Migrate data from a MongoDB database that uses the replica set architecture to a MongoDB database that uses the replica set or sharded cluster architecture - Data Transmission Service

This topic describes how to migrate data from an ApsaraDB for MongoDB replica set instance to an ApsaraDB for MongoDB replica set or sharded cluster instance by using Data Transmission Service (DTS).

Supported source and destination databases

Source database that uses the replica set architecture	Destination database that uses the replica set or sharded cluster architecture
ApsaraDB for MongoDB database	ApsaraDB for MongoDB database
Self-managed database that is hosted on an Elastic Compute Service (ECS) instance	Self-managed database that is hosted on an ECS instance
Self-managed database that is connected over Express Connect, VPN Gateway, or Smart Access Gateway	Self-managed database that is connected over Express Connect, VPN Gateway, or Smart Access Gateway
Self-managed database with a public IP address	Self-managed database with a public IP address

In this example, the source database is an ApsaraDB for MongoDB replica set instance and the destination database is an ApsaraDB for MongoDB replica set or sharded cluster instance. You can also follow the procedure to configure data migration tasks for other types of MongoDB databases.

Prerequisites

The source ApsaraDB for MongoDB replica set instance and the destination ApsaraDB for MongoDB replica set or sharded cluster instance are created. For more information, see Create a replica set instance and Create a sharded cluster instance.
Note
For more information about the supported database versions, see Overview of data migration 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.
If the destination ApsaraDB for MongoDB instance is a sharded cluster instance, databases and collections to be sharded are created, data sharding is configured, the balancer is enabled, and pre-sharding is performed in the destination ApsaraDB for MongoDB instance based on your business requirements. For more information, see Configure sharding to maximize the performance of shards and the What do I do if the data of a MongoDB database deployed in the sharded cluster architecture is not evenly distributed? section of the "FAQ" topic.
Note
After you configure sharding for a sharded cluster instance, the migrated data is distributed among different shards. This maximizes the performance of the sharded cluster. You can also enable the balancer and perform pre-sharding to prevent data skew.

Limits

Category	Description
Limits on the source database	Bandwidth requirements: The server on which the source database is deployed must have sufficient outbound bandwidth. Otherwise, the data migration speed decreases. The collections to be migrated 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 migrated and you need to edit collections in the destination database, such as renaming collections, up to 1,000 collections can be migrated in a single data migration task. If you run a task to migrate more than 1,000 collections, a request error occurs. In this case, we recommend that you configure multiple tasks to migrate the collections in batches or configure a task to migrate the entire database. If you want to migrate incremental data, make sure that the following requirements are met: The oplog feature is enabled. Otherwise, error messages are returned during the precheck, and the data migration task cannot be started. The oplogs of the source database must be retained for at least seven days. Otherwise, DTS may fail to obtain the oplogs, which causes the task to fail, or even data inconsistency and data loss. Make sure that you set the retention period of oplogs 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 schema migration and full data migration, do not perform schema change on databases or collections. Otherwise, the data migration task fails. If you perform only full data migration, do not write data to the source database during data migration. Otherwise, data inconsistency between the source and destination databases occurs. To ensure data consistency, we recommend that you select Schema Migration, Full Data Migration, and Incremental Data Migration as the migration types. You cannot migrate collections that contain time to live (TTL) indexes. If the database to be migrated contains TTL indexes, data inconsistency may occur between the source and destination databases due to inconsistent time zones and clocks of the source and destination databases.
Other limits	If the destination database is a sharded cluster database, take note of the following limits: Orphaned documents must be deleted. Otherwise, the migration performance is compromised. During data migration, if a `_id` conflict exists in the documents of the source and destination databases, data inconsistency may occur, or the data migration task may fail. Before you start the data migration task, you must add shard keys to the data to be migrated in the source database. If you cannot add shard keys to the data in the source database, you can migrate data from a MongoDB database without shard keys. For more information, see Migrate data from a MongoDB instance without a sharding key to a MongoDB sharded cluster instance. During the data migration, if you execute the INSERT statement to insert data into the data to be migrated, the data to be migrated must contain shard keys. If you execute the UPDATE statement to modify the data to be migrated, you cannot modify shard keys. To ensure compatibility, the version of the destination MongoDB database must be the same as or later than the version of the source MongoDB database. If the version of the destination database is earlier than the version of the source database, database compatibility issues may occur. DTS cannot migrate 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 migration. This may increase migration latency. Transaction information is not retained. When transactions are migrated to the destination database, the transactions are converted into a single record. Before you migrate data, evaluate the impact of data migration on the performance of the source and destination databases. We recommend that you migrate data during off-peak hours. During full data migration, 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 migration, concurrent INSERT operations cause fragmentation in the collections of the destination database. After full data migration is complete, the storage space for collections of the destination database is larger than that of the source database. Make sure that the precision settings for columns of the FLOAT or DOUBLE data type meet your business requirements. DTS uses the `ROUND(COLUMN,PRECISION)` function to retrieve values from columns of the FLOAT or DOUBLE data type. If you do not specify a precision, DTS sets the precision for columns of the FLOAT data type to 38 digits and the precision for columns of the DOUBLE data type to 308 digits. DTS attempts to resume data migration tasks that failed within the last seven days. Before you switch workloads to the destination database, you must stop or release the failed tasks. You can also execute the `REVOKE` statement to revoke the write permissions from the accounts that are used by DTS to access the destination database. Otherwise, the data in the source database overwrites the data in the destination database 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 MongoDB database. Make sure that the destination MongoDB database does not have the same primary key as the source database. The default primary key is _id. Otherwise, data may be lost. If the data in the destination database has the same primary key as that in the source database, clear the related data in the destination database without interrupting the services of DTS. For example, if the same primary key is _id, you can delete the data in the destination database that has the same _id as the source database.
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 migration task is running, the task fails. DTS calculates migration latency based on the timestamp of the latest migrated 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 migration latency may be inaccurate. If the latency of the migration 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 migrated, you can create a heartbeat table. The heartbeat table is updated or receives data every second.

Billing

Migration type	Instance configuration fee	Internet traffic fee
Schema migration and full data migration	Free of charge.	Charged only when data is migrated from Alibaba Cloud over the Internet. For more information, see Billing overview.
Incremental data migration	Charged. For more information, see Billing overview.

Migration types

Migration type	Description
Schema migration	DTS migrates the schemas of the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS supports schema migration for databases, collections, and indexes.
Full data migration	DTS migrates full data of objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS supports full data migration for databases and collections.
Incremental data migration	DTS migrates incremental data from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS migrates incremental data generated by the following operations: CREATE DATABASE, CREATE COLLECTION, and CREATE INDEX DROP DATABASE, DROP COLLECTION, and DROP INDEX RENAME COLLECTION The insert, update, and delete operations of documents on collections

Permissions required for database accounts

Database type	Schema migration	Full data migration	Incremental data migration
Source ApsaraDB for MongoDB instance	The read permissions on the source database and the config database.		The read permissions on the source database, the admin database, and the local database.
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.

For more information about how to create a database account and grant permissions to the database account for an ApsaraDB for MongoDB instance, see Manage the permissions of MongoDB database users.

Procedure

Go to the Data Migration Tasks page.
1. Log on to the Data Management (DMS) console.
2. In the top navigation bar, move the pointer over DTS.
3. Choose DTS (DTS) > Data Migration.
Note
- The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
- You can also go to the Data Migration page of the new DTS console.
From the drop-down list on the right side of Data Migration Tasks, select the region in which your data migration instance resides.
Note
If you use the new DTS console, you must select the region in which the data migration instance resides in the upper-left corner.

Click Create Task. In the Create Data Migration Task wizard, configure the source and destination databases. The following table describes the parameters.

Warning

After you configure the source and destination databases, we recommend that you read the Limits that are displayed in the upper part of the page. Otherwise, the task may fail or data inconsistency may occur.

Section	Parameter	Description
N/A	Task Name	The name of the task. DTS automatically generates a task name. We recommend that you specify an informative name to identify the task. You do not need to specify a unique task name.
Source Database	Select an existing DMS database instance	The database 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 select an existing instance, you must configure the following database information.
	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 migrate data across Alibaba Cloud accounts. In this example, No is selected.
	Architecture	The architecture in which the source instance is deployed. Select Replica Set. 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 that is used to access the database instance.
Destination Database	Select an existing DMS database instance	The database 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 select an existing instance, you must configure the following database information.
	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 in which the destination instance is deployed. Select an architecture based on your instance information. 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 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 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 that is used to access the database instance.

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 IP address 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 make sure that the ECS instance can access the database. If the self-managed database is hosted on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. 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 IP address whitelist of the database to allow DTS to access the database. For more information, see the CIDR blocks of DTS servers section of the Add the CIDR blocks of DTS servers topic.
Warning
If the public CIDR blocks of DTS servers are automatically or manually added to the whitelist of a database instance or to the security group rules of an ECS instance, security risks may arise. Therefore, before you use DTS to migrate 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 security group rules and forbidding unauthorized CIDR blocks, or connecting the database instance to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

Configure the objects to be migrated and advanced settings. The following table describes the parameters.

Parameter	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 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. For more information, see the Migration types section of this topic.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains collections that have the same names as collections in the source database. If the source and destination databases do not contain identical collection names, the precheck is passed. Otherwise, an error is returned during the precheck, and the data migration task cannot be started. Note You can use the object name mapping feature to rename the collections that are migrated to the destination database. You can use this feature if the source and destination databases contain identical collection names and the collections in the destination database cannot be deleted or renamed. 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 inconsistency may occur, 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 some columns are migrated, or the data migration task fails.
Capitalization of Object Names in Destination Instance	The capitalization of database names and collection names in the destination instance. By default, DTS default policy is selected. You can select other options to ensure that the capitalization of object names is consistent with 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 migrated.
Selected Objects	To rename an object that you want to migrate to the destination instance, right-click the object in the Selected Objects section. For more information, see Map the name of a single object. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time. Note If you use the object name mapping feature to rename an object, other objects that depend on the object may fail to be migrated. To specify WHERE conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Specify filter conditions.

Click Next: Advanced Settings to configure advanced settings.

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

Advanced Settings

Parameter	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.
Set Alerts	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.
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 1440. 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.
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	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 added.
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.

In the lower part of the page, click Next: Save Task Settings and Precheck.
You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameters to be specified when you call the relevant API operation to configure the DTS task.
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.
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.