Data synchronization between PolarDB-X 1.0 instances - Data Transmission Service

Prerequisites

The source and destination PolarDB-X 1.0 instances are created. The corresponding databases and tables are created in the destination PolarDB-X 1.0 instance based on the data structure of the objects to be synchronized from the source PolarDB-X 1.0 instance. For more information, see Create a PolarDB-X 1.0 instance and Create a database.

Note
The storage type of a PolarDB-X 1.0 instance must be RDS for MySQL, including custom RDS instances and separately purchased RDS instances. PolarDB for MySQL is not supported.
The destination instance has more storage space than the source instance.

Usage notes

Important

Two-way synchronization between PolarDB-X 1.0 instances is not supported.

Type	Description
Source database limitations	Tables to synchronize must have a primary key or a unique constraint to ensure each row is unique and prevent duplicate data in the destination database. If a table has only a unique constraint, schema synchronization is not supported for that table. We recommend that you use a primary key. Synchronization of tables with secondary indexes is not supported. If you synchronize data at the table level and perform editing operations such as column name mapping, a single synchronization task supports a maximum of 5,000 tables. If you exceed this limit, split the tables into multiple tasks or synchronize the entire database. Otherwise, a request error may occur when you submit the task. The binary logs of the ApsaraDB RDS for MySQL instances attached to the source PolarDB-X 1.0 instance must meet the following requirements: The binary logging feature must be enabled, and the `binlog_row_image` parameter must be set to `full`. Otherwise, the precheck fails, and the data synchronization task cannot start. For an incremental data synchronization task, you must retain the binary logs of the source database for at least 24 hours. For a task that includes both full and incremental data synchronization, the binary logs must be retained for at least 7 days. You can reduce the retention period to 24 hours after the full data synchronization is complete. If DTS cannot obtain the binary logs because the retention period is too short, the task may fail. In extreme cases, data inconsistency or loss can occur. Issues caused by a binary log retention period shorter than the DTS requirement are not covered by the DTS Service-Level Agreement (SLA). Limitations on operations in the source database: If you change the network type of the PolarDB-X 1.0 instance during data synchronization, you must update the connection information for the synchronization link. During synchronization, do not perform operations on the source instance such as scaling (for example, scaling its attached ApsaraDB RDS for MySQL instance, or changing the distribution of physical tables that correspond to logical tables in ApsaraDB RDS for MySQL even if the ApsaraDB RDS for MySQL instance is not scaled), migrating hot tables, changing the shard key, or making DDL changes. Otherwise, the data synchronization task will fail or data inconsistencies will occur. If you change the partition key of a source table, and the primary key of the corresponding destination table does not include the partition key, data loss may occur in the destination database. Reason: A PolarDB-X 1.0 instance has multiple ApsaraDB RDS for MySQL instances attached to it. DTS maintains independent synchronization links for each ApsaraDB RDS for MySQL instance. Changing a partition key causes data to be deleted from one ApsaraDB RDS for MySQL instance and inserted into another. Because these links execute independently, the order of the DELETE and INSERT operations can be disrupted. If the primary key of the destination database does not include the partition key, a DELETE operation that is executed later can incorrectly delete the newly inserted data. Recommendation: To prevent data loss from out-of-order operations, ensure the primary key of the destination table includes the partition key columns of the source table. Do not perform DDL operations during full data synchronization — this causes the task to fail. During the full synchronization phase, DTS queries the source database and may generate metadata locks, which can block DDL operations on the source. The source PolarDB-X 1.0 instance must be version 5.2 or later. During the operation of a DTS synchronization task, changing the type of a broadcast table in the source PolarDB-X 1.0 instance (for example, changing a broadcast table to a regular table or a sharded table) is not supported. To change the table type, stop the synchronization task first, and then reconfigure the synchronization task after the change is complete.
Other limitations	Incremental data synchronization relies on continuous XA transactions in the source PolarDB-X 1.0 instance to ensure data consistency. If this continuity is disrupted, for example when modifying synchronization objects or during disaster recovery of the incremental data capture module, uncommitted XA transactions may be lost. The storage of PolarDB-X 1.0 must be ApsaraDB RDS for MySQL. This includes privately customized RDS instances and separately purchased RDS instances. PolarDB for MySQL is not supported. Read-only instances of PolarDB-X 1.0 are not supported. The storage resources of PolarDB-X 1.0 must be horizontally split. Vertical splitting is not supported. Data synchronization from a PolarDB-X 1.0 instance is a distributed process. DTS creates a subtask for each attached ApsaraDB RDS for MySQL instance. You can view subtask status on the Task Topology page. Perform data synchronization during off-peak hours. Full data synchronization consumes read and write resources on both the source and destination databases, which increases their loads. Full data synchronization performs concurrent INSERT operations, which can cause table fragmentation in the destination database. As a result, the destination tablespace is larger than the source after full data synchronization. Do not use tools such as pt-online-schema-change to perform online DDL changes on synchronization objects in the source database. Otherwise, the synchronization task fails. During data synchronization, do not write data to the destination database by using tools other than DTS. Otherwise, data inconsistencies may occur. For example, if you use DMS to perform an online DDL change while other data is being written to the destination database, 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.

Billing

Synchronization type	Link configuration fee
Full synchronization	Free of charge.
Incremental synchronization	Paid. For more information, see Billing overview.

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 synchronization topologies and usage notes, see Synchronization topologies.

Supported SQL operations

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE

Permissions required for database accounts

Database	Required permissions	Authorization method
Source PolarDB-X 1.0 instance	Read permissions on the objects to be synchronized.	Manage accounts
Destination PolarDB-X 1.0 instance	Read and write permissions on the destination database to which the objects are synchronized.	Manage accounts

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 DMS console layout and style.
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.

Category	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	Database Type	Select PolarDB-X 1.0.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB-X 1.0 instance resides.
	Instance ID	Select the ID of the source PolarDB-X 1.0 instance.
	Database Account	Enter the database account for the source PolarDB-X 1.0 instance. For more information about the required permissions, see Permissions required for database accounts.
	Database Password	Enter the password for the specified database account.
Destination Database	Database Type	Select PolarDB-X 1.0.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination PolarDB-X 1.0 instance resides.
	Instance ID	Select the ID of the destination PolarDB-X 1.0 instance.
	Database Account	Enter the database account for the destination PolarDB-X 1.0 instance. For more information about the required permissions, see Permissions required for database accounts.
	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	By default, Incremental Data Synchronization is selected, which supports only Full Data Synchronization. Schema Synchronization is not supported. After the precheck is complete, DTS initializes the synchronization objects in the destination cluster with data from the source instance. This initial data 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 Object 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.
Case Policy for Destination Object Names	Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.
Source Objects	In the Source Objects box, click the objects, and then click to move them to the Selected Objects box. Note We recommend that you select objects at the table level. If you select an entire database as a synchronization object, changes to add or delete tables in the source database are not synchronized to the destination database.
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 SQL operations to synchronize for a specific database or table, right-click the object in the Selected Objects box and select the desired SQL operations. To filter data by using a WHERE clause, right-click the table to be synchronized in the Selected Objects box and specify the filter condition. For more information, see Set filter 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 Synchronization	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	Select an environment tag to identify the instance. Not required for this example.
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 notifications. For more information, see Configure monitoring and alerting during task configuration.

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.