Synchronize PostgreSQL to PolarDB for PostgreSQL - Data Transmission Service

This topic describes how to use Data Transmission Service (DTS) to synchronize data from an ApsaraDB RDS for PostgreSQL instance to a PolarDB for PostgreSQL cluster.

Prerequisites

You have created a destination PolarDB for PostgreSQL cluster. The available storage space of the destination cluster must be larger than the storage space occupied by the source ApsaraDB RDS for PostgreSQL instance. For more information, see Create a database cluster.
Note
For more information about the supported versions of the source and destination databases, see Overview of data synchronization solutions.
You have created a database in the PolarDB for PostgreSQL cluster to receive data. For more information, see Database management.

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
Source database limits	The tables to be synchronized must have a primary key or a UNIQUE constraint, and the fields must be unique. Otherwise, duplicate data may appear in the destination database. Note If the target table is not created by DTS (the Synchronization Types is not set to Schema Synchronization), you must ensure that the table has the same primary key or non-null UNIQUE constraint as the corresponding table in the source database. Otherwise, duplicate data may appear in the target database. The name of the database to be synchronized cannot contain hyphens (-), such as dts-testdata. If you synchronize objects at the table level and need to edit them, such as mapping column names, and the number of tables in a single task exceeds 5,000, split the tables into multiple tasks. You can also configure a task to synchronize the entire database. Otherwise, a request error may be reported after you submit the task. WAL logs: Enable WAL logs. Set the wal_level parameter to logical. For an incremental synchronization task, DTS requires that WAL logs in the source database are retained for more than 24 hours. For a task that includes both full and incremental synchronization, DTS requires that WAL logs are retained for at least 7 days. You can change the log retention period to more than 24 hours after the full synchronization is complete. If the task fails because DTS cannot obtain the WAL logs, or in extreme cases, data becomes inconsistent or is lost because the log retention period is shorter than required, the issue is not covered by the DTS Service-Level Agreement (SLA). If the source database has long-running transactions and the instance includes an incremental synchronization task, Write-Ahead Logging (WAL) logs before the long-running transactions are committed may not be cleared and can accumulate. This can cause insufficient disk space in the source database. Due to the limits of logical replication in the source database, if a single piece of data to be synchronized exceeds 256 MB after an incremental change, the synchronization instance may fail and cannot be recovered. You must reconfigure the synchronization instance. During initial schema synchronization and initial full data synchronization, do not perform Data Definition Language (DDL) operations that change the schema of databases or tables. Otherwise, the data synchronization task fails. Note During initial full data synchronization, DTS queries the source database. This creates metadata locks, which may block DDL operations on the source database. If you perform a major version upgrade on the source database while the synchronization instance is running, the instance fails and cannot be recovered. You must reconfigure the synchronization instance.
Other limits	A single data synchronization task can synchronize only one database. To synchronize multiple databases, configure a separate task for each database. Synchronization of TimescaleDB extension tables and tables with cross-schema inheritance is not supported. If a table to be synchronized contains fields of the SERIAL type, the source database automatically creates a Sequence for these fields. Therefore, when you configure Source Objects, if you set the Synchronization Types to Schema Synchronization, we recommend that you also select Sequence or synchronize the entire schema. Otherwise, the synchronization instance may fail. For full or incremental synchronization tasks, if the tables to be synchronized in the source database contain foreign keys, triggers, or event triggers, DTS temporarily sets the `session_replication_role` parameter to `replica` at the session level when the destination database account is a privileged account. 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 (while the `session_replication_role` parameter is set to `replica` during the full or incremental synchronization), data inconsistency may occur if cascading update or delete operations are performed on the source database. After the DTS sync task is released, you can change the `session_replication_role` parameter value back to `origin`. In the following three scenarios, you must run the `ALTER TABLE schema.table REPLICA IDENTITY FULL;` command on the tables to be synchronized before writing data to them. This ensures data consistency. During the execution of this command, do not perform table lock operations. Otherwise, the tables may be locked. If you skip the related check in the precheck, DTS automatically runs this command during the initialization of the instance. When the instance runs for the first time. When you select objects to synchronize at the schema level, and a new table is created in the schema or a table to be synchronized is rebuilt using the RENAME command. When you use the feature to modify synchronization objects. Note In the command, replace `schema` and `table` with the schema name and table name of the data to be synchronized. Perform this operation during off-peak hours. DTS creates the following temporary tables in the source database to obtain information such as DDL statements for incremental data, the schema of incremental tables, and heartbeat data. Do not delete these temporary tables during 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`. To ensure the accuracy of the displayed synchronization latency, DTS adds a heartbeat table named dts_postgres_heartbeat to the source database. During data synchronization, DTS creates a replication slot with the prefix `dts_sync_` in the source database to replicate data. Using this replication slot, DTS can obtain incremental logs from the source database within the last 15 minutes. When the data synchronization fails or the instance is released, DTS attempts to clean up the replication slot. Note If you change the password of the source database account used by the task or delete the DTS IP address whitelist from the source database during synchronization, the replication slot cannot be automatically cleaned up. In this case, you must manually clean up the replication slot in the source database to prevent it from accumulating and occupying disk space, which can make the source database unavailable. If a primary/secondary failover occurs in the source database, you must log on to the secondary database to perform the cleanup. Before synchronizing data, evaluate the performance of the source and destination databases. Perform data synchronization during off-peak hours. Otherwise, initial full data synchronization consumes read and write resources on both the source and destination databases, which may increase the database load. Initial full data synchronization runs concurrent INSERT operations, which causes fragmentation in the destination database tables. As a result, the tablespace in the destination instance is larger than that in the source instance after the initialization is complete. For table-level data synchronization, if no data other than that from DTS is written to the destination database, you can use Data Management (DMS) to perform online DDL operations. For more information, see Change schemas without locking tables. During DTS synchronization, do not allow data from sources other than DTS to be written to the destination database. Otherwise, data inconsistency between the source and destination databases occurs. For example, if data from other sources is written to the destination database while you use DMS to perform online DDL operations, data may be lost in the destination database. DTS validates data content but does not support the validation of metadata such as Sequences. You must validate metadata yourself. After you switch your business to the destination, new Sequences do not increment from the maximum value of the source Sequences. Before the switch, you must update the Sequence values in the destination database. For more information, see Update Sequence values in the destination database. 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 or adjusting its parameters may be performed. Note When parameters are adjusted, only the parameters of the DTS instance are modified. The parameters in the database are not modified. The parameters that may be modified include but are not limited to those described in Modify instance parameters.
Special cases	If the source instance is an RDS for PostgreSQL instance During synchronization, do not modify the connection address or zone of the RDS for PostgreSQL instance. Otherwise, the synchronization fails. If the source instance is a self-managed PostgreSQL database Make sure that the values of the max_wal_senders and max_replication_slots parameters are both greater than the sum of the number of replication slots currently in use and the number of DTS instances to be created with this self-managed PostgreSQL database as the source. When the source instance is Google Cloud Platform Cloud SQL for PostgreSQL, you must provide an account with `cloudsqlsuperuser` permissions for the Database Account of the source database. When you select objects to sync, you must select objects that this account has permission to manage, or grant the Owner permission for the objects to be synced to this account (for example, using the `GRANT <Owner of the object to be synced> TO <source database account used by the task>` command, you can enable this account to perform related operations as the owner of the object to be synced). Note An account with cloudsqlsuperuser permissions cannot manage data owned by another account with cloudsqlsuperuser permissions.

Billing description

Synchronization type	Task configuration fee
Schema synchronization and full data synchronization	Free of charge.
Incremental 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 cascade synchronization
One-way many-to-one synchronization

For more information about the synchronization topologies and their usage notes, see Synchronization topologies.

Supported synchronization objects

SCHEMA and TABLE.
Note
This includes PRIMARY KEY, UNIQUE KEY, FOREIGN KEY, DATATYPE (built-in data types), and DEFAULT CONSTRAINT.
VIEW, PROCEDURE (for PostgreSQL 11 or later), FUNCTION, RULE, SEQUENCE, EXTENSION, TRIGGER, AGGREGATE, INDEX, OPERATOR, and DOMAIN.

SQL operations that can be synchronized

Operation type	SQL statement
DML	`INSERT`, `UPDATE`, and `DELETE`
DDL	DDL operations can be synchronized only by data synchronization tasks that are created after . Important For data synchronization tasks created before May 12, 2023 (Singapore Time), you must create triggers and functions in the source database to capture DDL information before you configure the 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. DTS can synchronize the following DDL statements if the database account of the source database is a privileged account and the minor engine version of the ApsaraDB RDS for PostgreSQL instance is 20210228 or later. For information about how to update the minor engine version of an instance, see Update the minor engine version. `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 version of the source PostgreSQL database must be 11 or later.) `CREATE INDEX ON TABLE` Important You cannot synchronize additional information of DDL statements, such as `CASCADE` or `RESTRICT`. You cannot synchronize the DDL statements from a session that runs the `SET session_replication_role = replica` command. You cannot synchronize DDL statements that are executed by calling a `FUNCTION` or by other means. 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. DTS does not support synchronizing DDL statements that are executed directly within a plug-in through the Server Programming Interface (SPI).

Permissions required for database accounts

Database	Required permissions	Account creation and authorization
ApsaraDB RDS for PostgreSQL	A privileged account that is the owner of the selected database. Note If the source instance is an ApsaraDB RDS for PostgreSQL 9.4 instance and you need to synchronize only DML operations, the account only needs the `REPLICATION` permission.	Create an account and Create a database.
PolarDB for PostgreSQL	Owner permission on the destination `SCHEMA`. Note You can use the database owner that you specified when you created 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
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 PostgreSQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source ApsaraDB RDS for PostgreSQL instance resides.
	Across Alibaba Cloud Accounts	For synchronization within a single Alibaba Cloud account, select Not Cross-account.
	Instance ID	Select the ID of the source ApsaraDB RDS for PostgreSQL instance.
	Database Name	Enter the name of the database in the source ApsaraDB RDS for PostgreSQL instance that contains the objects to be synchronized.
	Database Account	Enter the database account for the source ApsaraDB RDS for PostgreSQL instance. For information about the required permissions, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.
	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.
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 database in the destination PolarDB for PostgreSQL cluster that is used to receive data.
	Database Account	Enter the database account for the destination PolarDB for PostgreSQL cluster. For information about the required permissions, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.

Click Test Connectivity and Proceed in the lower part of the page.
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.
- If the source or destination database is a self-managed database and its Access Method is not set to Alibaba Cloud Instance, click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

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.
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 schemas or tables as the objects to be synchronized. If you select tables as the objects to be synchronized, DTS does not migrate 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 SQL operations for synchronization at the database or table level, right-click the object to be synchronized in Selected Objects and select the desired SQL operations in the dialog box that appears. To set a WHERE condition to filter data, right-click the table that you want to synchronize in the Selected Objects pane and set the filter condition 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 for your business needs. This example does not require a selection.
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.