Data Transmission Service:Configure two-way data synchronization between an RDS PostgreSQL instance and a PolarDB for PostgreSQL cluster

Circular sync prevention

To prevent changes from looping back indefinitely, DTS creates a schema named dts in both the source and destination databases when a two-way synchronization instance runs. DTS uses this schema to track which changes originated from replication, so those changes are not re-replicated in the reverse direction.

Do not modify the dts schema while the task is running.

DDL synchronization direction

DDL operations are synchronized only in the forward direction (source to destination). DDL operations are ignored in the reverse direction.

Forward and reverse task rules

When both tasks are configured to synchronize the same objects:

• Only one task (typically the forward task) can synchronize both full data and incremental data. The other task synchronizes incremental data only.

• Data synchronized by the forward task is not used as source data for the reverse task.

Source database limits

Other limits

To query maximum sequence values in the source database before switching workloads:

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

The returned SQL statements cover all sequences in the source database. Run only the statements relevant to your requirements in the destination database.

Special cases

Step 1: Open the Data Synchronization page

Use one of the following methods:

Step 2: Create a task

Click Create Task to go to the task configuration page.

Optional: Click New Configuration Page in the upper-right corner if the page shows Back to Previous Version. Skip this step if Back to Previous Version is already displayed.

Some parameters differ between the new and previous configuration page versions. Use the new version.

Step 3: Configure source and destination databases

Step 4: Test connectivity and proceed

Click Test Connectivity and Proceed at the bottom of the page.

Make sure the CIDR blocks of DTS servers are added to the security settings of both the source and destination databases. See Add the CIDR blocks of DTS servers. For self-managed databases not set to Alibaba Cloud Instance, click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Step 5: Configure objects to synchronize

1. In the Configure Objects step, set the following parameters:

   Parameter	Description
   Synchronization Types	Select Schema Synchronization, Full Data Synchronization, and Incremental Data Synchronization. DTS synchronizes historical data first as the basis for subsequent incremental synchronization.
   Synchronization Topology	Select Two-way Synchronization.
   Exclude DDL Operations	Select Yesalert notification settings to exclude DDL synchronization, or No to include it. DDL operations are synchronized only in the forward direction. This parameter appears only when configuring the forward task.
   Global Conflict Resolution Policy	Select the policy for handling conflicts. See Supported conflict detection for details. The policy does not apply during latency windows when you pause and restart a task — data in the destination is overwritten by default during that window.
   Processing Mode of Conflicting Tables	Precheck and Report Errors: the precheck fails if the destination contains tables with the same names as source tables. Use Map object names to rename destination tables if they cannot be deleted or renamed. Ignore Errors and Proceed: skips the precheck for identical table names. During full data synchronization, existing destination records with matching primary or unique keys are kept. During incremental synchronization, they are overwritten. If schemas differ, data initialization may fail. Proceed with caution.
   Source Objects	Select objects from the Source Objects section and click the arrow icon to move them to the Selected Objects section. You can select columns, tables, or schemas.
   Selected Objects	Right-click an object to rename it in the destination (see Map object names), set WHERE filter conditions (see Specify filter conditions), or select which SQL operations to synchronize for that object. Click an object and then click the remove icon to remove it. Note that renaming an object may cause dependent objects to fail synchronization.

2. Click Next: Advanced Settings and configure the following:

   Parameter	Description
   Dedicated Cluster for Task Scheduling	By default, DTS uses the shared cluster. To improve stability, purchase a dedicated cluster. See What is a DTS dedicated cluster.
   Retry Time for Failed Connections	Time range for retrying failed connections after the task starts. Valid values: 10–1440 minutes. Default: 720 minutes. Set to more than 30 minutes. If DTS reconnects within this window, the task resumes. Otherwise, the task fails. If multiple tasks share the same source or destination database, the shortest retry window applies. DTS charges for the instance during retries.
   Retry Time for Other Issues	Time range for retrying failed DDL or DML operations. Valid values: 1–1440 minutes. Default: 10 minutes. Set to more than 10 minutes. This value must be less than Retry Time for Failed Connections.
   Enable Throttling for Full Data Synchronization	Limit read/write resource usage during full data synchronization. Configure Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s). Available only when Full Data Synchronization is selected.
   Enable Throttling for Incremental Data Synchronization	Limit resource usage during incremental synchronization. Configure RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).
   Environment Tag	Tag for identifying the DTS instance. Optional.
   Configure ETL	Select Yes to enable extract, transform, and load (ETL) and enter data processing statements. Select No to skip. See What is ETL? and Configure ETL in a data migration or data synchronization task.
   Monitoring and Alerting	Select Yes to receive notifications when the task fails or synchronization latency exceeds the threshold. Configure the alert threshold and notification settings. See Configure monitoring and alerting when you create a DTS task. Select No to skip.

3. Click Next Step: Data Verification to configure data verification. See Configure a data verification task.

Step 6: Save settings and run a precheck

Click Next: Save Task Settings and Precheck at the bottom of the page.

To preview the API parameters for this task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters before proceeding.

DTS runs a precheck before starting the task. The task starts only after the precheck passes.

If the precheck fails, click View Details next to each failed item, resolve the issue, and click Precheck Again.

If the precheck triggers an alert:

For alerts that cannot be ignored: click View Details, fix the issue, and rerun the precheck.

For alerts that can be ignored: click Confirm Alert Details > Ignore > OK > Precheck Again. Ignoring an alert may cause data inconsistency.

Step 7: Purchase a synchronization instance

Wait until Success Rate reaches 100%, then click Next: Purchase Instance.

On the buy page, configure the following:

Read and select Data Transmission Service (Pay-as-you-go) Service Terms, then click Buy and Start. In the confirmation dialog box, click OK.

The task appears in the task list. You can monitor its progress there.

Step 8: Wait for the forward task to start running

Wait until the Status of the forward synchronization task changes to Running and initial synchronization is complete.

Step 9: Configure the reverse synchronization task

1. Find the reverse synchronization task in the task list and click Configure Task in the Actions column.

2. Configure the reverse task by following Steps 3–6. Apply the constraints from Key differences between the forward and reverse tasks:

   Important

   - Swap source and destination: the source for the reverse task is the destination of the forward task (PolarDB for PostgreSQL), and the destination is the source of the forward task (ApsaraDB RDS for PostgreSQL). - Instance Region is fixed and cannot be changed. - Configure only the parameters that appear in the console — fewer parameters are shown for the reverse task. - Tables already synchronized to the destination by the forward task are excluded from the Processing Mode of Conflicting Tables check in the reverse direction. - Do not reuse the selected objects from the forward task for the reverse task. - Avoid using the object name mapping feature — it may cause data inconsistency.

3. Wait until Success Rate reaches 100%, then click Back.

Step 10: Verify two-way synchronization is running

Wait until the Status of both the forward and reverse synchronization tasks changes to Running. Two-way data synchronization is now active.