERT, UPDATE, DELETE
DDL	ADD COLUMN MODIFY COLUMN CHANGE COLUMN DROP COLUMN, DROP TABLE TRUNCATE TABLE RENAME TABLE Important The RENAME TABLE operation may cause data inconsistency between the source and destination databases. For example, if you select a table as the object to be synchronized and rename the table during data synchronization, the data of this table is not synchronized to the destination database. To prevent this situation, select the database to which this table belongs as the object to be synchronized 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 added to the objects to be synchronized.

Permissions required for the database accounts

Database	Required permissions	Method to create and grant permissions
Source RDS for MySQL	Read and write permissions on the synchronization objects	Create an account and Modify account permissions
Destination SelectDB	Cluster access permissions (Usage_priv) and database read and write permissions (Select_priv, Load_priv, Alter_priv, Create_priv, and Drop_priv)	Permission Management and Basic Permission Management

Note

If the source database account was not created and granted permissions in the RDS for MySQL console, you must ensure 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 task 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	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 MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source RDS for 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 for MySQL instance.
	Database Account	Enter the database account of the source RDS for MySQL instance. For information about the required permissions, see Permissions required for the 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	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 SelectDB.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination SelectDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	This example shows synchronization within the same Alibaba Cloud account. Select No.
	Instance ID	Select the ID of the destination SelectDB instance.
	Database Account	Enter the database account of the destination SelectDB instance. For information about the required permissions, see Permissions required for the 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 DTS server IP addresses to a whitelist.
- 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.

Configuration	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. Important When data is synchronized from MySQL to SelectDB, data types are converted. If you do not select Schema Synchronization, you must create tables that use the Unique or Duplicate model with the corresponding schemas in the destination SelectDB instance beforehand. For more information, see Data type mapping, Additional column information, and Data model.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks whether a table with the same name exists in the destination database. If a table with the same name does not exist, the check passes. If a table with the same name exists, the precheck fails and the data synchronization task does not start. Note If you cannot delete or rename the table with the same name in the destination database, you can map it to a different table name in the destination database. For more information, see Map schemas, tables, and columns. 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, which can pose risks to your business. For example: If the table schemas are the same and a record in the destination database has the same primary key or unique key value as a record in the source database, the record from the source database overwrites the record in the destination database. If the table schemas are different, the data may fail to be initialized, only some columns of data can be synchronized, or the synchronization may fail. 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 objects to synchronize 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 about how to set the name, see Map schemas, tables, and columns. If you select Schema Synchronization for Synchronization Types, select objects at the table level, 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, set the Value as needed, and then click OK. Note To select the SQL operations for incremental synchronization at the database or table level, right-click the synchronization object in the Selected Objects box and select the desired SQL operations in the dialog box that appears. To set a WHERE clause to filter data, right-click the table that you want to synchronize in the Selected Objects box and set the filter condition in the dialog box that appears. For more information about how to set the filter condition, see Set filter conditions. If you use the object name mapping feature, other objects that depend on the mapped object may fail to synchronize.

Click Next: Advanced Settings to configure advanced settings.

Configuration	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 instances, purchase a dedicated cluster. For more information, see What is a DTS dedicated 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 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 Synchronization	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 You can configure this parameter 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.
Environment Tag	You can select an environment tag to identify the instance if needed. This example does not require a selection.
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.
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 instance. 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.

Optional: After you complete the preceding configurations, click Next: Configure Database and Table Fields to set the Primary Key Column, Distribution Key, and Engine for the tables that you want to synchronize in the destination.
Note
- This step is available only if you select Schema Synchronization for Synchronization Types when you configure the task objects. You can set Definition Status to All and then modify the settings.
- For the Primary Key Column, you can select multiple columns to form a composite primary key. You must select one or more columns from the Primary Key Column as the Distribution Key.
- For tables that have neither a primary key nor a unique key, you must set Engine to duplicate. Otherwise, the instance may fail or data loss may occur.

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 the 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 task. 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.

Data type mapping

Category	MySQL data type	SelectDB data 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, data of the CHAR and VARCHAR(n) types is converted to VARCHAR(4*n) after being synchronized to SelectDB. If the data length is not specified, the default value of SelectDB, VARCHAR(65533), is used. If the data length exceeds 65533, the data is converted to STRING after being synchronized to SelectDB.
	BINARY VARBINARY	STRING
	TINYTEXT TEXT MEDIUMTEXT LONGTEXT	STRING
	TINYBLOB BLOB MEDIUMBOLB LONGBLOB	STRING
	ENUM	STRING
	SET	STRING
	JSON	STRING

Additional column information

Note

The following table describes the additional columns that DTS automatically adds or that you must manually add to the destination table that uses the Duplicate model.

Name	Data type	Default value	Description
_is_deleted	Int	0	Indicates whether the data is deleted. Insert: The value is 0. Update: The value is 0. Delete: The value is 1.
_version	Bigint	0	For full data synchronization, the value is 0. For incremental data synchronization, the value is the corresponding timestamp (in seconds) in the binary log of the source database.
_record_id	Bigint	0	For full data synchronization, the value is 0. For incremental data synchronization, the value is the record ID of the incremental log. This ID is the unique identifier of the log. Note The ID value is unique and increments.

Prerequisites

Precautions

The destination table uses the Unique engine

The destination table uses the Duplicate engine