Configure two-way data synchronization between ApsaraDB for MongoDB instances (sharded cluster architecture) - ApsaraDB for MongoDB

Data Transmission Service (DTS) supports two-way data synchronization between two ApsaraDB for MongoDB instances (sharded cluster architecture). 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 instances.

Prerequisites

The source and destination ApsaraDB for MongoDB instances are created. For more information, see Create a sharded cluster instance.
Note 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 is set to true. For more information, see Configure database parameters for an ApsaraDB for MongoDB instance.
The source and destination ApsaraDB for MongoDB instances are deployed in a sharded cluster architecture. You need to create databases and collections to be sharded in the source and destination ApsaraDB for MongoDB instances and configured sharding based on your business requirements. For more information, see Configure sharding to maximize the performance of shards.
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.
- The shard nodes in a sharded cluster instance must share the same account password and endpoint. For more information, see Apply for an endpoint for a shard or Configserver node.
- 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 instance (sharded cluster architecture).
  If you purchase a DTS instance before you configure a DTS task, you must specify the number of shards when you purchase the instance.

Usage notes


Category	Description
Limits on the source and destination databases	Bandwidth requirements: The server to which the source database is deployed must have a 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 want to edit collections, 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 For an incremental data synchronization task, the oplogs of the source database must be stored for more than 24 hours. If you perform both full data synchronization and incremental data synchronization, the oplogs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the oplogs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After the full data synchronization is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of oplogs in accordance with the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not guarantee service reliability or performance. During a data synchronization task, ApsaraDB for MongoDB sharded cluster instances involved in the task cannot be scaled. Otherwise, the task fails. If you synchronize data from a self-managed MongoDB database, the source instance cannot be a sharded cluster instance. The number of Mongos nodes in the source ApsaraDB for MongoDB sharded cluster instance cannot exceed 10.
Other limits	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 the source or the destination instance is located in a region outside the Chinese mainland, two-way data synchronization is supported only between instances located within the same region. For example, if the source instance resides in the Japan (Tokyo) region, data can be synchronized only within the Japan (Tokyo) region and cannot be synchronized to or from the Germany (Frankfurt) region in two-way synchronization scenarios. 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 usage of collections in the destination database is larger than that of collections in 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. 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. If a table is synchronized in both the forward and reverse synchronization and both the full data and incremental data of the table are synchronized in the forward synchronization, DTS synchronizes only the incremental data of the table in the reverse synchronization. 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 loss may occur in the destination database. If the destination database has the same primary key as the source database, clear the related data in the destination database without interrupting the services of DTS. 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 ApsaraDB for MongoDB balancer 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 balancer, see Manage the ApsaraDB for MongoDB balancer .

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

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 objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.
Full data synchronization	DTS synchronizes existing data of 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: database, collection, and index.
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

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 instance (sharded cluster architecture).

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 page of the new DTS console.
Note You can also log on to the Data Management (DMS) console. In the top navigation bar, click DTS. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
In the upper-left corner of the page, select the region in which the data synchronization instance resides.
In the upper-left corner of the page, select the region in which the destination instance resides.

Click Create Task. On the page that appears, configure the source and destination databases.

Warning After you select the source and destination instances, we recommend that you read the limits displayed in the upper part of the page. This helps you create and run the data synchronization task.


Section	Parameter	Description
N/A	Task Name	The name of the task. DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. You do not need to specify a unique task name.
Source Database	Database Type	The type of the source database. 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.
	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 to which the objects to be synchronized in the source ApsaraDB for MongoDB instance belong. If you did not change the name before, the default value is admin.
	Database Account	The database account of the source ApsaraDB for MongoDB instance. The account must have read permissions on the source, admin, and local databases.
	Database Password	The password of the database account.
	Shard Account	The shard account of the source ApsaraDB for MongoDB instance. For more information, see Apply for an endpoint for a shard or Configserver node.
	Shard Password	The shard password of the source ApsaraDB for MongoDB instance.
Destination 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 to which the synchronized objects in the destination ApsaraDB for MongoDB instance belong. If you did not change the name 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.
	Shard Account	The shard account of the destination ApsaraDB for MongoDB instance.
	Shard Password	The shard password of the destination ApsaraDB for MongoDB instance.

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. DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. 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 need to manually add the CIDR blocks of DTS servers to the whitelist of the self-managed database on the ECS instance to allow DTS to access the database. 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.
Warning
- If the CIDR blocks of DTS servers are automatically or manually added to the whitelist or 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.
- After the DTS task is complete or released, we recommend that you manually remove the CIDR blocks of DTS servers from the whitelist or ECS security group rules. You must remove the IP address whitelist group whose name contains dts from the whitelist of the Alibaba Cloud database instance or the security group rules of the ECS instance. You must also remove the CIDR blocks of DTS servers from the whitelist of the self-managed database on the ECS instance. For more information about the CIDR blocks that you must remove from the whitelist of the self-managed databases that are deployed in data centers or databases that are hosted on third-party cloud services, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.

Select objects for the task and configure advanced settings.

Basic Settings MongoDB: Basic Settings


Parameter or setting	Description
Task Stages	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 selected objects from the source instance 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 tables that have the same names as tables in the source database. If the source and destination databases do not contain tables that have identical table names, the precheck is passed. Otherwise, an error is returned during the precheck and the data synchronization task cannot be started. Note You can use the object name mapping feature to rename the tables that are synchronized to the destination database. You can use this feature if the source and destination databases contain identical table names and the tables in the destination database cannot be deleted or renamed. For more information, see Map object names . Ignore Errors and Proceed: skips the precheck for identical table 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 the source and destination databases have the same schemas, and a data record has the same primary key value as an existing data record in the destination database: During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained. During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten. If the source and destination databases have different schemas, initial data synchronization may fail. In this case, only specific columns are synchronized, or the data synchronization task fails.
Synchronization Topology	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, you can synchronize DDL operations only in the forward direction.
Conflict Resolution Policy	If you encounter the conflicts described in Conflict detection, 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.
Select Objects	Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section. Note You can select databases or collections as the objects to be synchronized.
Rename Databases and Tables	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 Map the name of a single object. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time.
Filter data	You can specify WHERE conditions to filter data. For more information, see Use SQL conditions to filter data.

Advanced Settings MongoDB: Advanced Settings


Parameter	Description
Capitalization of Object Names in Destination Instance	The capitalization of database names and collection names in the destination instance. By default, DTS default policy is selected. You can select other options to make sure that the capitalization of object names is consistent with that in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
Retry Time for Failed Connection	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 the 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 set different retry time ranges for multiple DTS tasks that have the same source or destination database, the shortest retry time range that is set takes precedence. If 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, or release the DTS instance at the earliest opportunity after the source and destination instances are released.

In the lower part of the page, click Next: Save Task Settings and Precheck.
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:
  In scenarios in which you cannot ignore the alert item, click View Details next to the failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
  In scenarios in which you can ignore the alert item, click Confirm Alert Details next to the failed item. 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 the instance 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. Pay-as-you-go: A pay-as-you-go instance is charged 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 pay-as-you-go instance to reduce costs.
	Instance Class	DTS provides several instance classes that have different performance in synchronization speed. You can select an instance class based on your business scenario. For more information, see Specifications 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 or one to three years. Note This parameter is displayed only if you select the subscription billing method.

Read and select the check box for 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 initial synchronization is complete and the data synchronization task in the forward direction is in the Running state. You can view the status of the data synchronization task on the Data Synchronization page.
On the Data Synchronization page, click Create Task. On the page that appears, configure the source and destination databases as described in Step 4.
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.

Select objects for the task and configure advanced settings. as described in Step 6. We recommend that you specify the same settings. The following table describes other settings.

Basic Settings


Parameter	Description
Processing Mode of Conflicting Tables	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.
Select Objects	We recommend that you select the same objects for the forward and reverse directions. You can also add or remove objects based on your business requirements.
Rename Databases and Tables	We recommend that you do not use this feature when you configure the task in the reverse direction. Otherwise, data inconsistency may occur.

In the lower part of the page, click Next: Save Task Settings and Precheck.
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:
  In scenarios in which you cannot ignore the alert item, click View Details next to the failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
  In scenarios in which you can ignore the alert item, click Confirm Alert Details next to the failed item. 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 value becomes 100%. Then, click Back.
After the second data synchronization task is configured, wait until both tasks enter the Running state. The two-way data synchronization tasks are configured.