Two-way synchronization between MySQL instances - Data Transmission Service

Prerequisites

You have created the source and destination ApsaraDB RDS for MySQL instances. For instructions, see Create an ApsaraDB RDS for MySQL instance.
The source and destination ApsaraDB RDS for MySQL instances should have the same storage capacity.

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 constraint checks and foreign key cascade operations at the session level. Data inconsistency may occur if cascade update or delete operations are performed on the source database while the task is running.

Type	Description
Source and destination database limits	If the tables to synchronize lack primary keys and unique constraints, enable Exactly-Once write in the task configuration. Otherwise, duplicate data may appear in the destination database. For more information, see Synchronize tables without primary keys or unique constraints. If you synchronize at the table level and need to edit mappings (such as column name mapping), each synchronization task supports up to 1,000 tables. If you exceed this limit, the task fails with an error. To fix this, split the tables across multiple tasks or configure a full-database synchronization task. Binary logs: ApsaraDB RDS for MySQL enables binary logging by default. Ensure that the binlog_row_image parameter is set to full. Otherwise, the precheck fails and the synchronization task cannot start. For instructions, see Configure instance parameters. Important If your source instance is a self-managed MySQL database, enable binary logging and set binlog_format to row and binlog_row_image to full. If your self-managed MySQL database is a dual-primary cluster (where both nodes act as primary and secondary), enable the log_slave_updates parameter so DTS can capture all binary log events. For instructions, see Create an account and configure binary logging for a self-managed MySQL database. The local binary logs for an ApsaraDB RDS for MySQL instance must be retained for at least three days (seven days is recommended). For a self-managed MySQL database, retain local binary logs for at least seven days. Otherwise, DTS may fail to retrieve binary logs, causing the task to fail. In extreme cases, this may cause data inconsistency or data loss. Issues caused by binary log retention periods shorter than DTS requires are not covered under the DTS SLA. Note To configure the retention period for local binary logs on an ApsaraDB RDS for MySQL instance, see Automatically delete local logs. Do not run DDL operations that change database or table schemas during schema synchronization or full synchronization. Otherwise, the synchronization task fails. Note During full synchronization, DTS queries the source database. This creates metadata locks that may block DDL operations on the source database. Data generated by changes that do not write to binary logs—such as data restored from physical backups or created by cascade operations—is not synchronized to the destination database. Note If this occurs, remove the affected database or table from the synchronization objects. Then add it back. You can do this only if your business allows it. For more information, see Modify synchronization objects. Ensure neither the source nor destination database contains invisible hidden columns in the columns to synchronize. Otherwise, the DTS task may fail or data may be lost. Note Run the `ALTER TABLE <table_name> ALTER COLUMN <column_name> SET VISIBLE;` command to make the hidden column visible. For more information, see Invisible Columns.
Other limits	Use the same version for the source and destination databases, or upgrade from a lower version to a higher version to ensure compatibility. Synchronizing from a higher version to a lower version may cause compatibility issues. If your source database uses online DDL operations in temporary table mode—including but not limited to multi-table merge scenarios—or adds function-based indexes to unique key columns, data loss or task failure may occur in the destination database. Resolvers defined by comment syntax do not support synchronous use. If a primary key or unique key conflict occurs while the task is running: If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database: During full data synchronization, DTS retains the destination record and skips the source record. During incremental synchronization, DTS overwrites the destination record with the source record. If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution. If you do not use DTS to synchronize table schemas, ensure field compatibility yourself. Otherwise, the task may fail or data may be lost. For example, if the source table uses the `text` data type and the destination column uses `varchar(255)`, large fields in the source table may be truncated. While a two-way synchronization task runs, DTS creates a database named dts in the destination databases of both forward and reverse tasks. This prevents circular synchronization. Do not modify this database while the task runs. Ensure the database account used by the task has read and write permissions on this database. Note The dts database may interfere with normal primary/secondary replication in MySQL. Check whether the dts database and its tables exist in the secondary database. If they do not, manually create them in the secondary database using the same schema as in the primary database. If your data includes four-byte characters—such as rare Chinese characters or emojis—the destination database and table must use the utf8mb4 charset. Note If you use DTS to synchronize table schemas, set the `character_set_server` parameter at the instance level in the destination database to utf8mb4. Assess the performance of both the source and destination databases before starting synchronization. Run synchronization during off-peak hours. Otherwise, full initialization consumes read and write resources on both databases and may increase database load. Full initialization runs INSERT operations concurrently. This fragments destination tables. After full initialization, the tablespace of the destination instance is larger than that of the source instance. If you synchronize one or more tables—not a full database—do not use tools like pt-online-schema-change to perform online DDL operations on the source tables. Otherwise, synchronization fails. You can use Data Management (DMS) to perform online DDL operations. For more information, see Online DDL without locking tables. DTS disables foreign key constraints by default when synchronizing to a destination database. Therefore, cascading operations and deletions in the source database are not synchronized to the destination database. A two-way synchronization task includes forward and reverse synchronization tasks. When you configure or reset the task, if the destination object of one task matches the synchronization object of the other task: Allow only one task to synchronize full and incremental data. The other task supports only incremental synchronization. Data from the source of the current task synchronizes only to the destination of the current task. It does not serve as source data for the other task. If a DDL operation fails in the destination database, the DTS task continues to run. Check the failed DDL in the task logs. For instructions, see View task logs. If you write columns with names that differ only in case to the same table in the destination MySQL database, unexpected results may occur because MySQL column names are case-insensitive. After data synchronization completes (the instance's Status is Completed), you should use the `ANALYZE TABLE <table_name>` command to confirm that all data is written to the target table. For example, after the HA failover mechanism is triggered in the target MySQL database, data might be written only to memory, which can cause data loss. If your ApsaraDB RDS for MySQL instance has Always-Encrypted enabled, full data synchronization is not supported. Note ApsaraDB RDS for MySQL instances with Transparent Data Encryption (TDE) enabled support schema synchronization, full data synchronization, and incremental data synchronization. To synchronize accounts from the source database, meet the prerequisites and review related considerations. For more information, see Migrate database accounts. If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters. Note Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.
Special cases	For a self-managed MySQL source database: If a primary/secondary switchover occurs in the source database during synchronization, the task fails. DTS calculates latency by comparing the timestamp of the last synchronized record with the current time. If no DML operations run for a long time in the source database, latency reporting may become inaccurate. If latency appears too high, run a DML operation in the source database to update the latency. Note If you select a full database for synchronization, create a heartbeat table. Update or write to this table every second. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command in the source database to advance the binary log offset. If the self-managed MySQL serves as the source database for both a bidirectional sync task and other tasks, you must add the line `log_slave_updates=1` to the `/etc/my.cnf` file of the source database and then restart the database. Otherwise, data changes from the reverse task will not be recorded in the binary log of the self-managed MySQL, causing data inconsistency in other tasks. If your source database is Amazon Aurora MySQL or another clustered MySQL instance, ensure the domain name or IP address used in the task configuration—and its DNS resolution—always points to a read/write (RW) node. Otherwise, synchronization may fail. For an ApsaraDB RDS for MySQL source database: Read-only instances—such as ApsaraDB RDS for MySQL 5.6 read-only instances—that do not record transaction logs cannot serve as source databases. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command in 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 statements

Operation type	SQL statement
DML	INSERT, UPDATE, DELETE
DDL	ALTER TABLE, ALTER VIEW CREATE FUNCTION, CREATE INDEX, CREATE PROCEDURE, CREATE TABLE, CREATE VIEW DROP INDEX, DROP TABLE RENAME TABLE Important The RENAME TABLE operation may cause data inconsistency. For example, if the synchronization object is only a specific table and you rename the table in the source instance during synchronization, the data of the table is not synchronized to the destination database. To prevent this issue, select the entire database to which the table belongs as the synchronization object 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 both included in the synchronization object. TRUNCATE TABLE

Important

DDL statements are synchronized only in forward tasks (from the source database to the destination database). In reverse tasks, DDL statements are not supported and are automatically filtered out.
During bidirectional synchronization, when you execute a DDL statement on the source database to modify a table schema, you must ensure that DML write operations on both the source and destination databases are consistent with the new schema. Otherwise, write operations may fail. For example, if you delete a column from the source database, the destination database may still attempt to write data based on the old schema that includes the deleted column.

Account permissions

Database	Permissions	Actions
Source ApsaraDB RDS for MySQL	The account must have read and write permissions on the source and destination databases. A privileged account for an ApsaraDB RDS for MySQL instance already has these permissions.	Create an account and reset a privileged account.
Destination ApsaraDB RDS for MySQL		Create an account and reset a privileged account.

Procedure

Go to the data synchronization task list page in the destination region. You can do this in one of two ways.
DTS console
1. Log on to the DTS console.
2. In the navigation pane on the left, click Data Synchronization.
3. In the upper-left corner of the page, select the region where the synchronization instance is located.
DMS console
Note
The actual steps may vary depending on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top menu bar, choose Data + AI > DTS (DTS) > Data Synchronization.
3. To the right of Data Synchronization Tasks, select the region of the synchronization instance.
Click Create Task to open the task configuration page.

Configure the source and destination databases.

Warning

After you select the source and destination instances, review the Limits at the top of the page. Otherwise, the task may fail or data inconsistency may occur.

Category	Parameter	Description
N/A	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select MySQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region of the source RDS MySQL instance.
	Replicate Data Across Alibaba Cloud Accounts	For this example, select No, as the database instance belongs to the current Alibaba Cloud account.
	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 synchronize data between two RDS MySQL instances or within the same RDS MySQL instance.
	Database Account	Enter the privileged account for the source RDS MySQL instance.
	Database Password	Enter the password for the specified database account.
	Encryption	Select Non-encrypted or SSL-encrypted as needed. If you set this to SSL-encrypted, you must enable SSL encryption for the RDS for MySQL instance beforehand. For more information, see Use a cloud certificate to quickly enable SSL link encryption.
Destination Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select MySQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region of the destination RDS MySQL instance.
	Replicate Data Across Alibaba Cloud Accounts	For this example, select No, as the database instance belongs to the current Alibaba Cloud account.
	RDS Instance ID	Select the ID of the destination RDS MySQL instance.
	Database Account	Enter the privileged account for the destination RDS MySQL instance.
	Database Password	Enter the password for the specified database account.
	Encryption	Select Non-encrypted or SSL-encrypted as needed. If you set this to SSL-encrypted, you must enable SSL encryption for the RDS for MySQL instance beforehand. For more information, see Use a cloud certificate to quickly enable SSL link encryption.

After completing the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
- Ensure that you add the CIDR blocks of the DTS servers (either automatically or manually) to the security settings of both the source and destination databases to allow access. For more information, see Add the IP address whitelist of DTS servers.
- If the source or destination is a self-managed database (i.e., the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the task objects.

On the Configure Objects page, specify the objects to synchronize.

Parameter	Description
Synchronization Types	DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization.
Method to Migrate Triggers in Source Database	Select a method to synchronize triggers based on your requirements. If the objects to be synchronized do not involve triggers, you can skip this parameter. For more information, see Configure how to synchronize or migrate triggers. Note You can configure this parameter only if you select Schema Synchronization for Synchronization Types.
Enable Migration Assessment	Assess whether the schemas of the source and destination databases, such as index length, stored procedures, and dependent tables, meet the requirements. You can select Yes or No based on your business requirements. Note You can configure this parameter only if you select Schema Synchronization for Synchronization Types. If you select Yes, the precheck may take longer. 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 a primary key or unique constraint, select Yes. For more information, see Synchronize tables without a primary key or unique constraint.
Exclude DDL Operations	Yes: DDL operations are not synchronized. No: DDL operations are synchronized. Important To ensure the stability of the two-way synchronization link, you can select whether to synchronize DDL operations only for the forward synchronization task (from the source database to the destination database). The reverse synchronization task (from the destination database to the source database) automatically filters out DDL operations.
Global Conflict Resolution Policy	If you encounter the preceding conflict types, select an appropriate conflict resolution policy based on your business requirements. TaskFailed (Fail and report an error upon conflict) The task will error out and stop if a conflict occurs. The task enters a failed state and requires manual intervention for recovery. Ignore (Keep the existing record in the destination instance) The system skips the conflicting statement and continues the synchronization. The existing record in the destination instance is retained. Overwrite (Overwrite the conflicting record in the destination instance) If a conflict occurs, the system overwrites the conflicting record in the destination instance with the data from the source. Note These policies do not apply during latency periods that occur after a task is paused or restarted. In such cases, the destination database data is overwritten by default.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks for tables with the same names in the destination database. If any tables with the same names are found, an error is reported during the precheck and the data synchronization task does not start. Otherwise, the precheck is successful. Note If you cannot delete or rename the table with the same name in the destination database, you can map it to a different name in the destination. For more information, see Database Table Column Name Mapping. Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example: If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database: During full data synchronization, DTS retains the destination record and skips the source record. During incremental synchronization, DTS overwrites the destination record with the source record. If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.
Whether to migrate Event	Specify whether to synchronize events from the source database as required. If you select Yes, you must also meet the relevant requirements and perform subsequent operations. For more information, see Synchronize or migrate events.
Capitalization of Object Names in Destination Instance	Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.
Source Objects	In the Source Objects box, click the objects, and then click to move them to the Selected Objects box. Note You can select objects at the database, table, or column level. If you select only tables or columns, DTS does not synchronize other object types (such as views, triggers, and stored procedures).
Selected Objects	To rename a single object in the destination instance, right-click the object in the Selected Objects box. For more information, see Map a single object name. To rename multiple objects in bulk, click Batch Edit in the upper-right corner of the Selected Objects box. For more information, see Map multiple object names in bulk. Note To select the SQL operations that you want to synchronize at the database or table level, right-click the object in the Selected Objects box and select the desired SQL operations in the dialog box that appears. To filter data by using a WHERE clause, right-click the table to be synchronized in the Selected Objects box, and then set a filter condition in the dialog box that appears. For more information, see Set filter conditions. If you use the object name mapping feature, other objects that depend on the mapped object may fail to be synchronized. If you need to configure an Independent Conflict Resolution Policy for incremental data synchronization, see Set an independent conflict resolution policy.

Click Next: Advanced Settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. 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 the source database uses Data Management (DMS) or gh-ost for online DDL changes, choose whether to synchronize the temporary tables generated during these operations. Important DTS tasks do not currently support online DDL changes performed by tools like pt-online-schema-change. Using such tools will cause the DTS task to fail. Yes: Synchronizes the temporary tables generated by online DDL changes. Note If the data of temporary tables generated by online DDL changes is too large, it may cause synchronization latency. No, Adapt to DMS Online DDL: Does not synchronize temporary tables generated by online DDL changes. Instead, it synchronizes only the original DDL statements executed in Data Management (DMS). Note This approach will cause table locks on the destination database. No, Adapt to gh-ost: Does not the synchronize temporary tables generated by online DDL changes. Instead, it synchronizes only the original DDL statements executed by gh-ost. You can use default or custom regular expressions for gh-ost shadow and trash tables. Note This approach will cause table locks on the destination database.
Whether to Migrate Accounts	Specify whether to synchronize account information from the source database based on your requirements. If you select Yes, also select the accounts to be synchronized and confirm their permissions. For more information such as authorization methods, see Migrate database accounts.
Retry Time for Failed Connections	If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails. Note If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances. DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.
Retry Time for Other Issues	If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.
Enable Throttling for Full Data Synchronization	During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s). Note This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
Enable Throttling for Incremental Data Synchronization	You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).
Environment Tag	Based on your requirements, you can select an environment tag to identify the instance. This parameter does not need to be configured in this example.
Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Choose whether DTS writes heartbeat SQL information to the source database while the instance is running. Yes: Does not write heartbeat SQL information to the source database. The DTS instance may display latency. No: Writes heartbeat SQL information to the source database. This may interfere with source database operations like physical backups and cloning.
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	Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert notifications. For more information, see Configure monitoring and alerting during task configuration.

Click Data Verification to configure a data verification task.
To use the data verification feature, see Configure data verification.

Save the task and perform a precheck.
- To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.
- If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.
- If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.
- If the precheck generates warnings:
  For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.
  For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.

Purchase the instance.

When the Success Rate reaches 100%, click Next: Purchase Instance.

On the Purchase page, select the billing method and specifications for the data synchronization instance. For more information, see the following table.

Category	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay when you create the instance. This is suitable for long-term needs, is more affordable than pay-as-you-go, and offers greater discounts for longer subscription durations. Pay-as-you-go: Billed hourly. This is suitable for short-term needs. You can release the instance immediately after use to save costs.
	Resource Group Configuration	The resource group to which the instance belongs. The default is default resource group. For more information, see What is Resource Management?.
	Link Specification	DTS provides synchronization specifications with different performance levels. The link specification affects the synchronization rate. You can choose one based on your business scenario. For more information, see Data synchronization link specifications.
	Synchronization Topology	Fixed as Two-way Synchronization.
	Subscription Duration	In subscription mode, select the duration and quantity for the subscription instance. You can choose from 1 to 9 months for a monthly subscription, or 1, 2, 3, or 5 years for a yearly subscription. Note This option appears only when the billing method is Subscription.

Read and select the checkbox for Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start, and then click OK in the OK dialog box.
You can monitor the task progress on the data synchronization page.

Configure the reverse synchronization task.
1. Wait for the forward synchronization task to initialize and for its Status to change to Running.
2. Locate the reverse synchronization task and click Configure Task.
3. Refer to Step 3 through Step 6 to configure the reverse synchronization task.
  Important
  When you configure the reverse synchronization task, you must select the correct source and destination instances. The source instance for reverse synchronization is the destination instance of the forward synchronization, and the destination instance for reverse synchronization is the source instance of the forward synchronization. You must also carefully confirm that instance information such as the database name, account, and password is consistent.
  You cannot modify the Instance Region of the source and destination databases for a reverse synchronization task. This task also requires fewer configuration parameters than a forward synchronization task. For the actual parameters, refer to the console.
  The Processing Mode of Conflicting Tables configuration for the reverse synchronization task does not check for tables synchronized to the destination instance by the forward synchronization task.
  The reverse synchronization task does not support synchronizing the objects specified 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 synchronization task. Otherwise, data inconsistency may occur.
4. When the Success Rate reaches 100%, click Back.
After the reverse synchronization task is configured, wait until the Status of both synchronization tasks is Running. This completes the two-way data synchronization configuration.