Migrate data from an ApsaraDB for MongoDB instance (replica set architecture) to another ApsaraDB for MongoDB instance (replica set architecture or sharded cluster architecture) - Data Transmission Service

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

Supported source and destination databases


Source database (replica set architecture)	Destination database (replica set architecture or sharded cluster architecture)
ApsaraDB for MongoDB	ApsaraDB for MongoDB
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 instance (replica set architecture) and the destination database is an ApsaraDB for MongoDB instance (replica set architecture or sharded cluster architecture). You can also follow the procedure to configure data migration tasks for other types of MongoDB databases.

Prerequisites

The source ApsaraDB for MongoDB instance (replica set architecture) and the destination ApsaraDB for MongoDB instance (replica set architecture or sharded cluster architecture) 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 deployed in a sharded cluster architecture, you need to create databases and collections to be sharded in the destination ApsaraDB for MongoDB instance and configure sharding based on your business requirements. For more information, see Configure sharding to maximize the performance of shards.
Note After you configure sharding for a cluster, the migrated data is distributed among different shards. This maximizes the performance of the sharded cluster.

Usage notes


Category	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 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 (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 split the collections to be migrated, configure multiple tasks to migrate the collections, or configure a task to migrate the entire database. If you need to migrate incremental data, make sure that the following requirements are met: The oplog feature is enabled. Otherwise, error messages are returned during precheck, and the data migration task cannot be started. If you perform only incremental data migration, the oplogs of the source database must be stored for more than 24 hours. If you perform both full data migration and incremental data migration, the oplogs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the oplogs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After full data migration is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of oplogs in accordance with the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not guarantee service reliability or performance. Limits on operations: During schema migration and full data migration, do not change the schemas of 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 may occur between the source and destination databases. To ensure data consistency, we recommend that you select Schema Migration, Full Data Migration, and Incremental Data Migration as the migration types.
Other limits	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. Transaction information is not retained. When transactions are migrated to the destination database, they 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 the FLOAT data type to 38 digits and the precision for 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, stop or release the failed tasks. You can also execute the `REVOKE` statement to revoke the write permissions from the accounts 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.
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 a long time, the migration latency may be inaccurate. If the latency of the migration task is 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. The heartbeat 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 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, collections, and indexes.
Incremental data migration	After full data migration is complete, 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

Permissions required for database accounts


Database	Schema migration	Full data migration	Incremental data migration
Source ApsaraDB for MongoDB instance	Read permissions on the source database		Read permissions on the source database, the admin database, and the local database
Destination ApsaraDB for MongoDB instance	dbAdminAnyDatabase permission, read and write permissions on the destination database, and 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 user permissions on MongoDB databases.

Procedure

Go to the Data Migration Tasks page.
1. Log on to the Data Management (DMS) console.
2. In the top navigation bar, click DTS.
3. In the left-side navigation pane, choose DTS (DTS) > Data Migration.
Note You can also configure the settings by using the new DTS console.
From the drop-down list to the right of Data Migration Tasks, select the region in which your data migration instance resides.
Note If you use the new DTS console, select the region in which your data migration instance resides in the upper-left corner.

Click Create Task. On the page that appears, configure the source and destination databases.

Warning After you select the source and destination instances, we recommend that you read the limits displayed at the top of the page. This helps you create and run the data migration task.


Section	Parameter	Description
N/A	Task Name	The name of the task. DTS automatically generates a task name. We recommend that you specify a descriptive name to identify the task. You do not need to specify a unique task name.
Source Database	Select Instance	The instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you use an existing instance, DTS automatically applies the parameter settings of the instance. If you do not use an existing instance, you must 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.
	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 achieve 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 to which the objects to be migrated in the source ApsaraDB for MongoDB instance belong. If you did not change the name 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 Permissions required for database accounts.
	Database Password	The password of the database account.
Destination Database	Select Instance	The instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you use an existing instance, DTS automatically applies the parameter settings of the instance. If you do not use an existing instance, you must 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 in which the destination instance is deployed. Select an architecture based on your business requirements. Valid values: Replica Set: You can deploy multiple types of nodes on a replica set instance to achieve 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 to which the objects to be migrated in the destination ApsaraDB for MongoDB instance belong. If you did not change the name 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 Permissions required for database accounts.
	Database Password	The password of the database account.

In the lower part of the page, click Test Connectivity and Proceed.
Warning
- 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. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. 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.
- If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database, Alibaba Cloud database instance, or ECS security group rules, 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 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.
- After the DTS task is complete or released, we recommend that you manually detect and remove the added CIDR blocks from the whitelist of the database, Alibaba Cloud database instance, or ECS security group rules.

Select objects for the task and configure advanced settings.


Parameter	Description
Task Stages	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 Incremental Data Migration is not selected, we recommend that you do not write data to the source instance during data migration. This ensures data consistency between the source and destination instances. For more information, see Migration types.
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	Specify 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 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 Available section. Click the icon and add the objects to the Selected 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 are dependent on the object may fail to be migrated. To specify WHERE conditions to filter data, right-click an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Use SQL conditions to filter data.

Click Next: Advanced Settings to configure advanced settings.


Parameter	Description
Set Alerts	Specifies whether to set alerts for the data migration task. If the task fails or the migration latency exceeds the threshold, the alert contacts will receive notifications. Valid values: No: does not set alerts. Yes: sets alerts. If you select Yes, you must also set the alert threshold and alert contacts.
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 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 migration task. Otherwise, the data migration task fails. Note If you set different retry time ranges for multiple data migration tasks that share the same source or destination database, the value that is set 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 and destination instances are released.
Configure ETL	Select Yes or No based on your business requirements. If you select Yes, you must enter domain-specific language (DSL) statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task.

In the lower part of the page, click Next: Save Task Settings and Precheck.
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 result, troubleshoot the issues. Then, run a precheck again.
- If an alert is triggered for an item during the precheck, click Confirm Alert Details next to the item and analyze the causes.
  You can troubleshoot the issues based on the causes and run a precheck again.
  If you do not need to troubleshoot the issues, 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, specify the Instance Class parameter for the data migration instance. The following table describes the parameter.


Section	Parameter	Description
New Instance Class	Instance Class	DTS provides several instance classes that have different performance in migration speed. You can select an instance class based on your business scenario. For more information, see Specifications of data migration instances.

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