Two-way data synchronization between ApsaraDB for MongoDB sharded cluster instances - Data Transmission Service

Data Transmission Service (DTS) supports two-way data synchronization between two ApsaraDB for MongoDB sharded cluster instances. This feature is suitable for scenarios such as active geo-redundancy (unit-based) and geo-disaster recovery. This topic describes how to configure two-way data synchronization between ApsaraDB for MongoDB sharded cluster instances.

Prerequisites

The source and destination ApsaraDB for MongoDB sharded cluster instances are created. For more information, see Create a sharded cluster instance.
Important
- Endpoints are assigned to the shards in the source ApsaraDB for MongoDB sharded cluster instance, and the shards share the same account and password. For more information, see Apply for an endpoint for a shard node or the ConfigServer node in a sharded cluster instance.
- For more information about the supported database versions, see Overview of data synchronization scenarios.
The available storage space of the destination ApsaraDB for MongoDB instance is 10% larger than the total size of the data in the source ApsaraDB for MongoDB instance. This is a recommended prerequisite.
The replication.oplogGlobalIdEnabled parameter of the shard and Configserver nodes of the source and destination instances is set to true. For more information, see Configure database parameters for an ApsaraDB for MongoDB instance.
Note
If the replication.oplogGlobalIdEnabled parameter is not set to true, the precheck fails or the two-way mongo must have gid error is returned.
The databases and collections to be sharded are created in the source and destination ApsaraDB for MongoDB sharded cluster instances, data sharding is configured, the balancer is enabled, and pre-sharding is performed based on your business requirements. For more information, see Configure sharding to maximize the performance of shards and the What do I do if the data of a MongoDB database deployed in the sharded cluster architecture is not evenly distributed? section of the "FAQ" topic.
Note
- After you configure sharding for a sharded cluster instance, the synchronized data is distributed among different shards. This maximizes the performance of the sharded cluster. After you enable the balancer for pre-sharding, data skew is prevented.
- In this topic, a DTS task is configured before a DTS instance is purchased. You do not need to specify the number of shards in the source ApsaraDB for MongoDB sharded cluster instance.
  If you purchase a DTS instance before you configure a DTS task, you must specify the number of shards when you purchase the instance.

Limits

Category	Description
Limits on the source and destination databases	Bandwidth requirements: The server to which the source database is deployed must have sufficient outbound bandwidth. Otherwise, the data synchronization speed is affected. The collections to be synchronized must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records. If you select collections as the objects to be synchronized and you need to edit collections in the destination database, such as renaming collections, up to 1,000 collections can be synchronized in a single data synchronization task. If you run a task to synchronize more than 1,000 collections, a request error occurs. In this case, we recommend that you configure multiple tasks to synchronize the collections in batches or configure a task to synchronize the entire database. The oplog feature must be enabled. Note The operation logs of the source database must be retained for at least seven days. Otherwise, DTS may fail to obtain the operation logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. Make sure that you set the retention period of operation logs based on the preceding requirements. Otherwise, the service level agreement (SLA) of DTS does not guarantee service reliability or performance. During a data synchronization task, MongoDB sharded cluster databases involved in the task cannot be scaled. Otherwise, the task fails. If the source database is a self-managed MongoDB database that uses the sharded cluster architecture, you can set the Access Method parameter only to Express Connect, VPN Gateway, or Smart Access Gateway or Cloud Enterprise Network (CEN) when you configure the DTS task. The number of Mongos nodes in the source MongoDB sharded cluster database cannot exceed 10. You cannot synchronize collections that contain time to live (TTL) indexes. If the database to be synchronized contains TTL indexes, data inconsistency may occur between the source and destination databases due to inconsistent time zones and time clocks of the source and destination databases. Make sure that no orphaned document exists in the source and destination databases. Otherwise, data inconsistency or even task failure may occur. For more information, see orphaned document in official MongoDB documentation and the How do I delete orphaned documents of a MongoDB database deployed in the sharded cluster architecture? section of the "FAQ" topic. The source and destination databases must be ApsaraDB for MongoDB instances with the same architecture, meaning that self-managed MongoDB or MongoDB with different architectures do not support two-way data synchronization.
Other limits	Before you start the data synchronization task, you must add sharding keys to the data to be synchronized in the source database. During the data synchronization, if you execute the INSERT statement to insert data into the data to be synchronized, the data to be synchronized must contain sharding keys. If you execute the UPDATE statement to modify the data to be synchronized, you cannot modify sharding keys. To ensure compatibility, the version of the destination MongoDB database must be the same as or later than the version of the source MongoDB database. If the version of the destination database is earlier than that of the source database, database compatibility issues may occur. If a collection of the destination database has a unique index or the `capped` attribute of a collection of the destination database is `true`, the collection supports only single-thread data writing and does not support concurrent replay during incremental data synchronization. This may increase synchronization latency. DTS cannot synchronize data from the admin or local database. Transaction information is not retained. When transactions are synchronized to the destination database, transactions are converted into a single record. Before you synchronize data, evaluate the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. During full data synchronization, DTS uses read and write resources of the source and destination databases. This may increase the loads on the database servers. During full data synchronization, concurrent INSERT operations cause fragmentation in the collections of the destination database. After full data synchronization is complete, the storage space for collections of the destination database is larger than that of the source database. During data synchronization, we recommend that you use only DTS to write data to the destination database. This prevents data inconsistency between the source and destination databases. For example, if you use tools other than DTS to write data to the destination database, data loss may occur in the destination database when you use DMS to perform online DDL operations. A two-way data synchronization instance contains a forward synchronization task and a reverse synchronization task. If an object is to be synchronized in both the forward and reverse synchronization tasks when you configure or reset the instance, the following rules apply: Only one of the tasks can synchronize both the full data and incremental data of the object. The other task synchronizes only the incremental data of the object. The source data of the current task can be synchronized only to the destination of the task. The synchronized data is not used as the source data of the other task. The data is concurrently written to the destination database. Therefore, the storage space occupied in the destination database is 5% to 10% larger than the size of the data in the source database. You must use the `db.$table_name.aggregate([{ $count:"myCount"}])` syntax to query the return value of a count operation on the destination MongoDB database. Make sure that the destination MongoDB database does not have the same primary key as the source database. The default primary key is _id. Otherwise, data may be lost. If the data in the destination database has the same primary key as that in the source database, clear the related data in the destination database without interrupting the services of DTS. For example, if the same primary key is _id, you can delete the data in the destination database that has the same _id as the source database. Make sure that the MongoDB balancer of the source database is disabled during full data synchronization. Do not enable the balancer until all full data synchronization is complete and incremental data synchronization starts. Otherwise, data inconsistency may occur. For more information about the MongoDB balancer, see Manage the ApsaraDB for MongoDB balancer. If data sharding is configured for the destination instance and you do not need to use the schema synchronization feature of DTS, do not select Schema Synchronization as one of the Synchronization Types in the Configure Objects and Advanced Settings step. Otherwise, data inconsistency may occur or the task may fail due to shard conflicts.

Billing

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

Supported two-way data synchronization topologies

DTS supports two-way data synchronization only between two ApsaraDB for MongoDB sharded cluster instances. DTS does not support two-way data synchronization among multiple ApsaraDB for MongoDB instances.

Supported conflict detection

To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes. If data records are updated on both nodes, DTS responds to conflicts based on the conflict resolution policy that you specify for the data synchronization task.

DTS checks and fixes conflicts to maximize the stability of two-way synchronization instances. DTS can detect the following types of conflicts:

Uniqueness conflicts caused by INSERT operations
INSERT operations that do not comply with the uniqueness constraint cannot be synchronized. For example, if a record with the same primary key value is inserted into the two synchronization nodes at almost the same time, one of the inserted records fails to be synchronized. The synchronization fails because a record with the same primary key value already exists on the other node.
Inconsistent records caused by UPDATE operations
- If the records to be updated do not exist in the destination instance, DTS converts the UPDATE operation into an INSERT operation. However, uniqueness conflicts may occur.
- The primary keys or unique keys of the records to insert may conflict with those of existing records in the destination instance.
Non-existent records to be deleted
The records to be deleted do not exist in the destination instance. In this case, DTS ignores the DELETE operation regardless of the conflict resolution policy that you specify.

Important

During two-way synchronization, the system time of the source and destination instances may be different. Synchronization latency may occur. For these reasons, DTS does not ensure that the conflict detection mechanism can prevent all data conflicts. To perform two-way synchronization, make sure that records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes.
DTS provides conflict resolution policies to prevent conflicts that may occur during data synchronization. You can select a conflict resolution policy when you configure two-way data synchronization.

Synchronization types

Synchronization type	Description
Schema synchronization	DTS synchronizes the schemas of the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.
Full data synchronization	DTS synchronizes all existing data of the selected objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS supports full data synchronization for the following types of objects: databases and collections.
Incremental data synchronization	DTS synchronizes incremental data from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance. Note DTS synchronizes incremental data generated by the following operations: CREATE COLLECTION and CREATE INDEX DROP COLLECTION and DROP INDEX RENAME COLLECTION The operations that are performed to insert, update, and delete documents in a collection.

Procedure

Important

In this topic, a DTS task is configured before a DTS instance is purchased. You do not need to specify the number of shards in the source ApsaraDB for MongoDB sharded cluster instance.

If you purchase a DTS instance before you configure a DTS task, you must specify the number of shards when you purchase the instance.

Go to the Data Synchronization Tasks page of the new DTS console.
Note
You can also log on to the Data Management (DMS) console. In the top navigation bar, move the pointer over DTS and choose DTS (DTS) > Data Synchronization.
In the top navigation bar, select the region in which you want to create the data synchronization task.

Click Create Task. In the Create Task wizard, configure the source and destination databases. The following table describes the parameters.

Warning

After you configure the source and destination databases, we recommend that you read the Limits that are displayed in the upper part of the page. Otherwise, the task may fail or data inconsistency may occur.

Section	Parameter	Description
N/A	Task Name	The name of the task. DTS automatically assigns a name to the task. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select an existing DMS database instance. (Optional. If you have not registered a DMS database instance, ignore this option and configure database settings in the section below.)	The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must configure parameters for the source database.
	Database Type	The type of the source instance. Select MongoDB.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source ApsaraDB for MongoDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
	Architecture	The architecture in which the source instance is deployed. Select Sharded Cluster.
	Instance ID	The ID of the source ApsaraDB for MongoDB instance.
	Authentication Database	The name of the authentication database that stores the database accounts and passwords of the source ApsaraDB for MongoDB instance. If you did not change the name of the authentication database before, the default value is admin.
	Database Account	The database account of the source ApsaraDB for MongoDB instance. The account must have the read permissions on the source, config, admin, and local databases.
	Database Password	The password of the database account.
	Shard Account	The account that is used to access the shards in the source ApsaraDB for MongoDB instance.
	Shard Password	The password that is used to access the shards in the source ApsaraDB for MongoDB instance.
Destination Database	Select an existing DMS database instance. (Optional. If you have not registered a DMS database instance, ignore this option and configure database settings in the section below.)	The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must configure parameters for the source database.
	Database Type	The type of the destination database. Select MongoDB.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination ApsaraDB for MongoDB instance resides.
	Architecture	The architecture in which the destination instance is deployed. Select Sharded Cluster.
	Instance ID	The ID of the destination ApsaraDB for MongoDB instance.
	Authentication Database	The name of the authentication database that stores the database accounts and passwords of the destination ApsaraDB for MongoDB instance. If you did not change the name of the authentication database before, the default value is admin.
	Database Account	The database account of the destination ApsaraDB for MongoDB instance. The account must have the dbAdminAnyDatabase permission, the read and write permissions on the destination database, and the read permissions on the local database.
	Database Password	The password of the database account.

In the lower part of the page, click Test Connectivity and Proceed.
If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you must make sure that the ECS instance can access the database. If the database is deployed on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the whitelist of the database to allow DTS to access the database. For more information, see Add the CIDR blocks of DTS servers.
Warning
If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database or instance, or to the ECS security group rules, security risks may arise. Therefore, before you use DTS to synchronize data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

Configure the objects to be synchronized and the advanced settings.

Parameter	Description
Synchronization Types	By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination cluster. The historical data is the basis for subsequent incremental synchronization.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains collections that have the same names as the collections in the source database. If the source and destination databases do not contain collections that have identical collection names, the precheck is passed. Otherwise, an error is returned during the precheck, and the data synchronization task cannot be started. Note If the source and destination databases have collections with identical names and the collections in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the collections that are synchronized to the destination database. For more information, see Rename an object to be synchronized. Ignore Errors and Proceed: skips the precheck for identical collection names in the source and destination databases. Warning If you select Ignore Errors and Proceed, data inconsistency may occur and your business may be exposed to potential risks. If a data record in the destination database has the same primary key value or unique key value as a data record in the source database, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained. Data may fail to be initialized, only specific columns are synchronized, or the data synchronization task fails.
Synchronization Topology	The synchronization topology of the data synchronization task. Select Two-way Synchronization.
Exclude DDL Operations	Yes: excludes DDL operations. No: synchronizes DDL operations. Note Limits on DDL synchronization directions: To ensure the stability of two-way data synchronization, DTS synchronizes DDL operations only in the forward direction.
Conflict Resolution Policy	If you encounter the conflicts described in the Supported conflict detection section of this topic, select a conflict resolution policy based on your business requirements. TaskFailed If a conflict occurs during data synchronization, the data synchronization task reports an error and exits the process. The task enters a failed state, and you must manually resolve the conflict. Ignore If a conflict occurs during data synchronization, the data synchronization task ignores the current statement and continues the process. The conflicting records in the destination database are used. Overwrite If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten. Note Only the Ignore policy is supported in this scenario.
Source Objects	Select one or more objects from the Source Objects section and click the icon to move the objects to the Selected Objects section. Note You can select databases or collections as the objects to be synchronized.
Selected Objects	To rename an object that you want to synchronize to the destination instance, right-click the object in the Selected Objects section. For more information, see the Map the name of a single object section of the Map object names topic. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see the Map multiple object names at a time section of the Map object names topic. Note To specify conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the conditions. You can specify conditions to filter data during the full data synchronization stage of a data synchronization task. You cannot specify conditions to filter data during the incremental data synchronization stage of a data synchronization task. For more information, see Set filter conditions. If you use the object name mapping feature to rename the database or collection to which the data is synchronized, other objects that depend on the database or collection may fail to be synchronized.

Click Next: Advanced Settings to configure advanced settings.

Data Verification Settings
For more information about how to configure the data verification feature, see Configure data verification.

Advanced Settings

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules tasks to shared clusters. You do not need to configure this parameter. If you want to improve the stability of data migration tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Set Alerts	Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts will receive notifications. Valid values: No: does not configure alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see Configure monitoring and alerting when you create a DTS task.
Retry Time for Failed Connections	The retry time range for failed connections. If the source or destination database fails to be connected after the data synchronization task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set this parameter to a value greater than 30. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Note If you specify different retry time ranges for multiple data synchronization tasks that have the same source or destination database, the shortest retry time range takes precedence. When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
Retry Time for Other Issues	The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set this parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Important The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Migration	During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server. Note This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter.
Enable Throttling for Incremental Data Synchronization	Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, no environment tag is selected.
Configure ETL	Specifies whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: configures the ETL feature. You can enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: does not configure the ETL feature.

Save the task settings and run a precheck.
- To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.
- If you do not need to view or have viewed the parameters, click Next: Save Task Settings and Precheck in the lower part of the page.
Note
- Before you can start the data synchronization task, DTS performs a precheck. You can start the data synchronization task only after the task passes the precheck.
- If the task fails to pass the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
- If an alert is generated for an item during the precheck, perform the following operations based on the scenario:
  If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issue. Then, run a precheck again.
  If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur and your business may be exposed to potential risks.
Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.

On the Purchase Instance page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay for your subscription when you create an instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use. You are offered lower prices for longer subscription durations. Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go instance, you can release the instance to reduce costs.
	Resource Group Settings	The resource group to which the instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides various synchronization specifications that support different performance. The synchronization speed varies based on the synchronization specifications that you select. You can select a synchronization specification based on your business requirements. For more information, see Instance classes of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, set the subscription duration and the number of instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years. Note This parameter is available only if you select the Subscription billing method.

Read and select the Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start to start the data synchronization task in the forward direction. You can view the progress of the task in the task list.
Wait until the data synchronization task in the forward direction enters the Running state. Find the data synchronization task in the reverse direction and click Configure Task.
Configure the data synchronization task in the reverse direction by repeating Step 3 to Step 7.
Important
- When you configure the data synchronization task in the reverse direction, you must select the correct source and destination instances. The source instance in the reverse direction is the destination instance in the forward direction. The destination instance in the reverse direction is the source instance in the forward direction. You must also make sure that the parameter settings, such as the database name, account, and password, are consistent.
- We recommend that you do not use the object name mapping feature when you configure the data synchronization task in the reverse direction. Otherwise, data inconsistency may occur.
- When you configure the source and destination databases of the data synchronization task in the reverse direction, the Instance Region parameter cannot be modified. The number of parameters that you need to configure for a data synchronization task in the reverse direction is less than that for a data synchronization task in the forward direction. Configure the parameters that are displayed in the console.
- When DTS checks for conflicting tables in the reverse direction, the tables that have been synchronized to the destination instance in the forward direction are ignored.
- You cannot select the objects that are selected in the data synchronization task in the forward direction for the data synchronization task in the reverse direction.
- The data synchronization task in the reverse direction ignores DDL operations.
Wait until the success rate becomes 100%. Then, click Back.
After the data synchronization task in the reverse direction is configured, wait until both tasks enter the Running state. Two-way data synchronization is configured.