Data Transmission Service:Synchronize data from a PolarDB for MySQL cluster to a self-managed Oracle database

Prerequisites

• The self-managed Oracle database runs in ARCHIVELOG mode. Archived log files must be accessible, and a suitable retention period must be set. For more information, see Managing Archived Redo Log Files.

• Supplemental logging is enabled for the self-managed Oracle database, including SUPPLEMENTAL_LOG_DATA_PK and SUPPLEMENTAL_LOG_DATA_UI. For more information, see Supplemental Logging.

• If the self-managed Oracle database is an Oracle Real Application Clusters (RAC) database, it must be connected to Alibaba Cloud through a virtual private cloud (VPC). Each Single Client Access Name (SCAN) IP of the Oracle RAC database and the virtual IP address (VIP) of each node must be connected to the VPC, and routing rules must be configured. This ensures that DTS tasks run as expected. For more information, see Connect an on-premises database to Alibaba Cloud and Connect a data center to DTS by using VPN Gateway.

Usage notes

Limits on the source database

• The tables to be synchronized must have PRIMARY KEY or UNIQUE constraints, and all fields must be unique. Otherwise, the destination may contain duplicate data records.

• If you select tables as the objects to be synchronized and you need to edit the tables (such as renaming tables or columns) in the destination database, you can synchronize up to 1,000 tables in a single data synchronization task. If a task synchronizes more than 1,000 tables, a request error occurs. To avoid this, configure multiple tasks to synchronize the tables, or configure a task to synchronize the entire database.

• To synchronize incremental data, you must enable the binary logging feature and set the loose_polar_log_bin parameter to on. Otherwise, error messages are returned during precheck and the data synchronization task cannot start. For more information about enabling binary logging and setting the loose_polar_log_bin parameter, see Enable binary logging and Modify parameters.

  Note - Enabling the binary logging feature for a PolarDB for MySQL cluster incurs charges for the storage space occupied by binary logs. - For an incremental data synchronization task, retain the binary logs of the source database for at least 24 hours. For a full and incremental data synchronization task, retain the binary logs for at least seven days. Otherwise, DTS may fail to obtain the binary logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After full data synchronization is complete, you can reduce the retention period to more than 24 hours. Make sure that you set the retention period based on the preceding requirements. Otherwise, the SLA of DTS does not guarantee service reliability or performance.

Other limits

• Read-only nodes of the source PolarDB for MySQL cluster cannot be synchronized.

• Before you synchronize data, evaluate the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. During initial full data synchronization, DTS uses read and write resources of the source and destination databases, which may increase the load on the database servers.

• During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data synchronization is complete, the tablespace of the destination database is larger than that of the source database.

• Do not use tools such as pt-online-schema-change to perform DDL operations on source tables during data synchronization. Otherwise, data synchronization may fail.

• If only DTS writes data to the destination database, you can use Data Management (DMS) to perform online DDL operations on source tables during data synchronization. For more information, see Perform lock-free operations.

• During data synchronization, we recommend that you use only DTS to write data to the destination database. This prevents data inconsistency between the source and destination databases. If you use tools other than DTS to write data to the destination database, data loss may occur when you use DMS to perform online DDL operations.

• If the self-managed Oracle database is an Oracle RAC database, you can only use a single virtual IP address (VIP) rather than a Single Client Access Name (SCAN) IP address when you configure the data synchronization task. After you specify the VIP, node failover of the Oracle RAC database is not supported.

Additional usage notes

DTS executes the CREATE DATABASE IF NOT EXISTS test statement in the source database as scheduled to move forward the binary log file position.

Billing

Supported synchronization topologies

• One-way one-to-one synchronization

• One-way one-to-many synchronization

• One-way cascade synchronization

• One-way many-to-one synchronization

For more information about the synchronization topologies supported by DTS, see Synchronization topologies.

SQL operations that can be synchronized

Procedure

1. Go to the Data Synchronization 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 Synchronization.

   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 Synchronization Tasks page of the new DTS console.

2. From the drop-down list to the right of Data Synchronization Tasks, select the region where the data synchronization instance resides.

   Note If you use the new DTS console, select the region in the top navigation bar.

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

   General settings

   Parameter	Description
   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 settings

   Parameter	Description
   Select Instance	Specifies whether to use an existing instance. If you select an existing instance, DTS automatically applies its parameter settings and you do not need to configure the corresponding parameters again. If you do not use an existing instance, you must configure parameters for the source database.
   Database Type	The type of the source database. Select PolarDB for MySQL.
   Access Method	The access method of the source database. Select Alibaba Cloud Instance.
   Instance Region	The region where the source PolarDB for MySQL cluster resides.
   Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
   PolarDB Cluster ID	The ID of the source PolarDB for MySQL cluster.
   Database Account	The database account of the source PolarDB for MySQL cluster. The account must have read permissions on the objects to be synchronized.
   Database Password	The password of the database account.

   Destination Database

   Parameter	Description
   Select Instance	Specifies whether to use an existing instance. If you select an existing instance, DTS automatically applies its parameter settings and you do not need to configure the corresponding parameters again. If you do not use an existing instance, you must configure parameters for the destination database.
   Database Type	The type of the destination database. Select Oracle.
   Access Method	The access method of the destination database. Select an access method based on the deployment of the destination database. In this example, Self-managed Database on ECS is selected. Important If you set this parameter to a different value, you must set up the required environment for the self-managed database. For more information, see Preparation overview.
   Instance Region	The region where the destination Oracle database resides.
   ECS Instance ID	The ID of the Elastic Compute Service (ECS) instance that hosts the self-managed Oracle database.
   Port number	The service port number of the self-managed Oracle database. Default value: 1521.
   Oracle Type	The architecture of the Oracle 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.
   Database Account	The account of the self-managed Oracle database.
   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. 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 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. You must also manually add the CIDR blocks of DTS servers to the whitelist of the self-managed database on the ECS instance. If the source or destination database is a self-managed database 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. - Adding CIDR blocks of DTS servers to the whitelist or ECS security group rules (automatically or manually) may introduce security risks. Before you use DTS to synchronize data, you must understand and acknowledge these potential risks and take preventive measures. These measures include but are not limited to: enhancing the security of your username and password, limiting exposed ports, authenticating API calls, regularly checking the whitelist or ECS security group rules and removing unauthorized CIDR blocks, or connecting the database to DTS through Express Connect, VPN Gateway, or Smart Access Gateway. - After the DTS task is complete or released, we recommend that you manually remove the CIDR blocks of DTS servers from the whitelist or ECS security group rules. Remove the IP address whitelist group whose name contains dts from the whitelist of the Alibaba Cloud database instance or the security group rules of the ECS instance. For more information about the CIDR blocks to remove from the whitelist of self-managed databases in data centers or on third-party cloud services, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.

5. Select objects for the task and configure advanced settings.

   Parameter	Description
   Task Stages	By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination database. The historical data serves as the basis for subsequent incremental synchronization.
   Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains tables with the same names as tables in the source database. If no identical table names exist, the precheck passes. Otherwise, an error is returned during the precheck, and the data synchronization task cannot start. Note If the source and destination databases contain identical table names and the tables in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the tables synchronized to the destination database. For more information, see Map object names. Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases. Warning If you select Ignore Errors and Proceed, data inconsistency may occur, and your business may be exposed to potential risks. If the source and destination databases have the same schemas and a data record has the same primary key value as an existing data record in the destination database: During full data synchronization, DTS does not synchronize the data record to the destination database and the existing data record in the destination database is retained. During incremental data synchronization, DTS synchronizes the data record to the destination database and the existing data record in the destination database is overwritten. If the source and destination databases have different schemas, initial data synchronization may fail. In this case, only some columns are synchronized, or the data synchronization task fails.
   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 that 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 add them to the Selected Objects section. Note You can select columns, tables, or databases as the objects to be synchronized. If you select tables or columns, DTS does not synchronize other objects such as views, triggers, or stored procedures to the destination database.
   Selected Objects	To rename an object that you want to synchronize 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.

6. Click Next: Advanced Settings to configure advanced settings.

   Parameter	Description
   Set Alerts	Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts receive notifications. Valid values: No (does not configure alerting) and Yes (configures alerting). If you select Yes, you must also specify 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. If the source or destination database fails to connect after the data synchronization task starts, DTS immediately retries the connection within the specified time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set this parameter to a value greater than 30. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Note If you set different retry time ranges for multiple DTS tasks that share the same source or destination database, the shortest retry time range takes precedence. DTS charges you for the DTS instance during retries. 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 instances 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) and No (does not configure the ETL feature).
   Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Specifies whether to write SQL operations on heartbeat tables to the source database while the DTS instance is running. Yes: does not write SQL operations on heartbeat tables. In this case, the DTS instance may display a latency. No: writes SQL operations on heartbeat tables. In this case, specific features such as physical backup and cloning of the source database may be affected.

7. In the lower part of the page, click Next: Save Task Settings and Precheck.

   Note - Before you can start the data synchronization task, DTS performs a precheck. You can start the data synchronization task only after the task passes the precheck. - If the task fails to pass the precheck, click View Details next to each failed item. Analyze the causes based on the check results, troubleshoot the issues, and then run a precheck again. - If an alert is generated for an item during the precheck, take one of the following actions based on your scenario: - If you cannot ignore the alert item, click View Details next to the failed item. Analyze the causes based on the check results, troubleshoot the issues, and then run a precheck again. - If you can ignore the alert item, click Confirm Alert Details next to the failed item. 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.

8. Wait until the success rate reaches 100%. Then, click Next: Purchase Instance.

9. On the Purchase Instance page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.

   Section	Parameter	Description
   New Instance Class	Billing Method	Subscription: You pay for the instance when you create it. The subscription billing method is more cost-effective than pay-as-you-go for long-term use. Pay-as-you-go: The instance is charged on an hourly basis. This billing method is suitable for short-term use. If you no longer need a pay-as-you-go instance, you can release it to reduce costs.
   	Instance Class	DTS provides several instance classes with different synchronization speeds. Select an instance class based on your business scenario. For more information, see Specifications of data synchronization instances.
   	Subscription Duration	If you select the subscription billing method, set the subscription duration and the number of instances to create. The subscription duration can be one to nine months or one to three years. Note This parameter is displayed only if you select the subscription billing method.

10. Read and select the check box for Data Transmission Service (Pay-as-you-go) Service Terms.

11. Click Buy and Start to start the data synchronization task. You can view the task progress in the task list.