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    Data Transmission Service:Two-way synchronization between PolarDB for PostgreSQL clusters

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    Data Transmission Service:Two-way synchronization between PolarDB for PostgreSQL clusters

    Last Updated:Feb 04, 2026

    This topic describes how to use Data Transmission Service (DTS) to implement two-way synchronization between PolarDB for PostgreSQL clusters.

    Prerequisites

    • You have created the source and destination PolarDB for PostgreSQL clusters. For more information, see Create a database cluster.

      Note

      • For the versions supported by the source and destination databases, see Overview of synchronization solutions.

      • We recommend that the destination PolarDB for PostgreSQL cluster has the same specifications as the source PolarDB for PostgreSQL cluster.

    • A database has been created in the target PolarDB for PostgreSQL cluster to receive data. For more information, see Database Management.

    • The wal_level parameter for both the source and target PolarDB for PostgreSQL clusters is set to logical. For more information, see Set cluster parameters.

    Notes

    Note

    • During schema synchronization, DTS synchronizes foreign keys from the source database to the destination database.

    • During full data synchronization and incremental data synchronization, DTS temporarily disables constraint checks and foreign key cascade operations at the session level. Data inconsistency may occur if cascade update or delete operations are performed on the source database while the task is running.

    Type

    Description

    Source database limits

    • In a PolarDB for PostgreSQL cluster, the tables to be synchronized must have a primary key or a non-null unique index.

    • If the source database has long-running transactions and the instance has an incremental synchronization task, the write-ahead log (WAL) that is generated before the long-running transactions are committed may accumulate. This can cause the disk space of the source database to become insufficient.

    • To ensure that the synchronization task runs as expected and to prevent logical replication from being interrupted by a primary/secondary switchover, the PolarDB for PostgreSQL cluster must support and enable Logical Replication Slot Failover.

      Note

      If the source PolarDB for PostgreSQL cluster does not support Logical Replication Slot Failover (for example, if the Database Engine of the cluster is PostgreSQL 14), a high-availability (HA) switchover in the source database may cause the synchronization instance to fail and become unrecoverable.

    • Due to the limits of logical replication in the source database, if a single piece of data to be synchronized exceeds 256 MB after an incremental change, the synchronization instance may fail and cannot be recovered. You must reconfigure the synchronization instance.

    • During schema synchronization and full data synchronization, do not perform Data Definition Language (DDL) operations that change the schema of databases or tables. Otherwise, the data synchronization task fails.

      Note

      During the full synchronization phase, DTS queries the source database, which acquires metadata locks. This may block DDL operations on the source database.

    Other limits

    • A single data synchronization task can synchronize only one database. To synchronize multiple databases, you must configure a separate task for each database.

    • DTS does not support synchronizing TimescaleDB extension tables, tables with cross-schema inheritance, or tables with unique indexes based on expressions.

    • Schemas created by installing plugins cannot be synchronized. You cannot obtain information about these schemas in the console when you configure the task.

    • In the following three scenarios, you must run the ALTER TABLE schema.table REPLICA IDENTITY FULL; command on the tables to be synchronized in the source database before you write data to them. This ensures data consistency. Do not lock the tables while running this command to prevent deadlocks. If you skip the related precheck items, DTS automatically runs this command during the initialization of the instance.

      • When the instance runs for the first time.

      • When you select Schema as the granularity for object selection, and a new table is created in the schema or a table to be synchronized is rebuilt using the RENAME command.

      • When you use the feature to modify synchronization objects.

      Note

      • In the command, replace schema and table with the actual schema name and table name.

      • We recommend that you perform this operation during off-peak hours.

    • If a table to be synced contains a field of the SERIAL type, the source database automatically creates a Sequence for that field. Therefore, when you configure Source Objects, if you select Schema Synchronization for the Synchronization Types, we recommend that you also select Sequence or synchronize the entire schema. Otherwise, the synchronization instance may fail to run.

    • During initial full data synchronization, DTS consumes read and write resources on both the source and destination databases, which may increase the database load. Therefore, evaluate the performance of the source and destination databases before you synchronize data, and perform the synchronization during off-peak hours (for example, when the CPU load of both databases is below 30%).

    • Because full data synchronization runs concurrent INSERT operations, it causes table fragmentation in the destination database. As a result, the table space in the destination database is larger than that in the source instance after full synchronization is complete.

    • During DTS synchronization, do not write data to the destination database from sources other than DTS. Otherwise, data inconsistency between the source and destination databases may occur.

    • A two-way synchronization instance includes a forward task and a reverse task. When you configure or reset a two-way synchronization instance, if the destination object of one task is the source object of the other task:

      • Only one task can synchronize full and incremental data. The other task can only synchronize incremental data.

      • The source data of the current task can only be synchronized to the destination of the current task. The synchronized data will not be used as the source data for the other task.

    • During data synchronization, DTS creates a replication slot with the dts_sync_ prefix in the source database to replicate data. This replication slot allows DTS to obtain incremental logs from the source database within the last 15 minutes. When the data synchronization fails or the synchronization instance is released, DTS attempts to automatically clear the replication slot.

      Note

      • If you change the password of the source database account used by the task or delete the DTS IP address from the whitelist of the source database during data synchronization, the replication slot cannot be automatically cleared. In this case, you must manually clear the replication slot in the source database. This prevents the slot from continuously accumulating and consuming disk space, which can make the source database unavailable.

      • If a failover occurs in the source database, you must log on to the secondary database to manually clear the slot.

      Amazon slot查询信息

    • DTS validates data content but does not support validation for metadata such as sequences. You must validate metadata yourself.

    • DTS creates the following temporary tables in the source database to obtain DDL statements for incremental data, the structure of incremental tables, and heartbeat information. Do not delete these temporary tables during synchronization. Otherwise, the DTS task becomes abnormal. The temporary tables are automatically deleted after the DTS instance is released.

      public.dts_pg_class, public.dts_pg_attribute, public.dts_pg_type, public.dts_pg_enum, public.dts_postgres_heartbeat, public.dts_ddl_command, public.dts_args_session, and public.aliyun_dts_instance.

    • After you switch your business to the destination instance, new sequences do not increment from the maximum value of the source sequence. Before the business switchover, you must query the maximum value of the corresponding sequence in the source database and then set it as the initial value for the corresponding sequence in the destination database. The following command queries the sequence values in the source database: 

      do language plpgsql $$
declare
  nsp name;
  rel name;
  val int8;
begin
  for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S'
  loop
    execute format($_$select last_value from %I.%I$_$, nsp, rel) into val;
    raise notice '%',
    format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1);
  end loop;
end;
$$;
      Note

      The SQL statements that are returned after you run the command contain all sequences of the source database. Run the SQL statements in the destination database as needed.

    • For a full or incremental synchronization task, if 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 is a privileged account or has superuser permissions. If the destination database account does not have these permissions, you must manually set the `session_replication_role` parameter to `replica` in the destination database. During this period (while `session_replication_role` is `replica`), if cascade update or delete operations occur in the source database, data inconsistency may occur. After the DTS task is released, you can change the `session_replication_role` parameter back to `origin`.

    • If the task fails, DTS technical support will attempt to recover it within 8 hours. During the recovery process, operations such as restarting the task or adjusting its parameters may be performed.

      Note

      When parameters are adjusted, only DTS task parameters are modified. Database parameters remain unchanged.The parameters that may be modified include but are not limited to those described in Modify instance parameters.

    • When you synchronize partitioned tables, you must include both the parent table and its child partitions as synchronization objects. Otherwise, data inconsistency may occur for the partitioned table.

      Note

      The parent table of a PostgreSQL partitioned table does not directly store data. All data is stored in the child partitions. The synchronization task must include the parent table and all its child partitions. Otherwise, data from the child partitions may not be synchronized, leading to data inconsistency between the source and destination.

    Billing

    Synchronization type

    Pricing

    Schema synchronization and full data synchronization

    Free of charge.

    Incremental data synchronization

    Charged. For more information, see Billing overview.

    Supported conflict detection

    To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated in only one of the database instances in the two-way synchronization. If data records are updated on both database instances, DTS the system will apply the conflict resolution policy configured in the task.

    DTS checks and fixes conflicts to maximize the stability of two-way synchronization tasks. DTS can detect the following types of conflicts:

    • Uniqueness conflicts caused by INSERT operations

      In two-way synchronization, if records with the same primary key are inserted into both database instances simultaneously (or in close succession), a uniqueness constraint conflict will be triggered. When the INSERT statement is synchronized to the peer instance, it will fail because a record with the same primary key value already exists.

    • Mismatched records in 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 record to be updated by an UPDATE operation causes a primary key or unique key conflict.

    • Non-existent records to be deleted

      The records to be deleted do not exist in the destination instance. In this case, DTS ignores the DELETE operation regardless of the conflict resolution policy that you specify.

    Important

    • Due to time differences and latency, DTS cannot guarantee 100% conflict prevention. To ensure consistency, update records with the same primary or unique key on only one database instance at a time.

    • DTS provides various conflict resolution strategies for the aforementioned data conflicts, which you can select while configuring two-way data synchronization.

    Supported objects to be synchronized

    • SCHEMA, TABLE

      Note

      This includes PRIMARY KEY, UNIQUE KEY, FOREIGN KEY, DATATYPE (built-in data types), and DEFAULT CONSTRAINT.

    • VIEW, PROCEDURE (PostgreSQL 11 or later), FUNCTION, RULE, SEQUENCE, EXTENSION, TRIGGER, AGGREGATE, INDEX, OPERATOR, DOMAIN

    Supported SQL operations

    Important

    Data Definition Language (DDL) operations can be synchronized only in the forward task, from the source database to the destination database. DDL operations are not supported in the reverse task, from the destination database to the source database, and are automatically filtered.

    Operation type

    SQL statement

    DML

    INSERT, UPDATE, and DELETE

    DDL

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

      Important

      • For data synchronization tasks created before May 12, 2023, you need to create triggers and functions in the source database to capture DDL information before you configure the synchronization task. For more information, see Implement PostgreSQL DDL incremental migration using triggers and functions.

      • You cannot synchronize data of the bit data type during incremental data synchronization.

    • The sync task supports the following DDL operations when the source database account is a privileged account:

      • 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 database engine of the source PolarDB for PostgreSQL cluster must be PostgreSQL 11 or later.)

      • CREATE INDEX ON TABLE

    Important

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

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

    • DDL statements that are run by calling methods such as FUNCTION cannot be synchronized.

    • If multiple SQL statements that are committed by the source database at the same time contain both DML and DDL statements, the DDL statements are not synchronized.

    • If multiple SQL statements that are committed by the source database at the same time contain DDL statements for objects that are not synchronized, the DDL statements are not synchronized.

    Database account permissions

    Database

    Required permissions

    Account creation and authorization

    Source and destination PolarDB for PostgreSQL clusters

    A privileged account that is the owner of the database.

    Create a database account and Database management.

    Procedure

    1. Go to the data synchronization task list page in the destination region. You can do this in one of two ways.

      DTS console

      1. Log on to the DTS console.

      2. In the navigation pane on the left, click Data Synchronization.

      3. In the upper-left corner of the page, select the region where the synchronization instance is located.

      DMS console

      Note

      The actual steps may vary depending on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.

      1. Log on to the DMS console.

      2. In the top menu bar, choose Data + AI > DTS (DTS) > Data Synchronization.

      3. To the right of Data Synchronization Tasks, select the region of the synchronization instance.

    2. Click Create Task to open the task configuration page.

    3. Configure the source and destination databases.

      Category

      Configuration

      Description

      None

      Task Name

      DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.

      Source Database

      Select Existing Connection

      • Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured.

        Note

        In the DMS console, this configuration item is Select a DMS database instance.

      • If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.

      Database Type

      Select PolarDB for PostgreSQL.

      Access Method

      Select Alibaba Cloud Instance.

      Instance Region

      Select the region where the source PolarDB for PostgreSQL cluster resides.

      Replicate Data Across Alibaba Cloud Accounts

      For this example, select No, as the database instance belongs to the current Alibaba Cloud account.

      Instance ID

      Select the ID of the source PolarDB for PostgreSQL cluster.

      Database Name

      Enter the name of the database that contains the objects to be synchronized in the source PolarDB for PostgreSQL cluster.

      Database Account

      Enter the database account of the source PolarDB for PostgreSQL cluster. For information about the permission requirements, see Database account permissions.

      Database Password

      Enter the password for the specified database account.

      Destination Database

      Select Existing Connection

      • Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured.

        Note

        In the DMS console, this configuration item is Select a DMS database instance.

      • If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.

      Database Type

      Select PolarDB for PostgreSQL.

      Access Method

      Select Alibaba Cloud Instance.

      Instance Region

      Select the region where the destination PolarDB for PostgreSQL cluster resides.

      Instance ID

      Select the ID of the destination PolarDB for PostgreSQL cluster.

      Database Name

      Enter the name of the database that is used to receive data in the destination PolarDB for PostgreSQL cluster.

      Database Account

      Enter the database account of the destination PolarDB for PostgreSQL cluster. For information about the permission requirements, see Database account permissions.

      Database Password

      Enter the password for the specified database account.

    4. After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page.

      Note

      Ensure that the IP address blocks of the DTS service are added to the security settings of the source and destination databases, either automatically or manually, to allow access from DTS servers. For more information, see Add the IP address whitelist of DTS servers.

    5. Configure the task objects.

      1. On the Configure Objects page, specify the objects to synchronize.

        Configuration

        Description

        Synchronization Types

        DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization.

        Synchronization Topology

        Select Two-way Synchronization.

        Exclude DDL Operations

        • If you select Yes, DDL operations are not synchronized.

        • Select No: synchronize DDL operations.

          Important

          To ensure the stability of the two-way synchronization link, you can select whether to synchronize DDL operations only for the forward task (from the source database to the destination database). DDL operations are automatically filtered out in the reverse task (from the destination database to the source database).

        Global Conflict Resolution Policy

        Select a conflict resolution policy as needed.

        • TaskFailed (Fail and report an error upon conflict)

          The task will error out and stop if a conflict occurs. The task enters a failed state and requires manual intervention for recovery.

        • Ignore (Keep the existing record in the destination instance)

          The system skips the conflicting statement and continues the synchronization. The existing record in the destination instance is retained.

        • Overwrite (Overwrite the conflicting record in the destination instance)

          If a conflict occurs, the system overwrites the conflicting record in the destination instance with the data from the source.

        Note

        • For information about the supported conflict types, see Supported conflict detection.

        • If the synchronization task is paused or restarted and a delay occurs, these policies do not take effect. The data in the destination is overwritten by default.

        Processing Mode of Conflicting Tables

        • Precheck and Report Errors: Checks for tables with the same names in the destination database. If any tables with the same names are found, an error is reported during the precheck and the data synchronization task does not start. Otherwise, the precheck is successful.

          Note

          If you cannot delete or rename the table with the same name in the destination database, you can map it to a different name in the destination. For more information, see Database Table Column Name Mapping.

        • Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database.

          Warning

          Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example:

          • If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database:

            • During full data synchronization, DTS retains the destination record and skips the source record.

            • During incremental synchronization, DTS overwrites the destination record with the source record.

          • If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.

        Capitalization of Object Names in Destination Instance

        Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.

        Source Objects

        In the Source Objects box, click the objects, and then click 向右 to move them to the Selected Objects box.

        Note

        • You can select schemas or tables as synchronization objects. If you select tables as synchronization objects, other objects, such as views, triggers, and stored procedures, are not synchronized to the destination database.

        • If a table to be synchronized contains SERIAL data type, and you select Schema Synchronization as the Synchronization Types, we recommend that you also select Sequence or entire schema synchronization.

        Selected Objects

        • To change the name of a synchronization object in the destination instance, right-click the synchronization object in Selected Objects and edit its name. For more information, see Library, Table, and Column Name Mapping.

        • To remove a selected synchronization object, click the object in the Selected Objects box and click image to move it to the Source Objects box.

        Note

        • If you use the object name mapping feature, other objects that depend on the mapped object may fail to be synchronized.

        • To set a WHERE clause to filter data, right-click the table to be synchronized in the Selected Objects box and set the filter condition in the dialog box that appears. For more information, see Set a filter condition.

        • To select the SQL operations for incremental synchronization, right-click the object to be synchronized in the Selected Objects box and select the desired SQL operations in the dialog box that appears.

      2. Click Next: Advanced Settings.

        Configuration

        Description

        Dedicated Cluster for Task Scheduling

        By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.

        Retry Time for Failed Connections

        If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails.

        Note

        • If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances.

        • DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.

        Retry Time for Other Issues

        If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails.

        Important

        The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.

        Enable Throttling for Full Data Synchronization

        During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s).

        Note

        Enable Throttling for Incremental Data Synchronization

        You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).

        Environment Tag

        You can select an environment tag to identify the instance based on your needs. In this example, no tag is selected.

        Configure ETL

        Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values:

        Monitoring and Alerting

        Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts.

      3. Click Data Verification to configure a data verification task.

        To use the data verification feature, see Configure data verification.

    6. Save the task and perform a precheck.

      • To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.

      • If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.

      Note

      • Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.

      • If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.

      • If the precheck generates warnings:

        • For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.

        • For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.

    7. Purchase the instance.

      1. When the Success Rate reaches 100%, click Next: Purchase Instance.

      2. On the Purchase page, select the billing method and link specifications for the data synchronization instance. For more information, see the following table.

        Category

        Parameter

        Description

        New Instance Class

        Billing Method

        • Subscription: You pay upfront for a specific duration. This is cost-effective for long-term, continuous tasks.

        • Pay-as-you-go: You are billed hourly for actual usage. This is ideal for short-term or test tasks, as you can release the instance at any time to save costs.

        Resource Group Settings

        The resource group to which the instance belongs. The default is default resource group. For more information, see What is Resource Management?.

        Instance Class

        DTS offers synchronization specifications at different performance levels that affect the synchronization rate. Select a specification based on your business requirements. For more information, see Data synchronization link specifications.

        Subscription Duration

        In subscription mode, select the duration and quantity of the instance. Monthly options range from 1 to 9 months. Yearly options include 1, 2, 3, or 5 years.

        Note

        This option appears only when the billing method is Subscription.

      3. Read and select the checkbox for Data Transmission Service (Pay-as-you-go) Service Terms.

      4. Click Buy and Start, and then click OK in the OK dialog box.

        You can monitor the task progress on the data synchronization page.

    8. Configure the reverse synchronization task.

      1. Wait for the initial synchronization of the forward task to complete until the Status is Running.

      2. In the Actions column of the reverse task, click Configure Task.

      3. Configure the reverse synchronization task by following Step 3 to Step 6.

        Important

        • When you configure the reverse synchronization task, you must select the correct source and destination instances. The source instance in the reverse task is the destination instance in the forward task. The destination instance in the reverse task is the source instance in the forward task. You must also carefully confirm that the instance information, such as the database name, account, and password, is consistent.

        • The Instance Region of the source and destination databases cannot be modified for the reverse synchronization task. Fewer parameters are available for configuration compared to the forward synchronization task. Configure the parameters that are displayed in the console.

        • The Processing Mode of Conflicting Tables for the reverse synchronization task does not check for tables that are synchronized to the destination instance by the forward synchronization task.

        • The reverse synchronization task does not support synchronizing objects that are not in the Selected Objects list of the forward task.

        • We recommend that you do not use the mapping feature when you configure the reverse task. Otherwise, data inconsistency may occur.

      4. When Success Rate is 100%, click Back.

    9. After the reverse synchronization task is configured, wait until the Status of both synchronization tasks is Running. The configuration of two-way data synchronization is complete.