Configure one-way synchronization between PolarDB for PostgreSQL clusters - Data Transmission Service

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

Prerequisites

A source and a destination PolarDB for PostgreSQL database cluster have been created. For more information, see Create a database cluster.
The wal_level parameter is set to logical for the source and destination PolarDB for PostgreSQL instances. For more information, see Set cluster parameters.
The available storage space of the destination database is larger than the used storage space of the source database.

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.

Type	Description
Limits on the source database	The tables that you want to synchronize from the source PolarDB for PostgreSQL cluster must have primary keys or non-null unique indexes. If the source database has long-running transactions and the instance includes an incremental synchronization task, the write-ahead log (WAL) generated before the long-running transactions are committed may accumulate and cannot be cleared. 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 subscription interruptions caused by primary/secondary switchovers, the PolarDB for PostgreSQL cluster must support and enable Logical Replication Slot Failover. Note If the source PolarDB for PostgreSQL cluster does not support the logical replication slot failover feature (for example, when Database Engine is set to PostgreSQL 14) and when you trigger a primary/secondary switchover, the data synchronization task may fail and it cannot be recovered. 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 during the runtime of the synchronization instance, the instance may fail and cannot be recovered. You must reconfigure the synchronization instance. During schema synchronization and initial full data synchronization, do not perform DDL operations to change the schemas of databases or tables. Otherwise, the data synchronization task fails.
Other limits	A single data synchronization task can synchronize only one database. If you want to synchronize multiple databases, you must configure a data synchronization task for each database. DTS does not support synchronization of tables that use the TimescaleDB extension or tables that have cross-schema inheritance relationships. In the following three scenarios, you must run the `ALTER TABLE schema.table REPLICA IDENTITY FULL;` command on the table to be synchronized in the source database before you write data to the table. This ensures data consistency. During the execution of this command, we recommend that you do not perform table locking operations. Otherwise, the table may be locked. If you skip the related check item in the precheck, DTS automatically runs this command when the instance is initialized. When the instance runs for the first time. When you synchronize data at the schema level, and a new table is created in the schema to be synchronized or a table to be synchronized is rebuilt using the RENAME command. When you use the feature of modifying synchronization objects. Note In the command, replace `schema` and `table` with the schema name and table name of the data to be synchronized. We recommend that you perform this operation during off-peak hours. If a table to be synchronized 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. During initial full data synchronization, DTS uses read and write resources of the source and destination databases. This increases the load on the database servers. Evaluate the performance of the source and destination databases before you perform data synchronization. We recommend that you perform data synchronization during off-peak hours. During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. After initial full data synchronization is complete, the tablespace of the destination instance is larger than that of the source instance. During data synchronization, do not write data to the destination database using other services. This prevents data inconsistency between the source and destination databases. During data synchronization, DTS creates a replication slot with the `dts_sync_` prefix in the source database to replicate data. DTS can obtain the incremental logs of the source database within 15 minutes from this replication slot. Note DTS automatically deletes the replication slot after the instance is released. If you change the password of the database account or delete the IP address of the DTS server from the whitelist of the source database during data synchronization, the replication slot cannot be automatically deleted. In this case, you must manually delete the replication slot from the source database to prevent it from occupying disk space and making the ApsaraDB RDS for PostgreSQL instance unavailable. When the synchronization task is released or fails, DTS automatically clears the replication slot. If a primary/secondary failover occurs on the ApsaraDB RDS for PostgreSQL instance, you must log on to the secondary database to manually clear the replication slot. DTS does not check the validity of metadata, such as sequences. You must manually check the validity of metadata. DTS creates the following temporary tables in the source database to obtain the DDL statements of incremental data, the schemas of incremental tables, and heartbeat information. Do not delete these temporary tables from the source database during data synchronization. Otherwise, the DTS task may become 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 workloads to the destination instance, the initial value of a new sequence is not the maximum value of the corresponding sequence in the source database. You must update the sequence value in the destination database before you switch your workloads. For more information, see Update the sequence value in the destination database. If the task performs full or incremental synchronization and the tables to be synchronized in the source database contain foreign keys, triggers, or event triggers, DTS temporarily sets the session_replication_role parameter to replica at the session level during full or incremental synchronization if the destination database account is a privileged account or an account with super privileges. If the destination database account does not have this permission, you must manually set the session_replication_role parameter to replica in the destination database. During this period (the value of the session_replication_role parameter is replica during full or incremental synchronization), if cascade update or delete operations exist in the source database, data inconsistency may occur. After the DTS task is released, you can change the value of the session_replication_role parameter back to origin. If an instance fails, DTS helpdesk will try to recover the instance within 8 hours. During the recovery process, operations such as restarting the instance and adjusting parameters may be performed. Note When parameters are adjusted, only the parameters of the DTS instance are modified. The parameters of the database are not modified. 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 tables as synchronization objects. Otherwise, data inconsistency may occur in the partitioned table. Note The parent table of a PostgreSQL partitioned table does not directly store data. All data is stored in the child tables. The synchronization task must include the parent table and all its child tables. Otherwise, data in the child tables may not be synchronized, leading to data inconsistency between the source and destination.

Billing

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

Supported synchronization topologies

One-way one-to-one synchronization
One-way one-to-many synchronization
One-way many-to-one synchronization

For more information about the synchronization topologies supported by DTS, see Synchronization topologies.

Supported objects to be synchronized

SCHEMA and TABLE.
Note
Including PRIMARY KEY, UNIQUE KEY, FOREIGN KEY, DATATYPE (built-in data type), and DEFAULT CONSTRAINT.
VIEW, PROCEDURE (PostgreSQL V11 or later.), FUNCTION, RULE, SEQUENCE, EXTENSION, TRIGGER, AGGREGATE, INDEX, OPERATOR, and DOMAIN.

SQL operations that support data synchronization

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 configuring the synchronization task. For more information, see Implement PostgreSQL DDL incremental migration through triggers and functions. 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.

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
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 task resides.
DMS console
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 + AI 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.

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

Category	Configuration	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	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select PolarDB for PostgreSQL.
	Access Method	Select Cloud Instance.
	Instance Region	Select the region where the source PolarDB for PostgreSQL cluster resides.
	Cross-account	In this example, data is synchronized within the same Alibaba Cloud account. Select No.
	Instance ID	Select the ID of the source PolarDB for PostgreSQL cluster.
	Database Name	Enter the name of the PolarDB for PostgreSQL database.
	Database Account	Enter the database account of the PolarDB for PostgreSQL cluster.
	Database Password	The password that is used to access the database.
Destination Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select PolarDB for PostgreSQL.
	Access Method	Select 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 destination PolarDB for PostgreSQL database.
	Database Account	Enter the privileged database account of the destination PolarDB for PostgreSQL cluster. For information about how to create and authorize an account, see Create a database account.
	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 DTS server IP addresses to a whitelist.

Configure the objects to be synchronized.

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

Configuration	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	Select One-way Synchronization.
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 instance 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 databases as objects to synchronize. If you select tables or columns as the objects to be synchronized, DTS does not synchronize other objects such as views, triggers, and stored procedures 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 rename an object that you want to synchronize to the destination instance, right-click the object in the Selected Objects section. For more information, see the "Map the name of a single object" section of the Map object names topic. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see the "Map multiple object names at a time" section of the Map object names topic. Note To select the SQL operations to be synchronized at the database or table level, right-click the object in the Selected Objects box and select the desired SQL operations in the dialog box that appears. To set WHERE conditions to filter data, right-click a table in the Selected Objects box and set the filter conditions in the dialog box that appears. For more information, see Set filter conditions.

Click Next: Advanced Settings to configure advanced settings.

Configuration	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 instances, 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 Synchronization	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 You can configure this parameter 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	You can select an environment tag to identify the instance. In this example, you do not need to select a tag.
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 instance. 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.

Purchase the instance.

Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.

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

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.