Two-way synchronization between MySQL databases - Data Transmission Service

Data Transmission Service (DTS) supports two-way data synchronization between MySQL databases. This topic describes the procedure for setting up two-way data synchronization between ApsaraDB RDS for MySQL instances as an example.

Prerequisites

Create the source and destination RDS MySQL instances.
We recommend using ApsaraDB RDS for MySQL instances with the same storage capacity for source and destination.

Limitations

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 cascading updates or deletions occur in the source database during synchronization, data inconsistencies may occur.

Type	Description
Source and destination database limitations	If the tables to be synchronized do not have a PRIMARY KEY constraint or a UNIQUE constraint, you must enable the Exactly-Once write feature when configuring the task. Otherwise, duplicate data may appear in the destination database. For more information, see Synchronize tables without a primary key or unique constraint. If you synchronize data at the table level and need to edit the objects, such as mapping table or column names, you can synchronize a maximum of 1,000 tables in a single task. If this limit is exceeded, an error is reported after you submit the task. In this case, split the tables into multiple synchronization tasks or configure a task to synchronize the entire database. Binary logs: Binary logging is enabled for RDS for MySQL instances by default. You must set the `binlog_row_image` parameter to full. Otherwise, an error is reported during the precheck and the data synchronization task cannot start. For more information about how to set instance parameters, see Set instance parameters. Important If the source instance is a self-managed MySQL database, you must enable binary logging and set `binlog_format` to `row` and `binlog_row_image` to `full`. If the source self-managed MySQL database is a primary/primary cluster where the two databases are the primary and secondary of each other, you must enable the `log_slave_updates` parameter. This ensures that DTS can obtain all binary logs. For more information, see Create a database account for a self-managed MySQL database and configure binary logging. Retain binary logs for at least 3 days (7 days recommended) on ApsaraDB RDS for MySQL instances and at least 7 days on self-managed MySQL databases. Otherwise, the DTS task may fail because DTS cannot obtain the binary logs. In extreme cases, data inconsistency or data loss may occur. Issues that are caused by a binary log retention period shorter than the required period are not covered by the DTS Service-Level Agreement (SLA). Note For more information about how to set the Retention Period of binary logs for an ApsaraDB RDS for MySQL instance, see Automatically delete local logs. During schema synchronization and 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 the full synchronization phase, DTS queries the source database, which acquires metadata locks. This may block DDL operations on the source database. During synchronization, DTS does not synchronize data changes that are not recorded in binary logs (such as data restored from a physical backup or data generated by cascade operations). 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. Ensure that both source and destination database tables designated for synchronization have no invisible columns. Otherwise, the DTS instance may fail or data loss may occur. Note Run `ALTER TABLE <table_name> ALTER COLUMN <column_name> SET VISIBLE;` to make the invisible columns visible. For more information, see Invisible Columns.
Other limitations	We recommend that you keep the versions of the source and destination databases the same, or synchronize data from an earlier version to a later version to ensure compatibility. If you synchronize data from a later version to an earlier version, database compatibility issues may occur. If you perform online DDL operations that use temporary tables on the source database, such as merging multiple tables, data loss may occur in the destination database or the synchronization task may fail. DTS does not support synchronizing parsers defined by using comment syntax. If a primary key or unique key conflict occurs while the synchronization task is running: 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 instance fails. Proceed with caution. If you do not use the schema synchronization feature provided by DTS, you must ensure column compatibility. Otherwise, the task may fail or data loss may occur. For example, if a source table column is of the `text` type and the destination column is of the `varchar(255)` type, data truncation may occur if the source table contains large objects. When a two-way synchronization task is running, DTS creates a database named dts in the destination databases of the forward and reverse tasks to prevent data loop synchronization. During the task runtime, do not modify this database and make sure that the database account used by the task has read and write permissions on this database. Note The dts database may affect the normal primary/secondary synchronization of MySQL. We recommend that you check whether the dts database and its related tables exist in the secondary database. If not, you must manually create them in the secondary database based on the schema of the dts database on the primary database. If the data to be synchronized contains content that requires 4-byte storage, such as uncommon characters or emojis, the destination database and tables must use the utf8mb4 charset. Note If you use DTS for schema synchronization, you must set the instance-level parameter `character_set_server` to utf8mb4 in the destination database. 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 consumes read and write resources on the source and destination databases, which may increase the database load. Initial full data synchronization runs concurrent INSERT operations, which causes fragmentation in the destination tables. As a result, the tablespace of the destination instance is larger than that of the source instance after initial full data synchronization is complete. If you synchronize one or more tables instead of an entire database, 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. You can use Data Management (DMS) to perform online DDL operations. For more information, see Change schemas without locking tables. By default, DTS removes foreign key constraints when it synchronizes data to a destination database. Therefore, cascade operations, such as deletes, in the source database are not synchronized to the destination database. A two-way synchronization instance includes a forward task and a reverse task. When you configure or reset a two-way synchronization instance, if the destination object of one task is the source object of the other task: Only one task can synchronize full and incremental data. The other task can only synchronize incremental data. The source data of the current task can only be synchronized to the destination of the current task. The synchronized data will not be used as the source data for the other task. If a DDL statement fails to be written to the destination database, the DTS task continues to run. You need to check the task logs for the failed DDL statement. For more information about how to view task logs, see View task logs. If source table columns that differ only in case are synchronized to the same destination MySQL table, the synchronization result may not be as expected. This is because MySQL column names are case-insensitive. After the data synchronization is complete (the Status of the instance is Completed), we recommend that you run `analyze table <table_name>` to confirm that all data is written to the destination table. For example, if an HA switchover is triggered in the destination MySQL database, data might be written only to memory, which can result in data loss. If the RDS MySQL instance has the always-confidential database (EncDB) feature enabled, full data synchronization is not supported. Note For RDS for MySQL instances with Transparent Data Encryption (TDE) enabled, schema synchronization, full data synchronization, and incremental data synchronization are supported. To synchronize accounts from the source database, you must also meet the prerequisites and understand the related considerations. For more information, see Migrate database accounts. If the task fails, DTS technical support will attempt to recover it within 8 hours. During the recovery process, operations such as restarting the task or adjusting its parameters may be performed. Note When parameters are adjusted, only DTS task parameters are modified. Database parameters remain unchanged.The parameters that may be modified include but are not limited to those described in Modify the parameters of a DTS instance.
Special cases	When the source database is a self-managed MySQL database: If a primary/secondary switchover occurs on the source database during synchronization, the sync task fails. The latency of DTS is calculated by comparing the timestamp of the last synchronized data record in the destination database with the current timestamp. If no DML operations are performed on the source database for a long time, the displayed latency may be inaccurate. If the displayed latency is too high, you can perform a DML operation on the source database to update the latency information. Note If you choose to synchronize the entire database, you can also create a heartbeat table. The heartbeat table is updated or written to every second. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command on the source database to advance the binary log offset. If a source self-managed MySQL database is used for both a bidirectional sync task and other tasks, you must add the `log_slave_updates=1` line to the `/etc/my.cnf` file and then restart the database. Otherwise, data changes from the reverse task are not recorded in the binlog of the source self-managed MySQL database, which causes data inconsistency in other tasks. If the source database is an Amazon Aurora MySQL instance or another cluster-mode MySQL instance, make sure that the domain name or IP address configured for the task and its resolved result always point to the read/write (RW) node address. Otherwise, the sync task may not run as expected. When the source database is an RDS for MySQL instance: RDS for MySQL instances that do not record transaction logs, such as read-only instances of RDS for MySQL 5.6, are not supported as a source. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command on the source database to advance the binary log offset.

Billing

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

Conflict detection

To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated in only one of the database instances in the two-way synchronization. If data records are updated on both database instances, DTS the system will apply the conflict resolution policy configured in the task.

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

Uniqueness conflicts caused by INSERT operations
In two-way synchronization, if records with the same primary key are inserted into both database instances simultaneously (or in close succession), a uniqueness constraint conflict will be triggered. When the INSERT statement is synchronized to the peer instance, it will fail because a record with the same primary key value already exists.
Mismatched records in 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 record to be updated by an UPDATE operation causes a primary key or unique key conflict.
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

Due to time differences and latency, DTS cannot guarantee 100% conflict prevention. To ensure consistency, update records with the same primary or unique key on only one database instance at a time.
DTS provides various conflict resolution strategies for the aforementioned data conflicts, which you can select while configuring two-way data synchronization.

Supported SQL

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 task from the source database to the destination database. DDL operations are not supported in the reverse task from the destination database to the source database and are automatically filtered out.

Database account permissions

Database	Required permissions	Creation and authorization
Source ApsaraDB RDS for MySQL	Read and write permissions on the source and destination databases. The privileged account of an ApsaraDB RDS for MySQL instance meets the requirements.	Create an account and Reset the permissions of a privileged account.
Destination ApsaraDB RDS for MySQL

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.

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.

Category	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 Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source RDS MySQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	RDS Instance ID	Select the ID of the source RDS MySQL instance. Note The source and destination RDS MySQL instances can be the same or different. You can use DTS to migrate data between two RDS MySQL instances or within the same RDS MySQL instance.
	Database Account	Enter the privileged account of the source RDS MySQL instance.
	Database Password	The password that is used to access the database.
	Encryption	Specifies whether to encrypt the connection to the database. You can select Non-encrypted or SSL-encrypted based on your business requirements. If you want to set this parameter to SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the DTS task. For more information, see Use a cloud certificate to enable SSL encryption.
Destination Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination RDS MySQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	RDS Instance ID	Select the ID of the destination RDS MySQL instance.
	Database Account	Enter the privileged account of the destination RDS MySQL instance.
	Database Password	The password that is used to access the database.
	Encryption	Specifies whether to encrypt the connection to the database. You can select Non-encrypted or SSL-encrypted based on your business requirements. If you want to set this parameter to SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the DTS task. For more information, see Use a cloud certificate to enable SSL encryption.

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.

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	Select a method to synchronize triggers as needed. If the objects that you are synchronizing do not involve triggers, you do not need to configure this parameter. For more information, see Configure how to synchronize or migrate triggers. Note This parameter is available only when Schema Synchronization is selected for Synchronization Types.
Enable Migration Assessment	Specify whether to assess if the schemas of the source and destination databases, such as index length, stored procedures, and dependent tables, meet the requirements. Note This parameter is available only when Schema Synchronization is selected for Synchronization Types. If you select Yes, the precheck time may increase. You can view the Assessment Result during the precheck phase. The assessment result does not affect the precheck result.
Synchronization Topology	Select Two-way Synchronization.
Enable Exactly-Once Write	If the objects to be synchronized include tables that do not have primary keys or UNIQUE constraints, select Yes. For more information, see Synchronize tables without primary keys or UNIQUE constraints.
Exclude DDL Operations	Yes: Exclude DDL operations from synchronization. No: Synchronize DDL operations. Important To ensure the stability of the two-way synchronization link, you can only choose whether to synchronize DDL operations for the forward task from the source database to the destination database. The reverse task from the destination database to the source database automatically filters out DDL operations.
Global Conflict Resolution Policy	If you encounter the previously described conflict types, select a conflict resolution policy as needed. TaskFailed If a conflict occurs during data synchronization, the data synchronization instance 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 instance 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 synchronization task is paused or restarted and experiences latency, the conflict resolution policies will not take effect during the latency period. By default, data on the destination instance will be overwritten.
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 instance fails. Proceed with caution.
Whether to migrate Event	Specify whether to synchronize events in the source database. You can configure this parameter based on your business requirements. If you select Yes, you must meet relevant requirements and complete subsequent operations. For more information, see Synchronize or migrate events.
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 SQL Operations: Right-click objects in Selected Objects to select SQL operations. Data Filters: Right-click tables in Selected Objects to set WHERE conditions. For more information about how to set this parameter, see Specify filter conditions. If you use the object name mapping feature, other objects that depend on the mapped object may fail to synchronize. To configure an Independent Conflict Resolution Policy for incremental synchronization, see Set an independent conflict resolution policy.

Click Next: Advanced Settings to configure advanced settings.

Parameter	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.
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	Select whether to synchronize the accounts from the source database. If you select Yes, you must also select the accounts to be synchronized and confirm their permissions. For more information about topics such as 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 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. 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	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	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.

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

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 an 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 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 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.

Configure the reverse synchronization task.
1. Wait for the initial synchronization of the forward synchronization task to be complete. The Status of the task changes to Running.
2. Find the reverse synchronization task and click Configure Task.
3. Configure the reverse synchronization task by following Step 3 to Step 6.
  Important
  For reverse synchronization, swap the source and destination instances used in the forward task. Ensure all details, such as the database name, account, and password, are verified and correct.
  The Instance Region of the source and destination databases for the reverse synchronization task cannot be modified. Fewer parameters are required than for the forward synchronization task. Please refer to the console for the actual configuration options.
  The Processing Mode of Conflicting Tables configuration for the reverse synchronization task does not check the tables that were synchronized to the destination instance by the forward synchronization task.
  The reverse synchronization task does not support the synchronization of objects in the Selected Objects list of the forward synchronization task.
  We recommend that you do not use the mapping feature when you configure the reverse task. Otherwise, data inconsistency may occur.
4. When the Success Rate is 100%, click Back.
After the reverse synchronization task is configured, wait for the Status of both synchronization tasks to become Running. The configuration of two-way data synchronization is complete.