Two-way data synchronization for PolarDB for MySQL - Data Transmission Service

This topic describes how to use Data Transmission Service (DTS) to synchronize data bidirectionally between PolarDB for MySQL clusters.

Prerequisites

The source and target PolarDB for MySQL clusters must be created. For more information, see Purchase an Enterprise Edition cluster and Purchase a subscription cluster.
Note
The source and target PolarDB for MySQL clusters should have the same storage capacity.
Binary logging must be enabled for the source and target PolarDB for MySQL clusters. For more information, see Enable binary logging.

Usage notes

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 limits	The tables to be synchronized must have a primary key or a UNIQUE constraint, and the fields must be unique. Otherwise, duplicate data may appear in the destination database. Note If a table to be synchronized in a two-way synchronization instance does not have a primary key or UNIQUE constraint, you can enable the Exactly-Once write feature. For more information, see Synchronize tables without primary keys or unique constraints. If you synchronize data at the table level and need to edit the tables, such as mapping column names, a single data synchronization task supports a maximum of 1,000 tables. If you exceed this limit, an error is reported when you submit the task. In this case, split the tables into multiple synchronization tasks or configure a task to synchronize the entire database. Binary logs: You must enable binary logging and set the `loose_polar_log_bin` parameter to `ON`. Otherwise, an error is reported during the precheck and the DTS instance fails to start. For more information about how to enable binary logging and modify parameters, see Enable binary logging and Set cluster and node parameters. Note Enabling binary logging for a PolarDB for MySQL cluster consumes storage space and incurs fees. The binary logs of the PolarDB for MySQL cluster must be retained for at least 3 days. We recommend that you retain them for 7 days. Otherwise, the DTS task may fail because DTS cannot obtain the binary logs. In extreme cases, this may cause data inconsistency or data loss. Issues caused by a binary log retention period shorter than the required period are not covered by the DTS Service-Level Agreement (SLA). Note For more information about how to set the retention period for binary logs of a PolarDB for MySQL cluster, see Modify the retention period. 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	You cannot synchronize data from a read-only node of the source PolarDB for MySQL cluster. You cannot synchronize OSS foreign tables from the source PolarDB for MySQL cluster. Synchronization of INDEX and PARTITION is not supported. Primary/secondary failover of the database instance is not supported during initial full data synchronization. If a failover occurs, reconfigure the synchronization task promptly. If your source database uses online DDL operations in temporary table mode—including but not limited to multi-table merge scenarios—or adds function-based indexes to unique key columns, data loss or task failure may occur in the destination database. If a primary key or unique key conflict occurs while the task is running: 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. Resolvers defined by comment syntax do not support synchronous use. If your data includes four-byte characters—such as rare Chinese characters or emojis—the destination database and table must use the utf8mb4 charset. Note If you use DTS to synchronize table schemas, set the `character_set_server` parameter at the instance level in the destination database to utf8mb4. Before you synchronize data, evaluate the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. Otherwise, initial full data synchronization consumes read and write resources on the source and destination databases, which may increase the database load. Initial full data synchronization runs concurrent INSERT operations, which causes fragmentation in the destination database tables. As a result, the tablespace of the destination instance is larger than that of the source instance after initial full data synchronization is complete. For table-level data synchronization, do not use tools such as pt-online-schema-change to perform online DDL operations on the synchronization objects in the source database. Otherwise, the synchronization fails. For table-level data synchronization, if no data other than the data from DTS is written to the destination database, you can use Data Management (DMS) to perform online DDL operations. For more information, see Change schemas without locking tables. During DTS synchronization, do not write data other than the data from DTS to the destination database. Otherwise, data inconsistency between the source and destination databases may occur. For example, if you use DMS to perform online DDL operations while other data is being written to the destination database, data may be lost in the destination database. If a DDL operation fails to be written to the destination database, the DTS task continues to run. You need to check the task logs for the failed DDL operation. For more information about how to view task logs, see View task logs. To synchronize accounts from the source database, meet the prerequisites and review related considerations. For more information, see Migrate database accounts. 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.
Other notes	Two-way synchronization between PolarDB for MySQL clusters: DTS currently supports two-way synchronization between only two PolarDB for MySQL clusters. Two-way synchronization among multiple PolarDB for MySQL clusters is not supported. DDL syntax synchronization direction limit. To ensure the stability and data consistency of the two-way synchronization link, only forward DDL synchronization is supported. Reverse DDL synchronization is not supported. When a two-way synchronization task is running, DTS creates a database named DTS in the destination databases of the forward and reverse tasks to prevent data loop synchronization. Do not modify this database while the task is running, and make sure that the database account used for the task has read and write permissions on this database. A two-way synchronization task includes forward and reverse synchronization tasks. When you configure or reset the task, if the destination object of one task matches the synchronization object of the other task: Allow only one task to synchronize full and incremental data. The other task supports only incremental synchronization. Data from the source of the current task synchronizes only to the destination of the current task. It does not serve as source data for the other task. DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command on the source database to advance the binary log offset.

Billing

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

Conflict detection

To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated in only one of the database instances in the two-way synchronization. If data records are updated on both database instances, DTS the system will apply the conflict resolution policy configured in the task.

DTS checks and fixes conflicts to maximize the stability of two-way synchronization tasks. DTS can detect the following types of conflicts:

Uniqueness conflicts caused by INSERT operations

In two-way synchronization, if records with the same primary key are inserted into both database instances simultaneously (or in close succession), a uniqueness constraint conflict will be triggered. When the INSERT statement is synchronized to the peer instance, it will fail because a record with the same primary key value already exists.
Mismatched records in UPDATE operations
- If the records to be updated do not exist in the destination instance, DTS converts the UPDATE operation into an INSERT operation. However, uniqueness conflicts may occur.
- The record to be updated by an UPDATE operation causes a primary key or unique key conflict.
Non-existent records to be deleted

The records to be deleted do not exist in the destination instance. In this case, DTS ignores the DELETE operation regardless of the conflict resolution policy that you specify.

Important

Due to time differences and latency, DTS cannot guarantee 100% conflict prevention. To ensure consistency, update records with the same primary or unique key on only one database instance at a time.
DTS provides various conflict resolution strategies for the aforementioned data conflicts, which you can select while configuring two-way data synchronization.

Supported SQL operations

Important

DDL operations are synchronized only in the forward direction (from the source database to the destination database). They are not supported in the reverse direction and are automatically filtered.

Operation type	SQL statement
DML	INSERT, UPDATE, DELETE
DDL	ALTER TABLE, ALTER VIEW CREATE FUNCTION, CREATE INDEX, CREATE PROCEDURE, CREATE TABLE, CREATE VIEW DROP INDEX, DROP TABLE RENAME TABLE Important The RENAME TABLE operation may cause data inconsistency. For example, if the synchronization object is only a specific table and you rename the table in the source instance during synchronization, the data of the table is not synchronized to the destination database. To prevent this issue, select the entire database to which the table belongs as the synchronization object when you configure the data synchronization task. Make sure that the databases to which the table belongs before and after the RENAME TABLE operation are both included in the synchronization object. TRUNCATE TABLE

Database account permissions

Database	Required permissions	Actions
Source PolarDB for MySQL cluster	Privileged account	Create and manage a database account and Manage the password of a database account.
Destination PolarDB for MySQL cluster	Privileged account

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.

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 a DMS database instance.	Choose whether to use an existing instance as needed. If you use an existing instance, the database information is automatically filled in. You do not need to enter it again. If you do not use an existing instance, configure the database information below. Note In the DMS console, click Add DMS Database Instance to add a database instance. For more information, see Add an ApsaraDB database instance and Add a third-party or self-managed database instance. In the DTS console, you can add a database to DTS on the Database Connections page or the new configuration page. For more information, see Manage data connections.
	Database Type	Select PolarDB for MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB for MySQL cluster is located.
	Replicate Data Across Alibaba Cloud Accounts	For this example, select No, as the database instance belongs to the current Alibaba Cloud account.
	PolarDB Cluster ID	Select the ID of the source PolarDB for MySQL cluster.
	Database Account	Enter the database account for the source PolarDB for MySQL cluster. For information about the required permissions, see Permissions required for database accounts.
	Database Password	Enter the password for the specified database account.
	Encryption	Select an option. For more information about SSL encryption, see Configure SSL encryption.
Destination Database	Select a DMS database instance.	Choose whether to use an existing instance as needed. If you use an existing instance, the database information is automatically filled in. You do not need to enter it again. If you do not use an existing instance, configure the database information below. Note In the DMS console, click Add DMS Database Instance to add a database instance. For more information, see Add an ApsaraDB database instance and Add a third-party or self-managed database instance. In the DTS console, you can add a database to DTS on the Database Connections page or the new configuration page. For more information, see Manage data connections.
	Database Type	Select PolarDB for MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination PolarDB for MySQL cluster is located.
	Replicate Data Across Alibaba Cloud Accounts	For this example, select No, as the database instance belongs to the current Alibaba Cloud account.
	PolarDB Cluster ID	Select the ID of the destination PolarDB for MySQL cluster. Note If the destination cluster is a multi-master cluster (Limitless) and its read/write mode is Read/Write (automatic read/write splitting), you must add the primary node for the destination database and table to the read/write address, or select all primary nodes. This is because multi-master clusters use a database- and table-based routing mechanism. Evaluate the routing impact before you select nodes. For more information, see Configure a database proxy.
	Database Account	Enter the database account for the destination PolarDB for MySQL cluster. For information about the required permissions, see Permissions required for database accounts.
	Database Password	Enter the password for the specified database account.
	Encryption	Select an option. For more information about SSL encryption, see Configure SSL encryption.

After completing the configuration, click Test Connection and Proceed at the bottom of the page.
If the source or destination database is an Alibaba Cloud database instance, such as an RDS for MySQL instance or an ApsaraDB for MongoDB instance, DTS automatically adds the IP address of the DTS service for the corresponding region to the whitelist of the Alibaba Cloud database instance. If the source or destination database is a self-managed database on an ECS instance, DTS automatically adds the IP address of the DTS service for the corresponding region to the security rules of the ECS instance. You must also ensure that the self-managed database does not restrict access from the ECS instance. If the database is deployed in a cluster on multiple ECS instances, you must manually add the IP address of the DTS service for the corresponding region to the security rules of each of the other ECS instances. If the source or destination database is a self-managed database in an IDC or a database from another cloud provider, you must manually add the IP address of the DTS service for the corresponding region to allow access from the DTS servers. For more information about the IP addresses of the DTS service, see IP address blocks of DTS servers.
Warning
Adding the public IP address blocks of the DTS service, either automatically or manually, may pose security risks. Using this product, you acknowledge that you understand and accept the potential security risks and that you must implement basic security measures. These measures include, but are not limited to, strengthening password security, limiting the ports open to each CIDR block, using authentication for internal API calls, and regularly checking and restricting unnecessary CIDR blocks. Alternatively, you can connect through a private network using a leased line, VPN Gateway, or Smart Access Gateway.

Configure the task objects.

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

Parameter	Description
Synchronization Types	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.
Method to Migrate Triggers in Source Database	Select a method to synchronize triggers. If the objects to be synchronized do not include triggers, you can skip this parameter. For more information, see Configure a method to synchronize or migrate triggers. Note This can be configured only when Synchronization Types is set to Schema Synchronization.
Synchronization Topology	Select Two-way Synchronization.
Enable Exactly-Once Write	If the synchronization objects include tables without primary keys or unique constraints, select Yes. For more information, see Synchronize tables that have no primary keys or unique constraints.
Exclude DDL Operations	Select Yes to exclude DDL operations. Select No to synchronize DDL operations. Important To ensure the stability of two-way synchronization, DDL operations can be synchronized only in the forward direction (from the source database to the destination database). DDL operations are automatically filtered out in the reverse direction.
Global Conflict Resolution Policy	Select a conflict resolution policy based on your business requirements. For information about supported conflict types, see Supported conflict detection. TaskFailed (Fail and report an error upon conflict) The task will error out and stop if a conflict occurs. The task enters a failed state and requires manual intervention for recovery. Ignore (Keep the existing record in the destination instance) The system skips the conflicting statement and continues the synchronization. The existing record in the destination instance is retained. Overwrite (Overwrite the conflicting record in the destination instance) If a conflict occurs, the system overwrites the conflicting record in the destination instance with the data from the source. Note If a synchronization task is paused or restarted and a latency occurs, these policies do not take effect during the latency period. By default, data in the destination database is overwritten.
Processing Mode of Conflicting 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.
Whether to migrate Event	Specify whether to synchronize events from the source database as required. If you select Yes, you must also meet the relevant requirements and perform subsequent operations. For more information, see Synchronize or migrate events.
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 If you use the object name mapping feature, other objects that depend on the mapped object might not be synchronized. To select the SQL operations to synchronize for a specific database or table, set filter conditions, add additional columns to the destination table, or configure an Independent Conflict Resolution Policy for incremental data synchronization, right-click the object in the Selected Objects box and configure the settings in the dialog box that appears. For more information, see Set filter conditions, Supported SQL operations, Add additional columns to a destination table, and Configure an independent conflict resolution policy.

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?.
Select the engine type of the destination database	Select the engine type for the destination database. InnoDB: The default storage engine. X-Engine: An online transaction processing (OLTP) database storage engine.
Copy the temporary table of the Online DDL tool that is generated in the source table to the destination database.	If the source database uses Data Management (DMS) or gh-ost for online DDL changes, choose whether to synchronize the temporary tables generated during these operations. Important DTS tasks do not currently support online DDL changes performed by tools like pt-online-schema-change. Using such tools will cause the DTS task to fail. Yes: Synchronizes the temporary tables generated by online DDL changes. Note If the data of temporary tables generated by online DDL changes is too large, it may cause synchronization latency. No, Adapt to DMS Online DDL: Does not synchronize temporary tables generated by online DDL changes. Instead, it synchronizes only the original DDL statements executed in Data Management (DMS). Note This approach will cause table locks on the destination database. No, Adapt to gh-ost: Does not the synchronize temporary tables generated by online DDL changes. Instead, it synchronizes only the original DDL statements executed by gh-ost. You can use default or custom regular expressions for gh-ost shadow and trash tables. Note This approach will cause table locks on the destination database.
Whether to Migrate Accounts	Select whether to synchronize account information from the source database based on your requirements. If you select Yes, you also need to select the account to synchronize, configure the hosts from which this account is allowed to log in, and confirm the permissions for the account. For more information, see Migrate database accounts.
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).
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.
Environment Tag	You can select an environment tag to identify the instance based on your requirements. In this example, you do not need to select one.
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.

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 an 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.

Configure the reverse synchronization task.
1. Wait until the forward synchronization task is initialized and its Status is Running.
2. Locate the reverse synchronization task and click Configure Task.
3. Refer to Step 3 through Step 6 to configure the reverse synchronization task.
  Important
  When you configure a reverse synchronization task, select the correct source and destination instances. For the reverse task, the source instance is the destination instance of the forward task, and the destination instance is the source instance of the forward task. Also, verify that the instance information, such as the database name, account, and password, is correct.
  You cannot modify the Instance Region for the source and destination databases of a reverse synchronization task. Fewer parameters are required compared to a forward synchronization task. Refer to the console for the specific parameters to configure.
  The Processing Mode of Conflicting Tables setting for a reverse synchronization task ignores tables that were synchronized by the forward task.
  A reverse synchronization task cannot synchronize objects that are in the Selected Objects list of the forward task.
  Do not use the mapping feature when you configure a reverse task. This can cause data inconsistencies.
4. When the Success Rate is 100%, click Back.
After the reverse synchronization task is configured, wait until the Status of both synchronization tasks is Running. This completes the configuration of two-way data synchronization.