Data migration between ApsaraDB RDS for PostgreSQL instances-Data Transmission Service(DTS)-阿里云帮助中心

This topic describes how to use Data Transmission Service (DTS) for schema migration, full data migration, and incremental data migration between ApsaraDB RDS for PostgreSQL instances. By combining these three migration types, you can migrate your database without downtime.

Prerequisites

You have created the source and destination ApsaraDB RDS for PostgreSQL instances. For more information, see Create an ApsaraDB RDS for PostgreSQL instance.
Note
- For information about the supported versions of the source and destination databases, see Overview of migration scenarios.
- To ensure compatibility, the destination database should run the same or a later version than the source database. Migrating data to an earlier version may cause compatibility issues.
The destination ApsaraDB RDS for PostgreSQL instance must have more storage space than the source ApsaraDB RDS for PostgreSQL instance uses.

Usage notes

Type	Description
Source database limits	The tables to be migrated must have primary keys or UNIQUE constraints, and the values in the key columns must be unique. Otherwise, data may be duplicated in the destination database. Note If the destination tables are not created by DTS (you did not select Schema Migration for Migration Types), make sure that the destination tables have the same primary keys or non-empty UNIQUE constraints as the source tables. Otherwise, data may be duplicated in the destination database. The names of the databases to be migrated cannot contain hyphens (-), such as dts-testdata. If you migrate data at the table level and need to edit the migration objects, such as mapping table or column names, a single data migration task can migrate a maximum of 1,000 tables. If you exceed this limit, an error is reported after you submit the task. In this case, split the tables into multiple migration tasks or configure a task to migrate the entire database. DTS does not support the migration of temporary tables, internal triggers, or some functions (C language functions and internal functions for PROCEDURE and FUNCTION) from the source database. DTS supports the migration of some custom data types (COMPOSITE, ENUM, or RANGE) and the following constraints: primary key, foreign key, unique, and CHECK. For incremental migration, note the following requirements for write-ahead logs (WALs): The wal_level parameter must be set to logical. For an incremental migration task, DTS requires that WALs in the source database are retained for more than 24 hours. For a task that includes both full migration and incremental migration, DTS requires that WALs are retained for at least 7 days. You can change the log retention period to more than 24 hours after the full migration is complete. If the task fails because DTS cannot obtain the required WALs, or in extreme cases, data inconsistency or data loss occurs, the issue is not covered by the DTS Service-Level Agreement (SLA) because the log retention period is shorter than required. Source database operation limits: During schema migration and full migration, do not perform DDL operations to change the schema of the database or tables. Otherwise, the data migration task fails. If you perform only full data migration, do not write new data to the source database. Otherwise, the data in the source and destination databases will be inconsistent. To ensure real-time data consistency, select Schema Migration, Full Data Migration, and Incremental Data Migration. Due to the 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. If the source database has long-running transactions and the instance performs incremental migration, the write-ahead logs (WALs) generated before the long-running transactions are committed cannot be cleared. This may cause the WALs to accumulate and exhaust the disk space of the source database. During the migration, if you perform a major engine version upgrade on the source database, the migration instance fails and cannot be recovered. You must reconfigure the migration instance.
Other limits	To ensure the migration task runs as expected and to prevent logical replication from being interrupted by a failover, you must enable Logical Replication Slot Failover for the ApsaraDB RDS for PostgreSQL instance. For more information, see Logical Replication Slot Failover. 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 the migration of tables created by the TimescaleDB extension, tables with cross-schema inheritance, or tables with unique indexes based on expressions. Schemas created by installing extensions cannot be migrated. You cannot obtain information about these schemas in the DTS console when you configure the task. If a table to be migrated contains a SERIAL column, the source database automatically creates a sequence for that column. Therefore, when you configure the Source Objects, if you select Schema Migration for Migration Types, select Sequence or migrate the entire schema. Otherwise, the migration instance may fail. If the migration instance performs incremental data migration, 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. This requirement applies in the following two 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 the instance initialization. When the instance runs for the first time. When the migration object granularity is Schema, and a new table is created in the schema to be migrated or a table to be migrated is rebuilt using the RENAME command. Note In the command, replace `schema` and `table` with the schema name and table name of the data to be migrated. We recommend that you perform this operation during off-peak hours. DTS validates data content but does not support the validation of metadata such as sequences. You must validate the metadata yourself. After you switch your business to the destination instance, new sequences do not start incrementing from the maximum value of the source sequences. You must update the sequence values in the destination database before the switchover. For more information, see Update the sequence values in the destination database. Data Transmission Service (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. During data migration, do not delete temporary tables in the source database. Otherwise, the data migration task may fail. 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`. For a full or incremental migration task, 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. To ensure the accuracy of the displayed migration latency for incremental data, DTS adds a heartbeat table named `dts_postgres_heartbeat` to the source database. During incremental data migration, DTS creates a replication slot with the `dts_sync_` prefix 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 migration fails or the migration instance is released, DTS attempts to automatically clear this 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 data migration, the replication slot cannot be automatically cleared. In this case, you must manually clear 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 failover occurs in the source database, you must log on to the secondary database to manually clear the slot. Before you migrate data, evaluate the performance of the source and destination databases. We recommend that you perform data migration during off-peak hours. Otherwise, DTS consumes read and write resources on both the source and destination databases during full data migration, which may increase the database load. Full data migration involves concurrent INSERT operations, which can cause table fragmentation in the destination database. As a result, the table storage space in the destination database will be larger than that in the source database after full migration is complete. Confirm whether the migration precision that DTS uses for columns of the FLOAT or DOUBLE data type meets your business requirements. DTS reads the values of these columns using `ROUND(COLUMN,PRECISION)`. If the precision is not explicitly defined, DTS uses a precision of 38 for FLOAT and 308 for DOUBLE. DTS attempts to recover failed migration tasks within seven days. Therefore, before you switch your business to the destination instance, make sure to end or release the task. Alternatively, 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 if the task is automatically recovered. If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters. Note Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed 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 The parent table of a PostgreSQL partitioned table does not store data directly. All data is stored in the child tables (partitions). The 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.
Special cases	If the source instance is an ApsaraDB RDS for PostgreSQL instance, do not change its connection address or zone during migration. Otherwise, the migration fails.

Migration types

Schema migration

DTS migrates the schema definitions of the migration objects from the source database to the destination database.
Full migration

DTS migrates all historical data of the specified migration objects from the source database to the destination database.
Incremental migration

After a full migration is complete, DTS migrates incremental data updates from the source database to the destination database. Incremental migration lets you smoothly migrate data without interrupting your self-managed applications.

Supported objects

schema and table

Note
This includes primary key, unique key, foreign key, DATATYPE (built-in data type), and default constraint.
view, procedure (for PostgreSQL 11 or later), function, rule, sequence, extension, trigger, aggregate, index, operator, and domain

Supported SQL operations

Operation type	SQL operations
DML	`INSERT`, `UPDATE`, and `DELETE`
DDL	Only data migration tasks created after October 1, 2020 (UTC+8)October 1, 2020 (Singapore Time) support DDL operation migration. Important For data migration tasks created before September 9, 2022 (UTC+8), create triggers and functions in the source database to capture DDL information before you configure the migration task. For more information, see DDL incremental migration for PostgreSQL using triggers and functions. Data of the `BIT` type cannot be migrated during incremental data migration. The migration task supports the following DDL statements. The database account for the source database must be a privileged account, and the minor engine version of the RDS PostgreSQL instance must be 20210228 or later. To upgrade the minor engine version, see Upgrade 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` (for source PostgreSQL 11 or later) `CREATE INDEX ON TABLE` Important Migration of additional information in DDL statements, such as `CASCADE` or `RESTRICT`, is not supported. Migration of DDL statements in sessions that use the `SET session_replication_role = replica` command is not supported. Migration is not supported for DDL executed by methods such as calling `FUNCTION`. If a single transaction in the source database contains both DML and DDL statements, the DDL statements are not migrated. If a single transaction in the source database contains DDL statements for objects that are not migrated, the DDL statements are not migrated. Migration of DDL statements that are executed directly within a plugin using the Server Programming Interface (SPI) is not supported.

Database account permissions

Database instance	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.	A privileged account that is the owner of the selected database. Note If the source ApsaraDB RDS for PostgreSQL instance runs PostgreSQL 9.4 and you need to migrate only DML operations, the account only needs the REPLICATION permission.
destination ApsaraDB RDS for PostgreSQL instance	The CREATE and USAGE permissions on the destination objects.	Schema owner permissions.

To create a database account for an ApsaraDB RDS for PostgreSQL instance and grant it permissions, see Create an account and Create a database.

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.

Warning

After you select the source and destination instances, we recommend that you carefully read the limits displayed at the top of the page. Otherwise, the task may fail or data inconsistency may occur.

Section	Parameter	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 PostgreSQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source ApsaraDB RDS for PostgreSQL instance is located.
	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 migrated.
	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	Enter the password for the database account.
	Encryption	Select a connection method based on your requirements. This example uses Non-encrypted. If you need to connect to the database by using SSL encryption, select SSL-encrypted and upload the CA Certificate, Client Certificate, and Private Key of Client Certificate as needed. You may also need to enter the Private Key Password of Client Certificate. Note If you select SSL-encrypted for a self-managed PostgreSQL database, you must upload the CA Certificate. If you need to use a client certificate, you must upload both the Client Certificate and the Private Key of Client Certificate, and enter the Private Key Password of Client Certificate. For information about the SSL encryption feature of an ApsaraDB RDS for PostgreSQL instance, see SSL encryption.
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 PostgreSQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination ApsaraDB RDS for PostgreSQL instance is located.
	Instance ID	Select the ID of the destination ApsaraDB RDS for PostgreSQL instance.
	Database Name	Enter the name of the database in the destination ApsaraDB RDS for PostgreSQL instance that will receive the migrated objects.
	Database Account	Enter the database account for the destination ApsaraDB RDS for PostgreSQL instance. For information about the required permissions, see Permissions required for database accounts.
	Database Password	Enter the password for the database account.
	Encryption	Select a connection method based on your requirements. This example uses Non-encrypted. If you need to connect to the database by using SSL encryption, select SSL-encrypted and upload the CA Certificate, Client Certificate, and Private Key of Client Certificate as needed. You may also need to enter the Private Key Password of Client Certificate. Note If you select SSL-encrypted for a self-managed PostgreSQL database, you must upload the CA Certificate. If you need to use a client certificate, you must upload both the Client Certificate and the Private Key of Client Certificate, and enter the Private Key Password of Client Certificate. For information about the SSL encryption feature of an ApsaraDB RDS for PostgreSQL instance, see SSL encryption.

After completing the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
- Ensure that you add the CIDR blocks of the DTS servers (either automatically or manually) to the security settings of both the source and destination databases to allow access. For more information, see Add the IP address whitelist of DTS servers.
- If the source or destination is a self-managed database (i.e., the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the task objects.

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

Parameter	Description
Migration Types	To perform a one-time full data migration, select both Schema Migration and Full Data Migration. To perform a migration with minimal downtime, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If you select Schema Migration, DTS migrates the schema of the tables to be migrated, including foreign keys, from the source database to the destination database. If you do not select Incremental Data Migration, do not write new data to the source instance during the 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.
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 at the schema or table level. If you select tables, other objects such as views, triggers, and stored procedures are not migrated. If a table to be migrated contains columns of the SERIAL type and you have selected Migration Types for Schema Migration, also select Sequence or migrate the entire schema.
Selected Objects	To change the name of a single migration object in the target instance, right-click the object in the Selected Objects box. For more information, see Map individual schema, table, and column names. To change the names of multiple migration objects in the target instance, click Selected Objects in the upper-right corner of the Batch Edit box. For more information, see Map multiple schema, table, and column names. Note If you use the object name mapping feature, other objects that depend on the renamed object may fail to migrate. To set a WHERE clause to filter data, right-click the table in the Selected Objects box and specify the filter condition in the dialog box that appears. For more information, see Set filter conditions. To select the incremental SQL operations to migrate 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.

Click Next: Advanced Settings to configure advanced parameters.

Parameter	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	You can select an environment tag to identify the instance based on your business requirements. This example does not require an environment tag.
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 contact. 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.