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Synchronize a replica set to a replica set or sharded cluster

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DTS can synchronize a MongoDB replica set to another replica set or a sharded cluster.

Supported source and destination databases

This section lists the supported source and destination databases for synchronizing data from a MongoDB replica set to a MongoDB replica set or sharded cluster. This topic demonstrates the configuration procedure using an ApsaraDB for MongoDB replica set instance as the source and an ApsaraDB for MongoDB replica set or sharded cluster instance as the destination. The procedure for other data sources is similar.

Source database

Destination database

ApsaraDB for MongoDB replica set instance

ApsaraDB for MongoDB replica set or sharded cluster instance

Self-managed MongoDB replica set on an ECS instance

Self-managed MongoDB replica set or sharded cluster on an ECS instance

Self-managed MongoDB replica set connected over Express Connect, VPN Gateway, or Smart Access Gateway

Self-managed MongoDB replica set or sharded cluster connected over Express Connect, VPN Gateway, or Smart Access Gateway

Prerequisites

Limitations

Type

Description

Source database limits

  • Bandwidth requirement: The server where your source database resides must have sufficient outbound bandwidth. Otherwise, synchronization speed may be affected.

  • The collections to synchronize must have a primary key or UNIQUE constraint, and the field values must be unique. Otherwise, duplicate data may appear in the destination database.

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

  • If you synchronize at the collection level and need to edit objects (such as rename collections), one synchronization task supports up to 1,000 collections. If you exceed this limit, the task fails with an error after submission. To resolve this, split the collections into batches and configure multiple tasks. Or configure a full-database synchronization task.

  • A single document in your source database cannot exceed 16 MB. Otherwise, the task fails.

  • Your source database cannot be Azure Cosmos DB for MongoDB or Amazon DocumentDB (elastic cluster).

  • Your source database must enable Oplog and retain Oplog for at least seven days. Or enable Change Streams and ensure DTS can subscribe to data changes in the last seven days through Change Streams. Otherwise, DTS may fail to capture data changes, causing task failure. In extreme cases, this may cause data inconsistency or loss. These issues are not covered by the DTS Service-level agreement (SLA).

    Important

    • We recommend using Oplog to capture data changes.

    • Only MongoDB 4.0 and later support Change Streams. Change Streams do not support two-way synchronization.

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

  • If the collections to synchronize contain TTL indexes, data inconsistency or latency may occur.

  • Source database operation limits:

    • During schema synchronization and full data synchronization, do not change the schema of databases or collections (including array-type data updates). Otherwise, the synchronization task may fail or cause data inconsistency between the source and destination databases.

    • If you perform only full data synchronization, do not write new data to the source instance. Otherwise, data inconsistency may occur between the source and destination databases.

Other limits

  • If the destination instance uses sharded cluster architecture:

    • Clear orphaned documents. Otherwise, synchronization performance may suffer. If documents with conflicting _id values appear during synchronization, data inconsistency or task failure may occur.

    • Before starting the task, add the shard key used by the destination to the data in the source. If you cannot add the shard key to the source, see Synchronize from MongoDB (no shard key) to MongoDB (sharded cluster architecture).

    • After starting the task, INSERT statements for the data to synchronize must include the shard key. UPDATE statements cannot modify the shard key.

  • If the destination instance uses ReplicaSet architecture:

    • When Access Method is Express Connect, VPN Gateway, or Smart Access Gateway or Cloud Enterprise Network (CEN), you must enter the address and port of the primary node for Domain Name or IP and Port Number, or configure a high-availability connection address. For more information about high-availability connection addresses, see Create an instance with a high-availability MongoDB source or destination database.

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

  • We recommend using the same MongoDB version for the source and destination databases. Or use a higher version for the destination to ensure compatibility. Synchronizing from a higher version to a lower version may cause compatibility issues.

  • DTS does not support connecting to MongoDB databases using SRV addresses.

  • DTS does not support synchronizing data from the admin, config, and local databases.

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

  • Transaction information is not preserved. Transactions in the source database become individual records in the destination database.

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

  • If your source MongoDB version is earlier than 3.6 and your destination MongoDB version is 3.6 or later, differences in execution plans may cause inconsistent field ordering in synchronized data. Field-value relationships remain unchanged. If your business uses text-matching queries on nested structures, assess the impact of inconsistent field ordering.

  • Evaluate the performance of both the source and destination databases before synchronization. Run synchronization during off-peak hours. Otherwise, full data initialization consumes read and write resources, increasing database load.

  • Full initialization runs INSERT operations concurrently. This creates fragmentation in the destination database collection. After full initialization, the collection space in the destination instance is larger than in the source instance.

  • Do not write data to the destination database except through DTS during synchronization. Otherwise, data inconsistency may occur between the source and destination databases. For example, if you use DMS to perform online DDL operations while other data is written to the destination database, data loss may occur.

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

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

  • Ensure the destination MongoDB does not have the same primary key (default is _id) as the source. Otherwise, data loss may occur. If the destination has the same primary key, clear related data in the destination without affecting your business (delete documents with the same _id value in the destination).

  • 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 your destination database is a MongoDB sharded cluster, ensure your business behavior meets MongoDB's requirements for sharded collections after switching traffic to this database.

  • If the source database is MongoDB 5.0 or later and the destination database version is earlier than 5.0, you cannot sync capped collections. If you attempt to sync them, the task fails or data inconsistency occurs between the source and destination databases. This is because, starting from MongoDB 5.0, the behavior of capped collections changed to allow operations such as explicit deletion and increasing document size during updates, and earlier versions of the database kernel cannot support these new features.

  • DTS does not support synchronizing time-series collections introduced in MongoDB 5.0 and later.

Special cases

If your source database is a self-managed MongoDB:

  • If a primary/secondary switchover occurs in the source database during synchronization, the task fails.

  • DTS calculates latency by comparing the timestamp of the last synchronized document with the current timestamp. If the source database has no updates for a long time, latency information may be inaccurate. If latency appears too high, run an update operation in the source database to refresh latency information.

Note

If you select full-database synchronization, create a heartbeat table. Update or write to this table every second.

Billing

Synchronization type

Pricing

Schema synchronization and full data synchronization

Free of charge.

Incremental data synchronization

Charged. For more information, see Billing overview.

Supported synchronization topologies

  • One-way one-to-one synchronization

  • One-way one-to-many synchronization

  • One-way many-to-one synchronization

  • One-way cascade synchronization

For details about these synchronization topologies and their considerations, see Synchronization topologies.

Synchronization types

Type

Description

Schema synchronization

Replicates the schemas of selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.

Full data synchronization

Replicates all historical data for the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.

Note

Full data synchronization is supported at the database and collection levels.

Incremental data synchronization

Replicates ongoing changes from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance after full data synchronization completes.

Oplog

Incremental data synchronization does not replicate databases created after the task starts. The following changes are replicated:

  • CREATE COLLECTION and CREATE INDEX

  • DROP DATABASE, DROP COLLECTION, and DROP INDEX

  • RENAME COLLECTION

    Note

    The RENAME COLLECTION operation is not replicated if the dropTarget option is set to true.

  • Document inserts, updates, and deletes within a collection.

    Note

    For document updates, it replicates only operations that use the $set command.

Change stream

The following changes are replicated:

  • DROP DATABASE and DROP COLLECTION

  • RENAME COLLECTION

    Note

    The RENAME COLLECTION operation is not replicated if the dropTarget option is set to true.

  • Document inserts, updates, and deletes within a collection.

    Note

    For document updates, it replicates only operations that use the $set command.

Procedure

  1. Go to the data synchronization task list page in the destination region. You can do this in one of two ways.

    DTS console

    1. Log on to the DTS console.

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

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

    DMS console

    Note

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

    1. Log on to the DMS console.

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

    3. To the right of Data Synchronization Tasks, select the region of the synchronization instance.

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

  3. Configure the source and destination databases.

    Warning

    After you select the source and destination instances, review the Limits 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

    • Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured.

      Note

      In the DMS console, this configuration item is Select a DMS database instance.

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

    Database Type

    Select MongoDB.

    Connection Type

    Select Alibaba Cloud Instance.

    Instance Region

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

    Replicate Data Across Alibaba Cloud Accounts

    For this example, select No, as the database instance belongs to the current Alibaba Cloud account.

    Architecture

    Select Replica Set.

    Migration Method

    Select a method for incremental data synchronization 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 self-managed MongoDB databases and ApsaraDB for MongoDB instances. This method offers lower latency for incremental synchronization tasks because logs are retrieved faster. 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 (non-elastic cluster) instance, you can select only ChangeStream.

      • If you set Architecture of the source database to Sharded Cluster, you do not need to specify Shard account and Shard password.

    Instance ID

    Select the ID of the source ApsaraDB for MongoDB instance.

    Authentication Database

    Enter the database name associated with the account of the source ApsaraDB for MongoDB instance. The default is admin.

    Database Account

    Enter the account of the source ApsaraDB for MongoDB database. The account must have read permission on the databases to be synchronized, the config database, the admin database, and the local database.

    Database Password

    Enter the password for the specified 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

    • Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured.

      Note

      In the DMS console, this configuration item is Select a DMS database instance.

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

    Database Type

    Select MongoDB.

    Connection Type

    Select Alibaba Cloud Instance.

    Instance Region

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

    Replicate Data Across Alibaba Cloud Accounts

    For this example, select No, as the database instance belongs to the current Alibaba Cloud account.

    Architecture

    Select the architecture type of the destination ApsaraDB for MongoDB instance.

    Instance ID

    Select the ID of the destination ApsaraDB for MongoDB instance.

    Authentication Database

    Enter the database name associated with the account of the destination ApsaraDB for MongoDB instance. The default is admin.

    Database Account

    Enter the account of the destination ApsaraDB for MongoDB database. The account must have the dbAdminAnyDatabase permission, the readWrite permission on the destination database, and the read permission on the local database.

    Database Password

    Enter the password for the specified 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.

    1. On the Configure Objects page, specify the objects to synchronize.

      Parameter

      Description

      Synchronization Types

      DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization.

      Note

      For more information, see Task steps.

      Processing Mode of Conflicting Tables

      • Precheck and Report Errors: Checks if the destination database contains collections with the same names. If a collection with a duplicate name is found, an error is reported during the precheck phase and the data synchronization task does not start. Otherwise, the precheck is successful.

        Note

        If you cannot delete or rename the collection with the same name in the destination database, you can use the object name mapping feature to change the collection name in the destination database. For more information, see Set the name of a synchronization object in the destination instance.

      • Ignore Errors and Proceed: Skips checking for collections with the same name in the destination database.

        Warning

        Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example:

        • If a record in the destination database has the same primary key or unique key value as a record in the source database, the record in the destination database is retained. The record from the source database is not synchronized to the destination database.

        • Data initialization may fail, only some data may be synchronized, or the synchronization may fail.

      Synchronization Topology

      Select One-way Synchronization.

      Capitalization of Object Names in Destination Instance

      You can configure the case policy for database and collection object names in the destination instance. By default, the DTS default policy is selected. Alternatively, you can match the default policy of the source or destination database. For more information, see Case policy for destination object names.

      Source Objects

      In the Source Objects box, click the objects, and then click 向右 to move them to the Selected Objects box.

      Note

      You can select objects at the DATABASE or COLLECTION level.

      Selected Objects

      • To set the name of a synchronization object in the destination instance, right-click the object in the Selected Objects box to modify it. For more information, see Object name mapping.

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

      Note

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

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

      • If you use the object name mapping feature to specify a database or collection to receive data, synchronization may fail for other objects that depend on the mapped object.

    2. Click Next: Advanced Settings.

      Parameter

      Description

      Dedicated Cluster for Task Scheduling

      By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.

      Retry Time for Failed Connections

      If the connection to the source or destination database fails after the synchronization task starts, 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 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails.

      Note

      • If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances.

      • DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.

      Retry Time for Other Issues

      If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails.

      Important

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

      Enable Throttling for Full Data Synchronization

      During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s).

      Note

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

      Specify whether the data types of the _id primary key are unique within each collection to be synchronized.

      Important

      • Select an option based on your actual data. An incorrect selection may lead to data loss.

      • This parameter is available only if Synchronization Types includes Full Data Synchronization.

      • Yes: The data types are unique. During the full synchronization phase, DTS does not scan the data types of the primary keys in the source data. For each collection, DTS synchronizes only the data corresponding to one primary key data type.

      • No: The data types are not unique. During the full synchronization phase, DTS scans the data types of the primary keys in the source data and synchronizes all the data.

      Enable Throttling for Incremental Data Synchronization

      You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).

      Environment Tag

      You can select an environment tag to identify the instance based on your requirements. In this example, no selection is needed.

      Configure ETL

      Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values:

      Monitoring and Alerting

      Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts.

    3. Click Data Verification to configure a data verification task.

      To use the data verification feature, see Configure data verification.

  6. Save the task and perform a precheck.

    • To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.

    • If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.

    Note

    • Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.

    • If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.

    • If the precheck generates warnings:

      • For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.

      • For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.

  7. Purchase the instance.

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

    2. On the Purchase page, select the billing method and link specifications for the data synchronization instance. For more information, see the following table.

      Category

      Parameter

      Description

      New Instance Class

      Billing Method

      • Subscription: You pay upfront for a specific duration. This is cost-effective for long-term, continuous tasks.

      • Pay-as-you-go: You are billed hourly for actual usage. This is ideal for short-term or test tasks, as you can release the instance at any time to save costs.

      Resource Group Settings

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

      Instance Class

      DTS offers synchronization specifications at different performance levels that affect the synchronization rate. Select a specification based on your business requirements. For more information, see Data synchronization link specifications.

      Subscription Duration

      In subscription mode, select the duration and quantity of the instance. Monthly options range from 1 to 9 months. Yearly options include 1, 2, 3, or 5 years.

      Note

      This option appears only when the billing method is Subscription.

    3. Read and select the checkbox for Data Transmission Service (Pay-as-you-go) Service Terms.

    4. Click Buy and Start, and then click OK in the OK dialog box.

      You can monitor the task progress on the data synchronization page.

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.

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