Synchronize a non-sharded database to a sharded cluster - Data Transmission Service

Prerequisites

Create a destination ApsaraDB for MongoDB sharded cluster instance. For more information, see Create a sharded cluster instance.
Note
- If the source is an ApsaraDB for MongoDB sharded cluster instance, an endpoint must be available for each shard, and all shards must use the same account and password. For instructions, see Apply for a shard endpoint.
- For a list of supported versions, see Synchronization scenario overview.
The destination ApsaraDB for MongoDB instance must have more storage space than the source ApsaraDB for MongoDB instance. The destination instance should have at least 10% more storage space.
Create the required databases and collections, configure sharding, enable the balancer, and perform pre-sharding in the destination ApsaraDB for MongoDB instance. For more information, see Configure sharding to maximize shard performance and How do I balance data distribution in a MongoDB sharded cluster instance?.

Note
Configuring sharding distributes data across multiple shards to maximize cluster performance. Enabling the balancer and performing pre-sharding helps prevent data skew.

Limitations

Type	Description
Source 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. 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. If your source MongoDB is a sharded cluster instance, the number of Mongos nodes cannot exceed 10. 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. 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. 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. To maintain real-time consistency, we recommend selecting schema, full, and incremental data synchronization. If your source MongoDB is a sharded cluster instance, 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. During synchronization, do not update the shard key field in the source. Otherwise, the DTS task may fail. If the collections to synchronize contain TTL indexes, data inconsistency or latency may occur. Ensure no orphaned documents exist in your MongoDB sharded cluster instance. 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). If your source MongoDB is a sharded cluster and the Balancer balances data, latency may occur.
Other limits	For collections added in the source during synchronization, setting a default value for the shard key is not supported. DTS does not support connecting to MongoDB databases using SRV addresses. 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 the source is not a sharded cluster and the destination is an Alibaba Cloud MongoDB sharded cluster, the task proceeds to the Configure Database and Table Fields step. If the target MongoDB instance (sharded cluster architecture) is a version earlier than 4.4, the default ShardKey value that you set in the Configure Database and Table Fields step takes effect. DTS then pads the source data with this default value and writes the data to the destination instance. If the target instance is version 4.4 or later, the default value that you set in the Configure Database and Table Fields step does not take effect, and DTS writes the source data to the destination instance by default. 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. DTS does not support synchronizing data from the admin, config, and local databases. 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. Transaction information is not preserved. Transactions in the source database become individual records in the destination database. 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 synchronization consumes read and write resources, increasing database load. Full data synchronization runs INSERT operations concurrently. This creates fragmentation in the destination database collection. After full synchronization, the collection storage space in the destination database is larger than in the source instance. Do not write data to the destination database except through DTS during synchronization. Otherwise, data inconsistency may occur between the source and destination databases. DTS attempts to recover failed synchronization tasks within seven days. Before switching traffic to the destination instance, end or release the task. Or revoke write permissions for the DTS account on the destination instance. This prevents automatic recovery from overwriting destination data with source data. 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). 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. If you switch traffic to the destination MongoDB database, ensure your business behavior meets MongoDB's requirements for sharded collections. DTS does not support synchronizing time-series collections introduced in MongoDB 5.0 and later. 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.
Special cases	If your source database is a self-managed MongoDB: If a primary/secondary switchover occurs in the source database during synchronization, the task fails. DTS calculates latency by comparing the timestamp of the last synchronized document with the current timestamp. If the source database has no updates for a long time, latency information may be inaccurate. If latency appears too high, run an update operation in the source database to refresh latency information. Note If you select full-database synchronization, create a heartbeat table. Update or write to this table every second.

Billing

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

Synchronization types

Type	Description
Schema synchronization	Replicates the schemas of selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.
Full data synchronization	Replicates all historical data for the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note Full data synchronization is supported at the database and collection levels.
Incremental data synchronization	Replicates ongoing changes from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance after full data synchronization completes. 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.

Database account permissions

Database	Required permissions	Authorization method
Source ApsaraDB for MongoDB instance	`read` permission on the databases you are synchronizing, and on the `config`, `admin`, and `local` databases.	Use DMS to manage MongoDB database accounts
Destination ApsaraDB for MongoDB instance	`dbAdminAnyDatabase` permission, `readWrite` permission on the destination database, and `read` permission on the `local` and `config` databases.	Use DMS to manage MongoDB database accounts

Procedure

Go to the data synchronization task list page in the destination region. You can do this in one of two ways.
DTS console
1. Log on to the DTS console.
2. In the navigation pane on the left, click Data Synchronization.
3. In the upper-left corner of the page, select the region where the synchronization instance is located.
DMS console
Note
The actual steps may vary depending on the mode and layout of the DMS console. For more information, see Simple mode console and Customize 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.

Section	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	In this example, synchronization occurs within a single Alibaba Cloud account. Select No.
	Architecture	In this example, select Replica Set. Replica Set: This architecture uses multiple nodes to provide high availability and read/write splitting. For more information, see replica set architecture. Sharded Cluster: This architecture provides Mongos, Shard, and ConfigServer components. You can customize the number and configurations of Mongos and Shard nodes. For more information, see sharded cluster architecture.
	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 for the database account of the source ApsaraDB for MongoDB instance. The default is admin.
	Database Account	Enter the database account for the source ApsaraDB for MongoDB instance. For information about the required permissions, see Permissions required for database accounts.
	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 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	In this example, select Sharded Cluster.
	Instance ID	Select the ID of the destination ApsaraDB for MongoDB instance.
	Authentication Database	Enter the name of the database for the database account on the target ApsaraDB for MongoDB instance. The default is admin.
	Database Account	Enter the database account for the destination ApsaraDB for MongoDB instance. For information about the required permissions, see Permissions required for database accounts.
	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.
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.
Synchronization Topology	Select One-way Synchronization.
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 synchronize databases or collections.
Selected Objects	To specify the name of the destination database in the ApsaraDB for MongoDB sharded cluster: Right-click the database under Selected Objects. In the Edit Schema dialog box, change the Schema Name to the desired database name in the destination ApsaraDB for MongoDB sharded cluster. For more information, see Map object names. To specify the name of the destination collection in the ApsaraDB for MongoDB sharded cluster: Right-click the collection under Selected Objects. In the Edit Table dialog box, change the Table Name to the desired collection name in the destination ApsaraDB for MongoDB sharded cluster. Important You can specify a destination collection only when you select collections as the synchronization objects. Note To set filter conditions (supported for full synchronization but not for incremental synchronization), right-click the table under Selected Objects and specify the conditions. For more information, see Set filter conditions. If you use the object name mapping feature to specify a destination database or collection, other objects that depend on the mapped object may fail to synchronize.

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. In this example, no tag is selected.
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.
Click Next: Configure Database and Table Fields.
1. In the row of the destination collection, click Set Default Value.
  
  Note
  If the Number of Shard Keys for a Table Name (collection) is 0, it means the destination collection does not have a sharding key, and you do not need to set a default value.
2. Select a Shard key default value type.
  
  Note
  Currently, only string and int are supported for Shard key default value type.
3. Set a Default Value for the ShardKey.
  Important
  - The default shard key value you set takes effect only if the version of the destination ApsaraDB for MongoDB sharded cluster is earlier than 4.4.
  - You must set default values for all shard keys of the objects to be synchronized. Otherwise, a warning is generated during the Precheck phase, which may even cause the task to fail.

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

Synchronization types

Oplog

Change stream

Database account permissions

Procedure

DTS console

DMS console