Data Transmission Service (DTS) supports two-way synchronization between MySQL instances.
Supported source and destination databases
The following table describes the types of source and destination MySQL databases that are supported by DTS. In this example, the source and destination ApsaraDB RDS for MySQL instances are used. You can also follow the procedure to configure data synchronization tasks for other types of MySQL databases.
Source database | Destination database |
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Prerequisites
- The source and destination ApsaraDB RDS for MySQL instances are created. For more information, see Create an ApsaraDB RDS for MySQL instance.
The source and destination ApsaraDB RDS for MySQL instances have the same storage capacity.
Usage notes
- During schema synchronization, DTS synchronizes foreign keys from the source database to the destination database.
- During full synchronization and incremental synchronization, DTS temporarily disables checking of foreign key constraints and foreign key cascade operations at the session level. If you perform the cascade update and delete operations on the source database during data synchronization, data inconsistency may occur.
Category | Description |
Limits on the source and destination databases |
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Other limits |
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Special cases | If the source database is a self-managed MySQL database, take note of the following items:
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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. |
Conflict detection
To ensure data consistency, make sure that data records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes. If data records are updated on both nodes, DTS responds to conflicts based on the conflict resolution policy that you specify for the data synchronization task.
DTS checks and fixes conflicts to maximize the stability of two-way synchronization instances. DTS can detect the following types of conflicts:
Uniqueness conflicts caused by INSERT operations
INSERT operations that do not comply with the uniqueness constraint cannot be synchronized. For example, if a record with the same primary key value is inserted into the two synchronization nodes at almost the same time, one of the inserted records fails to be synchronized. The synchronization fails because a record with the same primary key value already exists on the other node.
Inconsistent records caused by 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 primary keys or unique keys of the records to insert may conflict with those of existing records in the destination instance.
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.
During two-way synchronization, the system time of the source and destination instances may be different. Synchronization latency may occur. For these reasons, DTS does not ensure that the conflict detection mechanism can prevent all data conflicts. To perform two-way synchronization, make sure that records with the same primary key, business primary key, or unique key are updated only on one of the synchronization nodes.
DTS provides conflict resolution policies to prevent conflicts that may occur during data synchronization. You can select a conflict resolution policy when you configure two-way data synchronization.
Supported synchronization topologies
For more information about synchronization topologies that are supported by DTS, see Synchronization topologies.
SQL operations that can be synchronized
Operation type | SQL statement |
DML | INSERT, UPDATE, and DELETE |
DDL |
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DDL operations can be synchronized only in the forward direction. DDL operations cannot be synchronized from the destination database to the source database because DDL operations are ignored in the reverse direction.
Permissions required for database accounts
Database | Required permission | References |
Source ApsaraDB RDS for MySQL instance | Read and write permissions on the objects to be synchronized | Create databases and accounts for an ApsaraDB RDS for MySQL instance and Modify the permissions of a standard account on an ApsaraDB RDS for MySQL instance |
Destination ApsaraDB RDS for MySQL instance |
Procedure
Go to the Data Synchronization page of the new DTS console.
NoteYou can also log on to the Data Management (DMS) console. In the top navigation bar, click DTS. In the left-side navigation pane, choose .
In the upper-left corner of the page, select the region in which you want to create the data synchronization task.
Click Create Task. On the page that appears, configure the source and destination databases.
WarningAfter you configure the source and destination databases, we recommend that you read the limits that are displayed in the upper part of the page. Otherwise, the task may fail or data inconsistency may occur.
Section
Parameter or setting
Description
N/A
Task Name
The name of the task. DTS automatically assigns a name to the task. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database
Database Type
The type of the source database. Select MySQL.
Access Method
The access method of the source database. Select Alibaba Cloud Instance.
Instance Region
The region in which the source ApsaraDB RDS for MySQL instance resides.
Replicate Data Across Alibaba Cloud Accounts
Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
RDS Instance ID
The ID of the source ApsaraDB RDS for MySQL instance.
NoteThe source and destination ApsaraDB RDS for MySQL instances can be the same or different. You can use DTS to synchronize data between two ApsaraDB RDS for MySQL instances or within an ApsaraDB RDS for MySQL instance.
Database Account
The database account of the source ApsaraDB RDS for MySQL instance. For information about the permissions that are required for the account, see Permissions required for database accounts.
Database Password
The password of the database account.
Encryption
Specifies whether to encrypt the connection to the database. Select Non-encrypted or SSL-encrypted based on your business requirements. If you select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the data synchronization task. For more information, see Configure SSL encryption for an ApsaraDB RDS for MySQL instance.
Destination Database
Database Type
The type of the destination database. Select MySQL.
Access Method
The access method of the destination database. Select Alibaba Cloud Instance.
Instance Region
The region in which the destination ApsaraDB RDS for MySQL instance resides.
RDS Instance ID
The ID of the destination ApsaraDB RDS for MySQL instance.
Database Account
The database account of the destination ApsaraDB RDS for MySQL instance. For information about the permissions that are required for the account, see Permissions required for database accounts.
Database Password
The password of the database account.
Encryption
Specifies whether to encrypt the connection to the database. Select Non-encrypted or SSL-encrypted based on your business requirements. If you select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the data synchronization task. For more information, see Configure SSL encryption for an ApsaraDB RDS for MySQL instance.
In the lower part of the page, click Test Connectivity and Proceed.
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 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 ensure that the ECS instance can access the database. 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 whitelist of the database to allow DTS to access the database. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.
WarningIf 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.
Configure objects to synchronize and advanced settings.
Basic Settings
Parameter or setting
Description
Synchronization Type
By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of selected objects from the source instance to the destination instance. The historical data is the basis for subsequent incremental synchronization.
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 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.
NoteYou can use the object name mapping feature 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 Map object names.
Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases.
WarningIf you select Ignore Errors and Proceed, data inconsistency may occur, and your business may be exposed to potential risks.
If the source and destination databases have the same schemas, and a data record has the same primary key value as an existing data record in the destination database:
During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained.
During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten.
If the source and destination databases have different schemas, data may fail to be initialized, only some columns are synchronized, or the data synchronization task fails. Operate with caution.
Precheck and Report Errors: checks whether the destination database contains collections that have the same names as collections in the source database. If the source and destination databases do not contain collections that have identical collection names, the precheck is passed. Otherwise, an error is returned during the precheck and the data synchronization task cannot be started.
NoteYou can use the object name mapping feature to rename the collections that are synchronized to the destination database. You can use this feature if the source and destination databases contain collections that have identical names and the collections 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 collection names in the source and destination databases.
WarningIf you select Ignore Errors and Proceed, data inconsistency may occur, and your business may be exposed to potential risks.
DTS does not synchronize data records that have the same primary key values as data records in the destination database.
Data may fail to be initialized, only some columns are synchronized, or the data synchronization task fails.
Precheck and Report Errors: checks whether the destination database 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.
Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases.
WarningIf you select Ignore Errors and Proceed, data inconsistency may occur, and your business may be exposed to potential risks.
If the source and destination databases have the same schemas, and a data record has the same primary key value as an existing data record in the destination database:
During full data synchronization, DTS does not synchronize the data record to the destination database. The existing data record in the destination database is retained.
During incremental data synchronization, DTS synchronizes the data record to the destination database. The existing data record in the destination database is overwritten.
If the source and destination databases have different schemas, data may fail to be initialized, only some columns are synchronized, or the data synchronization task fails. Operate with caution.
Synchronization Topology
Select Two-way Synchronization.
Exclude DDL Statements
Yes: excludes DDL operations.
No: synchronizes DDL operations.
ImportantLimits on DDL synchronization directions: DDL operations can be synchronized only in the forward direction (from the source database to the destination database). DDL operations are ignored in the reverse direction (from the destination database to the source database). Therefore, you can set this parameter only when you configure the task in the forward direction.
Conflict Resolution Policy
If you encounter the conflicts described in the Conflict detection section of this topic, select a conflict resolution policy based on your business requirements.
TaskFailed
If a conflict occurs during data synchronization, the data synchronization task reports an error and exits the process. The task enters a failed state, and you must manually resolve the conflict.
Ignore
If a conflict occurs during data synchronization, the data synchronization task ignores the current statement and continues the process. The conflicting records in the destination database are used.
Overwrite
If a conflict occurs during data synchronization, the conflicting records in the destination database are overwritten.
NoteIf a latency occurs when you pause or restart a data synchronization task, the selected conflict resolution policy does not take effect during the latency. By default, data in the destination database is overwritten by the data that is synchronized during the latency.
Select Objects
Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section.
NoteYou can select columns, tables, or databases as the objects to be synchronized. If you select tables or columns as the objects to be synchronized, DTS does not synchronize other objects such as views, triggers, or stored procedures to the destination database.
Rename Databases and Tables
To rename an object that you want to synchronize to the destination instance, right-click the object in the Selected Objects section. For more information, see Map the name of a single object.
To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time.
Filter data
You can specify WHERE conditions to filter data. For more information, see Use SQL conditions to filter data.
Select the SQL operations to be synchronized
In the Selected Objects section, right-click an object. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information, see SQL operations that can be synchronized.
Advanced Settings
Parameter
Description
Set Alerts
Specifies whether to configure alerting for the data synchronization task. If the task fails or the synchronization latency exceeds the specified threshold, alert contacts will receive notifications. Valid values:
No: does not configure alerting.
Yes: configures alerting. In this case, you must also configure the alert threshold and alert contacts. For more information, see Configure monitoring and alerting when you create a DTS task.
Capitalization of Object Names in Destination Instance
The capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to ensure that the capitalization of object names is consistent with that in the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
Copy the temporary table of the Online DDL tool that is generated in the source table to the destination database
If you use Data Management (DMS) or the gh-ost tool to perform online DDL operations on the source database, you can specify whether to synchronize the data of temporary tables generated by online DDL operations.
ImportantYou cannot use tools such as pt-online-schema-change to perform online DDL operations on the source database. Otherwise, the DTS task fails.
Yes: DTS synchronizes the data of temporary tables generated by online DDL operations.
NoteIf online DDL operations generate a large amount of data, the data synchronization task may take an extended period of time to complete.
No, Adapt to DMS Online DDL: DTS does not synchronize the data of temporary tables generated by online DDL operations. Only the original DDL operations that are performed by using Data Management (DMS) are synchronized.
NoteIf you select this option, the tables in the destination database may be locked.
No, Adapt to gh-ost: DTS does not synchronize the data of temporary tables generated by online DDL operations. Only the original DDL operations that are performed by using the gh-ost tool are synchronized. You can use the default or custom regular expressions to filter out the shadow tables of the gh-ost tool and tables that are not required.
NoteIf you select this option, the tables in the destination database may be locked.
Specify the retry time range for failed connections
The retry time range for failed connections. If the source or destination database fails to be connected after the data synchronization task is started, DTS immediately retries a connection within the time range. Valid values: 10 to 1440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS reconnects to the source and destination databases within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails.
NoteIf you set different retry time ranges for multiple DTS tasks that have the same source or destination database, the shortest retry time range that is set takes precedence.
When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at your earliest opportunity after the source and destination instances are released.
Click Next: Save Task Settings and Precheck in the lower part of the page.
You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameter settings of the API operation that is called to configure the instance.
NoteBefore 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, click View Details next to each failed item. After you troubleshoot the issues based on the causes, run a precheck again.
If an alert is triggered for an item during the precheck:
If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.
If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur, and your business may be exposed to potential risks.
Wait until the success rate becomes 100%. Then, click Next: Purchase Instance.
On the Purchase Instance page, configure the Billing Method and Instance Class parameters for the data synchronization instance. The following table describes the parameters.
Section
Parameter
Description
New Instance Class
Billing Method
Subscription: You pay for the instance when you create an instance. The subscription billing method is more cost-effective than the pay-as-you-go billing method for long-term use.
Pay-as-you-go: A pay-as-you-go instance is charged on an hourly basis. The pay-as-you-go billing method is suitable for short-term use. If you no longer require a pay-as-you-go instance, you can release the pay-as-you-go instance to reduce costs.
Resource Group
The resource group on which the instance is run. Default value: default resource group. For more information, see What is Resource Management?.
Instance Class
DTS provides various synchronization specifications that provide different performance. The synchronization speed varies based on the synchronization specifications that you select. You can select a synchronization specification based on your business scenario. For more information, see Specifications of data synchronization instances.
Synchronization Topology
This value is set to Two-way Synchronization.
Subscription Duration
If you select the subscription billing method, set the subscription duration and the number of instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years.
NoteThis parameter is available only if you select the Subscription billing method.
Read and select the check box to agree to the Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start to start the data synchronization task. You can view the progress of the task in the task list.
Wait until initial synchronization is complete and the data synchronization task in the forward direction enters the Running state. You can view the state of the data synchronization task on the Data Synchronization page.
Find the data synchronization task in the reverse direction and click Configure Task.
On the Create Task page, configure the source and destination databases as described in Step 5.
ImportantThe source instance in the reverse direction corresponds to the destination instance in the forward direction. The destination instance in the reverse direction corresponds to the source instance in the forward direction. You must also make sure that the parameter settings such as the database name, account, and password are consistent.
Configure the basic and advanced settings as described in Step 7. We recommend that you use the same settings.Configure objects to synchronize and advanced settings. The following table describes other settings.
Basic Settings
Parameter or setting
Description
Processing Mode of Conflicting Tables
When DTS checks for conflicting tables in the reverse direction, the tables that have been synchronized to the destination instance are ignored.
Exclude DDL Operations
The task in the reverse direction ignores DDL operations.
Select Objects
We recommend that you select the same objects for the forward and reverse directions.
You can also add or remove objects based on your business requirements.
Rename Databases and Tables
We recommend that you do not use this feature when you configure the task in the reverse direction. Otherwise, data inconsistency may occur.
Click Next: Save Task Settings and Precheck in the lower part of the page.
You can move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters to view the parameter settings of the API operation that is called to configure the instance.
NoteBefore 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, click View Details next to each failed item. After you troubleshoot the issues based on the causes, run a precheck again.
If an alert is triggered for an item during the precheck:
If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.
If an alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur, and your business may be exposed to potential risks.
Wait until the success rate becomes 100%. Then, click Back.
After the second data synchronization task is configured, wait until both tasks enter the Running state. The two-way data synchronization tasks are configured.