Configure two-way data synchronization between PolarDB for PostgreSQL clusters

This topic describes how to configure two-way data synchronization between PolarDB for PostgreSQL clusters by using Data Transmission Service (DTS).

Prerequisites

The source and destination PolarDB for PostgreSQL clusters are created. For more information, see Create a cluster.
Note
- For information about the supported versions of the source and destination databases, see Overview of data synchronization scenarios.
- We recommend that the specifications of the destination PolarDB for PostgreSQL cluster are the same as those of the source PolarDB for PostgreSQL cluster.
A database is created in the destination PolarDB for PostgreSQL cluster to receive data. For more information, see Database management.
The wal_level parameter is set to logical for the source and destination PolarDB for PostgreSQL clusters. For more information, see Specify cluster parameters.

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 you perform the cascade update and delete operations on the source database during data synchronization, data inconsistency may occur.

Category	Description

Category	Description
Limits on the source database	The tables that you want to synchronize from the source PolarDB for PostgreSQL cluster must contain primary keys or UNIQUE NOT NULL indexes. If the source database has one or more long-running transactions and incremental data is synchronized in the data synchronization task, the write-ahead logging (WAL) logs generated before the long-running transactions in the source database are committed may be accumulated. As a result, the disk space of the source database may be insufficient. If you want to perform a primary/secondary switchover on the source PolarDB for PostgreSQL cluster, 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. During schema synchronization and full data synchronization, do not execute DDL statements to change the schemas of databases or tables. Otherwise, the data synchronization task fails.
Other limits	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. 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, such as when the CPU load of the source and destination databases is less than 30%. During full data synchronization, concurrent INSERT operations cause table fragmentation in the destination database. Therefore, after full data synchronization is complete, the size of the used tablespace of the destination database is larger than that of the source database. Data inconsistency between the source and destination databases may occur if data from other sources is written to the destination database during data synchronization. A two-way data synchronization instance contains a forward synchronization task and a reverse synchronization task. If an object is to be synchronized in both the forward and reverse synchronization tasks when you configure or reset the instance, the following rules apply: Only one of the tasks can synchronize both the full data and incremental data of objects. The other task synchronizes only the incremental data of the objects. The source data of the current task can be synchronized only to the destination database in the task. The synchronized data is not used as the source data of the other task. During data synchronization, DTS creates a replication slot in the source database to replicate data. 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 If the data synchronization task is released or fails, DTS automatically deletes the replication slot. If a primary/secondary switchover is performed on the source database, you must log on to the secondary database to manually delete the replication slot. 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 initial value of the sequences in the destination database. To query the maximum value of the sequences in the source database, execute the following statements: `do language plpgsql $$ declare nsp name; rel name; val int8; begin for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S' loop execute format($_$select last_value from %I.%I$_$, nsp, rel) into val; raise notice '%', format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1); end loop; end; $$;` 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 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 data synchronization task is released, you can change the value of the session_replication_role parameter back to origin. If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified. Note Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

Billing

Synchronization type	Task configuration fee

Synchronization type	Task configuration fee
Schema synchronization and full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

Conflict detection

To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes. If data records are updated on both nodes, DTS responds to conflicts based on the conflict resolution policy that you specify for the data synchronization task.

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

Uniqueness conflicts caused by INSERT operations
INSERT operations that do not comply with the uniqueness constraint cannot be synchronized. For example, if a record with the same primary key value is inserted into the two synchronization nodes at almost the same time, one of the inserted records fails to be synchronized. The synchronization fails because a record with the same primary key value already exists on the other node.
Inconsistent records caused by 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 primary keys or unique keys of the records to insert may conflict with those of existing records in the destination instance.
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

During two-way synchronization, the system time of the source and destination instances may be different. Synchronization latency may occur. For these reasons, DTS does not ensure that the conflict detection mechanism can prevent all data conflicts. To perform two-way synchronization, make sure that records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes.
DTS provides conflict resolution policies to prevent conflicts that may occur during data synchronization. You can select a conflict resolution policy when you configure two-way data synchronization.

SQL operations that can be synchronized

Important

DDL operations can be synchronized only in the forward direction from the source database to the destination database. DDL operations cannot be synchronized from the destination database to the source database because DDL operations are ignored in the reverse direction.

Operation type	SQL statement

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE Note When data is written to the destination AnalyticDB for PostgreSQL instance, the UPDATE statement is automatically converted to the REPLACE INTO statement. If the UPDATE statement is executed on primary keys, the UPDATE statement is converted to the DELETE and INSERT statements.
DDL	DDL operations can be synchronized only by the data synchronization tasks that are created after October 1, 2020. Important To use a data synchronization task created before May 12, 2023 to synchronize DDL operations, you must create triggers and functions in the source database to capture DDL information before you configure the data synchronization task. For more information, see Use triggers and functions to implement incremental DDL migration for PostgreSQL databases. You cannot synchronize data of the BIT type during incremental data synchronization. If the account of the source database is a privileged account, data synchronization tasks support the following DDL operations: 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 PolarDB for PostgreSQL cluster 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 DDL statements from a session for which the `SET session_replication_role = replica` statement is executed. DDL statements that are executed by calling methods such as FUNCTION cannot be synchronized. If multiple SQL statements committed by the source database at the same time contain both DML and DDL statements, DTS does not synchronize the DDL statements. If multiple SQL statements committed by the source database at the same time contain DDL statements for objects that are not synchronized, the DDL statements are not synchronized.

Permissions required for database accounts

Database type	Required permissions	References

Database type	Required permissions	References
Source and destination PolarDB for PostgreSQL clusters	Permissions of a privileged account. The account must be the owner of the database.	Create a database account and Database management

Procedure

Use one of the following methods to go to the Data Synchronization page and select the region in which the data synchronization instance resides.
DTS console
DMS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Synchronization.
3. In the upper-left corner of the page, select the region in which the data synchronization instance resides.
Note
The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data Development and choose DTS (DTS) > Data Synchronization.
3. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.
Optional. In the upper-right corner of the page, click New Configuration Page.
Note
- Skip this step if the Back to Previous Version button is displayed in the upper-right corner of the page.
- Specific parameters in the new and previous versions of the configuration page may be different. We recommend that you use the new version of the configuration page.

Configure the source and destination databases. The following table describes the parameters.

Section	Parameter	Description

Section	Parameter	Description
N/A	Task Name	The name of the DTS task. DTS automatically generates a task name. 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	Select Existing Connection	The database that you want to use. You can choose whether to use an existing database based on your business requirements. If you select an existing database, DTS automatically populates the parameters for the database. If you do not select an existing database, you must configure the following database information. Note In the DTS console, register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	The type of the source database. Select PolarDB for PostgreSQL.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source PolarDB for PostgreSQL cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Instance ID	The ID of the source PolarDB for PostgreSQL cluster.
	Database Name	The name of the database that contains the objects to be synchronized in the source PolarDB for PostgreSQL cluster.
	Database Account	The database account of the source PolarDB for PostgreSQL cluster. For more information about the permissions that are required for the database account, see the "Permissions required for database accounts" section of this topic.
	Database Password	The password that is used to access the database.
Destination Database	Select Existing Connection	The database that you want to use. You can choose whether to use an existing database based on your business requirements. If you select an existing database, DTS automatically populates the parameters for the database. If you do not select an existing database, you must configure the following database information. Note In the DTS console, register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	The type of the destination database. Select PolarDB for PostgreSQL.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination PolarDB for PostgreSQL cluster resides.
	Instance ID	The ID of the destination PolarDB for PostgreSQL cluster.
	Database Name	The name of the database used to receive data in the destination PolarDB for PostgreSQL cluster.
	Database Account	The database account of the destination PolarDB for PostgreSQL cluster. For more information about the permissions that are required for the database account, see the "Permissions required for database accounts" section of this topic.
	Database Password	The password that is used to access the database.

In the lower part of the page, click Test Connectivity and Proceed.
Note
Make sure that the CIDR blocks of DTS servers can be automatically or manually added to the security settings of the source and destination databases to allow access from DTS servers. For more information, see Add the CIDR blocks of DTS servers.

Configure the objects to be synchronized.

In the Configure Objects step, configure the objects that you want to synchronize.

Parameter	Description

Parameter	Description
Synchronization Types	The synchronization types. 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 the selected objects from the source database to the destination cluster. The historical data is the basis for subsequent incremental synchronization.
Synchronization Topology	The synchronization topology. Select Two-way Synchronization.
Enable Exactly-Once Write	Specifies whether to enable the Exactly-Once write feature. If the objects to be synchronized include tables that do not have primary keys and UNIQUE constraints, select Yes. For more information, see Synchronize tables without primary keys or UNIQUE constraints.
Exclude DDL Operations	Yes: excludes DDL operations. No: synchronizes DDL operations. Important DDL operations can be synchronized only in the forward direction from the source database to the destination database. DDL operations are ignored in the reverse direction from the destination database to the source database. Therefore, this parameter is displayed only when you configure the task in the forward direction.
Global Conflict Resolution Policy	Select an appropriate conflict resolution policy based on your actual business requirements. TaskFailed If a conflict occurs during data synchronization, the data synchronization task reports an error and exits the process. The task enters a failed state, and you must manually resolve the conflict. Ignore If a conflict occurs during data synchronization, the data synchronization task ignores the current statement and continues the process. The conflicting records in the destination database are used. Overwrite If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten. Note For more information about the types of conflicts that DTS can detect, see the "Conflict detection" section of this topic. If a latency occurs when you pause or restart a data synchronization task, the selected conflict resolution policy does not take effect during the latency. By default, data in the destination database is overwritten by the data that is synchronized during the latency.
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 If the source and destination databases contain tables with identical names and the tables in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the tables that are synchronized to the destination database. 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 schema and a data record in the destination database has the same primary key value or unique key value as a data record in the source 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. In this case, only some columns are synchronized, or the data synchronization task fails. Proceed with caution.
Capitalization of Object Names in Destination Instance	The capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to ensure that the capitalization of object names is consistent with that in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
Source Objects	Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section. Note You can select columns, tables, or schemas as objects to be synchronized.
Selected Objects	To rename an object to be synchronized in the destination database, right-click the object in the Selected Objects section. For more information, see Map object names. To remove a selected object, click the object in the Selected Objects section and then click the icon to move the object to the Source Objects section. Note If you use the object name mapping feature to rename an object, other objects that depend on the object may fail to be synchronized. To specify WHERE conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Specify filter conditions. To select SQL operations for incremental synchronization, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data synchronization tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Retry Time 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 this 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 specify different retry time ranges for multiple data synchronization tasks that have the same source or destination database, the shortest retry time range 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.
Retry Time for Other Issues	The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set this parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Important The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Migration	During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server. Note This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter.
Enable Throttling for Incremental Data Synchronization	Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, you do not need to configure this parameter.
Configure ETL	Specifies whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: configures the ETL feature. You can enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: does not configure the ETL feature.
Monitoring and Alerting	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: No: does not enable alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see the "Configure monitoring and alerting when you create a DTS task" section of the Configure monitoring and alerting topic.

Click Next Step: Data Verification to configure data verification.
For more information about how to use the data verification feature, see Configure a data verification task.

Save the task settings and run a precheck.
- To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.
- If you do not need to view or have viewed the parameters, click Next: Save Task Settings and Precheck in the lower part of the page.
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 data synchronization task fails the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, rerun the precheck.
- 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 issue. 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.
Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.

Purchase a data synchronization instance.

On the buy page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.

Section	Parameter	Description

Section	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay for a subscription when you create a data synchronization 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 billed 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 data synchronization instance, you can release the instance to reduce costs.
	Resource Group Settings	The resource group to which the data synchronization instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides instance classes that vary in synchronization speed. You can select an instance class based on your business requirements. For more information, see Instance classes of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, specify the subscription duration and the number of data synchronization 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.

Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the dialog box that appears, click OK.
You can view the progress of the task in the task list.

Wait until the initial synchronization is complete and the Status of the data synchronization task in the forward direction changes to Running.
Configure a data synchronization task in the reverse direction.
1. In the Actions column of the data synchronization task in the reverse direction, click Configure Task.
2. Configure the task by performing steps 4 to 7.
  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.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.Selected Objects
  - 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.
3. Wait until Success Rate becomes 100%. Then, click Back.
Wait until the Status of the synchronization tasks in both forward and reverse directions changes to Running. This indicates that the configuration of two-way data synchronization is complete.