Migrate MongoDB without shard keys to a MongoDB sharded cluster architecture - Data Transmission Service

Data Transmission Service (DTS) enables the migration of data from a source MongoDB that lacks shard key fields to a destination MongoDB featuring a sharded cluster architecture, while allowing for the specification of default shard key values. This discussion focuses on the migration process, using ApsaraDB for MongoDB with a ReplicaSet architecture as the source and ApsaraDB for MongoDB with a sharded cluster architecture as the target.

Prerequisites

The destination ApsaraDB for MongoDB (sharded cluster architecture) is now established. For more information, see how to create a sharded cluster instance.
Note
- If the source is a sharded cluster architecture within ApsaraDB for MongoDB, you must obtain an endpoint for the shard nodes and ensure the accounts and passwords for each shard are consistent. For more information, see Requesting an Endpoint for Shard or ConfigServer Nodes.
- For a list of supported versions, please refer to the Migration Solutions Overview.
The storage capacity of the destination ApsaraDB for MongoDB should exceed that of the source ApsaraDB for MongoDB by at least 10%.
To meet business requirements, you can create the necessary databases and collections within ApsaraDB for MongoDB, set up data sharding, activate the Balancer, and initiate pre-sharding. For more information, see optimizing shard performance through data sharding and addressing uneven data distribution in MongoDB sharded cluster architectures.

Note
Configuring data sharding can prevent data from being migrated to the same Shard, which could affect cluster performance. Enabling the Balancer and pre-sharding can help avoid data skew issues.

Notes

Type	Description
Limits on the source database	Bandwidth requirements: The server to which the source database belongs must have sufficient outbound bandwidth. Otherwise, the data migration speed is affected. The collections to be migrated must have a primary key or UNIQUE constraint, and the fields must be unique. Otherwise, duplicate data may appear in the destination database. If the granularity of the migration object is a collection and editing is required (such as collection name mapping), a single migration task supports migrating up to 1,000 collections. If the number exceeds the limit, a request error is displayed after the task is submitted. In this case, it is recommended to split the collections to be migrated, configure multiple tasks in batches, or configure a task to migrate the entire database. The size of a single data entry in the source database cannot exceed 16 MB. Otherwise, the data migration task fails. If the source MongoDB is a sharded cluster instance, the number of source Mongos nodes cannot exceed 10. If the source database is an Azure Cosmos DB for MongoDB cluster or an Amazon DocumentDB elastic cluster, only full data migration is supported. To perform incremental data migration, you must make sure that the following requirements are met: The oplog feature is enabled for the source database and operation logs of the source database are retained for at least seven days. Alternatively, change streams are enabled for the source database and DTS can use change streams to subscribe to data changes in the source database within the last seven days. Otherwise, DTS may fail to obtain the operation logs and the data migration task may fail. In exceptional circumstances, data inconsistency or loss may occur. The service level agreement (SLA) of DTS does not cover these issues. Important We recommend that you use the oplog feature to obtain data changes in the source database. You can use change streams to obtain data changes in the source database only if the source database runs MongoDB V4.0 or later. If the source database is an inelastic Amazon DocumentDB cluster, you must enable change streams for the database and set the Migration Method parameter to ChangeStream and the Architecture parameter to Sharded Cluster when you configure the data migration task. Limits on operations to perform on the source database: During schema migration and full data migration, do not perform schema change on databases or collections, including the update of the array type. Otherwise, the data migration task fails, or data inconsistency between the source and destination databases may occur. If you perform only full data migration, do not write new data to the source instance during data migration. Otherwise, data inconsistency occurs between the source and destination databases. To ensure real-time data consistency, it is recommended to select schema migration, full data migration, and incremental data migration. You cannot migrate collections that contain time to live (TTL) indexes. If the source database contains TTL indexes, data inconsistency may occur between the source and destination databases after the migration. Ensure that there are no orphaned documents in the MongoDB instance with a sharded cluster architecture. Otherwise, data inconsistency or even task failure may occur. For more information, see Orphaned document and How to clean orphaned documents in MongoDB (sharded cluster architecture)?. If the source database is a MongoDB with a sharded cluster architecture and the Balancer in the source database has balancing data behavior, it may cause latency in the instance.
Other limits	New collections added to the source during migration do not support setting default values for shard keys. If the source is a non-sharded cluster and the destination is Alibaba Cloud MongoDB (sharded cluster architecture), it will enter the Configure Database and Table Fields stage. If the version of the destination MongoDB (sharded cluster architecture) is lower than 4.4, the default value of the ShardKey set in the Configure Database and Table Fields stage will take effect. DTS will fill the original data with the default value you set and write it to the destination. If the destination MongoDB (sharded cluster architecture) is version 4.4 or later, the default value set in the Configure Database and Table Fields stage will not take effect, and DTS will write the original data to the destination by default. It is recommended that the database versions of the source and destination MongoDB remain consistent, or migrate from a lower version to a higher version to ensure compatibility. If migrating from a higher version to a lower version, database compatibility issues may occur. Data in the admin and local databases is not supported for migration. Transaction information is not retained. When transactions in the source database are migrated to the destination database, they are converted into individual records. Before migrating data, evaluate the performance of the source and destination databases. It is also recommended to perform data migration during off-peak hours. Otherwise, during full data migration, DTS occupies certain read and write resources of the source and destination databases, which may increase the load on the database servers. Because full data migration performs concurrent INSERT operations, it causes fragmentation in the collections of the destination database. Therefore, after full migration is completed, the storage space of the collections in the destination database will be larger than that of the collections in the source instance. DTS attempts to resume tasks that failed within the last seven days. Therefore, before switching workloads to the destination instance, you must stop or release the task, or revoke the write permissions from the accounts that are used by DTS to access the destination instance. This prevents the task from being automatically resumed and the source data from overwriting the destination data. Because DTS writes data concurrently, the storage space occupied by the destination is 5% to 10% larger than that of the source. 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 migration. This may increase migration latency. The count number of the destination MongoDB needs to be queried using the `db.$table_name.aggregate([{ $count:"myCount"}])` syntax. Ensure that the destination MongoDB does not have the same primary key as the source (the default is _id). Otherwise, data may be lost. If the destination has the same primary key as the source, delete the corresponding data in the destination without affecting your business (delete the document in the destination that has the same _id as the source). If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified. Note Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.
Special cases	When the source database is a self-managed MongoDB: During migration, if the source database performs a primary-secondary switch, the migration task will fail. Because the latency time of DTS is derived from the comparison between the timestamp of the last piece of data migrated to the destination database and the current timestamp, if the source database does not perform update operations for a long time, the latency information may be inaccurate. If the task shows excessive latency, you can perform an update operation on the source database to update the latency information. Note If the migration object is selected as the entire database, you can also create a heartbeat that periodically updates or writes data every second.

Billing overview

Migration type	Link configuration fee	Data transfer cost
Schema migration and full data migration	Free of charge.	Data transfer costs are incurred when data is migrated out of Alibaba Cloud through the Internet. In this example, migration occurs between Alibaba Cloud instances, so no data transfer cost is charged. For more information, see Billable items.
Incremental data migration	Charged. For details, see Billing overview.

Migration types

Migration type	Description
Schema migration	Migrate the structure of the migration object from the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB. Note Objects that support schema migration include DATABASE, COLLECTION, and INDEX.
Full migration	Migrate all historical data of the migration object from the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB. Note Full migration supports data in DATABASE and COLLECTION.
Incremental migration	On the basis of full migration, migrate the incremental updates from the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB. Using Oplog Incremental migration does not support databases created after the task starts running. The supported incremental updates are as follows: CREATE COLLECTION, INDEX DROP DATABASE, COLLECTION, INDEX RENAME COLLECTION Insert, update, and delete documents in the collection. Note When migrating incremental update documents, only operations updated using the `$set` command are supported. Using ChangeStream The supported incremental updates are as follows: DROP DATABASE, COLLECTION RENAME COLLECTION Insert, update, and delete documents in the collection. Note When migrating incremental update documents, only operations updated using the `$set` command are supported.

Permissions required for database accounts

Database	Schema migration	Full migration	Incremental migration
Source ApsaraDB for MongoDB	The read permission on the source database and the config database.		The read permission on the source database, the admin database, and the local database.
Destination ApsaraDB for MongoDB	The dbAdminAnyDatabase permission, the readWrite permission on the destination database, the read permission on the local database, and the read permission on the config database.

For guidance on creating and authorizing database accounts for both source and destination ApsaraDB for MongoDB, refer to Managing MongoDB Database Users with DMS.

Procedure

Use one of the following methods to go to the Data Migration page and select the region in which the data migration instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Migration.
3. In the upper-left corner of the page, select the region in which the data migration instance resides.
DMS console
Note
The actual operation may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data + AI > DTS (DTS) > Data Migration .
3. From the drop-down list to the right of Data Migration Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.
Optional. Click New Configuration Page in the upper-right corner of the page.
Note
- Skip this step if the Back to Previous Version button is displayed in the upper-right corner of the page.
- Specific parameters in the new and previous versions of the configuration page may be different. We recommend that you use the new version of the configuration page.

Configure the source and destination databases. The following table describes the parameters.

Category	Configuration	Description
None	Task Name	The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select Existing Connection	The instance that you want to use. You can choose whether to use 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 use an existing instance, you must configure the database information below. Note You can register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	Select MongoDB.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region in which the source ApsaraDB for MongoDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database instance of the current Alibaba Cloud account is used. Select No.
	Architecture	In this example, select Replica Set. Replica Set: Achieves high availability and read/write splitting by deploying multiple nodes. For more information, see ReplicaSet architecture. Sharded Cluster: Provides three components: Mongos, Shard, and ConfigServer. You can freely choose the number and configuration of Mongos and Shard. For more information, see Sharded cluster architecture. Note If Architecture is selected as Sharded Cluster, you also need to fill in the Shard account and Shard password.
	Migration Method	Select the method for incremental data migration based on your actual situation. Oplog (recommended): If the source database has enabled Oplog, this option is supported. Note Local self-managed MongoDB and ApsaraDB for MongoDB have Oplog enabled by default. When migrating incremental data using this method, the delay of the incremental migration task is small (the speed of pulling logs is fast). Therefore, it is recommended to select Oplog. ChangeStream: If the source database has enabled Change Streams (Change Streams), this option is supported. Note When the source database is Amazon DocumentDB (non-elastic cluster), only ChangeStream is supported. When the Architecture of the source database is selected as Sharded Cluster, there is no need to fill in the Shard account and Shard password.
	Instance ID	Select the instance ID of the source ApsaraDB for MongoDB.
	Authentication Database	Enter the name of the database to which the database account of the source ApsaraDB for MongoDB instance belongs. If it has not been modified, it is the default admin.
	Database Account	Enter the database account of the source ApsaraDB for MongoDB instance. For permission requirements, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.
	Encryption	Specifies whether to encrypt the connection to the source database. You can select Non-encrypted, SSL-encrypted, or Mongo Atlas SSL based on your business requirements. The options available for the Encryption parameter are determined by the values selected for the Access Method and Architecture parameters. The options displayed in the DTS console prevail. Note If the Architecture parameter is set to Sharded Cluster, and the Migration Method parameter is set to Oplog for the ApsaraDB for MongoDB database, the Encryption parameter SSL-encrypted is unavailable. If the source database is a self-managed MongoDB database that uses the Replica Set architecture, the Access Method parameter is not set to Alibaba Cloud Instance, and the Encryption parameter is set to SSL-encrypted, you can upload a certification authority (CA) certificate to verify the connection to the source database.
Destination Database	Select Existing Connection	The instance that you want to use. You can choose whether to use 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 use an existing instance, you must configure the database information below. Note You can register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	Select MongoDB.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region in which the destination ApsaraDB for MongoDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database instance of the current Alibaba Cloud account is used. Select No.
	Architecture	In this example, select Sharded Cluster.
	Instance ID	Select the instance ID of the destination ApsaraDB for MongoDB.
	Authentication Database	Enter the name of the database to which the database account of the destination ApsaraDB for MongoDB instance belongs. If it has not been modified, it is the default admin.
	Database Account	Enter the database account of the destination ApsaraDB for MongoDB. For permission requirements, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.
	Encryption	Specifies whether to encrypt the connection to the destination database. You can select Non-encrypted, SSL-encrypted, or Mongo Atlas SSL based on your business requirements. The options available for the Encryption parameter are determined by the values selected for the Access Method and Architecture parameters. The options displayed in the DTS console prevail. Note If the destination database is an ApsaraDB for MongoDB instance and the Architecture parameter is set to Sharded Cluster, the Encryption parameter SSL-encrypted is unavailable. If the destination database is a self-managed MongoDB database that uses the Replica Set architecture, the Access Method parameter is not set to Alibaba Cloud Instance, and the Encryption parameter is set to SSL-encrypted, you can upload a CA certificate to verify the connection to the destination database.

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

Configure the objects to be migrated.

On the Configure Objects page, configure the objects that you want to migrate.

Configuration	Description
Migration Types	To perform only full data migration, select Schema Migration and Full Data Migration. To ensure service continuity during data migration, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If you do not select Schema Migration, make sure a database and a table are created in the destination database to receive data and the object name mapping feature is enabled in Selected Objects. If you do not select Incremental Data Migration, we recommend that you do not write data to the source database during data migration. This ensures data consistency between the source and destination databases.
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 names, the precheck is passed. Otherwise, an error is returned during the precheck and the data migration task cannot be started. Note If the source and destination databases contain collections with identical names and the collections in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the collections that are migrated to the destination database. For more information, see Map object names. 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 consistency is not ensured and your business may be exposed to potential risks. DTS does not migrate data records that have the same primary keys as data records in the destination database. Data may fail to be initialized, only specific columns are migrated, or the data migration task fails.
Capitalization of Object Names in Destination Instance	The capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to make sure that the capitalization of object names is consistent with that of 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. Click the icon to add the objects to the Selected Objects section. Note The granularity for selecting migration objects is database or collection.
Selected Objects	To specify the database name in the destination MongoDB (sharded cluster architecture) for receiving data: Right-click the target database under Selected Objects, and in the Edit Schema dialog box that appears, rename the Schema Name to match the destination MongoDB database name within the sharded cluster architecture designated for data reception. For more information, see the referenced document. To specify the collection name in the destination MongoDB (sharded cluster architecture) for receiving data: Right-click the collection under Selected Objects, and in the Edit Table dialog box that appears, rename the Table Name to match the collection name in the destination MongoDB with a sharded cluster architecture for data reception. Important Only when the granularity of the migration object is a collection can you specify the collection for receiving data. Note If you need to establish filter conditions for data (available during the full migration phase but not during the incremental migration phase), right-click the table you want to migrate in Selected Objects, and configure the filter conditions in the ensuing dialog box. For more information, see Set filter conditions. Using the object name mapping function (specifying the database or collection for receiving data) may cause the migration of other objects that depend on this object to fail.

Click Next: Advanced Settings to configure advanced settings.

Configuration	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the data migration task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data migration tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Retry Time for Failed Connections	The retry time range for failed connections. If the source or destination database fails to be connected after the data migration task is started, DTS immediately retries a connection within the retry time range. Valid values: 10 to 1,440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS is reconnected to the source and destination databases within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails. Note If you specify different retry time ranges for multiple data migration tasks that share the same source or destination database, the value that is specified later 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 the earliest opportunity after the source database and destination instance are released.
Retry Time for Other Issues	The retry time range for other issues. For example, if DDL or DML operations fail to be performed after the data migration task is started, DTS immediately retries the operations within the retry 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 retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails. Important The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Migration	Specifies whether to 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 enable throttling for full data migration based on your business requirements. To configure throttling, you must configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters. This reduces the loads of the destination database server. Note You can configure this parameter only if you select Full Data Migration for the Migration Types parameter.
Only one data type for primary key _id in a single table	For the data to be migrated, whether the data type of the primary key `_id` is unique within the same collection. Note This configuration is only available when the Migration Types is set to Full Data Migration. Yes: Unique. DTS does not scan the data type of the primary key in the source data during the full migration phase from the source database. No: Indicates non-uniqueness. During the full migration phase, DTS scans the data type of the primary key in the source data slated for migration from the source database.
Enable Throttling for Incremental Data Migration	Specifies whether to enable throttling for incremental data migration. To configure throttling, you must configure the RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) parameters. This reduces the loads of the destination database server. Note You can configure this parameter only if you select Incremental Data Migration for the Migration Types parameter.
Environment Tag	You can select an environment tag to identify the instance based on your actual situation. 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.
Monitoring and Alerting	Specifies whether to configure alerting for the data migration task. If the task fails or the migration latency exceeds the specified threshold, the alert contacts receive notifications. Valid values: No: does not configure alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see the Configure monitoring and alerting when you create a DTS task section of the Configure monitoring and alerting topic.

Click Next Step: Data Verification to configure the data verification task.
For more information about how to use the data verification feature, see Configure a data verification task.
Click Next: Configure Database and Table Fields to set the default values for ShardKey.
1. Click Set Default Value in the corresponding row of the target collection.
  
  Note
  If the Number of Shard Keys for a Table Name (collection) is 0, this means the target collection does not contain a shard key and setting a default value is unnecessary.
2. Select the Shard key default value type.
  
  Note
  The current Shard key default value type supports only string and int data types.
3. Set the Default Value for the ShardKey.
  Important
  - The default value set for the ShardKey will only take effect if the version of the destination MongoDB (sharded cluster architecture) is lower than 4.4.
  - You must assign default values to all ShardKeys within the migration objects; failing to do so will trigger an alert during the Precheck phase and could potentially cause the task to fail.

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

Purchase an instance.

Wait until Success Rate becomes 100%. Then, click Next: Purchase Instance.

On the Purchase Instance page, configure the Instance Class parameter for the data migration instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Resource Group	The resource group to which the data migration instance belongs. Default value: default resource group. For more information, see What is Resource Management?
New Instance Class	Instance Class	DTS provides instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Instance classes of data migration instances.

Read and agree to Data Transmission Service (Pay-as-you-go) Service Terms by selecting the check box.
Click Buy and Start. In the message that appears, click OK.
You can view the progress of the task on the Data Migration page.

Prerequisites

Notes

Billing overview

Migration types

Using Oplog

Using ChangeStream

Permissions required for database accounts

Procedure

DTS console

DMS console