Configure two-way data synchronization between an ApsaraDB RDS for MySQL instance and a PolarDB for MySQL cluster

This topic describes how to configure two-way data synchronization between an ApsaraDB RDS for MySQL instance and a PolarDB for MySQL cluster by using Data Transmission Service (DTS).

Supported source databases

You can use DTS to synchronize data between a MySQL database and a PolarDB for MySQL cluster. The source MySQL database can belong to one of the following types. In this topic, an ApsaraDB RDS for MySQL instance is used to describe how to configure a data synchronization task. You can also follow the procedure to configure data synchronization tasks for other types of MySQL databases.

ApsaraDB RDS for MySQL instance
Self-managed database that is hosted on Elastic Compute Service (ECS)
Self-managed database that is connected over Express Connect, VPN Gateway, or Smart Access Gateway
Self-managed database that is connected over Database Gateway
Self-managed database that is connected over Cloud Enterprise Network (CEN)

Prerequisites

An ApsaraDB RDS for MySQL instance is created. For more information, see Create an ApsaraDB RDS for MySQL instance.
A PolarDB for MySQL cluster is created. For more information, see Purchase a pay-as-you-go cluster and Purchase a subscription cluster.
The available storage space of the source ApsaraDB RDS for MySQL instance is the same as that of the destination PolarDB for MySQL cluster.

Usage notes

Note

During schema synchronization, DTS synchronizes foreign keys from the source database to the destination database.
During full data synchronization and incremental data synchronization, DTS temporarily disables the constraint check and cascade operations on foreign keys at the session level. If you perform the cascade update and delete operations on the source database during data synchronization, data inconsistency may occur.

Category	Description

Category	Description
Limits on the source and destination databases	The tables to be synchronized must have PRIMARY KEY or UNIQUE constraints, and all fields must be unique. Otherwise, the destination database may contain duplicate data records. If you select tables as the objects to be synchronized and you need to modify the tables, such as renaming tables or columns, 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: For the MySQL database: By default, the binary logging feature is enabled. The binlog_row_image parameter must be set to full. Otherwise, error messages are returned during precheck and the data synchronization task cannot be started. For more information, see Modify instance parameters. Important If the source database is a self-managed MySQL database, you must enable the binary logging feature and set the binlog_format parameter to row and the binlog_row_image parameter to full. If the source database is a self-managed MySQL database deployed in a dual-primary cluster, you must set the log_slave_updates parameter to ON. This ensures that DTS can obtain all binary logs. For more information, see Create an account for a self-managed MySQL database and configure binary logging. For the PolarDB for MySQL cluster: You must enable the binary logging feature and set the loose_polar_log_bin parameter to on. Otherwise, error messages are returned during precheck and the data synchronization task cannot be started. The binary logs of the source database must be stored for at least seven days. Otherwise, the data synchronization task may fail because DTS cannot obtain the binary logs. 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. For more information about how to manage the binary log files of an ApsaraDB RDS for MySQL instance, see Manage binary log files.
Other limits	To ensure compatibility, the version of the destination database must be the same as or later than that of the source database. If the version of the destination database is earlier than that of the source database, database compatibility issues may occur. When DTS runs a two-way data synchronization task, DTS creates a database named dts in the destination database to prevent circular synchronization. When the task is running, do not modify the dts database. Before you synchronize data, evaluate the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. During initial full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers. During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. After initial full data synchronization is complete, the size of the used tablespace of the destination database is larger than that of the source database. If you select one or more tables instead of an entire database as the objects to be synchronized, do not use tools such as pt-online-schema-change to perform online DDL operations on the tables during data synchronization. Otherwise, the data synchronization task fails. You can use Data Management (DMS) to perform online DDL operations. For more information, see Perform lock-free DDL operations. Data inconsistency between the source and destination databases occurs if data from other sources is written to the destination database during data synchronization. For example, if you use DMS to execute online DDL statements while data from other sources is written to the destination database, data loss may occur in the destination database. By default, DTS disables FOREIGN KEY constraints for the destination database in a data synchronization task. Therefore, specific operations such as the cascade and delete operations of the source database are not synchronized to the destination database. A two-way data synchronization instance contains a forward synchronization task and a reverse synchronization task. If an object is to be synchronized in both the forward and reverse synchronization tasks when you configure or reset the instance, the following rules apply: Only one of the tasks can synchronize both the full data and incremental data of the object. The other task synchronizes only the incremental data of the object. The source data of the current task can be synchronized only to the destination of the task. The synchronized data is not used as the source data of the other task. You cannot enable throttling for full data migration. You cannot convert data from the DATETIME type to the VARCHAR type. If DDL statements fail to be executed in the destination database, the data synchronization task continues to run. You can view the DDL statements that fail to be executed in the task logs. For more information about how to view the task logs, see View task logs. If the source database is an ApsaraDB RDS for MySQL instance for which the EncDB feature is enabled, full data synchronization cannot be performed.
Special cases	If the source database is a self-managed MySQL database, take note of the following limits: If you perform a primary/secondary switchover on the source database when the data synchronization task is running, the task fails. DTS calculates synchronization latency based on the timestamp of the latest synchronized data in the destination database and the current timestamp in the source database. If no DML operation is performed on the source database for a long time, the synchronization latency may be inaccurate. If the latency of the data synchronization task is excessively high, you can perform a DML operation on the source database to update the latency. Note If you select an entire database as the object to be synchronized, you can create a heartbeat table. The heartbeat table is updated or receives data every second. DTS executes the CREATE DATABASE IF NOT EXISTS 'test' statement in the source database as scheduled to move forward the binary log file position. If the source database is an Apsara RDS for MySQL instance, take note of the following limit: An ApsaraDB RDS for MySQL instance that does not record transaction logs, such as a read-only ApsaraDB RDS for MySQL 5.6 instance, cannot be used as the source database.

Billing

Synchronization type	Task configuration fee

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 conflict detection

To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes. If data records are updated on both nodes, DTS responds to conflicts based on the conflict resolution policy that you specify for the data synchronization task.

DTS checks and fixes conflicts to maximize the stability of two-way synchronization instances. DTS can detect the following types of conflicts:

Uniqueness conflicts caused by INSERT operations
INSERT operations that do not comply with the uniqueness constraint cannot be synchronized. For example, if a record with the same primary key value is inserted into the two synchronization nodes at almost the same time, one of the inserted records fails to be synchronized. The synchronization fails because a record with the same primary key value already exists on the other node.
Inconsistent records caused by UPDATE operations
- If the records to be updated do not exist in the destination instance, DTS converts the UPDATE operation into an INSERT operation. However, uniqueness conflicts may occur.
- The primary keys or unique keys of the records to insert may conflict with those of existing records in the destination instance.
Non-existent records to be deleted
The records to be deleted do not exist in the destination instance. In this case, DTS ignores the DELETE operation regardless of the conflict resolution policy that you specify.

Important

During two-way synchronization, the system time of the source and destination instances may be different. Synchronization latency may occur. For these reasons, DTS does not ensure that the conflict detection mechanism can prevent all data conflicts. To perform two-way synchronization, make sure that records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes.
DTS provides conflict resolution policies to prevent conflicts that may occur during data synchronization. You can select a conflict resolution policy when you configure two-way data synchronization.

Supported synchronization topologies

DTS supports two-way data synchronization between an ApsaraDB RDS for MySQL instance and a PolarDB for MySQL cluster.

For more information about the synchronization topologies that are supported by DTS, see Synchronization topologies.

SQL operations that can be synchronized

Operation type	SQL statement

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	ALTER TABLE and ALTER VIEW CREATE FUNCTION, CREATE INDEX, CREATE PROCEDURE, CREATE TABLE, and CREATE VIEW DROP INDEX and DROP 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. TRUNCATE TABLE

Important

DDL operations can be synchronized only in the forward direction from the source database to the destination database. DDL operations cannot be synchronized from the destination database to the source database because DDL operations are ignored in the reverse direction.

Permissions required for database accounts

Database	Required permission	References

Database	Required permission	References
Source ApsaraDB RDS for MySQL instance	Permissions of a privileged account	Create accounts and databases
Destination PolarDB for MySQL cluster	Permissions of a privileged account	Create and manage a database account

Procedure

Note

This procedure is described based on the new version of the DTS console. In the event of discrepancies in operations between the DTS console and the DTS module in the Data Management Service (DMS) console, the DMS console takes precedence.

Go to the Data Synchronization Tasks page of the new DTS console.
Note
You can also log on to the Data Management (DMS) console. In the top navigation bar, move the pointer over DTS and choose DTS (DTS) > Data Synchronization.
In the top navigation bar, select the region in which you want to create the data synchronization task.

Click Create Task. In the Create Task wizard, configure the source and destination databases. The following table describes the parameters.

Warning

After you configure the source and destination databases, we recommend that you read the Limits that are displayed in the upper part of the page. Otherwise, the task may fail or data inconsistency may occur.

Section	Parameter	Description

Section	Parameter	Description
N/A	Task Name	The name of the task. DTS automatically assigns a name to the task. 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 a DMS database instance	The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must configure parameters for the source database.
	Database Type	The type of the source database. Select MySQL.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region where the source ApsaraDB RDS for MySQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether data is synchronized across Alibaba Cloud accounts. In this example, No is selected. Note You cannot configure two-way data synchronization across accounts.
	RDS Instance ID	The ID of the source ApsaraDB RDS for MySQL instance.
	Database Account	The database account of the source ApsaraDB RDS for MySQL instance. For information about the permissions that are required for the account, see Permissions required for database accounts.
	Database Password	The password of the database account.
	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 select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the data synchronization task. For more information, see Configure the SSL encryption feature.
Destination Database	Select a DMS database instance	The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must configure parameters for the source database.
	Database Type	The type of the destination database. Select PolarDB for MySQL.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination PolarDB for MySQL cluster resides.
	PolarDB instance ID	The ID of the destination PolarDB for MySQL cluster.
	Database Account	The database account of the destination PolarDB for MySQL cluster. For information about the permissions that are required for the account, see Permissions required for database accounts.
	Database Password	The password of the database account.

In the lower part of the page, click Test Connectivity and Proceed.
If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you must make sure that the ECS instance can access the database. If the database is deployed on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the whitelist of the database to allow DTS to access the database. For more information, see Add the CIDR blocks of DTS servers.
Warning
If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database or instance, or to the ECS security group rules, security risks may arise. Therefore, before you use DTS to synchronize data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

Configure the objects to be synchronized and advanced settings. The following table describes the parameters.

Basic Settings

Parameter or setting	Description

Parameter or setting	Description
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.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains tables that have the same names as tables in the source database. If the source and destination databases do not contain tables that have identical table names, the precheck is passed. Otherwise, an error is returned during the precheck, and the data synchronization task cannot be started. Note If the source and destination databases contain tables with identical names and the tables in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the tables that are synchronized to the destination database. For more information, see Map object names. Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases. Warning If you select Ignore Errors and Proceed, data inconsistency may occur and your business may be exposed to potential risks. If the source and destination databases have the same schema and a data record in the destination database has the same primary key value or unique key value as a data record in the source database: During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained. During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten. If the source and destination databases have different schemas, data may fail to be initialized. In this case, only some columns are synchronized, or the data synchronization task fails. Proceed with caution.
Synchronization Topology	Select Two-way Synchronization.
Exclude DDL Operations	Yes: does not synchronize DDL operations. No: synchronizes DDL operations. Important Limits on DDL synchronization directions: DDL operations can be synchronized only in the forward direction from the source database to the destination database. DDL operations are ignored in the reverse direction from the destination database to the source database. Therefore, you can set this parameter only when you configure the task in the forward direction.
Conflict Resolution Policy	If you encounter the conflicts described in the Supported conflict detection section of this topic, select a conflict resolution policy based on your business requirements. TaskFailed If a conflict occurs during data synchronization, the data synchronization task reports an error and exits the process. The task enters a failed state, and you must manually resolve the conflict. Ignore If a conflict occurs during data synchronization, the data synchronization task ignores the current statement and continues the process. The conflicting records in the destination database are used. Overwrite If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten. Note If a latency occurs when you pause or restart a data synchronization task, the selected conflict resolution policy does not take effect during the latency. By default, data in the destination database is overwritten by the data that is synchronized during the latency.
Select Objects	Select one or more objects from the Source Objects section and click the icon to move the objects to the Selected Objects section. Note You can select columns, tables, or databases as the objects to be synchronized. If you select tables or columns as the objects to be synchronized, DTS does not synchronize other objects such as views, triggers, and stored procedures to the destination database.
Rename Databases and Tables	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. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see the Map multiple object names at a time section of the Map object names topic.
Filter data	You can specify WHERE conditions to filter data. For more information, see Set filter conditions.
Select the SQL operations to be synchronized	In the Selected Objects section, right-click an object to be synchronized. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information, see SQL operations that can be synchronized.

Advanced Settings

Parameter	Description

Parameter	Description
Monitoring and Alerting	Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts will receive notifications. Valid values: No: does not configure alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see Configure monitoring and alerting when you create a DTS task.
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.
Copy the temporary table of the Online DDL tool that is generated in the source table to the destination database	If you use DMS or the gh-ost tool to perform online DDL operations on the source database, you can specify whether to synchronize the data of temporary tables generated by online DDL operations. Important You cannot use tools such as pt-online-schema-change to perform online DDL operations on the source database. Otherwise, the DTS task fails. Yes: DTS synchronizes the data of temporary tables generated by online DDL operations. Note If online DDL operations generate a large amount of data, the data synchronization task may take an extended period of time to complete. No, Adapt to DMS Online DDL: DTS does not synchronize the data of temporary tables generated by online DDL operations. Only the original DDL operations that are performed by using DMS are synchronized. Note If you select No, Adapt to DMS Online DDL, the tables in the destination database may be locked. No, Adapt to gh-ost: DTS does not synchronize the data of temporary tables generated by online DDL operations. Only the original DDL operations that are performed by using the gh-ost tool are synchronized. You can use the default or custom regular expressions to filter out the shadow tables of the gh-ost tool and tables that are not required. Note If you select No, Adapt to gh-ost, the tables in the destination database may be locked.
Retry Time for Failed Connection	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.

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 task fails to pass the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
- If an alert is generated for an item during the precheck, perform the following operations based on the scenario:
  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.
Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.

On the Purchase Instance page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.

Section	Parameter	Description

Section	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay for your subscription when you create an instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. You are offered lower prices for longer subscription durations. 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 instance, you can release the instance to reduce costs.
	Resource Group Settings	The resource group to which the instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides various synchronization specifications that support different performance. The synchronization speed varies based on the synchronization specifications that you select. You can select a synchronization specification based on your business requirements. For more information, see Specifications of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, set the subscription duration and the number of 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 the Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.
Wait until initial synchronization is complete and the data synchronization task in the forward direction enters the Running state. You can view the status of the data synchronization task on the Data Synchronization page.
Find the data synchronization task in the reverse direction and click Configure Task.
On the Create Task page, configure the source and destination databases, as described in Step 3.
Important
When you configure the data synchronization task in the reverse direction, you must select the correct source and destination instances. The source instance in the reverse direction corresponds to the destination instance in the forward direction. The destination instance in the reverse direction corresponds to the source instance in the forward direction. You must also make sure that the parameter settings, such as the database name, account, and password, are consistent.

Configure the objects to be synchronized and advanced settings. The following table describes the parameters., as described in Step 5. We recommend that you specify the same settings. The following table provides additional description.