Data Transmission Service:Migrate data from an ApsaraDB RDS for PPAS instance to a PolarDB for Oracle cluster

Prerequisites

• A PolarDB for Oracle cluster is created. For more information, see Create a PolarDB for PostgreSQL(Compatible with Oracle) cluster.
• The available storage space of the PolarDB for Oracle cluster is larger than the total size of the data in the ApsaraDB RDS for PPAS instance.
• If one or more uppercase letters exist in the names of the source database, tables, and fields in the ApsaraDB RDS for PPAS instance, you must enclose the names of the source database, tables, and fields in the PolarDB for Oracle cluster with double quotation marks (") when you create the objects in the cluster.
• To migrate incremental data from an ApsaraDB RDS for PPAS instance, you must grant the permissions of the superuser role to the database account.

Limits

• DTS uses read and write resources of the source and destination databases during full data migration. This may increase the loads of the database servers. If the database performance is unfavorable, the specification is low, or the data volume is large, database services may become unavailable. For example, DTS occupies a large amount of read and write resources in the following cases: a large number of slow SQL queries are performed on the source database, the tables have no primary keys, or a deadlock occurs in the destination database. 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. For example, you can migrate data when the CPU utilization of the source and destination databases is less than 30%.
• The tables to be migrated in the source database must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records.
• 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 a data migration task fails, DTS automatically resumes the task. Before you switch your workloads to the destination cluster, stop or release the data migration task. Otherwise, the data in the source database overwrites the data in the destination cluster after the task is resumed.
• 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 initial value of the sequences in the destination database. You can run the following commands to query the maximum value of the sequences in the source database.

  do language plpgsql $$
  declare
    nsp name;
    rel name;
    val int8;
  begin
    for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S'
    loop
      execute format($_$select last_value from %I.%I$_$, nsp, rel) into val;
      raise notice '%',
      format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1);
    end loop;
  end;
  $$;

• When you migrate data from an ApsaraDB RDS for PPAS instance to a PolarDB for Oracle cluster, take note of the following items:
  • Make sure that the specifications of the PolarDB for Oracle cluster are higher than or equal to those of the ApsaraDB RDS for PPAS instance. Otherwise, slow SQL queries or out of memory (OOM) errors may occur because of insufficient CPU or memory resources after you migrate the data to the PolarDB for Oracle cluster. For more information about recommended specifications of PolarDB for Oracle clusters, see Mapping between specifications of ApsaraDB RDS for PPAS instances and recommended specifications of PolarDB for Oracle clusters.
  • If you have business requirements for the number of connections and IOPS after migration, select appropriate specifications based on your business requirements. For more information, see Specifications of compute nodes.
  • If you use a cluster endpoint to connect your application to a PolarDB for Oracle cluster, read/write splitting is enabled. Then, the system forwards read requests to read-only PolarDB for Oracle nodes. This reduces the load on the PolarDB for Oracle cluster. For more information about how to obtain a cluster endpoint, see View or apply for an endpoint.
• During schema migration, DTS migrates foreign keys from the source database to the destination database.
• During full data migration and incremental data migration, DTS temporarily disables the constraint check and cascade operations on foreign keys at the session level. If you perform the cascade and delete operations on the source database during data migration, data inconsistency may occur.

Mapping between specifications of ApsaraDB RDS for PPAS instances and recommended specifications of PolarDB for Oracle clusters

Make sure that the specifications of the PolarDB for Oracle cluster are higher than or equal to the specifications of your ApsaraDB RDS for PPAS instance. Otherwise, slow SQL queries or OOM errors may occur because of insufficient CPU or memory resources after you migrate the data to the PolarDB for Oracle cluster. The following table lists the recommended specifications of PolarDB for Oracle clusters.

Migration types

Billing

Permissions required for database accounts

Log on to the source Oracle database, create an account for data collection, and grant permissions to the account.

To create a database account and grant permissions to the database account, perform the following operations:

To create a database account in a PolarDB for Oracle cluster, follow the instructions in Create database accounts

Procedure

1. 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

   • Operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Configure the DMS console based on your business requirements.
   • You can also go to the Data Migration Tasks page of the new DTS console.
2. From the drop-down list next to Data Migration Tasks, select the region in which the 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.
3. Click Create Task. On the page that appears, configure the source and destination databases.

   Section	Parameter	Description
   N/A	Task Name	DTS automatically generates a task name. We recommend that you specify an informative name to identify the task. You do not need to use a unique task name.
   Source Database	Database Type	The type of the source database. Select PostgreSQL. ApsaraDB RDS for PPAS is unavailable in the values of the Database Type parameter.
   	Access Method	The access method of the source database. Select Express Connect, VPN Gateway, or Smart Access Gateway.
   	Instance Region	The region in which the source ApsaraDB RDS for PPAS instance resides.
   	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to migrate data across Alibaba Cloud accounts. In this example, No is selected.
   	Connected VPC	The ID of the virtual private cloud (VPC) that is connected to the source database in the ApsaraDB RDS for PPAS instance.
   	IP Address	The private IP address of the ApsaraDB RDS for PPAS instance.
   	Port Number	The service port number of the ApsaraDB RDS for PPAS instance. The default port number is 3433.
   	Database Account	The database account of the ApsaraDB RDS for PPAS instance.
   	Database Password	The password of the database account.
   Destination Database	Database Type	The type of the destination database. Select PolarDB (Compatible with Oracle).
   	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
   	Instance Region	The region in which the destination PolarDB for Oracle cluster resides.
   	Instance ID	The ID of the destination PolarDB for Oracle cluster.
   	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 permissions that are required for this account, see the Permissions required for database accounts section of this topic.
   	Database Password	The password of the database account.

4. In the lower part of the page, click Test Connectivity and Proceed.
   Warning

   • 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 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. 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 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 whitelist of the database 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 or ECS security group rules.
5. Select objects for the task and configure advanced settings.
   • Basic 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 identical table 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 inconsistency may occur, 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 some columns are migrated or the data migration task fails. Proceed with caution.
     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 ensure that the capitalization of object names is consistent with the default capitalization of object names in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
     Source Objects	Select one or more objects from the Source Objects section and click the icon to move the objects to the Selected Objects section. Note The time field supports the TIMESTAMP data type. If the time field of the source database is 0, the time field of the destination database is automatically converted to null.
     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 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.

   • 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. For more information, see Configure monitoring and alerting when you create a DTS task.
     Retry Time for Failed Connection	The retry time range for failed connections. Valid values: 10 to 1440. Unit: minutes. Default value: 120. We recommend that you set the retry time range to more than 30 minutes. 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 share the same source or destination database, the value that is set 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 and destination databases are released.
     Configure ETL	Specifies whether to configure 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.

6. 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, click View Details next to each failed item. After you troubleshoot the issues based on the causes, 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 an 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.
7. Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.
8. On the Purchase Instance page, specify the Instance Class parameter for the data migration instance. The following table describes the parameter.

   Section	Parameter	Description
   Parameters	Instance Class	DTS provides several instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Specifications of data migration instances.

9. Read and select the check box for Data Transmission Service (Pay-as-you-go) Service Terms.
10. Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.