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

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

    Last Updated:Jul 10, 2026

    Data Transmission Service (DTS) enables real-time, two-way data synchronization between PolarDB for PostgreSQL clusters and PostgreSQL databases, such as ApsaraDB RDS for PostgreSQL instances and self-managed PostgreSQL databases. This topic demonstrates how to configure two-way data synchronization for an ApsaraDB RDS for PostgreSQL instance. The procedure is similar for other data sources.

    Prerequisites

    Usage 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 the constraint check and cascade operations on foreign keys at the session level. If cascading updates or deletions occur in the source database during synchronization, data inconsistencies may occur.

    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 inherent limitations of logical subscriptions in the source database, if a single data record to be synchronized exceeds 256 MB after an incremental change, the synchronization instance will fail and must be reconfigured.

    • Do not run DDL operations that change database or table schemas during schema synchronization or full synchronization. Otherwise, the synchronization task fails.

      Note

      During full synchronization, DTS queries the source database. This creates metadata locks that may block DDL operations on the source database.

    • Tables with generated columns in PostgreSQL 18 do not support synchronization. If you configure synchronization for such tables, DML operations on those tables will be blocked.

    Other limits

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

    • DTS does not support synchronizing TimescaleDB plug-in tables, tables with cross-schema inheritance, or tables with expression-based unique indexes.

    • 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 source tables before writing data to them to ensure data consistency. Do not perform table-locking operations while this command is running to prevent deadlocks. If you skip this check during the precheck, DTS automatically runs this command during instance initialization.

      • When the instance runs for the first time.

      • When you select objects at the schema level, and a new table is created in the schema or a table is rebuilt by using the RENAME command.

      • When you use the feature to modify synchronization objects.

      Note

      • Replace schema and table in the command with the names of the schema and table to be synchronized.

      • Perform this operation during off-peak hours.

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

    • 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 task includes forward and reverse synchronization tasks. When you configure or reset the task, if the destination object of one task matches the synchronization object of the other task:

      • Allow only one task to synchronize full and incremental data. The other task supports only incremental synchronization.

      • Data from the source of the current task synchronizes only to the destination of the current task. It does not serve as 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 generated within the last 15 minutes from the source database. When a data synchronization task fails or the instance is released, DTS attempts to automatically remove this replication slot.

      Note

      • If you change the password of the source database account or delete the IP whitelist for DTS during data synchronization, the replication slot will not be cleaned up automatically. Manually clean up the replication slot in the source database to prevent it from consuming disk space and making the source database unavailable.

      • If the source database fails over, log on to the standby database to perform the cleanup manually.

      Run the SQL statement SELECT * FROM pg_replication_slots; to view all replication slots in the source database. In the results, the DTS replication slot is the record where the slot_name value starts with dts_sync_ and the active value is true.

    • 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 schema of incremental tables, and heartbeat information. Do not delete these temporary tables during synchronization. Otherwise, the DTS task may fail. 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 database, sequences do not automatically continue from the maximum value of the corresponding sequences in the source database. Before the switchover, query the maximum value of the corresponding sequence in the source database and use it as the initial value for the corresponding sequence in the destination database. Run the following command to query 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 output by the preceding command include all sequences from the source database. Run them in the destination database based on your business requirements.

    • For full or incremental data synchronization tasks, if the source tables to be synchronized 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 in the destination database. During this period, if cascade update or delete operations occur in the source database while session_replication_role is set to replica, data inconsistency may occur. After the DTS task is released, change the session_replication_role parameter back to origin.

    • If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters.

      Note

      Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed 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 in the partitioned table may become inconsistent.

      Important

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

      • Synchronization of partitioned tables and inheritance tables (parent-child tables) across different databases is not supported. Ensure that the partitioned tables and all their partitions, and the parent tables and all their child tables, are in the same database.

    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

    Schemas and tables

    Note

    • This includes primary keys, unique keys, foreign keys, built-in data types, and default constraints.

    • When this database type is used as the source, the supported features may vary depending on the target database type. Refer to the console for specific details.

    Supported SQL operations

    Important

    • DDL statements are synchronized only in forward tasks (source to destination). Reverse tasks automatically filter out DDL statements.

    • When you execute DDL to modify a table schema during bidirectional synchronization, ensure DML writes on both databases are consistent with the new schema. For example, if you delete a column from the source, the destination may still write data based on the old schema that includes the deleted column, causing failures.

    Operation type

    SQL statement

    DML

    INSERT, UPDATE, DELETE

    DDL

    • DDL synchronization is supported only for data synchronization tasks created after October 1, 2020.

      Important

    • If the source database uses a privileged account, the data synchronization task supports the following DDL operations:

      • CREATE TABLE, 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 source PolarDB for PostgreSQL cluster must use a database engine of PostgreSQL 11 or later.)

      • CREATE INDEX ON TABLE

    Important

    • Additional clauses in DDL statements, such as CASCADE or RESTRICT, are not synchronized.

    • DDL statements are not synchronized from a session where the SET session_replication_role = replica command was executed.

    • DDL statements executed by calling a FUNCTION are not synchronized.

    • If a single commit in the source database contains both DML and DDL statements, the DDL statements are not synchronized.

    • If a commit includes a DDL statement that targets an object not configured for synchronization, the statement is not synchronized.

    Database account permissions

    Database type

    Required permissions

    References

    PolarDB for PostgreSQL

    A privileged account that owns the database.

    Create a database account and Database management.

    RDS PostgreSQL

    A privileged account that owns the database.

    Create an account and Create a database.

    Procedure

    1. Purchase a two-way synchronization instance. For more information, see Purchase a DTS instance.

      Important

      On the buy page, set Function to Data Synchronization, Source Instance to PolarDB for PostgreSQL, Destination Instance to PostgreSQL, and Synchronization Topology to Two-way Synchronization. Configure the other parameters based on your business requirements.

    2. 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 DMS console layout and style.

      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.

    3. Find the two-way synchronization instance that you created. In the Actions column of the forward task, click Configure Task.

    4. Configure the source and destination databases.

      Section

      Parameter

      Description

      N/A

      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

      This value is fixed to the region that you selected when you purchased the instance. You cannot change this value.

      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 instance 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 permission requirements, see Permission requirements for database accounts.

      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 PostgreSQL.

      Access Method

      Select Alibaba Cloud Instance.

      Instance Region

      This value is fixed to the region that you selected when you purchased the instance. You cannot change this value.

      Instance ID

      Select the instance ID of the destination ApsaraDB RDS for PostgreSQL instance.

      Database Name

      Enter the name of the database that is used to receive data in the destination ApsaraDB RDS for PostgreSQL instance.

      Database Account

      Enter the database account of the destination ApsaraDB RDS for PostgreSQL instance. For information about permission requirements, see Permission requirements for database accounts.

      Database Password

      Enter the password for the specified database account.

      Encryption

      Specifies whether to encrypt the connection to the source database. You can configure this parameter based on your business requirements. In this example, Non-encrypted is selected.

      If you want to establish an SSL-encrypted connection to the source database, perform the following steps: Select SSL-encrypted, upload CA Certificate, Client Certificate, and Private Key of Client Certificate as needed, and then specify Private Key Password of Client Certificate.

      Note

      • If you set Encryption to SSL-encrypted for a self-managed PostgreSQL database, you must upload CA Certificate.

      • If you want to use the client certificate, you must upload Client Certificate and Private Key of Client Certificate and specify Private Key Password of Client Certificate.

      • For information about how to configure SSL encryption for an ApsaraDB RDS for PostgreSQL instance, see SSL encryption.

    5. After completing the configuration, click Test Connectivity and Proceed at the bottom of the page.

      Note

      • Ensure that you add the CIDR blocks of the DTS servers (either automatically or manually) to the security settings of both the source and destination databases to allow access. For more information, see Add the IP address whitelist of DTS servers.

      • If the source or destination is a self-managed database (i.e., the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

    6. Configure the task objects.

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

        Parameter

        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.

        Exclude DDL Operations

        • Select Yes to exclude DDL operations from the synchronization.

        • Select No to synchronize DDL operations.

          Important

          To ensure the stability of the two-way synchronization link, you can choose whether to synchronize DDL operations only for the forward task (from the source database to the destination database). The reverse task (from the destination database to the source database) automatically excludes DDL operations.

        Global Conflict Resolution Policy

        Select a conflict resolution policy based on your business requirements.

        • 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 supported conflict types, see Supported conflict detection.

        • If a synchronization task is paused or restarted and latency occurs, these policies do not take effect during the latency period. By default, data in the destination database is overwritten.

        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 Object 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

        You can configure the case sensitivity policy for the names of migrated objects, such as databases, tables, and columns, in the destination instance. By default, DTS default policy is selected. You can also choose to keep the case sensitivity consistent with the default policy of the source or destination database. For more information, see Case sensitivity of object names in the destination database.

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

        • If the tables to be synchronized contain SERIAL columns and Synchronization Types is selected the Schema Synchronization, it is recommended that you also select Sequence or Full Schema Synchronization.

        Selected Objects

        • To change the name of an object in the destination instance, right-click the object in the Selected Objects box and modify its name. For more information, see Map table and column names.

        • To remove an object, click it in the Selected Objects box and then click image to move it back 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 synchronize.

        • To filter data by using a WHERE clause, right-click a table in the Selected Objects box and specify the filter conditions in the dialog box that appears. For more information, see Set filter conditions.

        • To select SQL operations for incremental synchronization, right-click an object in the Selected Objects box and select the required SQL operations in the dialog box that appears.

      2. Click Next: Advanced Settings.

        Parameter

        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 requirements. This example does not require this configuration.

        Configure ETL

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

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

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

    7. 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.

    8. When the Success Rate for the precheck is 100%, click Back.

    9. Configure the reverse synchronization task.

      1. Wait for the forward synchronization task to complete its initialization until its Status is Running.

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

      3. Repeat the procedures in Step 4 through Step 7 to configure the reverse synchronization task.

        Important

        • When you configure the data synchronization task in the reverse direction, you must select the correct source and destination instances. The source instance in the reverse direction is the destination instance in the forward direction. The destination instance in the reverse direction is the source instance in the forward direction. You must also make sure that the parameter settings such as the database name, account, and password are consistent.

        • When you configure the source and destination databases of the data synchronization task in the reverse direction, the Instance Region parameter cannot be modified. The number of parameters that you need to configure for a data synchronization task in the reverse direction is less than that for a data synchronization task in the forward direction. Configure the parameters that are displayed in the console.

        • When DTS checks for conflicting tables in the reverse direction, the tables that have been synchronized to the destination instance in the forward direction are ignored in the Processing Mode of Conflicting Tables.

        • You cannot select the Selected Objects of the data synchronization task in the forward direction for the data synchronization task in the reverse direction.

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

      4. When the Success Rate for the precheck is 100%, click Back.

    10. After the reverse synchronization task is configured, wait until the Status of both the forward and reverse tasks is Running. This indicates that the two-way data synchronization is successfully configured.