Migrate data between PolarDB for Oracle clusters - Data Transmission Service

This topic describes how to migrate data between PolarDB for Oracle clusters by using Data Transmission Service (DTS).

Prerequisites

The source and destination PolarDB for Oracle clusters are created. For more information, see Create a PolarDB for Oracle cluster.
The value of the wal_level parameter is set to logical for the source PolarDB for Oracle cluster. This setting ensures that logical encoding is supported in write-ahead logging (WAL). For more information, see Configure cluster parameters.

Limits


Category	Description
Limits on the source database	The server to which the source database belongs must have sufficient outbound bandwidth. Otherwise, the data migration speed decreases. 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. If you select tables as the objects to be migrated and you need to edit 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, make sure that the following requirements are met: The write-ahead logging (WAL) feature must be enabled. For incremental data migration, the WAL logs of the source database must be stored for more than 24 hours. For full data 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 and performance stated 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 may occur between the source and destination databases. 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 long-running transactions and the task contains incremental data migration, the WAL logs that are generated before the long-running transactions are submitted may not be cleared and therefore pile up, resulting in insufficient storage space in the source database.
Other limits	A data migration task can migrate data from only a single 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, 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 statement with your schema name and table name. To ensure that the latency of data migration is accurate, DTS adds a heartbeat table named `dts_postgres_heartbeat` to the source database. The following figure shows the schema of the heartbeat table. 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. Make sure 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 the FLOAT data type to 38 digits and the precision for 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, stop or release the data migration task. You can also execute the `REVOKE` statement to revoke the write permissions from the accounts 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 task is resumed.

Billing


Migration type	Instance configuration fee	Internet traffic fee
Schema migration and full data migration	Free of charge.	Charged only when data is migrated from Alibaba Cloud over the Internet. For more information, see Billing overview.
Incremental data migration	Charged. For more information, see Billing overview.

Migration types


Migration type	Description
Schema migration	DTS migrates the schemas of objects to the destination database. DTS supports schema migration for the following types of objects: table, view, synonym, trigger, stored procedure, stored function, package, and user-defined type. Note In this scenario, DTS does not support triggers. We recommend that you delete the triggers of the source database to prevent data inconsistency caused by triggers. For more information about, see Configure a data synchronization task for a source database that contains a trigger.
Full data migration	DTS migrates historical data of objects from the source database to the destination database. Note During schema migration and full data migration, do not perform DDL operations on the objects to be migrated. Otherwise, the objects may fail to be migrated.
Incremental data migration	DTS retrieves redo log files from the source database. Then, DTS migrates incremental data from the source database to the destination database by using the redo log files. Incremental data migration allows you to ensure service continuity when you migrate data between PolarDB for Oracle clusters.

SQL operations that can be incrementally migrated


Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE

Permissions required for database accounts


Database	Required permission
Source PolarDB for Oracle cluster	Permissions of a privileged account
Destination PolarDB for Oracle cluster	Permissions of the database owner Important The database owner is specified when you create a database.

For more information about how to create a database account and grant permissions to the database account, see the following topic:

PolarDB for Oracle cluster: Create database accounts

Procedure

Go to the Data Migration Tasks page.
1. Log on to the Data Management (DMS) console.
2. In the top navigation bar, click DTS.
3. In the left-side navigation pane, choose DTS (DTS) > Data Migration.
Note You can also configure the settings by using the new DTS console.
From the drop-down list to the right of Data Migration Tasks, select the region in which your data migration instance resides.

Note If you use the new DTS console, select the region in which your data migration instance resides in the upper-left corner.

Click Create Task. On the page that appears, configure the source and destination databases.

Warning After you select the source and destination instances, we recommend that you read the limits displayed at the top of the page. This helps you create and run the data migration task.


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 Instance	The instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you use an existing instance, DTS automatically applies the parameter settings of the instance. If you do not use an existing instance, you must set parameters for the source database.
	Database Type	The type of the source database. Select PolarDB for Oracle.
	Access Method	The access method of the source database. Select Public IP Address.
	Instance Region	The region in which the source PolarDB for Oracle cluster resides.
	Data Sources	Specifies whether to configure IP addresses and port numbers of multiple nodes in the source PolarDB for Oracle cluster. If you select Single Data Source, you must specify the Domain Name or IP Address and Port Number of the primary node in the source PolarDB for Oracle cluster. If you select Multiple Data Sources, you must specify the IP addresses and port numbers of multiple nodes, including the primary node, in the source PolarDB for Oracle cluster in the IP:Port of Data Sources field. Note If you select Multiple Data Sources and specify the IP:Port of Data Sources parameter, DTS can automatically switch to a new primary node in the PolarDB for Oracle cluster during a primary/secondary switchover. This is applicable to scenarios such as disaster recovery. In this example, Single Data Source is selected.
	Domain Name or IP Address	The endpoint of the primary node in the source PolarDB for Oracle cluster. Note You can run the ping command on your computer to obtain the IP address that is used to access the source PolarDB for Oracle cluster.
	Port Number	The service port number of the source database. Default value: 1521. Note In this example, the service port of the source database must be accessible over the Internet.
	IP:Port of Data Sources	The IP addresses and port numbers of multiple nodes, including the primary node, in the source PolarDB for Oracle cluster. Separate multiple IP:Port with commas (,).
	Oracle Type	The architecture of the source database. If you select Non-RAC Instance, you must specify the SID parameter. If you select RAC or PDB Instance, you must specify the Service Name parameter. In this example, Non-RAC Instance is selected.
	Database Name	The name of the source database in the PolarDB for Oracle cluster.
	Database Account	The database account of the source PolarDB for Oracle cluster. For information about the permissions that are required for the account, see Permissions required for database accounts.
	Database Password	The password of the database account.
Destination Database	Select Instance	The instance that you want to use. You can choose whether to use an existing instance based on your business requirements. If you use an existing instance, DTS automatically applies the parameter settings of the instance. If you do not use an existing instance, you must set parameters for the destination database.
	Database Type	The type of the destination database. Select PolarDB for Oracle.
	Access Method	The access method of the destination database. Select Public IP Address.
	Instance Region	The region in which the destination PolarDB for Oracle cluster resides.
	Data Sources	Specifies whether to configure IP addresses and port numbers of multiple nodes in the destination PolarDB for Oracle cluster. If you select Single Data Source, you must specify the Domain Name or IP Address and Port Number parameters of the primary node in the destination PolarDB for Oracle cluster. If you select Multiple Data Sources, you must specify the IP addresses and port numbers of multiple nodes, including the primary node, in the destination PolarDB for Oracle cluster in the IP:Port of Data Sources field. Note If you select Multiple Data Sources and specify the IP:Port of Data Sources parameter, DTS can automatically switch to a new primary node in the PolarDB for Oracle cluster during a primary/secondary switchover. This is applicable to scenarios such as disaster recovery. In this example, Single Data Source is selected.
	Domain Name or IP Address	The IP address of the primary node in the destination PolarDB for Oracle cluster. Note You can run the ping command on your computer to obtain the IP address that is used to access the destination PolarDB for Oracle cluster.
	Port Number	The service port number of the destination database. Default value: 1521.
	IP:Port of Data Sources	The IP addresses and port numbers of multiple nodes, including the primary node, in the destination PolarDB for Oracle cluster. Separate multiple IP:Port with commas (,).
	Database Name	The name of the destination database in the PolarDB for Oracle cluster.
	Database Account	The database account of the destination PolarDB for Oracle cluster. For more information about the permissions that are required for the account, see Permissions required for database accounts.
	Database Password	The password of the database account.

In the lower part of the page, click Test Connectivity and Proceed.
Warning
- If the source or destination database instance is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, or is a self-managed database hosted on Elastic Compute Service (ECS), DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the database instance or ECS security group rules. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. If the source or destination database is a self-managed database on data centers or is from other cloud service providers, you must manually add the CIDR blocks of DTS servers to allow DTS to access the database.
- If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database, Alibaba Cloud database instance, or ECS security group rules, 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 ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.
- After the DTS task is complete or released, we recommend that you manually detect and remove the added CIDR blocks from the whitelist of the database, Alibaba Cloud database instance, or ECS security group rules.

Select objects for the task and configure advanced settings.


Parameter	Description
Task Stages	To perform only full data migration, select Schema Migration and Full Data Migration. To ensure service continuity during data migration, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If Incremental Data Migration is not selected, we recommend that you do not write data to the source instance during data migration. This ensures data consistency between the source and destination instances.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains tables that have the same names as tables in the source database. If the source and destination databases do not contain tables that have the same names, the precheck is passed. Otherwise, an error is returned during the precheck and the data migration task cannot be started. Note You can use the object name mapping feature to rename the tables that are migrated to the destination database. You can use this feature if the source and destination databases contain tables that have identical names and the tables in the destination database cannot be deleted or renamed. 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 consistency is not ensured, and your business may be exposed to potential risks. If the source and destination databases have the same schema, DTS does not migrate data records that have the same primary keys as data records in the destination database. If the source and destination databases have different schemas, only specific columns are migrated or the data migration task fails.
Source Objects	Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section. Note You can select 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 an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Use SQL conditions to filter data. To select the SQL operations performed on a specific database or table, right-click an 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 can be migrated, see the SQL operations that can be incrementally migrated section of this topic.

Click Next: Advanced Settings to configure advanced settings.


Parameter	Description
Set Alerts	Specifies whether to set alerts for the data migration task. If the task fails or the migration latency exceeds the threshold, the alert contacts will receive notifications. Valid values: No: does not set alerts. Yes: sets alerts. If you select Yes, you must also set the alert threshold and alert contacts.
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 time range. Valid values: 10 to 1440. Unit: minutes. Default value: 120. We recommend that you set the parameter to a value greater than 30. If DTS reconnects 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 set different retry time ranges for multiple data migration tasks that have the same source or destination database, the shortest retry time range that is set takes precedence. When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
Configure ETL	Specify whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?. 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.
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, you can click the icon next to each failed item to view details.
  
  You can troubleshoot the issues based on the causes and run a precheck again.
  
  If you do not need to troubleshoot the issues, you can ignore failed items and run a precheck again.
Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.
Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.
Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.