Data synchronization between PolarDB-X 2.0 instances-Data Transmission Service(DTS)-阿里云帮助中心

Prerequisites

The source and destination PolarDB-X 2.0 instances are created.
The storage space of the destination PolarDB-X 2.0 instance must be greater than that of the source PolarDB-X 2.0 instance.

Considerations

Note

During schema synchronization, DTS synchronizes foreign keys from the source database to the destination database.
During full data synchronization and incremental data synchronization, DTS temporarily disables constraint checks and foreign key cascade operations at the session level. Data inconsistency may occur if cascade update or delete operations are performed on the source database while the task is running.

Type	Description
Source database limitations	Synchronized tables must have a primary key or a unique constraint. Otherwise, duplicate data may occur in the destination database. DTS does not support Enterprise Edition PolarDB-X 2.0 read-only instances as the source database. If you synchronize tables individually and perform edits, such as column name mapping, a single data synchronization task supports a maximum of 5,000 tables. To synchronize more than 5,000 tables, split them across multiple tasks or configure the task to synchronize the entire database. Otherwise, a request error may occur after you submit the task. Binary log requirements: You must enable the binary log feature in the PolarDB-X 2.0 console and set the binlog_row_image parameter to full. For more information, see Parameter settings. Otherwise, the precheck fails and the data synchronization task cannot start. For incremental data synchronization, you must retain the source database's binary logs for over 24 hours. For tasks that combine full and incremental synchronization, the retention period must be at least 7 days. After the full data synchronization is complete, you can reduce the retention period to 24 hours. The DTS SLA does not cover issues caused by an insufficient binary log retention period. If a table name in the source PolarDB-X 2.0 instance contains uppercase letters, DTS supports only schema synchronization for that table. DTS does not support synchronizing table groups (TABLEGROUP) or databases and tables that have the Locality attribute. DTS does not support synchronizing tables whose names are reserved keywords, such as `select`. DTS cannot synchronize partitions of databases in DRDS mode on a PolarDB-X 2.0 instance. Do not run DDL operations that change database or table schemas during schema synchronization or full synchronization. Otherwise, the synchronization task fails. Note During full synchronization, DTS queries the source database. This creates metadata locks that may block DDL operations on the source database.
Other limits	Evaluate the performance of your source and destination databases before starting data synchronization. Perform the synchronization during off-peak hours, as initial full data synchronization consumes significant resources and increases the database load. Initial full data synchronization runs concurrent INSERT operations, which can cause table fragmentation in the destination database. Consequently, the destination instance may occupy more tablespace than the source instance after synchronization. Do not use tools like pt-online-schema-change to perform online DDL operations on the objects being synchronized in the source database. Otherwise, the synchronization task fails. During data synchronization, ensure that only DTS writes data to the destination database. Writing data from other sources can cause data inconsistency between the source and destination databases. For example, if you perform online DDL changes by using DMS while another process is writing to the destination, data loss may occur. If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters. Note Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.
Precautions	DTS periodically updates the `dts_health_check`.`ha_health_check` table in the source database to advance the binary log file position.

Billing

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

Supported SQL operations

Operation type	SQL statement
DML	INSERT, UPDATE, DELETE
DDL	ALTER TABLE CREATE FUNCTION, CREATE INDEX, CREATE TABLE Note If the source database is a PolarDB-X 2.0 Enterprise Edition instance, CREATE FUNCTION and CREATE INDEX operations cannot be synchronized. DROP INDEX, DROP TABLE RENAME TABLE Important The RENAME TABLE operation may cause data inconsistency. For example, if you select only a specific table as a synchronization object and then rename that table in the source instance during synchronization, the data from that table is not synchronized to the destination database. To avoid this issue, select the entire database to which the table belongs as the synchronization object. This ensures that the table is still tracked after being renamed, as long as it remains within a synchronized database. TRUNCATE TABLE

Database account permissions

Database	Schema synchronization	Full data synchronization	Incremental data synchronization
Source PolarDB-X 2.0 instance	SELECT	SELECT	REPLICATION SLAVE, REPLICATION CLIENT, and SELECT on the objects to be synchronized.
Destination PolarDB-X 2.0 instance	Read and write permissions.

Procedure

Go to the data synchronization task list page in the destination region. You can do this in one of two ways.
DTS console
1. Log on to the DTS console.
2. In the navigation pane on the left, click Data Synchronization.
3. In the upper-left corner of the page, select the region where the synchronization instance is located.
DMS console
Note
The actual steps may vary depending on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top menu bar, choose Data + AI > DTS (DTS) > Data Synchronization.
3. To the right of Data Synchronization Tasks, select the region of the synchronization instance.
Click Create Task to open the task configuration page.

Configure the source and destination databases.

Warning

After you select the source and destination instances, review the Limits at the top of the page. Otherwise, the task may fail or data inconsistency may occur.

Section	Parameter	Description
N/A	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select PolarDB-X 2.0.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB-X 2.0 instance resides.
	Replicate Data Across Alibaba Cloud Accounts	For this example, which demonstrates synchronization within the same Alibaba Cloud account, select No.
	Instance ID	Select the ID of the source PolarDB-X 2.0 instance.
	Database Account	Enter the database account for the source PolarDB-X 2.0 instance. The account must have the REPLICATION SLAVE, REPLICATION CLIENT, and SELECT permissions on the objects to be synchronized. Note For information on how to grant permissions, see Account permission issues during data synchronization.
	Database Password	Enter the password for the specified database account.
Destination Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select PolarDB-X 2.0.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination PolarDB-X 2.0 instance resides.
	Instance ID	Select the ID of the destination PolarDB-X 2.0 instance.
	Database Account	Enter the database account for the destination PolarDB-X 2.0 instance. The account must have read and write permissions.
	Database Password	Enter the password for the specified database account.

After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
Ensure that the IP address blocks of the DTS service are added to the security settings of the source and destination databases, either automatically or manually, to allow access from DTS servers. For more information, see Add the IP address whitelist of DTS servers.

Configure the task objects.

On the Configure Objects page, specify the objects to synchronize.

Parameter	Description
Synchronization Type	DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization.
Processing Mode for Existing Destination Tables	Precheck and Report Errors: Checks for tables with the same names in the destination database. If any tables with the same names are found, an error is reported during the precheck and the data synchronization task does not start. Otherwise, the precheck is successful. Note If you cannot delete or rename the table with the same name in the destination database, you can map it to a different name in the destination. For more information, see Database Table Column Name Mapping. Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example: If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database: During full data synchronization, DTS retains the destination record and skips the source record. During incremental synchronization, DTS overwrites the destination record with the source record. If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.
Synchronization Topology	Select One-way Synchronization.
Source Objects	In the Source Objects box, click the objects, and then click to move them to the Selected Objects box. Note You can select objects at the database, table, or column level. If you select only tables or columns, DTS does not synchronize other object types (such as views, triggers, and stored procedures).
Selected Objects	To rename a single object in the destination instance, right-click the object in the Selected Objects box. For more information, see Map a single object name. To rename multiple objects in bulk, click Batch Edit in the upper-right corner of the Selected Objects box. For more information, see Map multiple object names in bulk. Note To select the SQL operations to synchronize at the database or table level, right-click the synchronization object in the Selected Objects box and select the desired SQL operations in the dialog box that appears. For a list of supported operations, see Supported SQL operations. To set a WHERE clause to filter data, right-click the table to be synchronized in the Selected Objects box and set the filter condition in the dialog box that appears. For instructions, see Filter task data by using SQL conditions.

Click Next: Advanced Settings.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.
Retry Time for Failed Connections	If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails. Note If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances. DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.
Retry Time for Other Issues	If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.
Enable Throttling for Full Data Migration	During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s). Note This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
Enable Throttling for Incremental Data Synchronization	You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).
Environment Tag	You can select an environment tag to identify the instance. This parameter is optional.
Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Choose whether DTS writes heartbeat SQL information to the source database while the instance is running. Yes: Does not write heartbeat SQL information to the source database. The DTS instance may display latency. No: Writes heartbeat SQL information to the source database. This may interfere with source database operations like physical backups and cloning.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert contact. For more information, see Configure monitoring and alerting during task configuration.

Click Data Verification to configure a data verification task.
To use the data verification feature, see Configure data verification.

Save the task and perform a precheck.
- To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.
- If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.
- If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.
- If the precheck generates warnings:
  For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.
  For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.

Purchase the instance.

When the Success Rate reaches 100%, click Next: Purchase Instance.

On the Purchase page, select the billing method and link specifications for the data synchronization instance. For more information, see the following table.

Category	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay upfront for a specific duration. This is cost-effective for long-term, continuous tasks. Pay-as-you-go: You are billed hourly for actual usage. This is ideal for short-term or test tasks, as you can release the instance at any time to save costs.
	Resource Group Settings	The resource group to which the instance belongs. The default is default resource group. For more information, see What is Resource Management?.
	Instance Class	DTS offers synchronization specifications at different performance levels that affect the synchronization rate. Select a specification based on your business requirements. For more information, see Data synchronization link specifications.
	Subscription Duration	In subscription mode, select the duration and quantity of the instance. Monthly options range from 1 to 9 months. Yearly options include 1, 2, 3, or 5 years. Note This option appears only when the billing method is Subscription.

Read and select the checkbox for Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start, and then click OK in the OK dialog box.
You can monitor the task progress on the data synchronization page.