One-way synchronization for a sharded cluster - Data Transmission Service

Prerequisites

Create a destination ApsaraDB for MongoDB replica set or sharded cluster instance. For instructions, see Create a replica set instance or Create a sharded cluster instance.
Important
- We recommend that the destination ApsaraDB for MongoDB instance has at least 10% more storage space than the source ApsaraDB for MongoDB instance.
- For information about supported versions, see Synchronization solutions overview.
Apply for endpoints for all shard nodes in the source ApsaraDB for MongoDB sharded cluster instance, and ensure that all shards use the same account and password. For instructions, see Apply for a shard endpoint.

If the destination MongoDB instance is a sharded cluster instance, make the necessary preparations depending on how you create the schema.
- Not using the schema synchronization feature of DTS
  
  In the Configure Objects step, do not select Schema Synchronization for Synchronization Types. In the destination ApsaraDB for MongoDB instance, create the databases and collections for sharding, and configure data sharding. We recommend that you enable the balancer and perform pre-sharding. For more information, see Configure data sharding to maximize shard performance and How do I handle uneven data distribution in a sharded MongoDB cluster?.
- Using the schema synchronization feature of DTS
  
  In the Configure Objects step, select Schema Synchronization for Synchronization Types. After schema synchronization, we recommend that you enable the balancer and perform pre-sharding. For more information, see How do I handle uneven data distribution in a sharded MongoDB cluster?.
Note
Configuring data sharding prevents data from being synchronized to a single shard, which ensures high cluster performance. Enabling the balancer and performing pre-sharding helps prevent data skew.

Limitations

Type	Description
Source and destination database limits	Bandwidth requirement: The server where your source database resides must have sufficient outbound bandwidth. Otherwise, synchronization speed may be affected. The collections to synchronize must have a primary key or UNIQUE constraint, and the field values must be unique. Otherwise, duplicate data may appear in the destination database. Field names in the data must not contain the "." (dot) character; otherwise, data inconsistencies may occur. The _id field in the collections to synchronize must have unique values. Otherwise, data inconsistency may occur. If you synchronize at the collection level and need to edit objects (such as rename collections), one synchronization task supports up to 1,000 collections. If you exceed this limit, the task fails with an error after submission. To resolve this, split the collections into batches and configure multiple tasks. Or configure a full-database synchronization task. A single document in your source database cannot exceed 16 MB. Otherwise, the task fails. Your source database cannot be Azure Cosmos DB for MongoDB or Amazon DocumentDB (elastic cluster). Your source database must enable Oplog and retain Oplog for at least seven days. Or enable Change Streams and ensure DTS can subscribe to data changes in the last seven days through Change Streams. Otherwise, DTS may fail to capture data changes, causing task failure. In extreme cases, this may cause data inconsistency or loss. These issues are not covered by the DTS Service-level agreement (SLA). Important We recommend using Oplog to capture data changes. Only MongoDB 4.0 and later support Change Streams. Change Streams do not support two-way synchronization. If your source database is Amazon DocumentDB (non-elastic cluster), manually enable Change Streams. When you configure the task, set Migration Method to ChangeStream and set Architecture to Sharded Cluster. If DTS uses Oplog for incremental synchronization, scaling the number of shards is not supported for a source MongoDB sharded cluster. Otherwise, DTS tasks may fail and cause data inconsistency. If your source instance is a self-managed MongoDB sharded cluster: Access Method supports only Express Connect, VPN Gateway, or Smart Access Gateway and Cloud Enterprise Network (CEN). If your MongoDB version is 8.0 or later and Migration Method is Oplog, ensure the Shard account used by the synchronization task has the `directShardOperations` permission. Add this permission using the command `db.adminCommand({ grantRolesToUser: "username", roles: [{ role: "directShardOperations", db: "admin"}]})`. Note Replace `username` in the command with the Shard account used by the synchronization task. If Migration Method is Oplog and the task includes full data synchronization, ensure the Mongos account for your source MongoDB sharded cluster has permission to run the `db.runCommand({"balancerStatus":1})` command. DTS checks whether the Balancer is disabled using this command during precheck. The number of Mongos nodes in your source MongoDB sharded cluster instance cannot exceed 10. If the collections to synchronize contain TTL indexes, data inconsistency or latency may occur. Ensure no orphaned documents exist in the source and destination instances. Otherwise, data inconsistency or task failure may occur. For more information, see orphaned documents and How to clean orphaned documents in MongoDB (sharded cluster architecture). Source database operation limits: During schema synchronization and full data synchronization, do not change the schema of databases or collections (including array-type data updates). Otherwise, the synchronization task may fail or cause data inconsistency between the source and destination databases. If you perform only full data synchronization, do not write new data to the source instance. Otherwise, data inconsistency may occur between the source and destination databases. While the synchronization instance is running, do not run commands that change data distribution on the objects to synchronize in the source database (for example, shardCollection, reshardCollection, unshardCollection, moveCollection, or movePrimary). Otherwise, data inconsistency may occur. If the Balancer in your source database balances data, latency may occur. DTS does not support connecting to MongoDB databases using SRV addresses. If the source database is MongoDB 5.0 or later and the destination database version is earlier than 5.0, you cannot sync `capped collection`s. If you attempt to sync them, the task fails or data inconsistency occurs between the source and destination databases. This is because, starting from MongoDB 5.0, the behavior of `capped collection`s changed to allow operations such as explicit deletion and increasing document size during updates, and earlier versions of the database kernel cannot support these new features.
Other limits	DTS does not support synchronizing data from the admin, config, and local databases. We recommend using the same MongoDB version for the source and destination databases. Or use a higher version for the destination to ensure compatibility. Synchronizing from a higher version to a lower version may cause compatibility issues. If the destination instance uses ReplicaSet architecture: When Access Method is Express Connect, VPN Gateway, or Smart Access Gateway or Cloud Enterprise Network (CEN), you must enter the address and port of the primary node for Domain Name or IP and Port Number, or configure a high-availability connection address. For more information about high-availability connection addresses, see Create an instance with a high-availability MongoDB source or destination database. When Access Method is Self-managed Database on ECS, enter the port of the primary node in Port Number. If the target MongoDB is a sharded cluster instance and you do not need to use the schema synchronization feature provided by DTS (for example, if data sharding is already configured on the target), do not select Schema Synchronization for Synchronization Types on the Configure Objects page. Otherwise, sharding conflicts may cause data inconsistency or task failure. Before starting the task, add the shard key used by the destination to the data in the source. After starting the task, INSERT statements for the data to synchronize must include the shard key. UPDATE statements cannot modify the shard key. Transaction information is not preserved. Transactions in the source database become individual records in the destination database. When DTS writes data to the destination collection, if a primary key or unique key conflict occurs, DTS skips the write statement and keeps existing data in the destination collection. If your source MongoDB version is earlier than 3.6 and your destination MongoDB version is 3.6 or later, differences in execution plans may cause inconsistent field ordering in synchronized data. Field-value relationships remain unchanged. If your business uses text-matching queries on nested structures, assess the impact of inconsistent field ordering. Evaluate the performance of both the source and destination databases before synchronization. Run synchronization during off-peak hours. Otherwise, full data initialization consumes read and write resources, increasing database load. Full initialization runs INSERT operations concurrently. This creates fragmentation in the destination database collection. After full initialization, the collection space in the destination instance is larger than in the source instance. If the destination collection has a unique index or its `capped` property is set to `true`, concurrent replay is not supported during incremental synchronization. Only single-threaded writes are allowed. This may increase task latency. Because DTS writes data concurrently, the storage space used by the destination is 5% to 10% larger than the source. To get the count of documents in the destination MongoDB, use the syntax `db.$table_name.aggregate([{ $count:"myCount"}])`. Ensure the destination MongoDB does not have the same primary key (default is _id) as the source. Otherwise, data loss may occur. If the destination has the same primary key, clear related data in the destination without affecting your business (delete documents with the same _id value in the destination). Disable the Balancer in your source MongoDB database during full synchronization. Keep it disabled until all subtasks reach the incremental phase. Otherwise, data inconsistency may occur. For more information about managing the Balancer, see Manage the MongoDB Balancer. If your destination database is a MongoDB sharded cluster, ensure your business behavior meets MongoDB's requirements for sharded collections after switching traffic to this database. DTS does not support synchronizing time-series collections introduced in MongoDB 5.0 and later. If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters. Note Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.

Billing

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

One-way data synchronization topologies

Data Transmission Service (DTS) currently supports one-way data synchronization only between two ApsaraDB for MongoDB instances that use a sharded cluster architecture. Synchronization among multiple ApsaraDB for MongoDB instances is not supported.

Synchronization types

Synchronization type	Description
Schema synchronization	Synchronizes the schema of the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.
Full data synchronization	Synchronizes the historical data of the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note Full data synchronization is supported for databases and collections.
Incremental data synchronization	Synchronizes incremental updates from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance after a full data synchronization. Oplog Incremental data synchronization does not replicate databases created after the task starts. The following changes are replicated: CREATE COLLECTION and CREATE INDEX DROP DATABASE, DROP COLLECTION, and DROP INDEX RENAME COLLECTION Note The RENAME COLLECTION operation is not replicated if the `dropTarget` option is set to `true`. Document inserts, updates, and deletes within a collection. Note For document updates, it replicates only operations that use the `$set` command. Change stream The following changes are replicated: DROP DATABASE and DROP COLLECTION RENAME COLLECTION Note The RENAME COLLECTION operation is not replicated if the `dropTarget` option is set to `true`. Document inserts, updates, and deletes within a collection. Note For document updates, it replicates only operations that use the `$set` command.

Clean up orphaned documents

Before you migrate data, you must clean up the orphaned documents from the source MongoDB database.

Important

If you do not clean up orphaned documents, migration performance may be degraded, and conflicting _id values may cause data migration errors.

ApsaraDB for MongoDB

Note

Running the cleanup script on an instance with a major version of MongoDB earlier than 4.2 or a minor version earlier than 4.0.6 causes an error. To view the current version of your instance, see MongoDB minor versions. To upgrade the major or minor version, see Upgrade the major version of a database and Upgrade the minor version of a database.

To clean up orphaned documents, use the cleanupOrphaned command. The usage of this command differs between MongoDB 4.4 and later and MongoDB 4.2 and earlier. The following sections describe the procedures.

MongoDB 4.4 and later

On a server that can connect to the sharded cluster instance, create a JavaScript (JS) script named cleanupOrphaned.js.

Note

This script cleans up orphaned documents from all collections in multiple databases across multiple shards. To clean up orphaned documents from a specific collection, you can modify the JS script.

// A list of shard names.
var shardNames = ["shardName1", "shardName2"];
// A list of databases to process.
var databasesToProcess = ["database1", "database2", "database3"];
shardNames.forEach(function(shardName) {
    // Iterate over the specified databases.
    databasesToProcess.forEach(function(dbName) {
        var dbInstance = db.getSiblingDB(dbName);
        // Get the names of all collections in the database instance.
        var collectionNames = dbInstance.getCollectionNames();
        // Iterate over each collection.
        collectionNames.forEach(function(collectionName) {
            // The full name of the collection.
            var fullCollectionName = dbName + "." + collectionName;
            // Build the cleanupOrphaned command.
            var command = {
                runCommandOnShard: shardName,
                command: { cleanupOrphaned: fullCollectionName }
            };
            // Run the 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 values of the shardNames and databasesToProcess parameters in the script.

shardNames: An array of IDs of the shards from which you want to clean up orphaned documents. You can obtain the shard IDs from the Shard List section on the Basic Information page of the instance. Example: d-bp15a3796d3a****.
databasesToProcess: An array of names for the databases that require cleanup.

In the directory where the cleanupOrphaned.js script is stored, run the following command.

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. Example: `s-bp14423a2a51****.mongodb.rds.aliyuncs.com`.
`<Primaryport>`	The port number of the mongos node of the sharded cluster instance. The default value is 3717.
`<database>`	The name of the authentication database. This is the database where the specified database account is defined.
`<username>`	The database account.
`<password>`	The password for the database account.
`output.txt`	The destination file for the execution results.

MongoDB 4.2 and earlier

On a server that can connect to the sharded cluster instance, create a JS script named cleanupOrphaned.js.

Note

This script cleans up orphaned documents from a specific collection in a specific database across multiple shards. To clean up orphaned documents from multiple collections, you can either run the script multiple times with a modified fullCollectionName parameter or modify the script to iterate through the 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 values of the shardNames and fullCollectionName parameters in the script.

shardNames: An array of IDs of the shards from which you want to clean up orphaned documents. You can obtain the shard IDs from the Shard List section on the Basic Information page of the instance. Example: d-bp15a3796d3a****.
fullCollectionName: The name of the collection to clean up, in the format database.collection.

In the directory where the cleanupOrphaned.js script is stored, run the following command.

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. Example: `s-bp14423a2a51****.mongodb.rds.aliyuncs.com`.
`<Primaryport>`	The port number of the mongos node of the sharded cluster instance. The default value is 3717.
`<database>`	The name of the authentication database. This is the database where the specified database account is defined.
`<username>`	The database account.
`<password>`	The password for the database account.
`output.txt`	The destination file for the execution results.

Self-managed MongoDB

On a server that can connect to your self-managed MongoDB database, download the cleanupOrphaned.js script file.

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

Modify the cleanupOrphaned.js script file. Replace test with the name of the database to clean up.

Important

If you have multiple databases, you must repeat Step 2 and Step 3 for each database.

function cleanupOrphaned(coll) {
  var nextKey = { };
  var result;
  while ( nextKey != null ) {
    result = db.adminCommand( { cleanupOrphaned: coll, startingFromKey: nextKey } );
    if (result.ok != 1)
       print("Unable to complete at this time: failure or timeout.")
    printjson(result);
    nextKey = result.stoppedAtKey;
  }
}
var dbName = 'test'
db = db.getSiblingDB(dbName)
db.getCollectionNames().forEach(function(collName) {
        cleanupOrphaned(dbName + "." + collName);
});

Run the following command to clean up orphaned documents from all collections in the specified database on a shard.

Note
You must repeat this step for each shard to clean up its orphaned documents.
```
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 authentication database. This is the database where the specified database account is defined.
- <username>: The database account.
- <password>: The password for the database account.
Example:

In this example, a self-managed MongoDB database has three shards. You must run the command on each shard to clean up its orphaned documents.
```
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

This topic provides an example of configuring a DTS task before purchasing the instance. In this scenario, you do not need to specify the number of shards for the source ApsaraDB for MongoDB sharded cluster. If you purchase a DTS instance before configuring the task, you must specify the correct number of shards during the purchase.

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

Configure the source and destination databases.

Category	Parameter	Description
N/A	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select MongoDB.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region of the source ApsaraDB for MongoDB instance.
	Replicate Data Across Alibaba Cloud Accounts	For this example, select No, as the database instance belongs to the current Alibaba Cloud account.
	Architecture	Select Sharded Cluster.
	Migration Method	Select a method for incremental data synchronization based on your requirements. Oplog (Recommended): This option is available if Oplog is enabled for the source database. Note Oplog is enabled by default for self-managed MongoDB databases and ApsaraDB for MongoDB instances. This method offers lower latency for incremental synchronization tasks because logs are retrieved faster. We recommend selecting Oplog. ChangeStream: This option is available if Change Streams are enabled for the source database. Note If the source database is an Amazon DocumentDB (non-elastic cluster) instance, you can select only ChangeStream. If you set Architecture of the source database to Sharded Cluster, you do not need to specify Shard account and Shard password.
	Instance ID	Select the ID of the source ApsaraDB for MongoDB instance.
	Authentication Database	Enter the name of the database to which the database account of the source ApsaraDB for MongoDB instance belongs. The default value is admin.
	Database Account	Enter the database account of the source ApsaraDB for MongoDB instance. The account must have read permissions on the databases to be synchronized, and on the config, admin, and local databases.
	Database Password	Enter the password for the specified database account.
	Shard account	Enter the account that is used to access the shards of the source ApsaraDB for MongoDB instance. Note If your source database is a self-managed MongoDB database, you must also specify the shard access information.
	Shard password	Enter the password for the shard account of the source ApsaraDB for MongoDB instance.
	Encryption	DTS supports three connection methods: Non-encrypted, SSL-encrypted, and Mongo Atlas SSL. The options for Encryption vary based on the selected Access Method and Architecture. The options displayed in the console prevail. Note A MongoDB database where the Architecture is Sharded Cluster and the Migration Method is Oplog does not support SSL-encrypted. If the source is a self-managed MongoDB database (Access Method is not Alibaba Cloud Instance) with a Replica Set architecture, and you select SSL-encrypted, DTS also allows you to upload a CA certificate to verify the connection.
Destination Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select MongoDB.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region of the destination ApsaraDB for MongoDB instance.
	Replicate Data Across Alibaba Cloud Accounts	For this example, select No, as the database instance belongs to the current Alibaba Cloud account.
	Architecture	Select the architecture of the destination ApsaraDB for MongoDB instance.
	Instance ID	Select the ID of the destination ApsaraDB for MongoDB instance.
	Authentication Database	Enter the name of the database to which the database account of the destination ApsaraDB for MongoDB instance belongs. The default value is admin.
	Database Account	Enter the database account of the destination ApsaraDB for MongoDB instance. The account must have the dbAdminAnyDatabase permission, the readWrite permission on the destination database, and the read permission on the local database.
	Database Password	Enter the password for the specified database account.
	Encryption	DTS supports three connection methods: Non-encrypted, SSL-encrypted, and Mongo Atlas SSL. The options for Encryption vary based on the selected Access Method and Architecture. The options displayed in the console prevail. Note MongoDB databases with an Architecture of Sharded Cluster do not support SSL-encrypted. If the destination is a self-managed MongoDB database (Access Method is not Alibaba Cloud Instance) with a Replica Set, and you select SSL-encrypted, DTS also allows you to upload a CA certificate to verify the connection.

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

Configure the task objects.

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

Parameter	Description
Synchronization Types	DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization. Important If the destination MongoDB instance is a sharded cluster and you do not need to use the schema synchronization feature of DTS (for example, data sharding is already configured on the destination), do not select Schema Synchronization. Otherwise, shard conflicts may cause data inconsistency or task failure.
Synchronization Topology	Select One-way Synchronization.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks if the destination database contains collections with the same names. If a collection with a duplicate name is found, an error is reported during the precheck phase and the data synchronization task does not start. Otherwise, the precheck is successful. Note If you cannot delete or rename the collection with the same name in the destination database, you can use the object name mapping feature to change the collection name in the destination database. For more information, see Set the name of a synchronization object in the destination instance. Ignore Errors and Proceed: Skips checking for collections with the same name in the destination database. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example: If a record in the destination database has the same primary key or unique key value as a record in the source database, the record in the destination database is retained. The record from the source database is not synchronized to the destination database. Data initialization may fail, only some data may be synchronized, or the synchronization may fail.
Capitalization of Object Names in Destination Instance	Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.
Source Objects	In the Source Objects box, click the objects, and then click to move them to the Selected Objects box. Note You can select objects at the DATABASE or COLLECTION level.
Selected Objects	To set the name of a synchronization object in the destination instance, right-click the object in the Selected Objects box to modify it. For more information, see Object name mapping. To remove a synchronization object, click it in the Selected Objects box, and then click to move it to the Source Objects box. Note To select incremental operations at the database or collection level, right-click the desired object in the Selected Objects box and make your selections in the dialog box that appears. To filter data (supported during full synchronization but not incremental synchronization), right-click the desired collection in the Selected Objects box and configure the settings in the dialog box that appears. For instructions, see Configure filter conditions. If you use the object name mapping feature to specify a database or collection to receive data, synchronization may fail for other objects that depend on the mapped object.

Click Next: Advanced Settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.
Retry Time for Failed Connections	If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails. Note If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances. DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.
Retry Time for Other Issues	If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.
Enable Throttling for Full Data Synchronization	During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s). Note This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
Only one data type for primary key _id in a table of the data to be synchronized	Specify whether the data types of the `_id` primary key are unique within each collection to be synchronized. Important Select an option based on your actual data. An incorrect selection may lead to data loss. This parameter is available only if Synchronization Types includes Full Data Synchronization. Yes: The data types are unique. During the full synchronization phase, DTS does not scan the data types of the primary keys in the source data. For each collection, DTS synchronizes only the data corresponding to one primary key data type. No: The data types are not unique. During the full synchronization phase, DTS scans the data types of the primary keys in the source data and synchronizes all the data.
Enable Throttling for Incremental Data Synchronization	You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).
Environment Tag	You can select an environment tag to identify the instance based on your business requirements. You do not need to select a tag in this example.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert notifications. For more information, see Configure monitoring and alerting during task configuration.

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

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

Purchase the instance.

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

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

Category	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay upfront for a specific duration. This is cost-effective for long-term, continuous tasks. Pay-as-you-go: You are billed hourly for actual usage. This is ideal for short-term or test tasks, as you can release the instance at any time to save costs.
	Resource Group Settings	The resource group to which the instance belongs. The default is default resource group. For more information, see What is Resource Management?.
	Instance Class	DTS offers synchronization specifications at different performance levels that affect the synchronization rate. Select a specification based on your business requirements. For more information, see Data synchronization link specifications.
	Subscription Duration	In subscription mode, select the duration and quantity of the instance. Monthly options range from 1 to 9 months. Yearly options include 1, 2, 3, or 5 years. Note This option appears only when the billing method is Subscription.

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

Prerequisites

Limitations

Billing

One-way data synchronization topologies

Synchronization types

Oplog

Change stream

Clean up orphaned documents

ApsaraDB for MongoDB

MongoDB 4.4 and later

MongoDB 4.2 and earlier

Self-managed MongoDB

Procedure

DTS console

DMS console