Two-way data synchronization between MySQL databases - Data Transmission Service

Data Transmission Service (DTS) supports two-way data synchronization between MySQL databases. This topic describes how to configure two-way data synchronization between ApsaraDB RDS for MySQL instances by using DTS.

Prerequisites

The source and destination ApsaraDB RDS for MySQL instances are created. For more information, see Create an ApsaraDB RDS for MySQL instance.
The source and destination ApsaraDB RDS for MySQL instances have the same storage capacity.

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
Limits on the source and destination databases	If the tables to be synchronized do not have primary keys or UNIQUE constraints, you must enable the Exactly-Once write feature when you configure the data synchronization task. Otherwise, the destination database may contain duplicate data records. For more information, see Synchronize tables without primary keys or UNIQUE constraints. 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 split the tables and configure multiple tasks to synchronize the tables in batches or configure a task to synchronize the entire database. The following requirements for binary logs must be met: 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. 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 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. During schema synchronization and full data synchronization, do not execute DDL statements to change the schemas of databases or tables. Otherwise, the data synchronization task fails.
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 source and destination databases to prevent circular synchronization. When the task is running, do not modify the dts databases. The dts database may affect the synchronization between the primary and secondary MySQL databases. We recommend that you check whether the dts database and related tables exist in the secondary database. If the dts database does not exist in the secondary database, you must manually create a dts database in the secondary database based on the schema of the dts database in the primary 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 load 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. 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. 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 task logs, see View task logs. Column names in MySQL databases are not case-sensitive. Therefore, if multiple columns in the source database have names that differ only in capitalization, data in the columns are written to the same column in the destination MySQL database during the synchronization. This can cause unexpected synchronization results. After data synchronization is complete, we recommend that you run the `analyze table <Table name>` command to check whether data is written to the destination table. For example, if a high-availability (HA) switchover is triggered in the source MySQL database, data may be written only to the memory. As a result, data loss occurs. 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 items: 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 self-managed MySQL database is the source database of a two-way data synchronization task and other DTS tasks, you must add the `log_slave_updates=1` code to the `my.cnf` file in the /etc directory of the source database. Then, restart the source database. Otherwise, the data changes in the reverse synchronization task are not recorded in the binary logs of the source self-managed MySQL database. In this case, data may be inconsistent between the source and destination databases of other DTS tasks. If the source database is an Apsara RDS for MySQL instance, take note of the following item: 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
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.

SQL operations that can be synchronized

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 migrated and rename the table during data migration, the data of this table is not migrated 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 type	Required permissions	References
Source ApsaraDB RDS for MySQL instance	Permissions of a privileged account	Create accounts and databases and Reset the permissions of a privileged account
Destination ApsaraDB RDS for MySQL instance	Permissions of a privileged account

Procedure

Go to the Data Synchronization Tasks page.
1. Log on to the Data Management (DMS) console.
2. In the top navigation bar, click DTS.
3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
Note
- 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.
- You can also go to the Data Synchronization Tasks page of the new DTS console.
On the right side of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Note
If you use the new DTS console, you must select the region in which the data synchronization instance resides in the top navigation bar.
Click Create Task to go to the task configuration page.
Optional. In the upper-right corner of the page, click New Configuration Page.
Note
- Skip this step if the Back to Previous Version button is displayed in the upper-right corner of the page.
- Specific parameters in the new and previous versions of the configuration page may be different. We recommend that you use the new version of the configuration page.

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 on the page. Otherwise, the task may fail or data inconsistency may occur.

Section	Parameter	Description
N/A	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 a DMS database instance.	In this example, no database instance is selected. Configure the following database information.
	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 in which the source ApsaraDB RDS for MySQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database instance of the current Alibaba Cloud account is used. Select No.
	RDS Instance ID	The ID of the source ApsaraDB RDS for MySQL instance. Note The source and destination ApsaraDB RDS for MySQL instances can be the same or different. You can use DTS to synchronize data between two ApsaraDB RDS for MySQL instances or within an ApsaraDB RDS for MySQL instance.
	Database Account	The privileged database account of the source ApsaraDB RDS for MySQL instance.
	Database Password	The password that is used to access the database instance.
	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 Configure the SSL encryption feature.
Destination Database	Select a DMS database instance.	In this example, no database instance is selected. Configure the following database information.
	Database Type	The type of the destination database. Select MySQL.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination ApsaraDB RDS for MySQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database instance of the current Alibaba Cloud account is used. Select No.
	RDS Instance ID	The ID of the destination ApsaraDB RDS for MySQL instance.
	Database Account	The privileged database account of the destination ApsaraDB RDS for MySQL instance.
	Database Password	The password that is used to access the database instance.
	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 Configure the SSL encryption feature.

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.

On the Select Objects page, configure the objects that you want to synchronize.

Parameter	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.
Method to Migrate Triggers in Source Database	The method used to synchronize triggers from the source database. You can select a synchronization method based on your business requirements. If no triggers are to be synchronized, you do not need to configure this parameter. For more information, see Synchronize or migrate triggers from the source database. Note This parameter is displayed only if the Synchronization Types parameter is set to Schema Synchronization.
Enable Migration Assessment	Specifies whether to enable synchronization assessment. You can check whether the schemas of the source and destination databases, such as the length of indexes, stored procedures, and dependent tables, meet the requirements. Select Yes or No based on your business requirements. Note This parameter is displayed only if the Schema Synchronization parameter is set to Synchronization Types. If you select Yes, the precheck time may increase. You can view the Assessment Result during the precheck. The assessment results do not affect the precheck results.
Synchronization Topology	The synchronization topology of the data synchronization instance. Select Two-way Synchronization.
Enable Exactly-Once Write	Specifies whether to enable the Exactly-Once write feature. If the objects to be synchronized include tables that do not have primary keys and UNIQUE constraints, select Yes. For more information, see Synchronize tables without primary keys or UNIQUE constraints.
Exclude DDL Operations	Yes: excludes DDL operations. No: synchronizes DDL operations. Important 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 configure this parameter only when you configure a data synchronization task in the forward direction.
Global 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.
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 migration, DTS does not migrate 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.
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 columns, tables, or databases as objects to synchronize. 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.
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. 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. Note To select 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. For more information about the SQL statements that can be synchronized, see the SQL operations that can be synchronized section of this topic. To specify WHERE conditions to filter data, right-click an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Set filter conditions. If you use the object name mapping feature to rename an object, other objects that depend on the object may fail to be synchronized. If you want to configure Independent Conflict Resolution Policy for incremental data synchronization, see Configure an independent conflict resolution policy.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules tasks to shared clusters. You do not need to configure this parameter. If you want to improve the stability of data migration tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
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 this option, 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 this option, the tables in the destination database may be locked.
Whether to Migrate Accounts	Specifies whether to synchronize the account information of the source database. You can configure this parameter based on your business requirements. If you select Yes, you must select the accounts to be synchronized and check their permissions. For more information such as the authorization methods, see Migrate database accounts.
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 Migration	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 This parameter is displayed 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	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, no environment 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. Yes: does not write SQL operations on heartbeat tables. In this case, 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 task. 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 contacts. For more information, see Configure monitoring and alerting when you create a DTS task.

Click Next Step: Verification Configurations to configure data verification.
For more information about how to use the data verification feature, see Configure data verification.

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.
Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.

Purchase a data synchronization instance.

On the buy page, configure the Billing Method and Instance Class parameters for the data synchronization instance. 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.
	Synchronization Topology	This value is set to Two-way Synchronization.
	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 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.

Configure a data synchronization task in the reverse direction.
1. Wait until the initial synchronization is complete and the Status of the data synchronization task in the forward direction changes to Running.
2. Find the data synchronization task in the reverse direction and click Configure Task.
3. Repeat Step 5 to Step 8 to configure the data synchronization task in the reverse direction.
  Important
  When you configure the reverse synchronization task, make sure that you select the correct source and destination instances. The source instance in the reverse direction is the destination instance in the forward direction. The destination instance in the reverse direction is the source instance in the forward direction. Make sure that the parameter settings, such as the database name, account, and password, are consistent.
  When you configure the source and destination databases of the reverse synchronization task, the values of the Instance Region parameter cannot be modified. The number of parameters that you need to configure for a reverse synchronization task is less than that for a forward synchronization task. Configure the parameters that are displayed in the console.
  When you configure Processing Mode of Conflicting Tables for a synchronization task in the reverse direction, make sure that the tables that have been synchronized to the destination instance in the forward direction are ignored.
  You cannot select the objects in the Selected Objects list of the data synchronization task in the forward direction for the data synchronization task in the reverse direction.
  We recommend that you do not use the name mapping feature when you configure the reverse synchronization task. Otherwise, data inconsistency may occur.
4. Wait until Success Rate becomes 100%. Then, click Back.
Wait until the Status of the synchronization tasks in both forward and reverse directions changes to Running. This indicates that the configuration of two-way data synchronization is complete.

Configure an independent conflict resolution policy

Note

You can configure an independent conflict resolution policy only for incremental data synchronization.

In the Selected Objects section, right-click a database or table that you want to synchronize.
Enable the independent conflict resolution policy.
- Database-level conflict resolution policy
  1. In the Independent Conflict Resolution Policy section of the dialog box that appears, set Enable Independent Conflict Resolution Policy to Yes.
  2. Select a value for Conflict Resolution Policy.
    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.
    UseMax
    If a conflict occurs during data synchronization, the two records in the conflicting columns are compared and the record that represents a larger value is written to the destination database. If this record does not exist or the field type of this record does not meet the requirements, the effect of UseMax is equivalent to that of Overwrite.
    UseMin
    If a conflict occurs during data synchronization, the two records in the conflicting columns are compared and the record that represents a smaller value is written to the destination database. If this record does not exist or the field type of this record does not meet the requirements, the effect of UseMin is equivalent to that of Ignore.
  3. In the Column Name field, enter the name of a column to which the independent conflict resolution policy applies.
    Note
    If the name of the column is changed by using the object name mapping feature, enter the new column name.
    You can click Add Column to the right of Columns for Conflict Detection to add more columns to which the independent conflict resolution policy applies.
- Table-level conflict resolution policy
  1. In the Columns section of the dialog box that appears, select Independent Conflict Resolution Policy.
  2. In the Columns for Conflict Detection column, select the columns to which the independent conflict resolution policy applies.
    Note
    By default, the columns with primary keys and UNIQUE constraints are selected.
    The global conflict resolution policy does not apply to the columns for which an independent conflict resolution policy is specified.
  3. Select a value for Conflict Resolution Policy.
    Note
    If the values of Columns for Conflict Detection are automatically selected by the system, you cannot select UseMax or UseMin for Conflict Resolution Policy.
    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.
    UseMax
    If a conflict occurs during data synchronization, the two records in the conflicting columns are compared and the record that represents a larger value is written to the destination database. If this record does not exist or the field type of this record does not meet the requirements, the effect of UseMax is equivalent to that of Overwrite.
    UseMin
    If a conflict occurs during data synchronization, the two records in the conflicting columns are compared and the record that represents a smaller value is written to the destination database. If this record does not exist or the field type of this record does not meet the requirements, the effect of UseMin is equivalent to that of Ignore.
Click OK.