How to migrate PolarDB for PostgreSQL (Oracle compatible) to a self-managed Oracle database - Data Transmission Service

Use Data Transmission Service (DTS) to migrate a PolarDB for PostgreSQL (Compatible with Oracle) cluster to a self-managed Oracle database.

Prerequisites

A source PolarDB for PostgreSQL (Compatible with Oracle) cluster has been created. For more information, see Create a PolarDB for PostgreSQL (Compatible with Oracle) cluster.
In the source PolarDB for PostgreSQL (Compatible with Oracle) cluster, the wal_level parameter must be set to logical. This setting adds the information required for logical decoding to the write-ahead logging (WAL). For more information, see Configure cluster parameters.

Limitations

Note

During full and incremental data migration:

Cascading updates or deletes in the source database can cause data inconsistency.
If the destination database has foreign keys or triggers, disable them. Otherwise, the migration task may fail.

Type	Description
Source database limitations	The server that hosts the source database must have sufficient outbound bandwidth. Insufficient bandwidth can affect the migration speed. Each table to be migrated must have a primary key or a unique constraint. All fields in the key or constraint must be unique to prevent duplicate data in the destination database. If you migrate objects at the table level and need to perform edits such as column name mapping, a single migration task supports a maximum of 1,000 tables. If you exceed this limit, an error is reported when you submit the task. In this case, you can split the tables into multiple tasks or configure a task to migrate the entire database. For incremental migration, you must configure write-ahead logging (WAL): The WAL feature must be enabled. For an incremental migration-only task, Data Transmission Service (DTS) requires the WAL logs of the source database to be retained for at least 24 hours. For a task that includes both full and incremental migration, DTS requires the logs to be retained for at least 7 days. You can change the retention period back to at least 24 hours after the full migration is complete. Setting a shorter retention period can cause the task to fail if DTS cannot obtain the required WAL logs, potentially leading to data inconsistency or data loss. The DTS Service Level Agreement (SLA) does not cover issues caused by a WAL retention period shorter than the requirement. Operational limitations on the source database: During the full data migration phase, do not perform any DDL operations that alter the schema of databases or tables. Otherwise, the migration task will fail. If you perform only full data migration, do not write new data to the source instance to avoid data inconsistency. To maintain real-time data consistency, we recommend selecting full data migration and incremental data migration. To ensure the migration task runs smoothly and to prevent logical subscription interruptions caused by a primary/secondary switchover, the source PolarDB for PostgreSQL(Compatible with Oracle) cluster must support logical replication slot failover and have it enabled. Note If the source PolarDB for PostgreSQL(Compatible with Oracle) cluster does not support logical replication slot failover (for example, if the cluster's database engine is Oracle syntax compatibility 2.0), a high-availability (HA) switchover on the source database can cause the migration instance to fail and become unrecoverable. Due to the limitations of logical subscriptions, if a migration instance that includes incremental data migration is running and the size of a single row to be migrated exceeds 256 MB after an incremental change, the migration instance fails irrecoverably, and you must reconfigure it. If long-running transactions exist in the source database during an incremental migration, write-ahead logging (WAL) records generated before these transactions commit cannot be cleared. This can cause WAL files to accumulate and exhaust the disk space of the source database.
Other limitations	Schema migration is not supported. Before you configure the migration task, you must create the corresponding databases and tables in the destination instance. A single data migration task can migrate only one database. To migrate multiple databases, you must configure a separate migration task for each one. DTS does not support migrating TimescaleDB extension tables, tables with cross-schema inheritance, or tables with expression-based unique indexes. These schemas will not be visible in the console when you configure the task. For tasks that include 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 for the tables in the following two scenarios. To prevent deadlocks, avoid table-locking operations while this command runs. If you skip the related checks during the precheck, DTS automatically runs this command when it initializes the instance. When the instance runs for the first time. When the migration object granularity is set to Schema and a new table is created in the schema or an existing table is rebuilt by 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. 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 structure of incremental tables, and heartbeats. Do not delete these temporary tables during the migration, as this will cause the task to fail. The 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 latency metrics for incremental data migration, DTS creates a heartbeat table named `dts_postgres_heartbeat` in the source database. During incremental data migration, DTS creates a replication slot prefixed with `dts_sync_` in the source database to replicate data. This replication slot allows DTS to obtain incremental logs from the source database from the last 15 minutes. When a data migration task fails or the migration instance is released, DTS attempts to automatically clean up the replication slot. Note If you change the password of the source database account used by the task or remove the DTS IP addresses from the IP address whitelist of the source database during migration, 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 log accumulation, which can exhaust disk space and cause the source database to become unavailable. If a primary/secondary switchover occurs on the source database, you must log on to the secondary database to manually clean up the replication slot. Concurrent INSERT operations during full data migration can cause table fragmentation, which may result in the destination database using more storage space than the source instance. Verify that the migration precision for columns of the FLOAT or DOUBLE data types meets your business requirements. DTS uses the `ROUND(COLUMN,PRECISION)` function to read values from these columns. If a precision is not explicitly defined, DTS uses a precision of 38 for FLOAT and 308 for DOUBLE. DTS attempts to resume a failed migration task for up to seven days. Before you switch business workloads to the destination instance, you must end or release the task, or revoke the write permissions of the account that DTS uses to access the destination instance by using the `revoke` command. This prevents an automatically resumed task from overwriting data in the destination instance. 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, you must include the parent table and all its child partitions as migration objects to prevent data inconsistency. Important In PolarDB for PostgreSQL(Compatible with Oracle), the parent table of a partitioned table does not store data directly. Data is stored in the child partitions. The migration task must include the parent table and all its child partitions to prevent data loss and inconsistency between the source and destination databases. Migration of partitioned tables or inherited tables (parent and child tables) that are distributed across different databases is not supported. Ensure that the partitioned tables and all their partitions, as well as parent tables and all their child tables, are in the same database.
Special cases	If the self-managed Oracle database uses a Real Application Clusters (RAC) architecture and needs to connect to an Alibaba Cloud VPC, you must connect both the SCAN IP address and the VIP address of each node to the Alibaba Cloud VPC and configure the necessary routes for the DTS task to run successfully. For more information, see Overview of solutions for connecting an on-premises data center to Alibaba Cloud and Connect a data center to DTS by using a VPN Gateway. Important When you configure the source Oracle database in the DTS console, enter only the SCAN IP address of the Oracle RAC in the Database address or Domain Name or IP field.

Billing

Migration type	Instance configuration fee	Internet traffic fee
Full data migration	Free of charge.	When the Access Method parameter of the destination database is set to Public IP Address, you are charged for Internet traffic. For more information, see Billing overview.
Incremental data migration	Charged. For more information, see Billing overview.

Migration types

Type

Description

Full data migration

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

Note

Do not perform DDL operations on the migration objects until the full data migration is complete. Otherwise, the migration will fail.

Incremental data migration

After a full data migration, DTS continuously replicates incremental updates to the destination database by polling for and capturing redo logs from the source database.

Incremental data migration enables a seamless migration with near-zero application downtime.

SQL operations for incremental migration

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	DTS migrates DDL operations only for data migration tasks created after October 1, 2020. Important For data migration tasks that were created before May 12, 2023, you must 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 by using triggers and functions. DTS does not support migrating data of the BIT type during incremental data migration. When the source database account is a privileged account, DTS supports the following DDL statements: 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 CREATE INDEX ON TABLE Important DTS does not migrate additional clauses in DDL statements, such as CASCADE or RESTRICT. DTS does not migrate DDL statements from a session in which the `SET session_replication_role = replica` command is executed. DTS does not migrate DDL statements executed by invoking a function. If a single commit in the source database contains both DML and DDL statements, DTS does not migrate the DDL statements. If a single commit in the source database includes DDL statements for unselected objects, DTS does not migrate these statements.

Database account permissions

Database	Permissions
PolarDB for PostgreSQL (Compatible with Oracle) cluster	Privileged account.
Self-managed Oracle database.	Schema owner permissions.

To create database accounts and grant permissions:

For a PolarDB for PostgreSQL (Compatible with Oracle) cluster, see Create database accounts.
For a self-managed Oracle database, see CREATE USER and GRANT.

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 PolarDB (Compatible with Oracle).
	Connection Type	In this example, Alibaba Cloud Instance is selected.
	Instance Region	Select the region where the source PolarDB for PostgreSQL (Compatible with Oracle) cluster is located.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database instance under the current Alibaba Cloud account is used. Select No.
	Instance ID	Select the ID of the source PolarDB for PostgreSQL (Compatible with Oracle) cluster.
	Database Name	Enter the name of the database in the source PolarDB for PostgreSQL (Compatible with Oracle) cluster that contains the objects to migrate.
	Database Account	Enter the database account for the source PolarDB for PostgreSQL (Compatible with Oracle) cluster. For the required permissions, see Permissions required for database accounts.
	Database Password	Enter the password for the 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 Oracle.
	Connection Type	Select Public IP Address.
	Instance Region	Select the region where the destination Oracle database is located.
	Hostname or IP address	Enter the endpoint of the primary node of the destination Oracle database.
	Port	Enter the service port of the destination database. The default value is 1521.
	Oracle Type	Non-RAC Instance: If you select this option, you must also specify the SID. RAC or PDB Instance: If you select this option, you must also specify the Service Name. In this example, Non-RAC Instance is selected.
	Database Account	Enter the account for the destination Oracle database. For the required permissions, see Permissions required for database accounts.
	Database Password	Enter the password for the database account.

After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page. In the CIDR Blocks of DTS Servers dialog box that appears, click Test Connectivity.

Note
Ensure that the IP address segments of the DTS service 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.

Parameter	Description
Migration Types	If you only need to perform a full migration, select Full Data Migration. To perform a migration with no downtime, select both Full Data Migration and Incremental Data Migration. Note 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 The granularity for selecting migration objects is schema, table, and column. If you select only tables or columns as migration objects, other objects such as views, triggers, and stored procedures are not migrated to the destination database.
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 filter data by using a WHERE clause, right-click a 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 SQL operations to migrate at the database or table level, right-click a migration 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 parameter is optional.
Configure ETL	Based on your business needs, select whether to configure the ETL feature to process data. Yes: Configures the ETL feature. You must also enter data processing statements in the text box. No: Does not configure 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 migration 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.