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 versions, see Overview of data synchronization scenarios.
  • The available storage space of the destination ApsaraDB for MongoDB instance is larger than the total size of the data in the source ApsaraDB for MongoDB instance.
  • 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 must create databases and collections to be sharded in the source and destination ApsaraDB for MongoDB instances and configure sharding based on your business needs. 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 will be distributed among different shards. This maximizes the performance of shards in the sharded cluster instance.
    • 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 scenario, you can configure a DTS task and then purchase a DTS instance. 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.

Limits

Category Description
Limits on the source and destination databases
  • Bandwidth requirements: The server to which the source database belongs must have a sufficient egress 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 synchronize 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 split the collections to synchronize, configure multiple tasks to synchronize the tables, or configure a task to synchronize the entire database.
  • The following requirements must be met:
    • The oplog feature must be enabled.
    • 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 7 days. After the full data synchronization is complete, you can set the retention period to more than 24 hours. Otherwise, Data Transmission Service (DTS) may fail to obtain the oplogs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. 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 ensure service reliability and performance

  • During a data synchronization task, ApsaraDB for MongoDB sharded cluster instances involved in the task cannot be scaled. Otherwise, the task fails.
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 the version 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, two-way data synchronization is supported between instances within the Japan (Tokyo) region. Two-way data synchronization between an instance in the Japan (Tokyo) region and another instance in the Germany (Frankfurt) region is not supported.
  • DTS cannot synchronize data from the admin or local database.
  • Transaction information is not retained. When transactions are synchronized to the destination database, they 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 of the database servers.
  • During full data synchronization, concurrent INSERT operations cause fragmentation in the collections of the destination database. After the 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.

Billing

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

Supported synchronization topologies

DTS supports two-way data synchronization only between two ApsaraDB for MongoDB instances (sharded cluster architecture). DTS does not support two-way data synchronization between 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 in 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 be inserted 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.

Notice
  • During two-way synchronization, the system time of the source and destination instances may be different. Synchronization latency may occur. For these reasons, DTS cannot guarantee 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 required objects from the source ApsaraDB for MongoDB instance to the destination ApsaraDB for MongoDB instance.
Full data synchronization DTS synchronizes historical data of the required 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 supports the following types of incremental updates:
  • CREATE COLLECTION and CREATE INDEX
  • DROP COLLECTION and DROP INDEX
  • RENAME COLLECTION

Procedure

Notice In this scenario, you can configure a DTS task and then purchase a DTS instance. 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.

  1. Go to the Data Synchronization page of the new DTS console.
    Note You can also log on to the Data Management console. In the top navigation bar, click DTS. Then, in the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
  2. In the upper-left corner of the page, select the region where the data synchronization instance resides.
    Region
  3. In the upper-left corner of the page, select the region where the destination instance resides.
  4. 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 ensures that you create and run the data synchronization task.
    Section Parameter Description
    None Task Name

    The task name that DTS automatically generates. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to use a unique task name.

    Source Database
    Database Type Select MongoDB.
    Access Method Select Cloud Instance.
    Instance Region The region where the source ApsaraDB for MongoDB instance resides.
    Architecture Select Sharded Cluster.
    Instance ID The ID of the source ApsaraDB for MongoDB instance.
    The name of the authentication database The name of the source database in the ApsaraDB for MongoDB instance.
    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 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 Select MongoDB.
    Access Method Select Cloud Instance.
    Instance Region The region where the destination ApsaraDB for MongoDB instance resides.
    Architecture Select Sharded Cluster.
    Instance ID The ID of the destination ApsaraDB for MongoDB instance.
    The name of the authentication database The name of the destination database in the ApsaraDB for MongoDB instance.
    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.
  5. In the lower part of the page, click Test Connectivity and Proceed.
    Note
    • You do not need to modify the security settings for ApsaraDB instances (such as ApsaraDB RDS for MySQL and ApsaraDB for MongoDB) and ECS-hosted databases. DTS automatically adds the CIDR blocks of DTS servers to the whitelists of ApsaraDB instances or the security group rules of ECS instances. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.
    • After data synchronization is complete, we recommend that you remove the CIDR blocks of DTS servers from the whitelists or security groups.
  6. Select the objects to migrate.
    • Basic SettingsBasic Settings
      Parameter Description
      Task Stages

      Incremental Data Synchronization is selected by default. 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 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 migrated 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 schema, and a data record has the same primary key 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 part of the columns are synchronized, or the data synchronization task fails.
      Synchronization Topology Select Two-way Synchronization.
      Exclude DDL Operations
      • To exclude DDL operations, select Yes.
      • To include DDL operations, select No.
        Note Limits on DDL synchronization direction: 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 needs.
      • TaskFailed

        If a conflict occurs during data synchronization, an error is reported and the task exits. The task fails and you must manually resume the task.

      • Ignore

        If a conflict occurs during data synchronization, the synchronization task ignores the current statement and the task continues. 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 This scenario supports only the Ignore policy.
      Select 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 synchronize.
      Rename Databases and Tables
      • To rename an object in 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 in the destination instance, 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 SettingsAdvanced Settings
      Parameter Description
      Capitalization of Object Names in Destination Instance

      Specifies 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 of the source or destination database. For more information,see Specify the capitalization of object names in the destination instance.

      Retry Time for Failed Connection
      Specifies the retry time range for failed connections. Valid values: 10 to 1440. Unit: minutes. Default value: 120. We recommend that you set the retry time range to more than 30 minutes. 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 an instance serves as the source or destination database of multiple data synchronization tasks, the less value that is specified for the instance 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 needs. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
  7. 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, you can click the Info icon icon next to each failed item to view details.
      • After you troubleshoot the issues based on the causes, you can run a precheck again.
      • If you do not need to troubleshoot the issues, you can ignore failed items and run a precheck again.
  8. Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.
  9. On the Purchase Instance page, specify the billing method and specifications for the data synchronization instance. The following table describes related parameters.
    Section Parameter Description
    Parameters 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. For short-term use, we recommend that you select the pay-as-you-go billing method. If you no longer need a pay-as-you-go instance, you can release the 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 available only if you select the subscription billing method.
  10. Read and select 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 initial synchronization is completed and the data synchronization task in the forward direction is in the Running state. You can view the state of the data synchronization task on the Data Synchronization page.
  13. On the Data Synchronization page, click Create Task. On the page that appears, configure the source and destination databases, as described in Step 4.
    Notice 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 corresponds to the destination instance in the forward direction. The destination instance in the reverse direction corresponds to 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.
  14. Select the objects to migrate., 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 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.
  15. 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, you can click the Info icon icon next to each failed item to view details.
      • After you troubleshoot the issues based on the causes, you can run a precheck again.
      • If you do not need to troubleshoot the issues, you can ignore failed items and run a precheck again.
  16. Wait until the Success Rate becomes 100%. Then, click Back.
  17. After the second data synchronization task is configured, wait until both tasks are in the Running state. The two-way data synchronization tasks are configured.