Two-way data synchronization between MongoDB replica sets - Data Transmission Service

Prerequisites

Source and destination ApsaraDB for MongoDB replica set instances are created. Create a replica set instance.

Important
Supported database versions are listed in Synchronization solution overview.
The destination ApsaraDB for MongoDB instance storage is at least 10% larger than the source ApsaraDB for MongoDB instance data size.
The replication.oplogGlobalIdEnabled parameter is set to true on both instances. Set database parameters.

Note
If not set to true, the precheck fails or returns the two-way mongo must have gid error.

Usage notes

Type	Description
Source and destination 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. 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. 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 SLA. Important We recommend using Oplog to capture data changes. Change Streams do not support two-way synchronization. If the collections to synchronize contain TTL indexes, data inconsistency or latency may occur. The source and destination databases must be ApsaraDB for MongoDB instances of the same architecture. Two-way synchronization is not supported for self-managed MongoDB instances or MongoDB instances of different architectures. 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. DTS does not support connecting to MongoDB databases using SRV addresses. If the source database is MongoDB 5.0 or later and the destination database version is earlier than 5.0, you cannot sync `capped collection`s. 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 collection`s 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.
Other limits	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. The architecture of the source and target ApsaraDB for MongoDB must be consistent. DTS does not support synchronizing data from the admin, config, and local databases. 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. 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. A two-way synchronization task includes forward and reverse synchronization tasks. When you configure or reset the task, if the destination object of one task matches the synchronization object of the other task: Allow only one task to synchronize full and incremental data. The other task supports only incremental synchronization. Data from the source of the current task synchronizes only to the destination of the current task. It does not serve as source data for the other task. 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). DTS does not support synchronizing time-series collections introduced in MongoDB 5.0 and later. 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.

Billing

Synchronization type	Pricing
Schema synchronization and full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

Supported two-way synchronization topologies

Currently, DTS supports two-way synchronization only between two ApsaraDB for MongoDB replica set instances. Synchronization among multiple ApsaraDB for MongoDB instances is not supported.

Supported conflict detection

To maintain data consistency, update records with the same primary key, business primary key, or unique key on only one node at a time.

DTS detects and resolves the following conflict types to ensure synchronization stability:

Uniqueness conflicts caused by INSERT operations

If an INSERT adds a record that already exists in the destination, DTS ignores the operation.
Mismatched records during UPDATE operations

If an UPDATE targets a non-existent record in the destination or causes a conflict, DTS ignores the operation.
DELETE operations on non-existent records

If a DELETE targets a non-existent record in the destination, DTS ignores the operation.

Important

DTS conflict detection cannot fully prevent data conflicts due to system time differences and synchronization latency. Your application must update records with the same primary key, business primary key, or unique key on only one node at a time.
The default Conflict Resolution Policy is Ignore and cannot be changed.

Synchronization types

Type	Description
Schema synchronization	Synchronizes selected object schemas from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.
Full data synchronization	Synchronizes all existing data of selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note Full data synchronization supports databases and collections.
Incremental data synchronization	After full data synchronization, synchronizes incremental data from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. 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.

Procedure

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.
Click Create Task to open the task configuration page.

Configure the source and destination databases.

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 of the source ApsaraDB for MongoDB instance.
	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 Oplog.
	Instance ID	Select the ID of the source ApsaraDB for MongoDB instance.
	Authentication Database	Enter the authentication database name for the source ApsaraDB for MongoDB instance account. Default: admin.
	Database Account	Enter the database account of the source ApsaraDB for MongoDB instance. The account requires read permissions on the synchronized databases and the `config`, `admin`, and `local` databases. Note Create and configure accounts as described in MongoDB database account permissions management.
	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 of the destination ApsaraDB for MongoDB instance.
	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.
	Instance ID	Select the ID of the destination ApsaraDB for MongoDB instance.
	Authentication Database	Enter the authentication database name for the destination ApsaraDB for MongoDB instance account. Default: admin.
	Database Account	Enter the database account of the destination ApsaraDB for MongoDB instance. The account requires dbAdminAnyDatabase, readWrite on the destination database, and read on the local database. Note Create and configure accounts as described in MongoDB database account permissions management.
	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.

After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
Ensure that the IP address blocks of the DTS service are added to the security settings of the source and destination databases, either automatically or manually, to allow access from DTS servers. For more information, see Add the IP address whitelist of DTS servers.

Configure the task objects.

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.
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 Two-way Synchronization.
Exclude DDL Operations	Select Yes: DDL operations are not synchronized. Select No: DDL operations are synchronized. Important DDL synchronization is supported in the forward direction only. Reverse DDL synchronization is not supported.
Conflict Resolution Policy	Select a conflict resolution policy. Only Ignore is supported: DTS keeps the destination data and discards the conflicting source change. When a conflict listed in Supported conflict detection occurs, DTS skips the statement and preserves the destination record.
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 to move them to the Selected Objects box. Note Objects can be selected 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 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.

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 This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
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	Optional. Select an environment tag to identify the instance.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
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. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert notifications. For more information, see Configure monitoring and alerting during task configuration.

Click Data Verification to configure a data verification task.
To use the data verification feature, see Configure data verification.

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.

Purchase the instance.

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

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.

Read and select the checkbox for Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start, and then click OK in the OK dialog box.
You can monitor the task progress on the data synchronization page.

Configure the reverse synchronization task.
1. Wait until the forward synchronization task completes initialization and its Status becomes Running.
2. Locate the reverse synchronization task and click Configure Task.
3. Repeat Step 3 to Step 6 to configure the reverse synchronization task.
  Important
  
  In the reverse task, source and destination are swapped: the source is the forward task's destination, and vice versa. Verify that the database name, account, and password match.
  
  We recommend that you do not change mapping names in the reverse task. Changing them may cause data inconsistency.
  
  The Instance Region cannot be changed in the reverse task. The reverse task has fewer parameters; use the values shown in the console.
  
  The Processing Mode of Conflicting Tables setting in the reverse task does not check collections already synchronized by the forward task.
  
  The reverse task cannot synchronize objects already in the forward task's Selected Objects list.
  
  The reverse synchronization task automatically excludes DDL operations.
4. When the Success Rate is 100%, click Back.
After configuring the reverse task, wait until both tasks show Status as Running. Two-way synchronization is now active.