This topic describes how to configure one-way synchronization between ApsaraDB RDS for PostgreSQL instances by using Data Transmission Service (DTS).

Prerequisites

  • The source and destination ApsaraDB RDS for PostgreSQL instances are created. For more information, see Create an ApsaraDB RDS for PostgreSQL instance.
    Note To ensure compatibility, the version of the destination database must be the same as or later than that of the source database.

    If the version of the destination database is earlier than that of the source database, database compatibility issues may occur.

  • The available storage space of the destination ApsaraDB RDS for PostgreSQL instance is larger than the total size of the data in the source ApsaraDB RDS for PostgreSQL instance.

Limits

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.
  • 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 WAL logs must be met:
    • The value of the wal_level parameter is set to logical.
    • If you perform only incremental data synchronization, the WAL logs of the source database must be stored for more than 24 hours. If you perform both full data synchronization and incremental data synchronization, the WAL logs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the WAL logs 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 WAL logs based on the preceding requirements. Otherwise, service reliability or performance in the Service Level Agreement (SLA) of DTS may not be guaranteed.

  • Limits on operations to be performed on the source database:

    If you want to perform a primary/secondary switchover on the source ApsaraDB RDS for PostgreSQL instance, the Logical Replication Slot Failover feature must be enabled. This prevents logical subscriptions from being interrupted and ensures that your data synchronization task can run as expected. For more information, see Logical Replication Slot Failover.

  • If the source database has long-running transactions and incremental data is synchronized in the data synchronization task, the write-ahead logging (WAL) logs generated before the long-running transactions in the source database are committed may be accumulated. As a result, the disk space of the source database may be insufficient.
Other limits
  • A single data synchronization task can synchronize data from only one database. To synchronize data from multiple databases, you must create a data synchronization task for each database.
  • During data synchronization, if you select a schema as the object to synchronize, take note of the following limits: If you create a table in the schema or run the RENAME command to rename the table, you must execute the ALTER TABLE schema.table REPLICA IDENTITY FULL; statement before you write data to the table. This ensures data consistency. When you execute this statement, we recommend that you do not lock the table. Otherwise, a deadlock occurs.
    Note
    • Replace the schema and table variables in the preceding sample statement with your actual schema name and table name.
    • We recommend that you perform this operation during off-peak hours.
  • DTS does not check the validity of metadata such as sequences. You must manually check the validity of metadata.
  • After your workloads are switched to the destination database, newly written sequences do not increment from the maximum value of the sequences in the source database. Therefore, you must query the maximum value of the sequences in the source database before you switch your workloads to the destination database. Then, you must specify the queried maximum value as the starting value of the sequences in the destination database. You can execute the following statements to query the maximum value of the sequences in the source database:
    do language plpgsql $$
    declare
      nsp name;
      rel name;
      val int8;
    begin
      for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S'
      loop
        execute format($_$select last_value from %I.%I$_$, nsp, rel) into val;
        raise notice '%',
        format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1);
      end loop;
    end;
    $$;
  • DTS creates the following temporary tables in the source database to obtain the DDL statements of incremental data, the schemas of incremental tables, and the heartbeat information. During data synchronization, do not delete temporary tables in the source database. Otherwise, exceptions occur. After the DTS instance is released, temporary tables are automatically deleted.

    public.DTS_PG_CLASS, public.DTS_PG_ATTRIBUTE, public.DTS_PG_TYPE, public.DTS_PG_ENUM, public.DTS_POSTGRES_HEARTBEAT, public.DTS_DDL_COMMAND, and public.DTS_ARGS_SESSION.

  • To ensure that the latency of data synchronization is accurate, DTS adds a heartbeat table to the source database. The name of the heartbeat table is dts_postgres_heartbeat.
  • During data synchronization, DTS creates a replication slot for the source database. The replication slot is prefixed with dts_sync_. DTS automatically clears historical replication slots every 120 minutes to reduce storage usage.
    Note
    • After the DTS instance is released, the replication slot is automatically deleted. If you change the password of the source database or delete the IP addresses of DTS from the IP address whitelist, the replication slot cannot be automatically deleted. In this case, you must delete the replication slot in the source database to prevent the replication slot from piling up.
    • If the data synchronization task is released or fails, DTS automatically clears the replication slot. If a primary/secondary switchover is performed on the source PostgreSQL database, you must log on to the secondary database to clear the replication slot.
    Replication slot information
  • 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 initial full data synchronization is complete, the tablespace of the destination database is larger than that of the source database.
  • If you use only DTS to write data to the destination database, you can use DMS to perform online DDL operations on source tables during data synchronization. For more information, see Change schemas without locking tables.
  • 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 Data Management (DMS) to perform online DDL operations.
  • If you use a privileged account or superuser account as the destination database account to perform full data synchronization or incremental data synchronization, and the tables to be synchronized in the source database contain foreign keys, triggers, or event triggers, DTS temporarily sets the session_replication_role parameter to replica at the session level. If the destination database account does not have the required permissions, you must manually set the session_replication_role parameter to replica. If the value of the session_replication_role parameter is replica and you perform the cascade update or delete operations on the source database during full data synchronization or incremental data synchronization, data inconsistency may occur. After the DTS synchronization task is released, you can change the value of the session_replication_role parameter back to origin.
Special casesIf the source instance is an ApsaraDB RDS for PostgreSQL instance, take note of the following limits:

During data synchronization, do not modify the endpoint or zone of the ApsaraDB RDS for PostgreSQL instance. Otherwise, the data synchronization task fails.

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 that are supported by DTS, see Synchronization topologies.

SQL operations that can be synchronized

Operation typeSQL statement
DMLINSERT, UPDATE, and DELETE

Procedure

  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 the data synchronization instance resides.
  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

    The name of the task. DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. You do not need to specify a unique task name.

    Source DatabaseDatabase TypeThe type of the source database. Select PostgreSQL.
    Access MethodThe access method of the source database. Select Alibaba Cloud Instance.
    Instance RegionThe region in which the source ApsaraDB RDS for PostgreSQL instance resides.
    Instance IDThe ID of the source ApsaraDB RDS for PostgreSQL instance.
    Database NameThe name of the source database in the ApsaraDB RDS for PostgreSQL instance.
    Database AccountA privileged account of the source ApsaraDB RDS for PostgreSQL instance. The account must be the owner of the database. For more information about how to create an account for an ApsaraDB RDS for PostgreSQL instance and grant permissions to this account, see Create an account on an ApsaraDB RDS for PostgreSQL instance and Create a database on an ApsaraDB RDS for PostgreSQL instance.
    Note If the database engine version of the source ApsaraDB RDS for PostgreSQL instance is 9.4 and you synchronize only DML operations, only the REPLICATION permission is required for the database account.
    Database Password

    The password of the database account.

    Destination DatabaseDatabase TypeThe type of the destination database. Select PostgreSQL.
    Access MethodThe access method of the destination database. Select Alibaba Cloud Instance.
    Instance RegionThe region in which the destination ApsaraDB RDS for PostgreSQL instance resides.
    Instance IDThe ID of the destination ApsaraDB RDS for PostgreSQL instance.
    Database NameThe name of the destination database in the ApsaraDB RDS for PostgreSQL instance.
    Database AccountThe database account of the destination ApsaraDB RDS for PostgreSQL instance. The account must have the owner permissions on schemas. For more information about how to create an account for an ApsaraDB RDS for PostgreSQL instance and grant permissions to this account, see Create an account on an ApsaraDB RDS for PostgreSQL instance.
    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. DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you need to manually add the CIDR blocks of DTS servers to the whitelist of the self-managed database on the ECS instance to allow DTS to access the database. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the whitelist of the database to allow DTS to access the database.
    Warning
    • If the CIDR blocks of DTS servers are automatically or manually added to the whitelist or ECS security group rules, security risks may arise. Therefore, before you use DTS to synchronize data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.
    • After the DTS task is complete or released, we recommend that you manually remove the CIDR blocks of DTS servers from the whitelist or ECS security group rules. You must remove the IP address whitelist group whose name contains dts from the whitelist of the Alibaba Cloud database instance or the security group rules of the ECS instance. You must also remove the CIDR blocks of DTS servers from the whitelist of the self-managed database on the ECS instance. For more information about the CIDR blocks that you must remove from the whitelist of the self-managed databases that are deployed in data centers or databases that are hosted on third-party cloud services, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.
  5. Select objects for the task and configure advanced settings.
    • Basic Settings
      ParameterDescription
      Task Stages

      By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of selected objects from the source instance to the destination cluster. The historical data is the basis for subsequent incremental synchronization.

      Note If you select Schema Synchronization, DTS synchronizes the schemas (including foreign keys) of the tables to be synchronized from the source database to the destination database.
      Processing Mode of Conflicting Tables
      • Precheck and Report Errors: checks whether the destination database contains tables that have the same names as tables in the source database. If the source and destination databases do not contain tables that have identical table names, the precheck is passed. Otherwise, an error is returned during the precheck and the data synchronization task cannot be started.

        Note You can use the object name mapping feature to rename the tables that are synchronized to the destination database. You can use this feature if the source and destination databases contain identical table names and the tables in the destination database cannot be deleted or renamed. For more information, see Map object names .
      • Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases.
        Warning If you select Ignore Errors and Proceed, data inconsistency may occur, and your business may be exposed to potential risks.
        • If the source and destination databases have the same schemas, and a data record has the same primary key value as an existing data record in the destination database:
          • During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained.
          • During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten.
        • If the source and destination databases have different schemas, initial data synchronization may fail. In this case, only specific columns are synchronized, or the data synchronization task fails.
      Synchronization Topology

      In this example, One-way Synchronization is selected.

      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 columns, tables, or schemas 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 select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information, 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, see Use SQL conditions to filter data.
      • If you use the object name mapping feature to rename an object, other objects that are dependent on the object may fail to be synchronized.
    • 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.
      • If DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements, or release the DTS instance at the earliest opportunity after the source and destination instances are released.
  6. In the lower part of the page, click Next: Save Task Settings and Precheck.
    Note
    • Before you can start the data synchronization task, DTS performs a precheck. You can start the data synchronization task only after the task passes the precheck.
    • If the task fails to pass the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
    • If an alert is generated for an item during the precheck, perform the following operations based on the scenario:
      • In scenarios in which you cannot ignore the alert item, click View Details next to the failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
      • In scenarios in which you can ignore the alert item, click Confirm Alert Details next to the failed item. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur and your business may be exposed to potential risks.
  7. Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.
  8. On the Purchase Instance page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.
    SectionParameterDescription
    New Instance ClassBilling 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 ClassDTS 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 DurationIf 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.
  9. Read and select the check box for Data Transmission Service (Pay-as-you-go) Service Terms.
  10. Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.