Synchronize data from PolarDB for MySQL to SelectDB - Data Transmission Service

SelectDB supports 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) helps you synchronize data from a PolarDB for MySQL cluster to a SelectDB instance to meet your big data analytics needs.

Prerequisites

A destination SelectDB instance is created. The available storage space of the instance is larger than the storage space used by the source PolarDB for MySQL cluster. For more information, see Create an instance.

Precautions

Type	Description
Limits on the source database	Requirements for synchronization objects: The tables to be synchronized must have PRIMARY KEY or UNIQUE constraints: Make sure that all fields must be unique. Otherwise, the destination database may contain duplicate data. The objects to be synchronized must contain tables that do not have PRIMARY KEY or UNIQUE constraints: We recommend that you select Schema Synchronization for the Synchronization Types parameter and duplicate for the Engine parameter in the Configurations for Databases, Tables, and Columns step when you configure an instance. Note During schema synchronization, DTS adds columns to the destination table. For more information, see Additional column information. If you select tables as the objects to be synchronized and you need to modify the tables, such as renaming tables or columns, in the destination database, you can synchronize up to 1,000 tables in a single data synchronization task. If you run a task to synchronize more than 1,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to synchronize the tables or configure a task to synchronize the entire database. The following requirements for binary logs must be met: 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 a DTS task cannot 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 retained at least three days. We recommend that you set the retention period of the binary logs to seven days. 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 in the Service Level Agreement (SLA) of DTS may not be guaranteed. Note For more information about how to set the Retention Period of the binary logs for an PolarDB for MySQL cluster, see the Modify the retention period section in the topic "Enable binary logging". 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.
Other limits	DTS does not synchronize the read-only nodes of the source PolarDB for MySQL cluster. DTS does not synchronize Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster. Synchronization of INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, and FK is not supported. Primary/secondary switchover of the database instance is not supported during the full synchronization phase of DTS. If a switchover occurs, reconfigure the synchronization task promptly. You can specify only the `bucket_count` parameter in the Selected Objects section. Note The `bucket_count` parameter value must be a positive integer. Default value: auto. You can synchronize data to only tables that use the Unique Key model or Duplicate key model in ApsaraDB for SelectDB instances. Unique key model If you synchronize data to the destination table that uses the Unique key model, make sure all unique keys of the destination table already are contained in the source table and the object to be synchronized. Otherwise, this may cause data inconsistency. Duplicate key model If you synchronize data to the destination table that uses the Duplicate key model, and if one of the following operations occurs, duplicate data may exist in the destination database. In this case, you can deduplicate the data based on additional columns such as _is_deleted, _version, and _record_id. Retry operation occurred in a data synchronization instance. Retry operation was executed in a data synchronization instance. Two or more DML operations were executed for the same row of data after a data synchronization instance started. Note When the destination table is the table that uses the Duplicate key model, DTS converts UPDATE or DELETE statements to INSERT statements. During data synchronization, do not create clusters in the destination ApsaraDB for SelectDB instance. Otherwise, the data synchronization task fails. You can restart the data synchronization instance to resume the data synchronization task that failed. ApsaraDB for SelectDB instances support only database names and table names that start with letters. If the name of a database or table that you want to synchronize does not start with a letter, you must use the object name mapping feature to rename the database or table. If the name of the object that you want to synchronize, such as a database, a table, or a column, contains Chinese characters, you must use the object name mapping feature to rename the object. For example, you can change the name from Chinese to English. Otherwise, the task may fail. You cannot modify DDL operations on multiple columns at a time or modify DDL operations on a table consecutively. During data synchronization, do not add backend nodes to the ApsaraDB for SelectDB database. Otherwise, the task fails. You can restart the data synchronization instance to resume the data synchronization task that failed. In a multi-table merge scenario where data from multiple source tables is synchronized to the same destination table, ensure that the source tables have the same schema. Otherwise, data inconsistency or task failure may occur. In a PolarDB for MySQL cluster, 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 VARCHAR fields in SelectDB to four times the length of the corresponding VARCHAR fields in the PolarDB for MySQL cluster. When you use DMS or gh-ost to perform online DDL changes on the source, DTS synchronizes only 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 at the destination. Note Synchronization of online DDL changes made using tools such as pt-online-schema-change on the source is not supported. If such changes exist on the source, data loss may occur at the destination, or the synchronization instance may fail. Before you synchronize data, evaluate the performance of the source and destination databases. We recommend that you synchronize 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 database loads. Initial full data synchronization runs concurrent INSERT operations, which causes table fragmentation in the destination database. Therefore, after the initial full data 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 operations on the synchronization objects in the source database. Otherwise, the synchronization fails. During data synchronization, if data is written to the destination database from sources other than DTS, data inconsistency may occur between the source and destination databases. DTS executes the CREATE DATABASE IF NOT EXISTS `test` statement in the source database as scheduled to move forward the binary log file position. During incremental synchronization, DTS uses a batch synchronization strategy to reduce the load on the destination. By default, for a single synchronization object, DTS writes data at most once every 5 seconds. Therefore, a normal synchronization latency of less than 10 seconds may occur for DTS synchronization tasks. To reduce this normal latency, modify the `selectdb.reservoir.timeout.milliseconds` parameter for the DTS instance in the console to adjust the batch time. The allowed range is [1000, 10000] milliseconds. Note When you adjust the batch time, a lower value increases the write frequency of DTS. This may increase the load and response time (RT) of the destination, which in turn increases the DTS synchronization latency. Adjust the batch 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.

Billing

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

SQL operations that support 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 database accounts

Database

Required permissions

Creation and authorization methods

Source PolarDB for MySQL

Read and write permissions on the objects to be synchronized

Create and manage a database account and Manage the password of a database account

Target: SelectDB

The USAGE_PRIV permission on the cluster and the SELECT_PRIV, LOAD_PRIV, ALTER_PRIV, CREATE_PRIV, and DROP_PRIV permissions on the database

Cluster Permission Management and Basic Permission Management

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 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 synchronization 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 of the source PolarDB for MySQL cluster. For 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 Secure Sockets Layer (SSL) encryption, 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 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 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 a PolarDB for MySQL cluster to SelectDB, data types are converted. If you do not select Schema Synchronization, you must create Unique or Duplicate model tables with the appropriate schemas in the destination SelectDB instance in advance. For more information, see Data type mappings, Additional column information, and Data models.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks for tables with the same name in the destination database. If no tables with the same name exist, the check passes. If tables with the same name exist, an error is reported during the precheck phase, 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 change the name of the table in the destination database. For more information, see Map database, table, and column names. 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 business risks. For example: If the table schemas are the same and DTS encounters a record in the destination database with the same primary key or unique key value as a record in the source database, DTS does not retain the record in the destination database. The record from the source database overwrites the record in the destination database. If the table schemas are different, data initialization may fail, only some columns may 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 databases or tables as synchronization objects.
Selected Objects	To rename an object that you want to synchronize to the destination instance, right-click the object in the Selected Objects section. For more information, see the "Map the name of a single object" section of the Map object names topic. If you select Schema Synchronization for the Synchronization Types parameter, you can select only tables as the objects to be synchronized and the `bucket_count` parameter must be set. Right-click a table to be migrated in the Selected Objects section, select Yes for the Enable Parameter Settings parameter in the Parameter Settings section that appears, specify Value based on your business requirements, and click OK. Note To select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize. To specify WHERE conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Specify filter conditions. If you use the object name mapping feature to rename an object, other objects that are dependent on the object may fail to be synchronized.

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. In this example, no tag is selected.
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. In the lower part of the page, click Next: Configure Database and Table Fields. In the dialog box that appears, specify Primary Key Column, Distribution Key, and Engine parameters for the tables that you want to synchronize.
Note
- This step is available only if you select Schema Synchronization for the Synchronization Types parameter. You can set Definition Status to All and modify the database and table fields.
- You can select multiple columns for the Primary Key Column parameter from the drop-down list. One or more columns specified for the Primary Key Column parameter can be selected for the Distribution Key parameter.
- If the table does not have primary keys or UNIQUE constraints, select duplicate for the Engine parameter. Otherwise, the data synchronization task 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 mappings

Category	Data type in PolarDB for MySQL	Data type in SelectDB
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 The zerofill attribute 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 the VARCHAR(4*n) type after it is synchronized to SelectDB. If you do not specify the data length, the default value VARCHAR(65533) of SelectDB is used. If the data length exceeds 65533, the data is converted to the STRING type after it is 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 need to manually add to the destination Duplicate model table.

Name	Data Type	Default Value	Description
_is_deleted	Int	0	Indicates whether the data is deleted. Insert: 0. Update: 0. Delete: 1.
_version	Bigint	0	Full synchronization data: 0. Incremental synchronization data: The corresponding UNIX timestamp (in seconds) from the source binary log.
_record_id	Bigint	0	Full synchronization data: 0. Incremental synchronization data: The record ID from the incremental log. This is the unique identifier for the log. Note The ID value is unique and increments.

Prerequisites

Precautions

Unique key model

Duplicate key model