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.
ImportantEndpoints 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 or Configserver node.
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.
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.
NoteAfter 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 |
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Other limits |
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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.
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:
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Procedure
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.
NoteYou can also log on to the Data Management (DMS) console. In the top navigation bar, move the pointer over DTS and choose in the left-side navigation pane.
In the upper-left corner of the page, 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.
WarningAfter you configure the source and destination databases, we recommend that you read the limits that are displayed on the page. Otherwise, the task may fail or data inconsistency may occur.
Section
Parameter
Description
N/A
Task Name
The name of the DTS task. DTS automatically generates a task name. 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.)
Select whether to use an existing instance.
If you select an existing instance, DTS automatically populates the parameters for the database.
If you do not select an existing instance, you must manually configure parameters for the 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.
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, 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.)
Select whether to use an existing instance.
If you select an existing instance, DTS automatically populates the parameters for the database.
If you do not select an existing instance, you must manually configure parameters for the 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.
WarningIf 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 advanced settings.
Parameter
Description
Synchronization Types
The 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 database. 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.
NoteIf the source and destination databases have identical collection 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.
WarningIf 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, and only specific columns are synchronized, or the data synchronization task fails.
Synchronization on Topology
The synchronization topology of the data synchronization task. Select Two-way Synchronization.
Exclude DDL Operations
Yes: excludes DDL operations.
No: synchronizes DDL operations.
NoteLimits 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.
NoteOnly 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.
NoteYou 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.
NoteTo 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
Select the dedicated cluster used to schedule the task
By default, DTS schedules tasks to shared clusters. You do not need to configure this parameter. You can purchase a dedicated cluster of specific specifications to run DTS data synchronization tasks. 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 contacts. For more information, see Configure monitoring and alerting.
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.
NoteIf 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.
The wait time before a retry when other issues occur in the source and destination databases.
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.
ImportantThe value of the The wait time before a retry when other issues occur in the source and destination databases. 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.
NoteThis 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.
NoteBefore 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 triggered for an item during the precheck:
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.
Pay-as-you-go: A pay-as-you-go instance is billed on an hourly basis. We recommend that you select the pay-as-you-go billing method 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
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 provide 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 Specifications of data synchronization instances.
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.
NoteThis parameter is displayed only if you select the subscription billing method.Subscription
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.
ImportantWhen 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.Processing Mode of Conflicting Tables
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.Selected Objects
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.Status