Configure one-way data synchronization between PolarDB for PostgreSQL (Compatible with Oracle) clusters - Data Transmission Service

PolarDB for PostgreSQL (Compatible with Oracle) is a next-generation cloud-native relational database developed by Alibaba Cloud. It is highly compatible with the Oracle syntax. You can use Data Transmission Service (DTS) to perform one-way data synchronization between PolarDB for PostgreSQL (Compatible with Oracle) clusters.

Prerequisites

The source PolarDB for PostgreSQL (Compatible with Oracle) cluster must be of the latest version. For more information about how to update a cluster, see Version management.
The tables to be synchronized in the source PolarDB for PostgreSQL (Compatible with Oracle) cluster must contain primary keys or UNIQUE NOT NULL indexes.
In the source PolarDB for PostgreSQL (Compatible with Oracle) cluster, the wal_level parameter is set to logical. This indicates that the information required for logical encoding is added to the write-ahead logging (WAL) logs. For more information, see Configure cluster parameters.

Usage notes

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 accurately displayed, DTS creates a heartbeat table in the source database. The name of the heartbeat table is dts_postgres_heartbeat. The following figure shows the schema and content of the table.
If you choose to synchronize a schema and create a table that uses the schema or run the RENAME command to rename the table to be synchronized, you must run the ALTER TABLE schema.table REPLICA IDENTITY FULL; command before you can write data to the table.
Note
Replace the schema and table variables in the preceding sample statement with the actual schema name and table name.
If the source database has one or more long-term transactions and incremental data is synchronized in the data synchronization task, the WAL logs generated before the long-term transactions in the source database are committed may be accumulated. As a result, the disk space of the source database may become insufficient.

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 support data synchronization

Operation	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	CREATE TABLE and DROP TABLE ALTER TABLE (This includes 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 partial DDL statements of a transaction are included in the data synchronization task, the DDL statements are not synchronized. If a DDL statement is executed from a session that is created by executing the SET session_replication_role = replica statement, the DDL statement is not synchronized. DDL statements that are executed by calling methods such as FUNCTION are 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-O, 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 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 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 instances.

Section	Parameter	Description
N/A	Synchronization Task Name	The task name that DTS automatically generates. We recommend that you specify a descriptive name to identify the task. You do not need to use a unique task name.
Source Instance Details	Instance 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 PostgreSQL (Compatible with Oracle) cluster.
	Database Name	The name of the source database.
	Database Account	The privileged account of the source PolarDB for PostgreSQL (Compatible with Oracle) cluster. For more information about how to create a privileged database account, see Create a database account.
	Database Password	The password of the database account.
Destination Instance Details	Instance 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. You cannot change the value of this parameter.
	PolarDB Instance ID	The ID of the destination PolarDB for PostgreSQL (Compatible with Oracle) cluster.
	Database Name	The name of the destination database.
	Database Account	The database account of the destination PolarDB for PostgreSQL (Compatible with Oracle) cluster. The account must have the permissions of the database owner. Important You can specify the database owner 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.
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 IP address 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, and you must make sure that the ECS instance can access the database. If the self-managed database is hosted on multiple ECS instances, you must manually add the CIDR blocks of DTS servers to the security group rules of each ECS instance. 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 IP address whitelist of the database to allow DTS to access the database. For more information, see Add the CIDR blocks of DTS servers.
Warning
If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database or instance, or to the ECS security group rules, security risks may arise. Therefore, before you use DTS to synchronize 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.

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

选择同步对象

Setting	Description
Processing Mode In Existed Target Table	Precheck and Intercept: checks whether the destination cluster 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 identical table names, the precheck is passed. Otherwise, an error is returned during the precheck and the data synchronization task cannot be started. Note If tables with the same names in the destination database cannot be deleted or renamed, you can rename the objects to be synchronized to avoid table name conflicts. For more information, see Rename an object to be synchronized. Ignore: skips the precheck for identical table names in the source and destination databases. Warning If you select Ignore, data inconsistency may occur between the source and destination databases. This may cause business loss. If the source and destination databases use the same schema, DTS does not synchronize the data records that have the same primary keys as the data records in the destination database during initial full data synchronization. However, DTS synchronizes these data records during incremental data synchronization. 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.
Select Object to Be Synchronized	Select one or more objects from the Available section and click the icon to add them to the Selected section. You can select tables or databases. Note If you select a database, all schema changes in the database are synchronized to the destination database. By default, after an object is synchronized to the destination database, the name of the object remains unchanged. If you want to change the names of the objects synchronized to the destination cluster, use the object name mapping feature. 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 Connection	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.

配置同步初始化

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 resolve this issue, see Configure a data synchronization or migration task for a source database that contains a trigger.
Initial Full Data Synchronization	DTS synchronizes historical data of objects from the source cluster to the destination 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.