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Data Transmission Service:Configure two-way data synchronization between ApsaraDB for MongoDB sharded cluster instances

Last Updated:Jan 15, 2024

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
  • 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 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 configure 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.

    • 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

CategoryDescription
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 typeTask configuration fee
Schema synchronization and full data synchronizationFree of charge.
Incremental data synchronizationCharged. 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

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.

  1. 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.

  2. In the upper-left corner of the page, select the region in which you want to create the data synchronization task.

  3. Click Create Task. On 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 select 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 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 shard nodes in the source ApsaraDB for MongoDB instance.

    Shard Password

    The password that is used to access shard nodes 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 select 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 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.

  4. 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 ensure that the ECS instance can 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. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.

    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.

  5. Configure the objects to be synchronized and the advanced settings.

    Parameter

    Description

    Synchronization Type

    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 instance. 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 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

      You can use the object name mapping feature to rename the collections that are synchronized to the destination database. You can use this feature if the source and destination databases contain collections that have identical names and the collections in the destination database cannot be deleted or renamed. 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.

      • DTS does not synchronize data records that have the same primary key values as data records in the destination database.

      • Data may fail to be initialized, only some 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 Rightwards arrow icon to add 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 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.

    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 Use SQL conditions to filter data.

    • If you use the object name mapping feature to rename the database or collection to which the data is synchronized, other objects that are dependent on the database or collection may fail to be synchronized.

  6. Click Next: Advanced Settings to configure advanced settings.

    • Data Verification Settings

      For more information about how to configure the data verification feature, see Enable 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 specified specifications to run DTS 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:

      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 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.

      • 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 the 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 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 Connection parameter.

      Enable Throttling for Full Data Migration

      During full data migration, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and BPS of Full Data Migration parameters for full data synchronization tasks to reduce the pressure on the destination database.

      Note

      This parameter can be configured only if Full Data Synchronization is selected as Synchronization Types.

      Enable Throttling for Incremental Data Synchronization

      You can configure the RPS of Incremental Data Migration and BPS of Incremental Data Migration parameters for incremental data synchronization tasks to reduce the pressure on the destination database.

      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, you do not need to configure this parameter.

      Configure ETL

      Specifies whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?. Valid values:
  7. Click Next: Save Task Settings and Precheck in the lower part of the page.

    You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameter settings of the API operation that is called to configure the instance.

    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 troubleshoot the issues based on the causes, 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 issues. 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.

  8. Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.

  9. 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.

    Resource Group

    The resource group on which the instance is run. 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 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, one year, two years, three years, or five years.

    Note

    This parameter is available only if you select the Subscription billing method.

  10. Read and select the check box to agree to the Data Transmission Service (Pay-as-you-go) Service Terms.

  11. 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.

  12. 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.

  13. 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.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.

  14. Wait until the success rate becomes 100%. Then, click Back.

  15. 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