Use DTS to migrate data between ApsaraDB RDS for PostgreSQL instances - ApsaraDB RDS

This topic describes how to perform schema migration, full data migration, and incremental data migration between ApsaraDB RDS for PostgreSQL instances by using Data Transmission Service (DTS). When you migrate data between ApsaraDB RDS for PostgreSQL instances, you can select all supported migration types to ensure service continuity.

Prerequisites

The source and destination ApsaraDB RDS for PostgreSQL instances are created. For more information, see Create an instance.
Note
- For information about the supported versions of the source and destination databases, see Overview of data migration scenarios.
- To ensure compatibility, the version of the destination database must be the same as or later than that of the source database. If the version of the destination database is earlier than that of the source database, database compatibility issues may occur.
The available storage space of the destination ApsaraDB RDS for PostgreSQL instance must be larger than the total size of data in the source ApsaraDB RDS for PostgreSQL instance.

Usage notes

Limit type	Description
Limits on the source database	The tables to be migrated must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records. The name of the source database cannot contain hyphens (-). For example, dts-testdata is invalid. If you select tables as the objects to be migrated and you need to edit the tables, such as renaming tables or columns in the destination database, up to 1,000 tables can be migrated in a single data migration task. If you run a task to migrate more than 1,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to migrate the tables, or configure a task to migrate the entire database. If you need to migrate incremental data, you must make sure that the following requirements are met: The value of the wal_level parameter must be set to logical. If you perform only incremental data migration, the write-ahead logging (WAL) logs of the source database must be stored for more than 24 hours. If you perform both full data migration and incremental data migration, the WAL logs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the WAL logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After full data migration is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of WAL logs based on the preceding requirements. Otherwise, the service reliability or performance in the Service Level Agreement (SLA) of DTS may not be guaranteed. Limits on operations to be performed on the source database: During schema migration and full data migration, do not perform DDL operations to change the schemas of databases or tables. Otherwise, the data migration task fails. If you perform only full data migration, do not write data to the source database during data migration. Otherwise, data inconsistency between the source and destination databases occurs. To ensure data consistency, we recommend that you select schema migration, full data migration, and incremental data migration as the migration types. If the source database has one or more long-running transactions and incremental data is migrated in the data migration task, the WAL logs that are generated before the long-running transactions are committed may be accumulated. As a result, the disk space of the source database may be insufficient.
Other limits	If you need to perform a primary/secondary switchover on the source ApsaraDB RDS for PostgreSQL instance, the Logical Replication Slot Failover feature must be enabled. This prevents logical subscriptions from being interrupted and ensures that your data migration task can run as expected. For more information, see Logical Replication Slot Failover. A single data migration task can migrate data from only one database. To migrate data from multiple databases, you must create a data migration task for each database. If you select a schema as the object to be migrated and create a table in the schema or execute the RENAME statement to rename a table in the schema during incremental data migration, you must execute the `ALTER TABLE schema.table REPLICA IDENTITY FULL;` statement before you write data to the table. Note Replace the `schema` and `table` variables in the preceding sample statement with the actual schema name and table name. 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 starting value of the sequences in the destination database. You can execute the following statements to query the maximum value of the sequences in the source database: `do language plpgsql $$ declare nsp name; rel name; val int8; begin for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S' loop execute format($_$select last_value from %I.%I$_$, nsp, rel) into val; raise notice '%', format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1); end loop; end; $$;` DTS creates the following temporary tables in the source database to obtain the DDL statements of incremental data, the schemas of incremental tables, and the heartbeat information. During data migration, do not delete temporary tables in the source database. Otherwise, exceptions occur. After the DTS instance is released, temporary tables are automatically deleted. `public.dts_pg_class`, `public.dts_pg_attribute`, `public.dts_pg_type`, `public.dts_pg_enum`, `public.dts_postgres_heartbeat`, `public.dts_ddl_command`, and `public.dts_args_session`. If you run a full or incremental data migration task, the tables to be migrated from the source database contain foreign keys, triggers, or event triggers, and the account of the destination database is a privileged account or an account that has the permissions of the superuser role, DTS temporarily sets the session_replication_role parameter to replica at the session level during full or incremental data migration. If the account of the destination database does not have the permissions, you must manually set the session_replication_role parameter to replica in the destination database. After the session_replication_role parameter is set to replica during full or incremental data migration, if a cascade update or delete operation is performed in the source database, data inconsistency may occur. After the data migration task is released, you can change the value of the session_replication_role parameter to origin. To ensure that the latency of incremental data migration is accurate, DTS creates a heartbeat table named `dts_postgres_heartbeat` in the source database. During incremental data migration, DTS creates a replication slot in the source database to replicate data. The replication slot is prefixed with `dts_sync_`. By using this replication slot, DTS can obtain the incremental logs of the source database within the last 15 minutes. Note After the DTS instance is released, the replication slot is automatically deleted. If you modify the password of the source database or delete the IP addresses of DTS from the IP address whitelist, the replication slot cannot be automatically deleted. In that case, you must manually delete the replication slot from the source database to prevent the replication slot from piling up. If the data migration task is released or fails, DTS automatically deletes the replication slot. If a primary/secondary switchover is performed on the source ApsaraDB for PostgreSQL instance, you must log on to the secondary instance to delete the replication slot. Before you migrate data, evaluate the impact of data migration on the performance of the source and destination databases. We recommend that you migrate data during off-peak hours. During full data migration, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. During full data migration, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data migration is complete, the tablespace of the destination database is larger than that of the source database. You must ensure that the precision settings for columns of the FLOAT or DOUBLE data type meet your business requirements. DTS uses the `ROUND(COLUMN,PRECISION)` function to retrieve values from columns of the FLOAT or DOUBLE data type. If you do not specify a precision, DTS sets the precision for columns of the FLOAT data type to 38 digits and the precision for columns of the DOUBLE data type to 308 digits. DTS attempts to resume data migration tasks that failed within the last seven days. Before you switch workloads to the destination database, you must stop or release the failed tasks. You can also execute the `REVOKE` statement to revoke the write permissions from the accounts that are used by DTS to access the destination database. Otherwise, the data in the source database will overwrite the data in the destination database after the failed task is resumed.
Special cases	During data migration, do not modify the endpoint or zone of the source ApsaraDB RDS for PostgreSQL instance. Otherwise, the data migration task fails.

Migration types

Schema migration
DTS migrates the schemas of the selected objects from the source database to the destination database.
Full data migration
DTS migrates the historical data of required objects from the source database to the destination database.
Incremental data migration
After full data migration is complete, DTS migrates incremental data from the source database to the destination database. Incremental data migration allows data to be migrated smoothly without interrupting the services of self-managed applications during data migration.

SQL operations that support incremental migration

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	DDL operations can be migrated only in the data migration tasks that are created after October 1, 2020. Important To run a data migration task created before May 12, 2023 to migrate DDL operations, you must create a trigger and a function in the source database to capture DDL information before you configure the data migration task. For more information, see Use triggers and functions to implement incremental DDL migration for PostgreSQL databases. You cannot migrate data of the BIT type during incremental data migration. If the account of the source database is a privileged account and the minor engine version of the source ApsaraDB RDS for PostgreSQL instance is 20210228 or later, the following DDL statements can be migrated in data migration tasks. For information about how to update the minor engine version of an ApsaraDB RDS for PostgreSQL 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 ApsaraDB RDS for PostgreSQL instance must be PostgreSQL V11 or later.) CREATE INDEX ON TABLE Important You cannot migrate additional information of DDL statements, such as CASCADE or RESTRICT. You cannot migrate DDL statements from a session in which the `SET session_replication_role = replica` statement is executed. You cannot migrate DDL statements that are executed by invoking functions. If the SQL statements submitted by the source database at a time contain both DML and DDL statements, DTS does not migrate the DDL statements. If the SQL statements submitted by the source database at a time contain DDL statements that are not to be migrated, DTS does not migrate the DDL statements.

Permissions required for database accounts

Database type	Schema migration	Full data migration	Incremental data migration
Source ApsaraDB RDS for PostgreSQL instance	The USAGE permission on the pg_catalog schema	The SELECT permission on the objects to be migrated	Permissions of a privileged account that is the owner of the database Note If the source ApsaraDB RDS for PostgreSQL instance runs PostgreSQL 9.4 and you want to migrate only DML operations, only the REPLICATION permission is required for the database account.
Destination ApsaraDB RDS for PostgreSQL instance	The CREATE and USAGE permissions on the objects to be migrated	The permissions of the schema owner

For more information about how to create a database account on an ApsaraDB RDS for PostgreSQL instance and grant permissions to the database account, see Create an account and Create a database.

Procedure

Go to the Data Migration Tasks page.
1. Log on to the Data Management (DMS) console.
2. In the top navigation bar, move the pointer over DTS.
3. Choose DTS (DTS) > Data Migration.
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.
- You can also go to the Data Migration page of the new DTS console.
From the drop-down list on the right side of Data Migration Tasks, select the region in which your data migration instance resides.
Note
If you use the new DTS console, you must select the region in which the data migration instance resides in the upper-left corner.

Click Create Task. On the Create Data Migration Task page, configure the source and destination databases. The following table describes the parameters.

Warning

After you configure the source and destination databases, we recommend that you read the Limits that are displayed in the upper part of the page. Otherwise, the task may fail or data inconsistency may occur.

Section	Parameter	Description
N/A	Task Name	The name of the task. DTS automatically generates a task name. We recommend that you specify an informative name to identify the task. You do not need to specify a unique task name.
Source Database	Select an existing DMS database instance	The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must configure the following database information.
	Database Type	The type of the source database. Select PostgreSQL.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source ApsaraDB RDS for PostgreSQL instance resides.
	Instance ID	The ID of the source ApsaraDB RDS for PostgreSQL instance.
	Database Name	The name of the database that contains the objects to be migrated from the source ApsaraDB RDS for PostgreSQL instance.
	Database Account	The database account of the source ApsaraDB RDS for PostgreSQL instance. For information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
	Database Password	The password that is used to access the database instance.
	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 an existing DMS database instance	The database instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you select an existing instance, DTS automatically populates the parameters for the database. If you do not select an existing instance, you must configure the following database information.
	Database Type	The type of the destination database. Select PostgreSQL.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination ApsaraDB RDS for PostgreSQL instance resides.
	Instance ID	The ID of the destination ApsaraDB RDS for PostgreSQL instance.
	Database Name	The name of the database to which objects are migrated in the destination ApsaraDB RDS for PostgreSQL instance.
	Database Account	The database account of the destination ApsaraDB RDS for PostgreSQL instance. For information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
	Database Password	The password that is used to access the database instance.
	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.

In the lower part of the page, click Test Connectivity and Proceed.
If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, DTS automatically adds the CIDR blocks of DTS servers to the IP address whitelist of the instance. If the source or destination database is a self-managed database hosted on an Elastic Compute Service (ECS) instance, DTS automatically adds the CIDR blocks of DTS servers to the security group rules of the ECS instance, and you must make sure that the ECS instance can access the database. If the self-managed database is hosted on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. If the source or destination database is a self-managed database that is deployed in a data center or provided by a third-party cloud service provider, you must manually add the CIDR blocks of DTS servers to the IP address whitelist of the database to allow DTS to access the database. For more information, see the CIDR blocks of DTS servers section of the Add the CIDR blocks of DTS servers topic.
Warning
If the public CIDR blocks of DTS servers are automatically or manually added to the whitelist of a database instance or to the security group rules of an ECS instance, security risks may arise. Therefore, before you use DTS to migrate data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or security group rules and forbidding unauthorized CIDR blocks, or connecting the database instance to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.

Configure the objects to be migrated and advanced settings. The following table describes the parameters.

Parameter	Description
Migration Types	To perform only full data migration, select Schema Migration and Full Data Migration. To migrate data without service downtime, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If you select Schema Migration, DTS migrates the schemas of the tables to be migrated from the source database to the destination database. The schemas include foreign keys. If you do not select Incremental Data Migration, we recommend that you do not write data to the source database during data migration. This ensures data consistency between the source and destination databases.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains tables that use 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 migration 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 migrated 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 the following potential risks: If the source and destination databases have the same schema, and a data record has the same primary key as an existing data record in the destination database, the following scenarios may occur: During full data migration, DTS does not migrate the data record to the destination database. The existing data record in the destination database is retained. During incremental data migration, DTS migrates 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, only specific columns are migrated or the data migration task fails. Proceed with caution.
Source Objects	Select one or more objects from the Source Objects section. Click the icon to add the objects to the Selected Objects section. Note You can select columns, tables, or schemas as the objects to be migrated. If you select tables or columns as the objects to be migrated, DTS does not migrate other objects, such as views, triggers, or stored procedures, to the destination database.
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 Map the name of a single object. 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 to rename an object, other objects that are dependent on the object may fail to be migrated. 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 migrate SQL operations performed on a specific database or table, right-click the object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to migrate. For more information about the SQL operations that support incremental migration, see the SQL operations that support incremental migration section of this topic.

Click Next: Advanced Settings to configure advanced settings.

Parameter	Description
Monitoring and Alerting	Specifies whether to configure alerting for the data migration task. If the task fails or the migration latency exceeds the specified threshold, the alert contacts receive notifications. Valid values: No: does not configure 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.
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 make sure that the capitalization of object names is consistent with that of the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
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 migration task is started, DTS immediately retries a connection within the retry time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS is reconnected to the source and destination databases within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails. Note If you specify different retry time ranges for multiple data migration tasks that share the same source or destination database, the value that is specified later 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 the earliest opportunity after the source database and destination instance are released.
Retry Time for Other Issues	The retry time range for other issues. For example, if DDL or DML operations fail to be performed after the data migration task is started, DTS immediately retries the operations within the retry time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set the parameter to a value greater than 10. If the failed operations are successfully performed within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration 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.
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.

In the lower part of the page, click Next: Save Task Settings and Precheck.
You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameters to be specified when you call the relevant API operation to configure the DTS task.
Note
- Before you can start the data migration task, DTS performs a precheck. You can start the data migration task only after the task passes the precheck.
- If the task fails to pass the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
- 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 issues. Then, run a precheck again.
  If the 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 Success Rate becomes 100%. Then, click Next: Purchase Instance.

On the Purchase Instance page, configure the Instance Class parameter for the data migration instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Resource Group	The resource group to which the data migration instance belongs. Default value: default resource group. For more information, see What is Resource Management?
New Instance Class	Instance Class	DTS provides instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Instance classes of data migration instances.

Read and agree to Data Transmission Service (Pay-as-you-go) Service Terms by selecting the check box.
Click Buy and Start. In the message that appears, click OK.
You can view the progress of the task on the Data Migration page.