Two-way synchronization between ApsaraDB for MongoDB instances (sharded cluster architecture)

Data Transmission Service (DTS) enables two-way synchronization between ApsaraDB for MongoDB instances with sharded cluster architecture, suitable for a range of scenarios including active geo-redundancy and geo-disaster recovery. This topic describes the steps to configure two-way data synchronization.

Prerequisites

The source and destination ApsaraDB for MongoDB sharded cluster instances are created. For more information, see how to create a sharded cluster instance.
Important
- All shard nodes within the source ApsaraDB for MongoDB sharded cluster instance, including the source instance for the reverse task, must obtain connection addresses, and the account passwords for each shard need to be consistent. For more information, see how to obtain connection addresses for Shard or ConfigServer nodes.
- For a list of supported versions, refer to the Overview of synchronization solutions.
We recommend that the storage space for the destination ApsaraDB for MongoDB instance should be at least 10% larger than the space used by the source ApsaraDB for MongoDB instance.
You need to configure the replication.oplogGlobalIdEnabled parameter to true on both the Shard and ConfigServer nodes for the source and destination instances. For more information, see Set database parameters.

Note

If true is not selected, the instance precheck will fail, or the error two-way mongo must have gid will appear.
You need to create the necessary databases and collections for sharding in both the source and destination ApsaraDB for MongoDB instances based on your business requirements, set up data sharding, activate the Balancer (which must remain active during incremental synchronization for the source ApsaraDB for MongoDB instance), and carry out pre-sharding. For more information, see how to optimize shard performance through data sharding and how to address uneven data distribution in a MongoDB sharded cluster.
Note
- Configuring data sharding helps prevent data from being synchronized to the same shard, which could degrade cluster performance. Enabling the Balancer and pre-sharding can mitigate data skew issues.
- This configuration scenario uses the setup of a DTS task prior to purchase as an example. There is no need to specify the number of shards for the source ApsaraDB for MongoDB with sharded cluster architecture.
  
  If you purchase before configuring the DTS task, you must enter the correct number of shards when purchasing the DTS task.

Precautions

Category	Description

Category	Description
Limits on the source and destination databases	Bandwidth requirements: The server to which the source database is deployed must have sufficient outbound bandwidth. Otherwise, the data synchronization speed is affected. The collections to be synchronized must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records. The _id field in the collection to be synchronized must be unique. Otherwise, data inconsistency may occur. If you select collections as the objects to be synchronized and you need to edit collections in the destination database, such as renaming collections, up to 1,000 collections can be synchronized in a single data synchronization task. If you run a task to synchronize more than 1,000 collections, a request error occurs. In this case, we recommend that you configure multiple tasks to synchronize the collections in batches, or configure a task to synchronize the entire database. The size of a single data entry to be synchronized from the source database cannot exceed 16 MB. Otherwise, the task fails. The source database cannot be an Azure Cosmos DB for MongoDB cluster or an Amazon DocumentDB elastic cluster. The oplog feature must be enabled for the source database and must retain log data for at least seven days. Alternatively, change streams must be enabled to ensure that DTS can subscribe to data changes in the source database within the last seven days. Otherwise, DTS may fail to obtain data changes in the source database and data synchronization fails. In some circumstances, data inconsistency or data loss may occur. Issues that arise in such circumstances are not covered by the service level agreement (SLA) of DTS. Important We recommend that you use the oplog to record data changes in the source database. Only MongoDB 4.0 and later allow you to use change streams to obtain data changes in the source database. Two-way synchronization is not supported when you use change streams to obtain data changes in the source database. If the source database is a non-elastic Amazon DocumentDB cluster, you must enable change streams and set the Migration Method parameter to ChangeStream and the Architecture parameter to Sharded Cluster. During a data synchronization task, MongoDB sharded cluster databases involved in the task cannot be scaled. Otherwise, the task fails. The number of Mongos nodes in the source MongoDB sharded cluster database cannot exceed 10. You cannot synchronize collections that contain time to live (TTL) indexes. If the source database contains TTL indexes, data inconsistency may occur between the source and destination databases after the synchronization. Make sure that no orphaned document exists in the source and destination databases. Otherwise, data inconsistency or even task failure may occur. For more information, see Glossary of MongoDB and How do I delete orphaned documents of a MongoDB database deployed in the sharded cluster architecture? The source and destination databases must be ApsaraDB for MongoDB instances with the same architecture. Two-way synchronization is not supported for self-managed MongoDB databases or MongoDB databases with different architectures. Limits on operations to be performed on the source database: During schema synchronization and full data synchronization, do not change the schemas of the databases or collections, including the update of array type. Otherwise, the data synchronization task fails or data inconsistency may occur between the source and destination databases. If you perform only full data synchronization, do not write data to the source database during data synchronization. Otherwise, data inconsistency occurs between the source and destination databases. If the balancer of the source database is enabled to balance data, the DTS task may be delayed.
Other limits	Before you start the data synchronization task, you must add sharding keys to the data to be synchronized in the source database. During the data synchronization, if you execute the INSERT statement to insert data into the data to be synchronized, the data to be synchronized must contain sharding keys. If you execute the UPDATE statement to modify the data to be synchronized, you cannot modify sharding keys. To ensure compatibility, the version of the destination MongoDB database must be the same as or later than the version of the source MongoDB database. If the version of the destination database is earlier than that of the source database, database compatibility issues may occur. If a collection of the destination database has a unique index or the `capped` attribute of a collection of the destination database is `true`, the collection supports only single-thread data writing and does not support concurrent replay during incremental data synchronization. This may increase synchronization latency. DTS cannot synchronize data from the admin or local database. Transaction information is not retained. When transactions are synchronized to the destination database, transactions are converted into a single record. 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 full data synchronization, DTS uses read and write resources of the source and destination databases. This may increase the loads on the database servers. During full data synchronization, concurrent INSERT operations cause fragmentation in the collections of the destination database. After full data synchronization is complete, the storage space for collections of the destination database is larger than that of the source database. Data inconsistency between the source and destination databases occurs if data from other sources is written to the destination database during data synchronization. For example, if you use DMS to execute online DDL statements while data from other sources is written to the destination database, data loss may occur in the destination database. 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 objects. The other task synchronizes only the incremental data of the objects. The source data of the current task can be synchronized only to the destination database in the task. The synchronized data is not used as the source data of the other task. The data is concurrently written to the destination database. Therefore, the storage space occupied in the destination database is 5% to 10% larger than the size of the data in the source database. You must use the `db.$table_name.aggregate([{ $count:"myCount"}])` syntax to query the return value of a count operation on the destination MongoDB database. Make sure that the destination MongoDB database does not have the same primary key as the source database. The default primary key is _id. Otherwise, data may be lost. If the data in the destination database has the same primary key as that in the source database, clear the related data in the destination database without interrupting the services of DTS. For example, if the same primary key is _id, you can delete the data in the destination database that has the same _id as the source database. Make sure that the MongoDB balancer of the source database is disabled during full data synchronization. Do not enable the balancer until all full data synchronization is complete and incremental data synchronization starts. Otherwise, data inconsistency may occur. For more information about the MongoDB balancer, see Manage the ApsaraDB for MongoDB balancer. If data sharding is configured for the destination instance and you do not need to use the schema synchronization feature of DTS, do not select Schema Synchronization as one of the Synchronization Types in the Configure Objects and Advanced Settings step. Otherwise, data inconsistency may occur or the task may fail due to shard conflicts. If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified. Note Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Billing

Synchronization type	Task configuration fee

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

Supported two-way synchronization architectures

Currently, DTS supports two-way synchronization exclusively between two ApsaraDB for MongoDB instances with sharded cluster architecture. It does not support two-way synchronization across multiple ApsaraDB for MongoDB instances.

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.

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
If the record that you want to insert into the destination instance by executing the INSERT statement conflicts with an existing record, DTS automatically ignores the INSERT operation.
Inconsistent records caused by UPDATE operations
If the record that you want to update by executing the UPDATE statement does not exist in the destination instance or conflicts with another record, DTS automatically ignores the UPDATE operation.
Non-existent records to be deleted
If the record that you want to delete from the destination instance by executing the DELETE statement does not exist, DTS automatically ignores the DELETE operation.

Important

During two-way synchronization, the system time of the source and destination instances may be different and synchronization latency may occur. Therefore, 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.
By default, DTS sets Conflict Resolution Policy to Ignore to resolve the preceding conflicts during data synchronization. You cannot change the value of Conflict Resolution Policy.

Task step description

Synchronization type	Description

Synchronization type	Description
Schema synchronization	Synchronize the structure of the synchronization objects in the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB.
Full synchronization	Synchronize all historical data of the synchronization objects in the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB. Note Supports full synchronization of DATABASE and COLLECTION.
Incremental synchronization	On the basis of full synchronization, synchronize the incremental updates of the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB. Note Incremental synchronization does not support newly created databases after the task starts running. The supported incremental updates are as follows: CREATE COLLECTION, INDEX DROP COLLECTION, INDEX RENAME COLLECTION Operations to insert, update, and delete documents in the collection. When synchronizing incremental update document data, only operations updated using the `$set` command are supported.

Procedure

Important

This configuration scenario uses the pre-purchase configuration of a DTS task as an example. There is no need to specify the number of shards for the source ApsaraDB for MongoDB (sharded cluster architecture).

If you purchase before configuring the DTS task, you must enter the correct number of shards when purchasing the DTS task.

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
DMS 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 instance resides.
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 Development 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.
Optional. Click New Configuration Page in the upper-right corner of the 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.

Category	Configuration	Description

Category	Configuration	Description
None	Task Name	The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select Existing Connection	The database that you want to use. You can choose whether to use an existing database based on your business requirements. If you select an existing database, DTS automatically populates the parameters for the database. If you do not select an existing database, you must configure the following database information. Note In the DTS console, register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	Select MongoDB.
	Access Method	Select Cloud Instance.
	Instance Region	Select the region where the source ApsaraDB for MongoDB resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Architecture Type	Select Sharded Cluster Architecture.
	Migration Method	Select Oplog.
	Instance ID	Select the source ApsaraDB for MongoDB instance ID.
	Authentication Database Name	Enter the database name to which the database account of the source ApsaraDB for MongoDB instance belongs. If not modified, the default is admin.
	Database Account	Enter the database account of the source ApsaraDB for MongoDB, which must have read permissions on the databases to be synchronized, the config database, the admin database, and the local database.
	Database Password	The password that is used to access the database.
	Shard account	Enter the Shard account of the source ApsaraDB for MongoDB.
	Shard password	Enter the Shard password of the source ApsaraDB for MongoDB.
	Encryption	Specifies whether to encrypt the connection to the source database. You can select Non-encrypted, SSL-encrypted, or Mongo Atlas SSL based on your business requirements. The options available for the Encryption parameter are determined by the values selected for the Access Method and Architecture parameters. The options displayed in the DTS console prevail. Note If the Architecture parameter is set to Sharded Cluster, and the Migration Method parameter is set to Oplog for the ApsaraDB for MongoDB database, the Encryption parameter SSL-encrypted is unavailable. If the source database is a self-managed MongoDB database that uses the Replica Set architecture, the Access Method parameter is not set to Alibaba Cloud Instance, and the Encryption parameter is set to SSL-encrypted, you can upload a certification authority (CA) certificate to verify the connection to the source database.
Destination Database	Select Existing Connection	The database that you want to use. You can choose whether to use an existing database based on your business requirements. If you select an existing database, DTS automatically populates the parameters for the database. If you do not select an existing database, you must configure the following database information. Note In the DTS console, register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	Select MongoDB.
	Access Method	Select Cloud Instance.
	Instance Region	Select the region where the destination ApsaraDB for MongoDB resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Architecture Type	Select Sharded Cluster Architecture.
	Instance ID	Select the destination ApsaraDB for MongoDB instance ID.
	Authentication Database Name	Enter the database name to which the database account of the destination ApsaraDB for MongoDB instance belongs. If not modified, the default is admin.
	Database Account	Enter the database account of the destination ApsaraDB for MongoDB, which must have dbAdminAnyDatabase permissions, readWrite permissions on the destination database, and read permissions on the local database.
	Database Password	The password that is used to access the database.
	Encryption	Specifies whether to encrypt the connection to the destination database. You can select Non-encrypted, SSL-encrypted, or Mongo Atlas SSL based on your business requirements. The options available for the Encryption parameter are determined by the values selected for the Access Method and Architecture parameters. The options displayed in the DTS console prevail. Note If the destination database is an ApsaraDB for MongoDB instance and the Architecture parameter is set to Sharded Cluster, the Encryption parameter SSL-encrypted is unavailable. If the destination database is a self-managed MongoDB database that uses the Replica Set architecture, the Access Method parameter is not set to Alibaba Cloud Instance, and the Encryption parameter is set to SSL-encrypted, you can upload a CA certificate to verify the connection to the destination database.

In the lower part of the page, click Test Connectivity and Proceed.
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 the CIDR blocks of DTS servers.

Configure the objects to be synchronized.

In the Configure Objects step, configure the objects that you want to synchronize.

Configuration	Description

Configuration	Description
Synchronization Types	The synchronization types. By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination cluster. The historical data is the basis for subsequent incremental synchronization.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains collections that have the same names as the collections in the source database. If the source and destination databases do not contain collections that have identical collection 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 have collections with identical names and the collections in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the collections that are synchronized to the destination database. For more information, see Rename an object to be synchronized. Ignore Errors and Proceed: skips the precheck for identical collection 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 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, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained. Data may fail to be initialized, only specific columns are synchronized, or the data synchronization task fails.
Synchronization Topology	Select Two-way Synchronization.
Filter DDL	Select Yes: Do not synchronize DDL operations. Select No: Synchronize DDL operations. Note Limits on DDL synchronization directions. To ensure the stability of the two-way synchronization link, only forward synchronization tasks support synchronizing DDL. Reverse synchronization tasks do not support synchronizing DDL.
Conflict Resolution Policy	If the above Supported Conflict Detection occurs, select an appropriate conflict resolution strategy based on your business needs. 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 This scenario only supports Ignore.
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 The selection granularity of synchronization objects is DATABASE and COLLECTION.
Selected Objects	To rename an object to be synchronized in the destination database, right-click the object in the Selected Objects section. For more information, see Map object names. To remove a selected object, click the object in the Selected Objects section and then click the icon to move the object to the Source Objects section. Note If you need to select incremental synchronization operations at the database or collection level, right-click the objects to be synchronized in the Selected Objects and make selections in the dialog box that appears. If you need to set filter conditions for data (filter conditions are supported during the full synchronization phase but not during the incremental synchronization phase), right-click the tables to be synchronized in the Selected Objects and set them in the dialog box that appears. For more information, see Set filter conditions. If the object name mapping feature is used (specifying the database or collection to receive data), it may cause other objects that depend on this object to fail to synchronize.

Click Next: Advanced Settings to configure advanced settings.

Configuration	Description

Configuration	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data synchronization tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Retry Time for Failed Connections	The retry time range for failed connections. If the source or destination database fails to be connected after the data synchronization task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set this parameter to a value greater than 30. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Note If you specify different retry time ranges for multiple data synchronization tasks that have the same source or destination database, the shortest retry time range takes precedence. When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
Retry Time for Other Issues	The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set this parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Important The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data 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.
Only one data type for primary key _id in a single table	Whether the data type for primary key `_id` in a collection of the data to be synchronized is unique. Valif value: Note This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter. Yes: The data type is unique. During full data synchronization, DTS does not scan the data type for primary key `_id` of the data to be synchronized from the source database. No: The data type is not unique. During full data synchronization, DTS scans the data type for primary key `_id` of the data to be synchronized from the source database.
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 based on your actual needs. This example does not require selection.
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 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

Section	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay for a subscription when you create a data synchronization instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go data synchronization instance, you can release the instance to reduce costs.
	Resource Group Settings	The resource group to which the data synchronization instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides instance classes that vary in synchronization speed. You can select an instance class based on your business requirements. For more information, see Instance classes of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, specify the subscription duration and the number of data synchronization instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years. Note This parameter is available only if you select the Subscription billing method.

Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the dialog box that appears, click OK.
You can view the progress of the task in the task list.

Configure the reverse synchronization task.
1. Wait for the forward synchronization task to complete its initialization until the Status is Running.
2. Find the reverse synchronization task and click Configure Task.
3. For configuring the reverse synchronization task, see Step 4 through Step 7.
  Important
  
  When configuring the reverse task, select the correct source and destination instances. In reverse synchronization, the source instance is the destination instance in forward synchronization, and the destination instance is the source instance in forward synchronization. Confirm the consistency of instance information, such as database name, account, and password.
  
  It is not recommended to change the mapping name when configuring the reverse task, as it may cause data inconsistency.
  
  The Instance Region for both the source and destination databases in a reverse synchronization task is fixed and cannot be altered. Additionally, reverse synchronization tasks require the configuration of fewer parameters than forward synchronization tasks. For more details, please consult the console interface.
  
  The Processing Mode of Conflicting Tables setting for the reverse synchronization task does not verify the tables that the forward synchronization task has synchronized to the destination instance.
  
  The reverse synchronization task cannot synchronize objects listed under Selected Objects from the forward task.
  
  The reverse synchronization task will automatically filter DDL operations.
4. Success Rate displays 100%. Click Back.
After setting up the second synchronization task, wait until the Status for both synchronization tasks shows as Running. This indicates that the two-way data synchronization configuration is complete.