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 instance versions, see Overview of data synchronization scenarios.
Endpoints are assigned to all shard nodes in the source ApsaraDB for MongoDB sharded cluster instance, and the shard nodes share the same account and password. For more information about how to apply for an endpoint, see Apply for an endpoint for a shard or ConfigServer component.
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 of 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.

Usage notes

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. 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 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? 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	If the destination database is a replica set instance, take note of the following limits: If the destination database is connected over Express Connect, VPN Gateway, or Smart Access Gateway, Public IP Address, or Cloud Enterprise NetworkCEN, set the Domain Name or IP and Port Number to the IP address and the port number of primary node, or configure a high-availability endpoint. For more information about high-availability endpoints, see Create a DTS task in which the source or destination database is a high-availability MongoDB database. If the destination database is connected over Self-managed Database on ECS, set the Port Number to the port number of primary node. 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. 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
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 Types	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. Use oplog A DTS task does not synchronize incremental data from databases that are created after the task starts to run. DTS synchronizes incremental data generated by the following operations: CREATE COLLECTION and CREATE INDEX DROP DATABASE, DROP COLLECTION, and DROP INDEX RENAME COLLECTION The operations that are performed to insert, update, and delete documents in a collection. Note When a DTS task synchronizes incremental data of a file, Only `$set` command can be synchronously run. Use change streams DTS synchronizes incremental data generated by the following operations: DROP DATABASE and DROP COLLECTION RENAME COLLECTION The operations that are performed to insert, update, and delete documents in a collection. Note When a DTS task synchronizes incremental data of a file, Only `$set` command can be synchronously run.

Delete orphaned documents

Before you synchronize data, you must delete the orphaned documents from the source MongoDB 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 the data that you do not want to synchronize may be synchronized.

ApsaraDB for MongoDB instances

Note

An error occurs if a cleanup script is executed to delete orphaned documents from an ApsaraDB for MongoDB instance whose major version is earlier than 4.2 or an ApsaraDB for MongoDB instance whose minor version is earlier than 4.0.6. For information about how to view the current version of an ApsaraDB for MongoDB instance, see Release notes for the minor versions of ApsaraDB for MongoDB. For information about how to update the minor version or major version of an ApsaraDB for MongoDB instance, see Upgrade the major version of an instance and Update the minor version of an instance.

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 that you can configure.

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.
`<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 that you can configure.

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.
`<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.

Use one of the following methods to go to the Data Synchronization page and select the region in which the data synchronization instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Synchronization.
3. In the upper-left corner of the page, select the region in which the data synchronization instance resides.
DMS console
Note
The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data + AI and choose DTS (DTS) > Data Synchronization.
3. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.
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.

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 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	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 where the source ApsaraDB for MongoDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Architecture	The architecture in which the source instance is deployed. Select Sharded Cluster.
	Migration Method	The method used to synchronize incremental data from the source database. Select a method based on your business requirements. Valid values: Oplog (recommended): This option is available if the oplog feature is enabled for the source database. Note By default, the oplog feature is enabled for both self-managed MongoDB databases and ApsaraDB for MongoDB instances. This feature allows you to synchronize incremental data at a low latency because of a fast log pulling speed. Therefore, we recommend that you select Oplog for the Migration Method parameter. ChangeStream: This option is available if change streams are enabled for the source database. For more information, see Change Streams. Note If the source database is an inelastic Amazon DocumentDB cluster, you can set the Migration Method parameter only to ChangeStream. If you select Sharded Cluster for the Architecture parameter, you do not need to configure the Shard account and Shard password parameters.
	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 admin is used.
	Database Account	The database account of the source ApsaraDB for MongoDB instance. The account must have read permissions on the source database, the config database, the admin database, and the local database.
	Database Password	The password that is used to access the database.
	Shard account	The account that is used to access the shard nodes 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 shard nodes in the source ApsaraDB for MongoDB instance.
Destination Database	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.
	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	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 where the destination ApsaraDB for MongoDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	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 admin is used.
	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 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.

Click Test Connectivity and Proceed in the lower part of the page.
Note
- Make sure that the CIDR blocks of DTS servers can be automatically or manually added to the security settings of the source and destination databases to allow access from DTS servers. For more information, see Add the CIDR blocks of DTS servers.
- If the source or destination database is a self-managed database and its Access Method is not set to Alibaba Cloud Instance, click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the objects to be synchronized.

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

Parameter	Description
Synchronization Types	The synchronization types. By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination cluster. The historical data is the basis for subsequent incremental synchronization. Note For more information about the synchronization types, see the "Synchronization types" section of this topic.
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	The synchronization topology. Select One-way Synchronization.
Capitalization of Object Names in Destination Instance	The capitalization of database names and collection 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 the default capitalization of object names in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
Source Objects	Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section. Note You can select databases or collections as the objects to be synchronized.
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 To synchronize incremental data by databases or collections, right-click the Selected Objects and select the operations in the dialog box that appears. To filter data in a table to be synchronized, right-click the table in the Selected Objects section. In the dialog box that appears, configure filter conditions. You can specify conditions to filter data during the full data synchronization stage of a data synchronization task. You cannot specify conditions to filter data during the incremental data synchronization stage of a data synchronization task. For more information, see Specify filter conditions. If you use the object name mapping feature to rename the database or collection to which the data is synchronized, other objects that depend on the database or collection may fail to be synchronized.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data synchronization 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	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, you do not need to configure this parameter.
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
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.