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Data Transmission Service:Configure one-way synchronization between ApsaraDB RDS for PostgreSQL instances

Last Updated:Oct 27, 2023

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

Category

Description

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 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 full data synchronization is completed, 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, the 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 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 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 in the preceding sample statement with the 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 can obtain the incremental logs of the source database within the last 15 minutes by using this replication slot.

    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.

    Amazon slot查询信息
  • 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 of the database servers.

  • During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. Therefore, after initial full data synchronization is complete, the size of the used tablespace of the destination database is larger than that of the source database.

  • If you use only DTS to write table 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. For example, 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 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 cases

  • If 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 type

SQL statement

DML

INSERT, UPDATE, and DELETE

DDL

  • DDL operations can be synchronized only in the data synchronization tasks that are created after October 1, 2020.

    Important
  • If the database account of the source database is a privileged account and the minor engine version of the ApsaraDB RDS for PostgreSQL instance is 20210228 or later, DTS can synchronize the following DDL statements. For information about how to update the minor engine version of an instance, see Update the minor engine version of an ApsaraDB RDS for PostgreSQL instance.

    • CREATE TABLE and DROP TABLE

    • ALTER TABLE, including RENAME TABLE, ADD COLUMN, ADD COLUMN DEFAULT, ALTER COLUMN TYPE, DROP COLUMN, ADD CONSTRAINT, ADD CONSTRAINT CHECK, and ALTER COLUMN DROP DEFAULT

    • TRUNCATE TABLE (The major engine version of the source ApsaraDB RDS for PostgreSQL instance must be PostgreSQL 11 or later.)

    • CREATE INDEX ON TABLE

    Important
    • You cannot synchronize additional information of DDL statements, such as CASCADE or RESTRICT.

    • You cannot synchronize the DDL statements from a session for which the SET session_replication_role = replica command is executed.

    • If the SQL statements submitted by the source database at a time contain both DML and DDL statements, DTS does not synchronize the DDL statements.

    • If the SQL statements submitted by the source database at a time contain DDL statements on objects not to be synchronized, DTS does not synchronize the DDL statements.

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 you want to create the data synchronization task.

  3. Click Create Task. On the page that appears, configure the source and destination databases.

    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

    Database Type

    The type of the source database. Select PostgreSQL.

    Access Method

    The access method of the source database. Select Alibaba Cloud Instance.

    Instance Region

    The region in which the source ApsaraDB RDS for PostgreSQL instance resides.

    Instance ID

    The ID of the source ApsaraDB RDS for PostgreSQL instance.

    Database Name

    The name of the source database in the ApsaraDB RDS for PostgreSQL instance.

    Database Account

    A 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 and Create a database.

    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 Database

    Database Type

    The type of the destination database. Select PostgreSQL.

    Access Method

    The access method of the destination database. Select Alibaba Cloud Instance.

    Instance Region

    The region in which the destination ApsaraDB RDS for PostgreSQL instance resides.

    Instance ID

    The ID of the destination ApsaraDB RDS for PostgreSQL instance.

    Database Name

    The name of the destination database in the ApsaraDB RDS for PostgreSQL instance.

    Database Account

    The 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.

    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 objects to synchronize and advanced settings.

    • Basic 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.

      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, data may fail to be initialized, only some columns are synchronized, or the data synchronization task fails. Operate with caution.

      • 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.

      • 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.

      • 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, only some columns are synchronized, or the data synchronization task fails. Operate with caution.

      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

      Parameter

      Description

      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:

      Specify the retry time range 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.

  6. 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.

  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.

    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.

  9. Read and select the check box to agree to the 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.