DocsICP FilingConsole
官方文档
首页

Migrate a MongoDB replica set to a replica set or sharded cluster

更新时间：
复制 MD 格式
产品详情
我的收藏

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 by using Data Transmission Service (DTS).

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 provides an example configuration for a source ApsaraDB for MongoDB replica set and a destination ApsaraDB for MongoDB instance (replica set or sharded cluster). The configuration process for other data sources is similar.

Prerequisites

  • You have created a source ApsaraDB for MongoDB replica set instance and a destination ApsaraDB for MongoDB replica set or sharded cluster instance. 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 destination ApsaraDB for MongoDB instance should have 10% more storage space than the source ApsaraDB for MongoDB instance.

  • If the destination ApsaraDB for MongoDB instance is a sharded cluster, you must 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 based on your business needs. For more information, see Set up data sharding to fully utilize shard performance and How to handle uneven data distribution in a MongoDB sharded cluster.

    Note

    Configuring data sharding prevents all data from being migrated to a single shard, ensuring optimal cluster performance; enabling the balancer and performing pre-sharding prevents data skew.

Notes

Type

Description

Source database limitations

  • Bandwidth requirement: The source database server must have sufficient egress bandwidth. Otherwise, data migration speed is affected.

  • Collections to be migrated must have a primary key or unique constraint, and the constrained fields must contain unique values. Otherwise, duplicate data may be created in the destination database.

  • Field names in the data must not contain the "." (dot) character; otherwise, data inconsistencies may occur.

  • If you migrate data at the collection level and need to edit the collections, such as by mapping collection names, a single migration task can migrate a maximum of 1,000 collections. If you exceed this limit, the task reports an error upon submission. In this case, you should split the collections into multiple batches and configure a separate task for each batch, or configure a task to migrate the entire database.

  • A single document in the source database cannot exceed 16 MB. Otherwise, the migration task will fail.

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

  • To perform incremental data migration:

    The source database must have the oplog enabled with at least seven days of retention. Alternatively, change streams must be enabled, and DTS must be able to subscribe to data changes from the source database within the last seven days by using change streams. If these requirements are not met, the migration task may fail because it cannot obtain data changes from the source. In extreme cases, this can lead to data inconsistency or loss. Issues arising from this are not covered by the DTS Service Level Agreement (SLA).

    Important

    • We recommend using the oplog to obtain data changes from the source database.

    • Only MongoDB 4.0 and later versions support obtaining data changes through change streams.

    • If the source database is an Amazon DocumentDB (non-elastic) cluster, you must manually enable change streams. When you configure the task, set the Migration Method to ChangeStream and the Architecture to Sharded Cluster.

  • Operational limitations on the source database:

    • During the schema migration and full data migration phases, do not perform schema changes on databases or collections, including updating data in arrays. Such changes can cause the migration to fail or lead to data inconsistency between the source and destination databases.

    • If you perform only full data migration, do not write new data to the source instance. Otherwise, the source and destination databases become inconsistent. 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 may occur or instance latency may increase.

Other limitations

  • If the destination instance has a sharded cluster architecture:

    • You must clear orphaned documents because they can affect migration performance. If documents with _id conflicts are encountered during migration, data inconsistency or task failure can occur.

    • Before starting the task, you must add a shard key to the source data for each sharded collection in the destination. If you cannot add a shard key to the source data, see Migrate data from a MongoDB instance without a shard key to a MongoDB sharded cluster instance.

    • After the task starts, the INSERT command must include the shard key. The UPDATE command cannot change the shard key.

  • If the destination instance has a replica set architecture:

    • When the Access Method is Express Connect, VPN Gateway, or Smart Access Gateway, Public IP Address, or Cloud Enterprise Network (CEN), you must set Domain Name or IP and Port Number to the primary node's address and port, or configure a high-availability connection address. For more information about high-availability connection addresses, see Create a Source or Destination Instance for a High-availability MongoDB Database.

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

  • DTS does not support connecting to a MongoDB database by using an SRV record.

  • Keep the MongoDB versions of the source and destination databases consistent, or migrate from an earlier version to a later version to ensure compatibility. Migrating from a later version to an earlier version may cause compatibility issues.

  • DTS does not support migrating data from the admin, config, or local databases.

  • If the destination collection has a unique index or its capped attribute is set to true, the collection does not support concurrent replay (only single-threaded writing is supported) during incremental data migration. This may increase task latency.

  • DTS does not preserve transaction information. It converts transactions from the source database into individual statements in the destination database.

  • When DTS writes data to a destination collection, if a primary key or unique key conflict occurs, DTS skips the conflicting data write statement and retains the existing data in the destination collection.

  • If the source database is a MongoDB version earlier than 3.6 and the destination database is MongoDB 3.6 or later, field order in the migrated data may differ from the source. The field-value pairs remain correct. This is caused by differences in the database engine's execution plan. If your application logic involves text matching on nested structures, evaluate the potential impact of this field order change.

  • Before you migrate data, evaluate the performance of both the source and destination databases and perform the migration during off-peak hours. During full data migration, DTS consumes read and write resources, which may increase the database load.

  • During full data migration, DTS performs concurrent INSERT operations, which can cause fragmentation in the destination collections. As a result, the destination collections may occupy more storage space than those in the source instance.

  • Verify that the migration precision DTS uses for FLOAT or DOUBLE columns meets your business requirements. DTS reads values of these types by using ROUND(COLUMN,PRECISION). If you do not explicitly define 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 the service cutover to the target instance, you must end or release the task, or use the revoke command to revoke the write permissions of the account that DTS uses to access the target instance. This prevents source data from overwriting data on the target instance if the task is automatically resumed.

  • Because DTS writes data concurrently, the destination database uses 5% to 10% more storage space than the source database.

  • Use the db.$table_name.aggregate([{ $count:"myCount"}]) syntax to query the count in the destination MongoDB.

  • Ensure that the destination MongoDB database does not have documents with the same primary key (the _id field by default) as the source database. Otherwise, data loss may occur. If such documents exist, delete those with conflicting _id values from the destination database before migration, provided that it does not affect your business.

  • 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 its requirements for sharded collections.

  • You cannot migrate capped collections when the source database is MongoDB 5.0 or later and the destination database is an earlier version. This can cause the task to fail or result in data inconsistency. This is because the behavior of capped collections changed in MongoDB 5.0, which allows explicit deletions and document size increases on update. Earlier database kernels are not compatible with these features.

  • Time series collections, introduced in MongoDB 5.0, are not supported for migration.

Special cases

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

  • A primary/secondary switchover on the source database during migration causes the task to fail.

  • DTS calculates latency by comparing the timestamp of the last record migrated to the destination with the current timestamp. If the source database has not been updated for a long time, the reported latency may be inaccurate. If the task shows excessive latency, perform a small update on the source database to refresh the latency value.

Note

If you migrate an entire database, you can also create a heartbeat table that is updated at regular intervals, such as 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

Migrate the schema of the migration objects from the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB.

Note

Schema migration is supported for databases, collections, and indexes.

full data migration

Migrate all existing data of the migration objects from the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB.

Note

Full data migration is supported for data in databases and collections.

incremental data migration

In addition to the full migration, you can migrate incremental updates from the source ApsaraDB for MongoDB to the destination ApsaraDB for MongoDB.

Oplog

Incremental migration does not support 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

    Note

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

  • RENAME COLLECTION

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

    Note

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

Change stream

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 by 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

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

To create database accounts and grant permissions 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.

    • Replica Set: This architecture uses multiple nodes to ensure high availability and enable read/write splitting. For more information, see Replica set architecture.

    • Sharded Cluster: This architecture 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 an oplog is enabled for the source database.

      Note

      An 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 selecting 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

    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 information about 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 vary based on the selected Access Method and Architecture. The options displayed in the console prevail.

    Note

    • A MongoDB database where the Architecture is Sharded Cluster and the Migration Method is Oplog does not support SSL-encrypted.

    • If the source 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 allows you to upload 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: This architecture uses multiple nodes to ensure high availability and enable read/write splitting. For more information, see Replica set architecture.

    • Sharded Cluster: This architecture 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

    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 information about 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 vary based on the selected Access Method and Architecture. The options displayed in the console prevail.

    Note

    • MongoDB databases with an Architecture of Sharded Cluster do not support SSL-encrypted.

    • If the destination is a self-managed MongoDB database (Access Method is not Alibaba Cloud Instance) with a Replica Set, and you select SSL-encrypted, DTS also allows you to upload 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.

    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 the object that you want to migrate, and then click 向右小箭头 to move it to the Selected Objects box.

    Note

    You can select objects at the DATABASE or COLLECTION level.

    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.

    1. 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

      In the data to be migrated, is the data type of the primary key _id uniform within a single collection?

      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 data types of primary keys in the source data. For 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 data types of primary keys in the source data 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. This is optional for this example.

      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 contact. If a migration fails or the latency exceeds the threshold, the system sends an alert notification.

    2. 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 encounter task latency and data inconsistency even when there are no application writes?

  • Cause: This issue occurs because of a conflict between the automatic deletion mechanism of a TTL index on a MongoDB collection and the data synchronization mechanism of DTS. This conflict can lead to task latency and data inconsistency in your synchronization/migration task.

    • Redundant DELETEs reduce efficiency: When the source TTL index deletes expired data, it writes a DELETE record to the Oplog. DTS replays this DELETE on the destination. If the destination TTL index already deleted the same data, MongoDB returns an unexpected affected-row count, triggering exception handling and slowing the migration.

    • Data inconsistency from asynchronous TTL deletion: TTL indexes do not delete data in real time. Expired data may still exist on the source while the destination has already deleted it, causing inconsistency.

      Example:

      The MongoDB Oplog or ChangeStream records only the updated fields for an UPDATE operation, not the full document. If an UPDATE 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, 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.

Previous：Migrate a standalone instance to any architectureNext：Migrate data from a self-managed MongoDB database (replica set architecture) to an ApsaraDB for MongoDB instance (replica set or sharded cluster architecture)