How do I synchronize data from a MongoDB instance without a sharding key to a MongoDB sharded cluster instance? - Data Transmission Service

Data Transmission Service (DTS) allows you to synchronize data from a source MongoDB instance without a sharding key to a MongoDB instance that is deployed in a sharded cluster architecture. You can specify the default values of the sharding keys in the destination MongoDB instance. This topic describes how to synchronize data from an ApsaraDB for MongoDB replica set instance to an ApsaraDB for MongoDB sharded cluster instance by using DTS.

Prerequisites

The destination ApsaraDB for MongoDB sharded cluster instance is created. For more information, see Create a sharded cluster instance.
Note
- If the source ApsaraDB for MongoDB instance is deployed in a sharded cluster architecture, make sure that endpoints are assigned to the shards in the source ApsaraDB for MongoDB instance and the shards share the same account and password. For more information, see Apply for an endpoint for a shard or Configserver node.
- For more information about the supported database versions, see Overview of data synchronization scenarios.
The available storage space of the destination ApsaraDB for MongoDB instance is 10% larger than the total size of the data in the source ApsaraDB for MongoDB instance. This is a recommended prerequisite.
The databases and collections to be sharded in the destination ApsaraDB for MongoDB instance are created, sharding is configured based on your business requirements, and the balancer is enabled for pre-sharding. For more information, see Configure sharding to maximize the performance of shards and the How do I balance data distribution in a MongoDB sharded cluster instance? section in the "FAQ" topic.
Note
After you configure sharding for a sharded cluster instance, the synchronized data is distributed among different shards. This maximizes the performance of the sharded cluster. After you enable the balancer for pre-sharding, data skew is prevented.

Limits

Category	Description
Limits on the source database	Bandwidth requirements: The server to which the source database is deployed must have sufficient outbound bandwidth. Otherwise, the data synchronization speed is affected. The collections to be synchronized must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records. If you select collections as the objects to be synchronized and you want to modify the collections in the destination database, such as renaming collections, you can synchronize up to 1,000 collections 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. If the source MongoDB instance is deployed in a sharded cluster architecture, the number of Mongos nodes in the instance cannot exceed 10. The operation logging feature must be enabled for the destination ApsaraDB for MongoDB instance. Otherwise, error messages are returned during the precheck and the data synchronization task cannot be started. Note The operation logs of the source database must be retained for at least seven days. Otherwise, DTS may fail to obtain the operation logs, which causes the task to fail, or even data inconsistency and data loss. Make sure that you set the retention period of operation logs based on the preceding requirements. Otherwise, the service level agreement (SLA) of DTS does not guarantee service reliability or performance. Limits on operations to be performed on the source database: During the schema synchronization and full data synchronization stages of a data synchronization task, do not change the schemas of the databases or collections. Otherwise, the data synchronization task fails. If you perform only full data synchronization, do not write data to the source database during data synchronization. Otherwise, data inconsistency may occur between the source and destination databases. To ensure data consistency, we recommend that you select schema synchronization, full data synchronization, and incremental data synchronization as the synchronization types. You cannot synchronize collections that contain time to live (TTL) indexes. If the database to be synchronized contains TTL indexes, data inconsistency may occur between the source and destination databases due to inconsistent time zones and time clocks of the source and destination databases. Make sure that orphaned documents are deleted in the destination MongoDB sharded cluster instance. Otherwise, data inconsistency may occur, or even the task may fail. For more information, see Glossary of MongoDB official documentation and the How do I delete orphaned documents of a MongoDB database deployed in the sharded cluster architecture? section of the "FAQ" topic.
Other limits	You cannot specify the default values of the sharding keys for the new collections that are added to the source ApsaraDB for MongoDB instance when the data synchronization is in progress. If the source ApsaraDB for MongoDB instance is not deployed in a sharded cluster architecture and the destination ApsaraDB for MongoDB instance is deployed in a sharded cluster architecture, you must configure the required parameters in the Configure Database and Table Fields step. If the version of the destination ApsaraDB for MongoDB sharded cluster instance is earlier than 4.4, the default values of the sharding keys that you configure in the Configure Database and Table Fields step take effect. DTS adds the default values to the original data before the data is written to the destination ApsaraDB for MongoDB instance. If the version of the destination ApsaraDB for MongoDB sharded cluster instance is 4.4 or later, the default values of the sharding keys that you configure in the Configure Database and Table Fields step do not take effect. By default, DTS writes the original data to the destination ApsaraDB for MongoDB instance. 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. 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. 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 the 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. During data synchronization, we recommend that you use only DTS to write data to the destination database. This prevents data inconsistency between the source and destination databases. DTS attempts to resume data synchronization tasks that failed within the last seven days. Before you switch workloads to the destination instance, you must stop or release the failed tasks. You can also revoke the write permissions from the accounts that are used by DTS to access the destination instance. Otherwise, the data in the source instance overwrites the data in the destination instance after a failed task is resumed. 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 ApsaraDB for MongoDB database. Make sure that the destination ApsaraDB for MongoDB instance does not have the same primary key as that in 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.
Special cases	If the source database is a self-managed MongoDB database, take note of the following limits: If you perform a primary/secondary switchover on the source database when the data synchronization task is running, the task fails. DTS calculates the synchronization latency based on the timestamp of the latest synchronized data in the destination database and the current timestamp in the source database. If no update operation is performed on the source database for an extended period of time, the synchronization latency may be inaccurate. If the latency of the synchronization task is excessively high, you can perform an update operation on the source database to update the latency. Note If you select an entire database as the object to be synchronized, you can create a heartbeat table. The heartbeat table is updated or receives data every second.

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.

Synchronization types

Synchronization type	Description
Schema synchronization	DTS synchronizes the schemas of objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.
Full data synchronization	DTS synchronizes all existing data of objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS supports full data synchronization for the following types of objects: database and collection.
Incremental data synchronization	DTS synchronizes incremental data from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS does not support incremental data synchronization 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

Permissions required for database accounts

Database	Required permission	References
Source ApsaraDB for MongoDB instance	The read permission on the source, admin, and local databases.	Manage the permissions of MongoDB database users
Destination ApsaraDB for MongoDB instance	The dbAdminAnyDatabase permission, the read and write permissions on the destination database, and the read permissions on the local database and the config database.	Manage the permissions of MongoDB database users

Procedure

Go to the Data Synchronization Tasks page.
1. Log on to the DMS console.
2. In the top navigation bar, click DTS.
3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
Note
- The actual operation may vary depending on the mode and layout of DMS. For more information, see Simple mode and Customize the layout and style of the DMS console.
- You can also go to the Data Synchronization Tasks page of the new DTS console.
From the drop-down list to the right of Data Synchronization Tasks, select the region in which you want to create the data synchronization task.
Note
If you use the new DTS console, select the region in which you want to create the data synchronization task in the upper-left corner of the page.

Click Create Task. On the Create Task page, configure the source and destination databases. The following table describes the parameters.

Section	Parameter	Description
N/A	Task Name	The name of the task. DTS automatically assigns a name to the task. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select an existing DMS database instance. (Optional. If you have not registered a DMS database instance, ignore this option and configure database settings in the section below.)	The database instance that you want to use. You can choose whether to select an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
	Database Type	The type of the source database. Select MongoDB.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source ApsaraDB for MongoDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
	Architecture	The architecture of the source ApsaraDB for MongoDB instance. In this example, Replica Set is selected. Valid values: Replica Set: You can deploy multiple types of nodes on a replica set instance to implement high availability and read/write splitting. For more information, see Replica set instances. Sharded Cluster: A sharded cluster instance consists of three types of components: mongos, shard, and Configserver nodes. You can customize the configurations and number of mongos and shard nodes. For more information, see Sharded cluster instances.
	Instance ID	The ID of the source ApsaraDB for MongoDB instance.
	Authentication Database	The name of the authentication database that stores the database accounts and passwords of the source ApsaraDB for MongoDB instance. If you did not change the name of the authentication database before, the default value is admin.
	Database Account	The database account of the source ApsaraDB for MongoDB instance. For information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
	Database Password	The password of the database account.
Destination Database	Select an existing DMS database instance. (Optional. If you have not registered a DMS database instance, ignore this option and configure database settings in the section below.)	The database instance that you want to use. You can choose whether to select an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must manually configure parameters for the database.
	Database Type	The type of the destination database. Select MongoDB.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination ApsaraDB for MongoDB instance resides.
	Architecture	The architecture of the destination ApsaraDB for MongoDB instance. In this example, Sharded Cluster is selected.
	Instance ID	The ID of the destination ApsaraDB for MongoDB instance.
	Authentication Database	The name of the authentication database that stores the database accounts and passwords of the destination ApsaraDB for MongoDB instance. If you did not change the name of the authentication database before, the default value is admin.
	Database Account	The database account of the destination ApsaraDB for MongoDB instance. For more information about the required permissions, see the Permissions required for database accounts section of this topic.
	Database Password	The password of the database account.

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

Configure the objects to be synchronized and the advanced settings.

Parameter	Description
Synchronization Types	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 selected objects from the source instance to the destination instance. The historical data is the basis for subsequent incremental synchronization.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains collections that have the same names as 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 You can use the object name mapping feature to rename the collections that are synchronized to the destination database. You can use this feature if the source and destination databases contain collections that have identical names and the collections in the destination database cannot be deleted or renamed. 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. DTS does not synchronize data records that have the same primary key values as data records in the destination database. Data may fail to be initialized, only some columns are synchronized, or the data synchronization task fails.
Synchronization Topology	The synchronization topology of the data synchronization task. Select One-way Synchronization.
Capitalization of Object Names in Destination Instance	The capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to ensure that the capitalization of object names is consistent with that in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
Source Objects	Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section. Note You can select databases or collections as the objects to be synchronized.
Selected Objects	To specify the database to which the data is synchronized in the destination ApsaraDB for MongoDB sharded cluster instance, perform the following operations: Right-click a database in the Selected Objects section. In the Edit Schema dialog box, enter the name of the database to which the data is synchronized in the destination ApsaraDB for MongoDB sharded cluster instance in the Schema Name field. For more information, see Map object names. To specify the collection to which the data is synchronized in the destination ApsaraDB for MongoDB sharded cluster instance, perform the following operations: Right-click a collection in the Selected Objects section. In the Edit Table Name dialog box, enter the name of the collection to which the data is synchronized in the destination ApsaraDB for MongoDB sharded cluster instance in the Table Name field. Important You can select a collection to receive data only if you select collections as the objects to be synchronized. Note To specify conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the 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 Set 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 are dependent on the database or collection may fail to be synchronized.

Click Next: Advanced Settings to configure advanced settings.

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

Advanced Settings

Parameter	Description
Select the dedicated cluster used to schedule the task	By default, DTS schedules tasks to shared clusters. You do not need to configure this parameter. You can purchase a dedicated cluster of specified specifications to run DTS synchronization tasks. For more information, see What is a DTS dedicated cluster.
Set Alerts	Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts will receive notifications. Valid values: No: does not configure alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert contacts. For more information, see Configure monitoring and alerting for a new DTS task.
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 the 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 set different retry time ranges for multiple DTS tasks that have the same source or destination database, the shortest retry time range that is set takes precedence. When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
The wait time before a retry when other issues occur in the source and destination databases.	The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set the parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Important The value of the The wait time before a retry when other issues occur in the source and destination databases parameter must be smaller than the value of the Retry Time for Failed Connection parameter.
Enable Throttling for Full Data Migration	During full data migration, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and BPS of Full Data Migration parameters for full data synchronization tasks to reduce the pressure on the destination database. Note This parameter can be configured only if Full Data Synchronization is selected as Synchronization Types.
Enable Throttling for Incremental Data Synchronization	You can configure the RPS of Incremental Data Migration and BPS of Incremental Data Migration parameters for incremental data synchronization tasks to reduce the pressure on the destination database.
Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, no environment tag is selected.
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.

Click Next: Configure Database and Table Fields to specify the default values of the sharding keys.
1. Find the destination collection that you want to configure and click Set Default Value in the Actions column.
  Note
  If the value of the Number of Shard Keys parameter of a collection is 0, the collection does not have a sharding key. You can skip this step.Table Name
2. Configure the Shard key default value type parameter.
  Note
  You can select string or int as the value of the Shard key default value type parameter.
3. Configure the Default Value parameter.
  Important
  - The Default Value parameter that you configure takes effect only if the version of the destination ApsaraDB for MongoDB sharded cluster instance is earlier than 4.4.
  - You must specify the default value of each sharding key for the collections to be synchronized. Otherwise, an alert is triggered during the precheck and the data synchronization task may fail.Precheck
Click Next: Save Task Settings and Precheck in the lower part of the page.
You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameter settings of the API operation that is called to configure the instance.
Note
- Before you can start the data synchronization task, DTS performs a precheck. You can start the data synchronization task only after the task passes the precheck.
- If the task fails to pass the precheck, click View Details next to each failed item. After you troubleshoot the issues based on the causes, run a precheck again.
- If an alert is triggered for an item during the precheck:
  If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.
  If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur, and your business may be exposed to potential risks.
Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.

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

Section	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay for the instance when you create an instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. Pay-as-you-go: A pay-as-you-go instance is charged 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 instance, you can release the pay-as-you-go instance to reduce costs.
	Resource Group	The resource group on which the instance is run. Default value: default resource group. For more information, see What is Resource Management?.
	Instance Class	DTS provides various synchronization specifications that provide different performance. The synchronization speed varies based on the synchronization specifications that you select. You can select a synchronization specification based on your business scenario. For more information, see Specifications of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, set the subscription duration and the number of instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years. Note This parameter is available only if you select the Subscription billing method.

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