One-way data synchronization from ApsaraDB for MongoDB sharded cluster instances - Data Transmission Service

Data Transmission Service (DTS) allows you to synchronize data from an ApsaraDB for MongoDB sharded cluster instance to an ApsaraDB for MongoDB replica set or sharded cluster instance. This topic describes how to synchronize data between ApsaraDB for MongoDB instances.

Prerequisites

The destination ApsaraDB for MongoDB replica set or sharded cluster instance is created. For more information, see Create a replica set instance or Create a sharded cluster instance.
Important
- We recommend that you use a destination ApsaraDB for MongoDB instance whose available storage space is 10% larger than the total size of the data in the source ApsaraDB for MongoDB instance.
- For more information about the supported database versions, see Overview of data synchronization scenarios.
Endpoints are assigned to the shards in the source ApsaraDB for MongoDB sharded cluster instance, and the shards share the same account and password. For more information, see Apply for an endpoint for a shard or Configserver node.
In the destination ApsaraDB for MongoDB sharded cluster instance, databases and collections to be sharded are created, data sharding is configured, the balancer is enabled, and pre-sharding is performed based on your business requirements. For more information, see Configure sharding to maximize the performance of shards and the What do I do if the data of a MongoDB database deployed in the sharded cluster architecture is not evenly distributed? section in the "FAQ" topic.
Note
- After you configure sharding for a sharded cluster instance, the synchronized data is to be distributed among different shards. This maximizes the performance of the sharded cluster. You can also enable the balancer and perform pre-sharding to prevent data skew.
- In this example, a DTS task is configured before a DTS instance is purchased. You do not need to specify the number of shards in the source ApsaraDB for MongoDB sharded cluster instance.
  If you purchase a DTS instance before you configure a DTS task, you must specify the number of shards when you purchase the instance.

Limits

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.
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 oplog feature must be enabled.
Note
The operation logs of the source database must be retained for at least seven days. Otherwise, DTS may fail to obtain the operation logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. Make sure that you set the retention period of operation logs based on the preceding requirements. Otherwise, the service level agreement (SLA) of DTS does not guarantee service reliability or performance.
During a data synchronization task, MongoDB sharded cluster databases involved in the task cannot be scaled. Otherwise, the task fails.
If the source database is a self-managed MongoDB database that uses the sharded cluster architecture, you can set the Access Method parameter only to Express Connect, VPN Gateway, or Smart Access Gateway or Cloud Enterprise Network (CEN) when you configure the DTS task.
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 database to be synchronized contains TTL indexes, data inconsistency may occur between the source and destination databases due to inconsistent time zones and time clocks of the source and destination databases.
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 orphaned document in official MongoDB documentation and the How do I delete orphaned documents of a MongoDB database deployed in the sharded cluster architecture? section of the "FAQ" topic.

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.
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.
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.
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.

Billing

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

Supported one-way data synchronization topologies

DTS supports one-way data synchronization between only two ApsaraDB for MongoDB sharded cluster instances. DTS does not support one-way data synchronization among multiple ApsaraDB for MongoDB instances.

Synchronization types

Synchronization type	Description
Schema synchronization	DTS synchronizes the schemas of the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.
Full data synchronization	DTS synchronizes the historical data of the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS supports full data synchronization for databases and collections.
Incremental data synchronization	DTS synchronizes incremental data from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS synchronizes incremental data generated by the following operations: CREATE COLLECTION and CREATE INDEX DROP COLLECTION and DROP INDEX RENAME COLLECTION

Delete orphaned documents

Before you synchronize data, you must delete the orphaned documents from the source database.

Important

If you do not delete the orphaned documents, the synchronization performance is compromised. In addition, some documents may have duplicate _id values and data that you do not want to synchronize may be synchronized.

ApsaraDB for MongoDB instances

The cleanupOrphaned command is required to delete orphaned documents. The method of running this command varies based on the version of the MongoDB database.

MongoDB 4.4 and later

Create a JavaScript script file named cleanupOrphaned.js on a server that can connect to the sharded cluster instance.

Note

This script is used to delete orphaned documents from all collections in multiple databases in multiple shards. If you want to delete orphaned documents from a specific collection, you can modify some of the parameters in the script file.

// The names of shards.
var shardNames = ["shardName1", "shardName2"];
// The databases from which you want to delete orphaned documents.
var databasesToProcess = ["database1", "database2", "database3"];

shardNames.forEach(function(shardName) {
    // Traverse the specified databases.
    databasesToProcess.forEach(function(dbName) {
        var dbInstance = db.getSiblingDB(dbName);
        // Obtain the names of all collections of the specified databases.
        var collectionNames = dbInstance.getCollectionNames();
        
        // Traverse all collections.
        collectionNames.forEach(function(collectionName) {
            // The complete collection name.
            var fullCollectionName = dbName + "." + collectionName;
            // Build the cleanupOrphaned command.
            var command = {
                runCommandOnShard: shardName,
                command: { cleanupOrphaned: fullCollectionName }
            };

            // Run the cleanupOrphaned command.
            var result = db.adminCommand(command); 
            if (result.ok) {
                print("Cleaned up orphaned documents for collection " + fullCollectionName + " on shard " + shardName);
                printjson(result);
            } else {
                print("Failed to clean up orphaned documents for collection " + fullCollectionName + " on shard " + shardName);
            }
        });
    });
});

You must modify the shardNames and databasesToProcess parameters in the script file. The following content describes the two parameters:

shardNames: the IDs of the shards from which you want to delete orphaned documents. You can view the IDs in the Shard List section on the Basic Information page of the sharded cluster instance. Example: d-bp15a3796d3a****.
databasesToProcess: the names of the databases from which you want to delete orphaned documents.

Run the following command in the directory in which the cleanupOrphaned.js script file is stored:

mongo --host <Mongoshost> --port <Primaryport>  --authenticationDatabase <database> -u <username> -p <password> cleanupOrphaned.js > output.txt

The following table describes the parameters.

Parameter	Description
`<Mongoshost>`	The endpoint of the mongos node of the sharded cluster instance. Format: `s-bp14423a2a51****.mongodb.rds.aliyuncs.com`.
`<Primaryport>`	The port number of the mongos node of the sharded cluster instance. Default value: 3717.
`<database>`	The name of the database to which the database account belongs.
`<username>`	The database account of the sharded cluster instance.
`<password>`	The password of the database account.
`output.txt`	The output.txt file that is used to store execution results.

MongoDB 4.2 and earlier

Create a JavaScript script file named cleanupOrphaned.js on a server that can connect to the sharded cluster instance.
Note
This script is used to delete orphaned documents from a specific collection in a database in multiple shards. If you want to delete orphaned documents from multiple collections, you can modify the fullCollectionName parameter in the script file and run the script multiple times. Alternatively, you can modify the script file to traverse all collections.
```
function cleanupOrphanedOnShard(shardName, fullCollectionName) {
    var nextKey = { };
    var result;

    while ( nextKey != null ) {
        var command = {
            runCommandOnShard: shardName,
            command: { cleanupOrphaned: fullCollectionName, startingFromKey: nextKey }
        };

        result = db.adminCommand(command);
        printjson(result);

        if (result.ok != 1 || !(result.results.hasOwnProperty(shardName)) || result.results[shardName].ok != 1 ) {
            print("Unable to complete at this time: failure or timeout.")
            break
        }

        nextKey = result.results[shardName].stoppedAtKey;
    }

    print("cleanupOrphaned done for coll: " + fullCollectionName + " on shard: " + shardName)
}

var shardNames = ["shardName1", "shardName2", "shardName3"]
var fullCollectionName = "database.collection"

shardNames.forEach(function(shardName) {
    cleanupOrphanedOnShard(shardName, fullCollectionName);
});
```
You must modify the shardNames and fullCollectionName parameters in the script file. The following content describes the two parameters:
- shardNames: the IDs of the shards from which you want to delete orphaned documents. You can view the IDs in the Shard List section on the Basic Information page of the sharded cluster instance. Example: d-bp15a3796d3a****.
- fullCollectionName: You must replace this parameter with the name of the collection from which you want to delete orphaned documents. Format: database name.collection name.

Run the following command in the directory in which the cleanupOrphaned.js script file is stored:

mongo --host <Mongoshost> --port <Primaryport>  --authenticationDatabase <database> -u <username> -p <password> cleanupOrphaned.js > output.txt

The following table describes the parameters.

Parameter	Description
`<Mongoshost>`	The endpoint of the mongos node of the sharded cluster instance. Format: `s-bp14423a2a51****.mongodb.rds.aliyuncs.com`.
`<Primaryport>`	The port number of the mongos node of the sharded cluster instance. Default value: 3717.
`<database>`	The name of the database to which the database account belongs.
`<username>`	The database account of the sharded cluster instance.
`<password>`	The password of the database account.
`output.txt`	The output.txt file that is used to store execution results.

Self-managed MongoDB databases

Download the cleanupOrphaned.js script file on a server that can connect to the self-managed MongoDB database.

wget "https://docs-aliyun.cn-hangzhou.oss.aliyun-inc.com/assets/attach/120562/cn_zh/1564451237979/cleanupOrphaned.js"

Replace test in the cleanupOrphaned.js file with the name of the database from which you want to delete orphaned documents.
Important
If you want to delete orphaned documents from multiple databases, repeat Step 2 and Step 3.
Run the following command on a shard to delete the orphaned documents from all collections in the specified database:
Note
You must repeat this step for each shard.
```
mongo --host <Shardhost> --port <Primaryport>  --authenticationDatabase <database> -u <username> -p <password> cleanupOrphaned.js
```
Note
- <Shardhost>: the IP address of the shard.
- <Primaryport>: the service port of the primary node in the shard.
- <database>: the name of the database to which the database account belongs.
- <username>: the account that is used to log on to the self-managed MongoDB database.
- <password>: the password that is used to log on to the self-managed MongoDB database.
Example:
In this example, a self-managed MongoDB database has three shards, and you must delete the orphaned documents from each shard.
```
mongo --host 172.16.1.10 --port 27018  --authenticationDatabase admin -u dtstest -p 'Test123456' cleanupOrphaned.js
```
```
mongo --host 172.16.1.11 --port 27021 --authenticationDatabase admin -u dtstest -p 'Test123456' cleanupOrphaned.js
```
```
mongo --host 172.16.1.12 --port 27024  --authenticationDatabase admin -u dtstest -p 'Test123456' cleanupOrphaned.js
```

Procedure

Important

In this example, a DTS task is configured before a DTS instance is purchased. You do not need to specify the number of shards in the source ApsaraDB for MongoDB sharded cluster instance.

If you purchase a DTS instance before you configure a DTS task, you must specify the number of shards when you purchase the instance.

Go to the Data Synchronization Tasks page.
1. Log on to the Data Management (DMS) console.
2. In the top navigation bar, click DTS.
3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
Note
- Operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
- You can also go to the Data Synchronization Tasks page of the new DTS console.
On the right side of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Note
If you use the new DTS console, you must select the region in which the data synchronization instance resides in the top navigation bar.

Click Create Task. In the Create Task wizard, configure the source and destination databases.

Section	Parameter	Description
N/A	Task Name	The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select an existing DMS database instance. (Optional. If you have not registered a DMS database instance, ignore this option and configure database settings in the section below.)	Select whether to use an existing instance. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
	Database Type	The type of the source database. Select MongoDB.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source ApsaraDB for MongoDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
	Architecture	The architecture in which the source instance is deployed. Select Sharded Cluster.
	Instance ID	The ID of the source ApsaraDB for MongoDB instance.
	Authentication Database	The name of the authentication database that stores the database accounts and passwords of the source ApsaraDB for MongoDB instance. If you did not change the name of the authentication database before, the default value is admin.
	Database Account	The database account of the source ApsaraDB for MongoDB instance. The account must have the read permissions on the source, admin, and local databases.
	Database Password	The password of the database account.
	Shard Account	The account that is used to access the shards in the source ApsaraDB for MongoDB instance. Note If the source database is a self-managed MongoDB database, you must specify the information that is used to access the shards.
	Shard Password	The password that is used to access the shards in the source ApsaraDB for MongoDB instance.
Destination Database	Select an existing DMS database instance. (Optional. If you have not registered a DMS database instance, ignore this option and configure database settings in the section below.)	Select whether to use an existing instance. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
	Database Type	The type of the destination database. Select MongoDB.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination ApsaraDB for MongoDB instance resides.
	Architecture	The architecture in which the destination ApsaraDB for MongoDB instance is deployed.
	Instance ID	The ID of the destination ApsaraDB for MongoDB instance.
	Authentication Database	The name of the authentication database that stores the database accounts and passwords of the destination ApsaraDB for MongoDB instance. If you did not change the name of the authentication database before, the default value is admin.
	Database Account	The database account of the destination ApsaraDB for MongoDB instance. The account must have the dbAdminAnyDatabase permission, the read and write permissions on the destination database, and the read permissions on the local database.
	Database Password	The password of the database account.

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

Configure the objects to be synchronized and the advanced settings.

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 database. The historical data is the basis for subsequent incremental synchronization.
Synchronization Topology	The synchronization topology of the data synchronization task. Select One-way 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 identical collection 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, and only specific columns are synchronized, or the data synchronization task fails.
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 move the objects to the Selected Objects section. Note You can select databases or collections as the objects to be synchronized.
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.

Click Next: Advanced Settings to configure advanced settings.

Data Verification Settings
For more information about how to configure the data verification feature, see Configure data verification.

Advanced Settings

Parameter	Description
Select the dedicated cluster used to schedule the task	By default, DTS schedules tasks to shared clusters. You do not need to configure this parameter. You can purchase a dedicated cluster of specific specifications to run DTS data synchronization tasks. 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.
The wait time before a retry when other issues occur in the source and destination databases.	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 The wait time before a retry when other issues occur in the source and destination databases. parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Migration	During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server. Note This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter.
Enable Throttling for Incremental Data Synchronization	Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, no environment tag is added.
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 configure alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert contacts. For more information, see Configure monitoring and alerting.

Save the task settings and run a precheck.
- To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.
- If you do not need to view or have viewed the parameters, click Next: Save Task Settings and Precheck in the lower part of the page.
Note
- Before you can start the data synchronization task, DTS performs a precheck. You can start the data synchronization task only after the task passes the precheck.
- If the task fails to pass the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
- If an alert is triggered for an item during the precheck:
  If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issue. Then, run a precheck again.
  If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur and your business may be exposed to potential risks.
Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.

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

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

Read and select the Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.