Configure one-way data synchronization between PolarDB for Oracle clusters - PolarDB

PolarDB for Oracle is a next-generation cloud-native relational database service that is developed by Alibaba Cloud. PolarDB for Oracle is highly compatible with the Oracle database engine. This topic describes how to configure one-way data synchronization between PolarDB for Oracle clusters by using Data Transmission Service (DTS).

Prerequisites

The source PolarDB for Oracle cluster uses the latest version. For more information, see Version Management.
The tables to be synchronized contain primary keys or UNIQUE NOT NULL indexes.
The value of the wal_level parameter is set to logical for the source PolarDB for Oracle cluster. This setting ensures that logical decoding is supported in write-ahead logging (WAL). For more information, see Configure cluster parameters

Limits

DTS uses read and write resources of the source and destination RDS instances during initial full data synchronization. This may increase the loads of the RDS instances. If the instance 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 RDS instance, the tables have no primary keys, or a deadlock occurs in the destination RDS instance. Before data synchronization, evaluate the impact of data synchronization on the performance of the source and destination RDS instances. We recommend that you synchronize data during off-peak hours. For example, you can synchronize data when the CPU utilization of the source and destination RDS instances is less than 30%.
A data synchronization task can synchronize data from only one database. To synchronize data from multiple databases, you must create a data synchronization task for each database.
To ensure that the latency of data synchronization 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.
If you select a schema as the object to be synchronized and create a table in the schema or execute the RENAME statement to rename the table, 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.
If the source database has long-running transactions and the task contains incremental data synchronization, 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.

Billing

Synchronization type	Task configuration fee
Schema synchronization and full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

SQL operations that can be synchronized

Type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	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 CREATE INDEX ON TABLE Note DDL statements are not synchronized in the following scenarios: Additional information such as CASCADE and RESTRICT in DDL statements is not synchronized. If a transaction contains both DML and DDL statements, the DDL statements are not synchronized. If only some DDL statements of a transaction are included in the objects of the data synchronization task, the DDL statements are not synchronized. If a DDL statement is executed in a session that is created by executing the SET session_replication_role = replica statement, the DDL statement is not synchronized. If no schema is defined in a DDL statement, the DDL statement is not synchronized. In this case, the public schema is specified in the SHOW search_path statement. If a DDL statement contains IF NOT EXISTS, the DDL statement is not synchronized.

Procedure

Purchase a data synchronization instance. For more information, see Purchase a DTS instance.
Note On the buy page, set both the Source Instance and Destination Instance parameters to PolarDB, and set the Synchronization Topology parameter to One-way Synchronization.
Log on to the DTS console.
Note If you are redirected to the Data Management (DMS) console, you can click the icon in the lower-right corner to go to the previous version of the DTS console.
In the left-side navigation pane, click Data Synchronization.
In the upper part of the Data Synchronization Tasks page, select the region in which the destination instance resides.
Find the data synchronization instance and click Configure Task in the Actions column.

Configure the source and destination databases.

Configure the source and destination instances

Section	Parameter	Description
N/A	Task Name	The task name that DTS automatically generates. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to use a unique task name.
Source Database	Database Type	The value of this parameter is set to PolarDB Instance and cannot be changed.
	Instance Region	The source region that you selected on the buy page. You cannot change the value of this parameter.
	PolarDB Instance ID	The ID of the source PolarDB for Oracle cluster.
	Database Name	The name of the source database.
	Database Account	The privileged account of the source PolarDB for Oracle cluster. For more information about how to create a privileged database account, see Create database accounts.
	Database Password	The password of the database account.
Destination Database	Database Type	The value of this parameter is set to PolarDB and cannot be changed.
	Instance Region	The destination region that you selected on the buy page. The value of this parameter cannot be changed.
	PolarDB Instance ID	The ID of the destination PolarDB for Oracle cluster.
	Database Name	The name of the destination database.
	Database Account	The database account of the destination PolarDB for Oracle cluster. The account must have the permissions of the database owner. Important The database owner is specified when you create a database.
	Database Password	The password of the database account.

In the lower-right corner of the page, click Set Whitelist and Next.
Note
- You do not need to modify the security settings for ApsaraDB instances (such as ApsaraDB RDS for MySQL and ApsaraDB for MongoDB) and ECS-hosted databases. DTS automatically adds the CIDR blocks of DTS servers to the whitelists of ApsaraDB instances or the security group rules of Elastic Compute Service (ECS) instances. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.
- After data synchronization is complete, we recommend that you remove the CIDR blocks of DTS servers from the whitelists or security groups.

Select the processing mode of conflicting tables and the objects to be synchronized.

Setting	Description
Select the 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 destination database does not contain tables that have the same names as those in the source database, the precheck is passed. Otherwise, an error is returned during precheck and the data synchronization task cannot be started. Note The object name mapping feature can be used to rename the tables that are synchronized to the destination database. You can use this feature if the source and destination databases contain identical table names and the tables in the destination database cannot be deleted or renamed. For more information, see Rename an object to be synchronized. 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. During initial data synchronization, DTS does not synchronize the data records that have the same primary keys as the data records in the destination database. This occurs if the source and destination databases have the same schema. However, DTS synchronizes these data records during incremental data synchronization. If the source and destination databases have different schemas, data may fail to be initialized. In this case, only some columns are migrated, or the data migration task fails.
Select the objects to be synchronized	Select one or more objects from the Available section and click the icon to add the objects to the Selected section. You can select tables or databases as the objects to be synchronized. Note If you select a database as the object to be synchronized, all schema changes in the database are synchronized to the destination database. By default, after an object is synchronized to the destination instance, the name of the object remains unchanged. You can use the object name mapping feature to rename the objects that are synchronized to the destination cluster. For more information, see Rename an object to be synchronized.
Rename Databases and Tables	You can use the object name mapping feature to rename the objects that are synchronized to the destination instance. For more information, see Object name mapping.
Retry Time for Failed Connections	By default, if DTS fails to connect to the source or destination database, DTS retries within the next 720 minutes (12 hours). You can specify the retry time based on your needs. If DTS reconnects to the source and destination databases within the specified time, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Note When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time based on your business needs. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.

In the lower-right corner of the page, click Next.

Select the initial synchronization types.

Initial synchronization type	Description
Initial schema synchronization	DTS synchronizes the schemas of the required objects to the destination PolarDB for Oracle cluster. DTS supports initial schema synchronization for the following types of objects: table, view, synonym, trigger, stored procedure, function, package, and user-defined type. Important However, if an object contains triggers, data will become inconsistent between the source and destination databases. For more information about how to solve this issue, see Configure a data synchronization task for a source database that contains a trigger.
Initial full data synchronization	DTS synchronizes historical data of objects from the source PolarDB for Oracle cluster to the destination PolarDB for Oracle cluster. Important During data synchronization, do not perform DDL operations on the objects to be synchronized. Otherwise, the objects may fail to be synchronized.

In the lower-right corner of the page, click 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, you can click the icon next to each failed item to view details.
  After you troubleshoot the issues based on the details, initiate a new precheck.
  If you do not need to troubleshoot the issues, ignore the failed items and initiate a new precheck.
Close the Precheck dialog box after the following message is displayed: Precheck Passed. Then, the data synchronization task starts.
Wait until initial synchronization is complete and the data synchronization task enters the Synchronizing state.
You can view the status of the data synchronization task on the Synchronization Tasks page.