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    ApsaraDB for MongoDB:Migrating replica sets to replica sets or sharded clusters

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    ApsaraDB for MongoDB:Migrating replica sets to replica sets or sharded clusters

    Last Updated:Apr 13, 2026

    This topic describes how to use DTS to migrate data from an ApsaraDB for MongoDB replica set instance to an ApsaraDB for MongoDB replica set or sharded cluster.

    Supported source and destination databases

    Source database (replica set)

    Destination database (replica set or sharded cluster)

    ApsaraDB for MongoDB

    ApsaraDB for MongoDB

    Self-managed database on an ECS instance

    Self-managed database on an ECS instance

    Self-managed database connected using Express Connect, VPN Gateway, or Smart Access Gateway

    Self-managed database connected using Express Connect, VPN Gateway, or Smart Access Gateway

    Self-managed database with a public IP address

    Self-managed database with a public IP address

    This topic describes the configuration process using an ApsaraDB for MongoDB instance (replica set) and an ApsaraDB for MongoDB instance (replica set or sharded cluster) as an example. The configuration process is similar for other data sources.

    Prerequisites

    • A source ApsaraDB for MongoDB replica set instance and a destination ApsaraDB for MongoDB instance (replica set or sharded cluster) are created. For more information, see Create a replica set instance and Create a sharded cluster instance.

      Note

      For information about supported versions, see Migration solutions overview.

    • The storage space of the destination ApsaraDB for MongoDB instance should be 10% larger than the storage space that is used by the source ApsaraDB for MongoDB instance.

    • If the destination ApsaraDB for MongoDB instance is a sharded cluster, create the databases and collections that require sharding, configure data sharding, enable the balancer, and perform pre-sharding in the destination ApsaraDB for MongoDB instance as needed. For more information, see Set up data sharding to make full use of shard performance and How to handle uneven data distribution in a MongoDB sharded cluster.

      Note

      Configuring data sharding prevents data from being migrated to a single shard. This ensures optimal cluster performance. Enabling the balancer and performing pre-sharding prevent data skew.

    Usage notes

    Type

    Description

    Source database limits

    • Bandwidth requirements: The server that hosts the source database must have sufficient outbound bandwidth. Otherwise, the migration speed is affected.

    • The collections to be migrated must have primary keys or UNIQUE constraints, and the fields must be unique. Otherwise, duplicate data may exist in the destination database.

    • If you migrate data at the collection level and need to edit the collections, such as mapping collection names, a single data migration task can migrate a maximum of 1,000 collections. If you exceed this limit, an error is reported after you submit the task. In this case, split the collections into multiple migration tasks, or configure a task to migrate the entire database.

    • A single piece of data to be migrated from the source database cannot exceed 16 MB. Otherwise, the task fails.

    • If the source database is Azure Cosmos DB for MongoDB or an Amazon DocumentDB elastic cluster, only full migration is supported.

    • To perform incremental migration:

      For incremental migration, you must enable Oplog on the source database and ensure that the Oplog is retained for at least seven days. Alternatively, you can enable Change Streams and ensure that DTS can subscribe to data changes in the source database from the last seven days through Change Streams. Otherwise, the task may fail because it cannot obtain data changes from the source. In extreme cases, data inconsistency or data loss may occur. Issues caused by this are not covered by the Service-Level Agreement (SLA).

      Important

      • Obtain data changes from the source database using the Oplog.

      • Only MongoDB 4.0 and later support obtaining data changes through Change Streams.

      • When the source database is Amazon DocumentDB (non-elastic cluster), you must manually enable Change Streams, select Migration Method as ChangeStream, and select Architecture as Sharded Cluster.

    • Source database operation limits:

      • During schema migration and full data migration, do not change the schema of databases or collections. This includes updating data of the array type. Otherwise, the data migration task may fail, or data inconsistency may occur between the source and destination databases.

      • If you perform only a full data migration, do not write new data to the source instance. Otherwise, data inconsistency occurs between the source and destination databases. To maintain real-time data consistency, select Schema Migration, Full Data Migration, and Incremental Data Migration.

    • If a collection to be migrated contains a Time to Live (TTL) index, data inconsistency or instance latency may occur.

    Other limits

    • If the destination instance is a sharded cluster:

      • Purge orphaned documents. Otherwise, migration performance is affected. If documents with conflicting _id values are found during migration, data inconsistency or task failure may occur.

      • Before the task starts, add sharding keys to the source data that correspond to the sharding keys in the destination. If you cannot add sharding keys to the source, see Migrate data from a MongoDB instance without sharding keys to a MongoDB sharded cluster.

      • After the task starts, the data to be migrated must include a sharding key when you use the INSERT command. You cannot change the sharding key when you use the UPDATE command.

    • If the destination instance is a replica set:

      • When the Access Method is Express Connect, VPN Gateway, or Smart Access Gateway, Public IP Address, or Cloud Enterprise Network (CEN), you must enter the address and port of the primary node in the Domain Name or IP and Port Number fields, or configure a high-availability connection endpoint. For more information about high-availability connection endpoints, see Create an Instance Where the Source or Destination Database Is a High-Availability MongoDB Instance.

      • When Access Method is Self-managed Database on ECS, you must enter the port of the primary node for Port Number.

    • Connecting to a MongoDB database using an SRV record is not supported.

    • Keep the versions of the source and destination MongoDB databases the same, or migrate from an earlier version to a later version to ensure compatibility. If you migrate from a later version to an earlier version, compatibility issues may occur.

    • Data in the admin, config, and local databases cannot be migrated.

    • If a destination collection has a unique index or its capped property is set to true, concurrent replay is not supported for the collection during incremental migration. Only single-threaded writes are supported. This may increase task latency.

    • Transaction information is not retained. Transactions from the source database are converted into individual records in the destination database.

    • When Data Transmission Service (DTS) writes data to a destination collection, if a primary key or unique key conflict occurs, DTS skips the corresponding write statement and retains the existing data in the destination collection.

    • If the source MongoDB version is earlier than 3.6 and the destination MongoDB version is 3.6 or later, the order of fields in the data may be inconsistent before and after migration. This is due to differences in the database engine's execution plan. The field-value pairs remain consistent. If your business involves text-matching queries on nested structures, assess the potential impact of the inconsistent field order on your business.

    • Assess the performance of the source and destination databases before migration. Migrate data during off-peak hours. During full data migration, DTS consumes read and write resources on both databases, which can increase the database workload.

    • Full data migration involves concurrent INSERT operations, which cause fragmentation in the destination collections. After the full migration is complete, the storage space used by the destination collections will be larger than that of the source collections.

    • Confirm whether the migration precision that DTS provides for columns of the FLOAT or DOUBLE data type meets your business requirements. DTS reads the values of these columns using ROUND(COLUMN,PRECISION). If you do not specify the precision, DTS migrates FLOAT values with a precision of 38 digits and DOUBLE values with a precision of 308 digits.

    • DTS attempts to resume failed migration tasks within seven days. Before you switch your business to the destination instance, you must end or release the task, or run the revoke command to revoke the write permissions of the account that DTS uses to access the destination instance. This prevents the source data from overwriting the data in the destination instance if the task is automatically resumed.

    • Because DTS writes data concurrently, the storage space used by the destination database is 5% to 10% larger than that of the source database.

    • To query the count of documents in the destination MongoDB database, use the db.$table_name.aggregate([{ $count:"myCount"}]) syntax.

    • Ensure that the destination MongoDB database does not have the same primary keys (by default, _id) as the source database. Otherwise, data loss occurs. If the destination database has the same primary keys as the source, clear the relevant data from the destination database without affecting your business. This means deleting documents in the destination database that have the same `_id` as documents in the source.

    • If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters.

      Note

      Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.

    • If the destination database is a MongoDB sharded cluster, after you switch your business to this database, you must ensure that your business operations comply with the requirements for sharded collections in the MongoDB database.

    • You cannot migrate a capped collection if the source database is MongoDB 5.0 or later and the destination database is a version earlier than 5.0. This can cause the migration task to fail or lead to data inconsistency between the source and destination databases. This is because the behavior of capped collection changed in MongoDB 5.0. The new behavior allows explicit deletions and allows documents to increase in size during updates. Earlier versions of the database kernel are not compatible with these new features.

    • Time-series collections introduced in MongoDB 5.0 and later are not supported for migration.

    Special cases

    If the source database is a self-managed MongoDB instance:

    • A primary/secondary switchover in the source database during migration will cause the migration task to fail.

    • The latency reported by DTS is calculated by comparing the timestamp of the last migrated data record with the current timestamp. If the source database has not been updated for a long time, the latency information may be inaccurate. If the task shows a high latency, you can perform an update operation on the source database to refresh the latency information.

    Note

    If you choose to migrate the entire database, you can also create a heartbeat table. The heartbeat table is updated or written to every second.

    Billing

    Migration type

    Instance configuration fee

    Internet traffic fee

    Schema migration and full data migration

    Free of charge.

    When the Access Method parameter of the destination database is set to Public IP Address, you are charged for Internet traffic. For more information, see Billing overview.

    Incremental data migration

    Charged. For more information, see Billing overview.

    Migration types

    Type

    Description

    Schema migration

    DTS migrates the schemas of 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 all existing data for the selected 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

    After the full data migration, DTS replicates ongoing data changes from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.

    Use oplog

    DTS does not replicate data from databases that are created after the task starts. The following incremental updates are supported:

    • CREATE COLLECTION and CREATE INDEX

    • DROP DATABASE, DROP COLLECTION, and DROP INDEX

    • RENAME COLLECTION

      Note

      RENAME COLLECTION operations that include the dropTarget option set to true are not supported.

    • Inserting, updating, and deleting documents in a collection.

      Note

      For incremental document updates, only changes made using the $set command are replicated.

    Use change streams

    The following incremental updates are supported:

    • DROP DATABASE and DROP COLLECTION

    • RENAME COLLECTION

      Note

      RENAME COLLECTION operations that include the dropTarget option set to true are not supported.

    • Inserting, updating, and deleting documents in a collection.

      Note

      For incremental document updates, only changes made using the $set command are replicated.

    Database account permissions

    Database

    Schema migration

    Full data migration

    Incremental data migration

    Source ApsaraDB for MongoDB instance

    Read permission on the database to be migrated and the config database.

    Read permission on the database to be migrated, the admin database, and the local database.

    Destination ApsaraDB for MongoDB instance

    The dbAdminAnyDatabase permission, the readWrite permission on the destination database, and the read permission on the local database.

    To create and authorize database accounts for the source and destination ApsaraDB for MongoDB instances, see Manage MongoDB database users with DMS.

    Procedure

    1. Navigate to the migration task list page for the destination region using one of the following methods.

      From the DTS console

      1. Log on to the Data Transmission Service (DTS) console.

      2. In the navigation pane on the left, click Data Migration.

      3. In the upper-left corner of the page, select the region where the migration instance is located.

      From the DMS console

      Note

      The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.

      1. Log on to the Data Management (DMS) console.

      2. In the top menu bar, choose Data + AI > Data Transmission (DTS) > Data Migration.

      3. To the right of Data Migration Tasks, select the region where the migration instance is located.

    2. Click Create Task to navigate to the task configuration page.

    3. Configure the source and destination databases.

      Warning

      After you select the source and destination instances, we recommend that you carefully read the limits displayed at the top of the page. Otherwise, the task may fail or data inconsistency may occur.

      Category

      Parameter

      Description

      N/A

      Task Name

      DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.

      Source Database

      Select Existing Connection

      • To use a database instance that has been added to the system (created or saved), select the desired database instance from the drop-down list. The database information below will be automatically configured.

        Note

        In the DMS console, this parameter is named Select a DMS database instance..

      • If you have not registered the database instance with the system, or do not need to use a registered instance, manually configure the database information below.

      Database Type

      Select MongoDB.

      Connection Type

      Select Cloud Instance.

      Instance Region

      Select the region where the source ApsaraDB for MongoDB instance is located.

      Replicate Data Across Alibaba Cloud Accounts

      In this example, a database instance under the current Alibaba Cloud account is used. Select No.

      Architecture

      Select Replica set architecture.

      • replica set architecture: A deployment model that uses multiple nodes to ensure high availability and enable read/write splitting. For more information, see replica set architecture.

      • sharded cluster architecture: A deployment model that consists of mongos, shard, and ConfigServer components. You can customize the number and configurations of mongos and shard nodes. For more information, see sharded cluster architecture.

      Migration Method

      Select a method for incremental data migration based on your requirements.

      • Oplog (Recommended):

        This option is available if Oplog is enabled for the source database.

        Note

        Oplog is enabled by default for both self-managed MongoDB databases and ApsaraDB for MongoDB instances. This method offers lower latency for incremental data migration due to faster log pulling. Therefore, we recommend that you select Oplog.

      • ChangeStream: This option is available if Change Streams are enabled for the source database.

        Note

        • If the source database is an Amazon DocumentDB instance (non-elastic cluster), you can only select ChangeStream.

        • If you set Architecture to Sharded Cluster for the source database, you do not need to enter a Shard account or Shard password.

      Instance ID

      Select the instance ID of the source ApsaraDB for MongoDB instance.

      Authentication Database Name

      Enter the name of the database to which the database account of the source ApsaraDB for MongoDB instance belongs. The default value is admin.

      Database Account

      Enter the database account of the source ApsaraDB for MongoDB instance. For permission requirements, see Database account permissions.

      Database Password

      Enter the password for the database account.

      Encryption

      DTS supports three connection methods: Non-encrypted, SSL-encrypted, and Mongo Atlas SSL. The options for Encryption depend on the Access Method and Architecture. The options displayed in the console prevail.

      Note

      • For a MongoDB database where Architecture is set to Sharded Cluster and Migration Method is set to Oplog, SSL-encrypted is not supported.

      • If the source database is a self-managed MongoDB database (Access Method is not Alibaba Cloud Instance) with a Replica Set architecture, and you select SSL-encrypted, DTS also supports uploading a CA certificate to verify the connection.

      Destination Database

      Select Existing Connection

      • To use a database instance that has been added to the system (created or saved), select the desired database instance from the drop-down list. The database information below will be automatically configured.

        Note

        In the DMS console, this parameter is named Select a DMS database instance..

      • If you have not registered the database instance with the system, or do not need to use a registered instance, manually configure the database information below.

      Database Type

      Select MongoDB.

      Connection Type

      Select Cloud Instance.

      Instance Region

      Select the region where the destination ApsaraDB for MongoDB instance is located.

      Replicate Data Across Alibaba Cloud Accounts

      In this example, a database instance under the current Alibaba Cloud account is used. Select No.

      Architecture

      Select an architecture based on your business requirements. Valid values:

      • replica set architecture: A deployment model that uses multiple nodes to ensure high availability and enable read/write splitting. For more information, see replica set architecture.

      • sharded cluster architecture: A deployment model that consists of mongos, shard, and ConfigServer components. You can customize the number and configurations of mongos and shard nodes. For more information, see sharded cluster architecture.

      Instance ID

      Select the instance ID of the destination ApsaraDB for MongoDB instance.

      Authentication Database Name

      Enter the name of the database to which the database account of the destination ApsaraDB for MongoDB instance belongs. The default value is admin.

      Database Account

      Enter the database account of the destination ApsaraDB for MongoDB instance. For permission requirements, see Database account permissions.

      Database Password

      Enter the password for the database account.

      Encryption

      DTS supports three connection methods: Non-encrypted, SSL-encrypted, and Mongo Atlas SSL. The options for Encryption depend on the Access Method and Architecture. The options displayed in the console prevail.

      Note

      • For a MongoDB database where Architecture is set to Sharded Cluster, SSL-encrypted is not supported.

      • If the destination database is a self-managed MongoDB database (Access Method is not Alibaba Cloud Instance) with a Replica Set architecture, and you select SSL-encrypted, DTS also supports uploading a CA certificate to verify the connection.

    4. After completing the configuration, click Test Connectivity and Proceed at the bottom of the page.

      Note

      • Ensure that you add the CIDR blocks of the DTS servers (either automatically or manually) to the security settings of both the source and destination databases to allow access. For more information, see Add the IP address whitelist of DTS servers.

      • If the source or destination is a self-managed database (i.e., the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

    5. Configure the task objects.

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

        Parameter

        Description

        Migration Types

        • If you only need to perform a full migration, select both Schema Migration and Full Data Migration.

        • To perform a migration with no downtime, select Schema Migration, Full Data Migration, and Incremental Data Migration.

        Note

        • If you do not select Schema Migration, you must ensure that a database and tables to receive the data exist in the destination database. You can also use the object name mapping feature in the Selected Objects box as needed.

        • If you do not select Incremental Data Migration, do not write new data to the source instance during data migration to ensure data consistency.

        For more information, see Migration types.

        Processing Mode of Conflicting Tables

        • Precheck and Report Errors: Checks whether collections with the same names exist in the destination database. If no collections with the same names exist, the precheck is passed. If collections with the same names exist, an error is reported during the precheck, and the data migration task does not start.

          Note

          If a collection in the destination database has the same name but cannot be easily deleted or renamed, you can change the name of the collection in the destination database. For more information, see Object name mapping.

        • Ignore Errors and Proceed: Skips the check for collections with the same names.

          Warning

          Selecting Ignore Errors and Proceed may cause data inconsistency and business risks. For example:

          • If a record in the destination database has the same primary key value as a record in the source database, the record in the destination database is kept. The record from the source database is not migrated to the destination database.

          • Data initialization may fail, only some data may be migrated, or the migration may fail.

        Capitalization of Object Names in Destination Instance

        You can configure the case policy for the names of migrated databases and collections in the destination instance. By default, DTS default policy is selected. You can also choose to align with the default policies of the source or destination database. For more information, see Object name case policy for the destination.

        Source Objects

        In the Source Objects box, click an object to migrate, and then click the 向右小箭头 icon to move it to the Selected Objects box.

        Note

        You can select objects at the DATABASE or COLLECTION level for migration.

        Selected Objects

        • To set the name of a migration object in the destination instance, or to specify the object that receives data in the destination instance, right-click the migration object in the Selected Objects box to make changes. For more information, see Object name mapping.

        • To remove a selected migration object, click the object in the Selected Objects box, and then click image to move it to the Source Objects box.

        Note

        • To select incremental migration operations at the database or collection level, right-click the migration object in the Selected Objects box and make your selection in the dialog box that appears.

        • To set conditions to filter data (supported during full data migration but not incremental data migration), right-click the collection in the Selected Objects box and configure the settings in the dialog box that appears. For instructions, see Set filter conditions.

        • If you use the object name mapping feature to specify a database or collection to receive data, the migration of other objects that depend on this object might fail.

      2. Click Next: Advanced Settings to configure advanced parameters.

        Parameter

        Description

        Dedicated Cluster for Task Scheduling

        By default, DTS schedules tasks on a shared cluster. You do not need to select one. If you want more stable tasks, you can purchase a dedicated cluster to run DTS migration tasks.

        Retry Time for Failed Connections

        After the migration task starts, if the connection to the source or destination database fails, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1440 minutes. We recommend that you set the duration to more than 30 minutes. If DTS reconnects to the source and destination databases within the specified duration, the migration task automatically resumes. Otherwise, the task fails.

        Note

        • For multiple DTS instances that share the same source or destination, the network retry time is determined by the setting of the last created task.

        • Because you are charged for the task during the connection retry period, we recommend that you customize the retry time based on your business needs, or release the DTS instance as soon as possible after the source and destination database instances are released.

        Retry Time for Other Issues

        After the migration task starts, if a non-connectivity issue, such as a DDL or DML execution exception, occurs in the source or destination database, DTS reports an error and immediately begins to retry the operation. The default retry duration is 10 minutes. You can customize the retry time to a value from 1 to 1440 minutes. We recommend that you set the duration to more than 10 minutes. If the related operations succeed within the specified retry duration, the migration task automatically resumes. Otherwise, the task fails.

        Important

        The value of Retry Time for Other Issues must be less than the value of Retry Time for Failed Connections.

        Enable Throttling for Full Data Migration

        During full migration, DTS consumes read and write resources on the source and destination databases, which may increase the database load. If required, you can enable throttling for the full migration task. You can set Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) to reduce the load on the destination database.

        Note

        • This configuration item is available only if you select Full Data Migration for Migration Types.

        • You can also adjust the full migration speed after the migration instance is running.

        Only one data type for primary key _id in a table of the data to be synchronized

        Specifies whether the data type of the primary key _id is unique within a single collection to be migrated.

        Important

        • Select an option based on your requirements. Otherwise, data loss may occur.

        • This parameter is available only if you select Full Data Migration for Migration Types.

        • Yes: The data type is unique. During full data migration, DTS does not scan the primary key data types in the source database. In a single collection, DTS migrates only the data corresponding to one primary key data type.

        • No: The data type is not unique. During full data migration, DTS scans the primary key data types in the source database and migrates all data.

        Enable Throttling for Incremental Data Migration

        If required, you can also choose to set speed limits for the incremental migration task. You can set RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) to reduce the load on the destination database.

        Note

        • This configuration item is available only if you select Incremental Data Migration for Migration Types.

        • You can also adjust the incremental migration speed after the migration instance is running.

        Environment Tag

        You can select an environment tag to identify the instance. In this example, this parameter is optional.

        Configure ETL

        Based on your business needs, select whether to configure the ETL feature to process data.

        • Yes: Configures the ETL feature. You must also enter data processing statements in the text box.

        • No: Does not configure the ETL feature.

        Monitoring and Alerting

        Select whether to set alerts and receive alert notifications based on your business needs.

        • No: Does not set an alert.

        • Yes: Configure alerts by setting an alert threshold and an alert notifications. If a migration fails or the latency exceeds the threshold, the system sends an alert notification.

      3. Click Next: Data Validation to configure a data validation task.

        For more information about the data validation feature, see Configure data validation.

    6. Save the task and run a precheck.

      • To view the parameters for configuring this instance when you call the API operation, move the pointer over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the bubble that appears.

      • If you do not need to view or have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.

      Note

      • Before the migration task starts, DTS performs a precheck. The task starts only after it passes the precheck.

      • If the precheck fails, click View Details next to the failed check item, fix the issue based on the prompt, and then run the precheck again.

      • If a warning is reported during the precheck:

        • For check items that cannot be ignored, click View Details next to the failed item, fix the issue based on the prompt, and then run the precheck again.

        • For check items that can be ignored, you can click Confirm Alert Details, Ignore, OK, and Precheck Again to skip the alert item and run the precheck again. If you choose to ignore a warning, it may cause issues such as data inconsistency and pose risks to your business.

    7. Purchase the instance.

      1. When the Success Rate is 100%, click Next: Purchase Instance.

      2. On the Purchase page, select the link specification for the data migration instance. For more information, see the following table.

        Category

        Parameter

        Description

        New Instance Class

        Resource Group Settings

        Select the resource group to which the instance belongs. The default value is default resource group. For more information, see What is Resource Management?

        Instance Class

        DTS provides migration specifications with different performance levels. The link specification affects the migration speed. You can select a specification based on your business scenario. For more information, see Data migration link specifications.

      3. After the configuration is complete, read and select Data Transmission Service (Pay-as-you-go) Service Terms.

      4. Click Buy and Start. In the OK dialog box that appears, click OK.

        You can view the progress of the migration task on the Data Migration Tasks list page.

        Note

        • If the migration task does not include incremental migration, it stops automatically after the full migration is complete. After the task stops, its Status changes to Completed.

        • If the migration task includes incremental migration, it does not stop automatically. The incremental migration task continues to run. While the incremental migration task is running, the Status of the task is Running.

    FAQ

    • Why do I see task latency and data inconsistency even when there are no active application writes?

      • Cause: This issue is caused by a conflict between the automatic deletion mechanism of a TTL index on a MongoDB collection and the data synchronization mechanism of DTS. This conflict causes task latency and data inconsistency.

        • Missed DELETE operations during incremental writes reduce efficiency: When the TTL index on the source instance deletes expired data, it generates a DELETE record in the Oplog. DTS then synchronizes this DELETE operation. If the TTL index on the destination instance has already deleted the same data, the DELETE operation from DTS will not find the data to delete. The MongoDB engine then returns an unexpected number of affected rows. This triggers an exception handling process and reduces migration efficiency.

        • Data inconsistency caused by asynchronous deletion of expired data: A TTL index does not delete data in real time. Expired data might still exist on the source instance when it has already been deleted on the destination instance. This causes data inconsistency.

          Example:

          The MongoDB Oplog or ChangeStream records only the updated fields for an UPDATE operation. It does not record the full document before and after the update. Therefore, if an UPDATE operation cannot find the target data on the destination, DTS ignores the operation.

          Timing

          Source instance

          Destination instance

          1

          Service inserts data

          2

          DTS synchronizes the INSERT operation

          3

          Data has expired but is not yet deleted by the TTL index

          4

          Service updates the data (for example, updates the TTL index field to change the expiration time)

          5

          TTL index deletes the data

          6

          DTS synchronizes the UPDATE, but the data is not found. The operation is ignored.

          As a result, this document is missing from the destination MongoDB instance.

      • Solution: To resolve this issue, temporarily modify the expiration time of the TTL index on the target during the synchronization/migration task. This ensures both synchronization efficiency and data consistency. For detailed steps, see Best practices for synchronizing or migrating collections with TTL indexes from a MongoDB source.