This topic describes how to synchronize incremental data in real time from a PolarDB-X 1.0 instance to a PolarDB for MySQL cluster by using Data Transmission Service (DTS).

Prerequisites

  • A PolarDB-X 1.0 instance is created. For more information, see Create a PolarDB-X 1.0 instance and Create a database.
    Note The storage type of the PolarDB-X 1.0 instance must be ApsaraDB RDS for MySQL, including custom instances and purchased instances. PolarDB for MySQL cannot be used as the storage type.
  • The destination PolarDB for MySQL cluster is created. For more information, see Purchase a pay-as-you-go cluster and Purchase a subscription cluster.
  • The available storage space of the destination cluster is larger than the total size of the data in the source instance.

Usage notes

Note
  • During schema synchronization, DTS synchronizes foreign keys from the source database to the destination database.
  • During full synchronization and incremental synchronization, DTS temporarily disables checking of foreign key constraints and foreign key cascade operations at the session level. If you perform the cascade update and delete operations on the source database during data synchronization, data inconsistency may occur.
CategoryDescription
Limits on the source database
  • The tables 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. The tables that only have UNIQUE constraints do not support schema synchronization. Therefore, we recommend that you synchronize the tables that have PRIMARY KEY constraints. Tables that have secondary indexes cannot be synchronized.
  • If you select tables as the objects to be synchronized and you want to edit the tables (such as renaming tables or columns) in the destination database, you can synchronize up to 5,000 tables in a single data synchronization task. If you run a task to synchronize more than 5,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to synchronize the tables in batches or configure a task to synchronize the entire database.
  • The following requirements for binary logs of the ApsaraDB RDS for MySQL instances attached to the PolarDB-X 1.0 instance must be met:
    • The binary logging feature is enabled. The value of the binlog_row_image parameter is set to full. Otherwise, error messages are returned during the precheck and the data synchronization task cannot be started.
    • For an incremental data synchronization task, the binary logs of the source database must be retained for at least 24 hours. For a full and incremental data synchronization task, the binary logs of the source database must be retained for at least seven days. Otherwise, DTS may fail to obtain the binary logs and the task may fail. In exceptional circumstances, data may be inconsistent or lost. After 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 binary logs in accordance with the preceding requirements. Otherwise, the service reliability and performance stated in the Service Level Agreement (SLA) of DTS cannot be achieved.

  • Limits on operations to be performed on the source database:
    • If you change the network type of the PolarDB-X 1.0 instance during data synchronization, you must modify the network connection information of the data synchronization task after you change the network type.
    • During data synchronization, do not scale the source instance, including the ApsaraDB RDS for MySQL instance attached to the source instance, or change the distribution of physical databases for which a logical database or logical table has been configured in the ApsaraDB RDS for MySQL instance. In addition, do not migrate frequently-updated table, change the shard keys, or perform online DDL operations on the source instance. Otherwise, the data synchronization task fails or data inconsistency occurs.ApsaraDB RDS for MySQL
  • The storage type of the PolarDB-X 1.0 instance must be ApsaraDB RDS for MySQL, including custom instances and purchased instances. PolarDB for MySQL cannot be used as the storage type.
  • PolarDB-X 1.0 storage resources can be split only by using horizontal splitting. Both databases and tables can be split. Vertical splitting is not supported.
  • Read-only instances at the PolarDB-X 1.0 compute layers are not supported.
Other limits
  • When DTS synchronizes data from a PolarDB-X 1.0 instance, data is distributed across the attached ApsaraDB RDS for MySQL instances. DTS runs a subtask for each ApsaraDB RDS for MySQL instance. The running state of the subtask is displayed in Task Topology.
  • 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 initial full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers.
  • During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data synchronization is complete, the size of used tablespace of the destination database is larger than that of the source database.
  • We recommend that you do not use pt-online-schema-change to perform DDL operations on objects. Otherwise, data synchronization may fail.
  • 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 typeTask configuration fee
Schema synchronization and full data synchronizationFree of charge.
Incremental data synchronizationCharged. For more information, see Billing overview.

Supported synchronization topologies

  • One-way one-to-one synchronization
  • One-way one-to-many synchronization
  • One-way cascade synchronization
  • One-way many-to-one synchronization

For more information about the synchronization topologies supported by DTS, see Synchronization topologies.

SQL operations that can be synchronized

Operation typeSQL statement
DMLINSERT, UPDATE, and DELETE

Permissions required for database accounts

DatabaseRequired permissionReferences
Source PolarDB-X 1.0 instanceRead permissions on the objects to be synchronized Manage accounts
Destination PolarDB for MySQL clusterRead and write permissions on the destination cluster to which the objects are to be synchronized Create a database account

Procedure

  1. Go to the Data Synchronization Tasks page.
    1. Log on to the Data Management (DMS) console.
    2. In the top navigation bar, click DTS.
    3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
    Note
  2. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
    Note If you use the new DTS console, you must select the region in which the data synchronization instance resides in the top navigation bar.
  3. 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.
    SectionParameterDescription
    N/ATask Name

    DTS automatically generates a task name. We recommend that you specify an informative name to identify the task. You do not need to use a unique task name.

    Source DatabaseDatabase TypeThe type of the source database. Select PolarDB-X 1.0.
    Access MethodThe access method of the source instance. Select Alibaba Cloud Instance.
    Instance RegionThe region in which the source PolarDB-X 1.0 instance resides.
    Instance IDThe ID of the source PolarDB-X 1.0 instance.
    Database AccountThe database account of the source PolarDB-X 1.0 instance. For information about the permissions that are required for the account, see Permissions required for database accounts.
    Database Password

    The password of the database account.

    Destination DatabaseDatabase TypeThe type of the destination database. Select PolarDB for MySQL.
    Access MethodThe access method of the destination database. Select Alibaba Cloud Instance.
    Instance RegionThe region in which the destination PolarDB for MySQL cluster resides.
    PolarDB Cluster IDThe ID of the destination PolarDB for MySQL cluster.
    Database AccountThe database account of the destination PolarDB for MySQL cluster. For information about the permissions that are required for the account, see Permissions required for database accounts.
    Database Password

    The password of the database account.

  4. In the lower part of the page, click Test Connectivity and Proceed.
    Warning
    • 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.
    • 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. 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.
  5. Select objects for the task and configure advanced settings.
    Parameter or settingDescription
    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 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 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, data may fail to be initialized. In this case, only some columns are synchronized or the data synchronization task fails.
    Capitalization of Object Names in Destination Instance

    The capitalization of database names, table names, and column 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.

    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 tables as the objects to be synchronized.

    If you select an entire database as the object to be synchronized, DTS does not synchronize the changes made to create tables in or delete tables from the source database to the destination database.

    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 select the SQL operations performed on a specific database or table, right-click an object to be synchronized in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information about the SQL operations that can be synchronized, see SQL operations that can be synchronized.
    • To specify WHERE conditions to filter data, right-click an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information about how to specify the conditions, see Use SQL conditions to filter data.
  6. Click Next: Advanced Settings to configure advanced settings.
    ParameterDescription
    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 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.
    • 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 the earliest opportunity after the source and destination instances are released.
    Configure ETL
    Specifies whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?. Valid values:
  7. 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 where 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 where 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.
  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.
    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.
  10. Read and select the check box for Data Transmission Service (Pay-as-you-go) Service Terms.
  11. Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.