Migrate data between PolarDB for PostgreSQL clusters - Data Transmission Service

Prerequisites

You have created a destination PolarDB for PostgreSQL cluster with more storage space than the source PolarDB for PostgreSQL cluster currently uses. For more information about how to create a cluster, see Create a database cluster.
You have set the wal_level parameter of the source PolarDB for PostgreSQL cluster to logical. For more information about how to configure this parameter, see Configure cluster parameters.
You have created a database in the destination PolarDB for PostgreSQL cluster to receive migrated data. For more information about how to create a database, see Database management.

Notes

Note

During schema migration, DTS migrates foreign keys from the source database to the destination database.
During full data migration and incremental data migration, DTS temporarily disables constraint checks and foreign key cascade operations at the session level. If cascade update or delete operations occur in the source database while the task is running, data inconsistency may occur.

Type	Description
Source database limits	In a PolarDB for PostgreSQL cluster, the tables to be migrated must have a primary key or a non-null unique index. If the source database has long-running transactions and the instance has an incremental migration task, the write-ahead log (WAL) that is generated before the long-running transactions are committed may accumulate. This can cause the disk space of the source database to become insufficient. To ensure that the migration task runs as expected and to prevent logical replication from being interrupted by a primary/secondary switchover, the PolarDB for PostgreSQL cluster must support and enable Logical Replication Slot Failover. Note If the source PolarDB for PostgreSQL cluster does not support Logical Replication Slot Failover (for example, if the Database Engine of the cluster is PostgreSQL 14), a high-availability (HA) switchover in the source database may cause the migration instance to fail and become unrecoverable. Limits on operations in the source database: During schema migration and full migration, do not perform DDL operations that change the database or table structure. Otherwise, the data migration task fails. If you perform only full data migration, do not write new data to the source instance. Otherwise, data inconsistency between the source and destination databases occurs. To maintain real-time data consistency, we recommend that you select schema migration, full data migration, and incremental data migration. Due to the inherent limits of logical replication in the source database, if a single piece of data to be migrated exceeds 256 MB after an incremental change, the migration instance may fail and cannot be recovered. You must reconfigure the migration instance.
Other limits	A single data migration task can migrate only one database. To migrate multiple databases, configure a separate data migration task for each database. DTS does not support migrating TimescaleDB extension tables, tables with cross-schema inheritance, or tables with unique indexes based on expressions. Schemas created by installing plugins cannot be migrated. You cannot obtain information about these schemas in the console when you configure the task. If the migration instance includes an incremental data migration task, you must run the `ALTER TABLE schema.table REPLICA IDENTITY FULL;` command on the tables to be migrated in the source database before you write data to them. This ensures data consistency. Run this command in the following scenarios. During the execution of this command, do not perform table lock operations to avoid deadlocks. If you skip the related check in the precheck, DTS automatically runs this command during instance initialization. When the instance runs for the first time. When the migration object is a schema, and a new table is created in the schema or an existing table is rebuilt using the RENAME command. Note In the command, replace `schema` and `table` with the actual schema name and table name. We recommend that you perform this operation during off-peak hours. If a table to be migrated 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 Migration for the Migration Types, we recommend that you also select Sequence or migrate the entire schema. Otherwise, the migration instance may fail. Because full data migration runs concurrent INSERT operations, it causes table fragmentation in the destination database. As a result, the table space in the destination database is larger than that in the source database after full migration is complete. DTS attempts to automatically recover failed tasks within seven days. Therefore, before you switch your business to the destination instance, you must end or release the task, or use the `revoke` command to revoke the write permissions of the account that DTS uses to access the destination instance. This prevents the source data from overwriting the data in the destination instance after the task is automatically recovered. DTS validates data content but does not support validation for metadata such as sequences. You must validate metadata yourself. DTS creates the following temporary tables in the source database to obtain DDL statements for incremental data, the structure of incremental tables, and heartbeat information. Do not delete these temporary tables during migration. Otherwise, the DTS task becomes 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`. During incremental data migration, DTS creates a replication slot with the prefix `dts_sync_` in the source database to replicate data. DTS uses this replication slot to obtain incremental logs from the source database within the last 15 minutes. When the data migration fails or the migration instance is released, DTS attempts to automatically clean up this replication slot. Note If you change the password of the source database account or remove the DTS IP address from the source database's IP address whitelist during data migration, the replication slot cannot be automatically cleaned up. In this case, you must manually clean up the replication slot in the source database. This prevents the slot from continuously accumulating and consuming disk space, which can make the source database unavailable. If a failover occurs on the source database, you must log on to the secondary database to manually clean up the slot. After a service switchover to the destination database, new sequences do not start incrementing from the maximum sequence value of the source database. You must update the sequence value in the destination database before the switchover. For more information, see Update the sequence value of the destination database. For full or incremental migration tasks, if the tables to be migrated 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 if the destination database account is a privileged account or has superuser permissions. If the destination database account does not have these permissions, you must manually set the session_replication_role parameter to replica in the destination database. During this period (when session_replication_role is replica), if cascade update or delete operations occur in the source database, data inconsistency may occur. After the DTS migration task is released, you can change the session_replication_role parameter back to origin. If the task fails, DTS technical support will attempt to recover it within 8 hours. During the recovery process, operations such as restarting the task or adjusting its parameters may be performed. Note When parameters are adjusted, only DTS task parameters are modified. Database parameters remain unchanged.The parameters that may be modified include but are not limited to those described in Modify instance parameters. When migrating partitioned tables, include both the parent table and its child tables as synchronization objects. Otherwise, data inconsistency may occur for the partitioned table. Note In PostgreSQL, the parent table of a partitioned table does not directly store data. All data is stored in the child tables. A sync 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

Migration type	Link configuration fee	Data transfer cost
Schema migration and full data migration	Free.	Free in this example.
Incremental data migration	Charged. For more information, see Billing overview.	Free in this example.

Supported objects

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

SQL operations supported for incremental migration

Operation type	SQL operation
DML	INSERT, UPDATE, and DELETE
DDL	DDL operations can be migrated only in data migration tasks that are created after October 1, 2020. Important For data migration tasks created before May 12, 2023, you must create a trigger and a function 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. Data of the bit type is not supported during incremental data migration. If the source database account is a privileged account, the following DDL operations are supported in migration tasks: 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 source PostgreSQL version must be PostgreSQL 11 or later.) CREATE INDEX ON TABLE Important Additional information in DDL statements, such as CASCADE or RESTRICT, is not supported. DDL statements in a session where the `SET session_replication_role = replica` command is run are not supported. DDL statements that are run by calling a FUNCTION or other methods are not supported. If multiple SQL statements submitted from the source database at one time contain both DML and DDL statements, the DDL statements are not migrated. If multiple SQL statements submitted from the source database at one time contain DDL statements for objects that are not to be migrated, the DDL statements are not migrated.

Required database account permissions

Database	Required permissions	How to create and grant permissions
Source PolarDB for PostgreSQL	Privileged account	Create a database account
Target PolarDB for PostgreSQL	Privileged account	Create a database account

Procedure

Navigate to the migration task list page for the destination region using one of the following methods.
From the DTS console
1. Log on to the Data Transmission Service (DTS) console.
2. In the navigation pane on the left, click Data Migration.
3. In the upper-left corner of the page, select the region where the migration instance is located.
From the DMS console

Note
The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.
1. Log on to the Data Management (DMS) console.
2. In the top menu bar, choose Data + AI > Data Transmission (DTS) > Data Migration.
3. To the right of Data Migration Tasks, select the region where the migration instance is located.
Click Create Task to navigate to the task configuration page.

Configure the source and destination databases.

Category	Configuration	Description
N/A	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Select Existing Connection	To use a database instance that has been added to the system (created or saved), select the desired database instance from the drop-down list. The database information below will be automatically configured. Note In the DMS console, this parameter is named Select a DMS database instance.. If you have not registered the database instance with the system, or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select PolarDB for PostgreSQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB for PostgreSQL cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	This example involves migration 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 database in the source PolarDB for PostgreSQL cluster that contains the objects to be migrated.
	Database Account	Enter the database account of the source PolarDB for PostgreSQL cluster. For required permissions, see Required database account permissions.
	Database Password	Enter the password for the specified database account.
Destination Database	Select Existing Connection	To use a database instance that has been added to the system (created or saved), select the desired database instance from the drop-down list. The database information below will be automatically configured. Note In the DMS console, this parameter is named Select a DMS database instance.. If you have not registered the database instance with the system, or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select PolarDB for PostgreSQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	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 will receive the migrated data.
	Database Account	Enter the database account of the destination PolarDB for PostgreSQL cluster. For required permissions, see Required database account permissions.
	Database Password	Enter the password for the specified database account.

After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page.

Note
Ensure that the DTS service IP address segments are 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 task objects.

On the Configure Objects page, configure the objects that you want to migrate.

Configuration	Description
Migration Types	If you only need to perform a full migration, select both Schema Migration and Full Data Migration. To perform a migration with no downtime, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If you do not select Schema Migration, you must ensure that a database and tables to receive the data exist in the destination database. You can also use the object name mapping feature in the Selected Objects box as needed. If you do not select Incremental Data Migration, do not write new data to the source instance during data migration to ensure data consistency.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks whether tables with the same names exist in the destination database. If no tables with the same names exist, the precheck is passed. If tables with the same names exist, an error is reported during the precheck, and the data migration task does not start. Note If a table in the destination database has the same name but cannot be easily deleted or renamed, you can change the name of the table in the destination database. For more information, see Object name mapping. Ignore Errors and Proceed: Skips the check for tables with the same names. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and business risks. For example: If the table schemas are consistent and a record in the destination database has the same primary key value as a record in the source database: During full migration, DTS keeps the record in the destination database. The record from the source database is not migrated. During incremental migration, DTS does not keep the record in the destination database. The record from the source database overwrites the record in the destination database. If the table schemas are inconsistent, only some columns of data may be migrated, or the migration may fail. Proceed with caution.
Capitalization of Object Names in Destination Instance	You can configure the case sensitivity policy for the names of migrated objects, such as databases, tables, and columns, in the destination instance. By default, DTS default policy is selected. You can also choose to keep the case sensitivity consistent with the default policy of the source or destination database. For more information, see Case sensitivity of object names in the destination database.
Source Objects	In the Source Objects box, click the objects to migrate, and then click to move them to the Selected Objects box. Note You can select objects to migrate at the schema or table level. If you select tables as the migration objects, other objects such as views, triggers, and stored procedures are not migrated to the destination database. If a table to be migrated contains a SERIAL field, and you select Schema Migration for Migration Types, we recommend that you also select Sequence or migrate the entire schema.
Selected Objects	To rename an object that you want to migrate to the destination instance, right-click the object in the Selected Objects section. For more information, see Individual table column mapping. To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time. Note If you use the object name mapping feature, migration of other objects that depend on this object may fail. To set a WHERE clause to filter data, right-click the table to be migrated in Selected Objects and configure the filter condition in the dialog box that appears. For more information, see Set filter conditions.

Click Next: Advanced Settings to configure advanced parameters.

Configuration	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules tasks on a shared cluster. You do not need to select one. If you want more stable tasks, you can purchase a dedicated cluster to run DTS migration tasks.
Retry Time for Failed Connections	After the migration task starts, if the connection to the source or destination database fails, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1440 minutes. We recommend that you set the duration to more than 30 minutes. If DTS reconnects to the source and destination databases within the specified duration, the migration task automatically resumes. Otherwise, the task fails. Note For multiple DTS instances that share the same source or destination, the network retry time is determined by the setting of the last created task. Because you are charged for the task during the connection retry period, we recommend that you customize the retry time based on your business needs, or release the DTS instance as soon as possible after the source and destination database instances are released.
Retry Time for Other Issues	After the migration task starts, if a non-connectivity issue, such as a DDL or DML execution exception, occurs in the source or destination database, DTS reports an error and immediately begins to retry the operation. The default retry duration is 10 minutes. You can customize the retry time to a value from 1 to 1440 minutes. We recommend that you set the duration to more than 10 minutes. If the related operations succeed within the specified retry duration, the migration task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than the value of Retry Time for Failed Connections.
Enable Throttling for Full Data Migration	During full migration, DTS consumes read and write resources on the source and destination databases, which may increase the database load. If required, you can enable throttling for the full migration task. You can set Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) to reduce the load on the destination database. Note This configuration item is available only if you select Full Data Migration for Migration Types. You can also adjust the full migration speed after the migration instance is running.
Enable Throttling for Incremental Data Migration	If required, you can also choose to set speed limits for the incremental migration task. You can set RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) to reduce the load on the destination database. Note This configuration item is available only if you select Incremental Data Migration for Migration Types. You can also adjust the incremental migration speed after the migration instance is running.
Environment Tag	Based on your needs, select an environment label to identify the instance. This example does not require a label.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	Select whether to set alerts and receive alert notifications based on your business needs. No: Does not set an alert. Yes: Configure alerts by setting an alert threshold and an alert notifications. If a migration fails or the latency exceeds the threshold, the system sends an alert notification.

Click Next: Data Validation to configure a data validation task.

For more information about the data validation feature, see Configure data validation.

Save the task and run a precheck.
- To view the parameters for configuring this instance when you call the API operation, move the pointer over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the bubble that appears.
- If you do not need to view or have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before the migration task starts, DTS performs a precheck. The task starts only after it passes the precheck.
- If the precheck fails, click View Details next to the failed check item, fix the issue based on the prompt, and then run the precheck again.
- If a warning is reported during the precheck:
  
  For check items that cannot be ignored, click View Details next to the failed item, fix the issue based on the prompt, and then run the precheck again.
  
  For check items that can be ignored, you can click Confirm Alert Details, Ignore, OK, and Precheck Again to skip the alert item and run the precheck again. If you choose to ignore a warning, it may cause issues such as data inconsistency and pose risks to your business.

Purchase the instance.

When the Success Rate is 100%, click Next: Purchase Instance.

On the Purchase page, select the link specification for the data migration instance. For more information, see the following table.

Category	Parameter	Description
New Instance Class	Resource Group Settings	Select the resource group to which the instance belongs. The default value is default resource group. For more information, see What is Resource Management?
New Instance Class	Instance Class	DTS provides migration specifications with different performance levels. The link specification affects the migration speed. You can select a specification based on your business scenario. For more information, see Data migration link specifications.

After the configuration is complete, read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the OK dialog box that appears, click OK.

You can view the progress of the migration task on the Data Migration Tasks list page.
Note
- If the migration task does not include incremental migration, it stops automatically after the full migration is complete. After the task stops, its Status changes to Completed.
- If the migration task includes incremental migration, it does not stop automatically. The incremental migration task continues to run. While the incremental migration task is running, the Status of the task is Running.