Two-Way Synchronization from MySQL to MySQL

Supported Source and Destination Databases

**Table 1** Supported databases
Instance Role in Current Cloud	Source DB	Destination DB
Active 1	RDS for MySQL (5.5, 5.6, 5.7, and 8.0)	On-premises MySQL databases (MySQL 5.5, 5.6, 5.7, and 8.0) ECS-hosted MySQL databases (MySQL 5.5, 5.6, 5.7, and 8.0) Other cloud MySQL databases (MySQL 5.5, 5.6, 5.7, and 8.0) RDS for MySQL (5.5, 5.6, 5.7, and 8.0)
Active 2	On-premises MySQL databases (MySQL 5.5, 5.6, 5.7, and 8.0) ECS-hosted MySQL databases (MySQL 5.5, 5.6, 5.7, and 8.0) Other cloud MySQL databases (MySQL 5.5, 5.6, 5.7, and 8.0) RDS for MySQL (5.5, 5.6, 5.7, and 8.0)	RDS for MySQL (5.5, 5.6, 5.7, and 8.0)

The version of the destination database must be the same as that of the source database.
Only whitelisted users can use this function. To use this function, submit a service ticket. In the upper right corner of the management console, choose Service Tickets > Create Service Ticket.

Database Account Permission Requirements

To start a synchronization task, the source and destination database users must meet the requirements in the following table. Different types of synchronization tasks require different permissions. For details, see Table 2. DRS automatically checks the database account permissions in the pre-check phase and provides handling suggestions.

**Table 2** Database account permissions
Type	Forward Task	Backward Task
Source database user	The user must have the following minimum permissions: SELECT, SHOW VIEW, EVENT, LOCK TABLES, REPLICATION SLAVE, and REPLICATION CLIENT	The user must have the following minimum permissions: REPLICATION SLAVE and REPLICATION CLIENT
Destination database user	The user must have the following minimum permissions: SELECT, CREATE, DROP, DELETE, INSERT, UPDATE, ALTER, CREATE VIEW, CREATE ROUTINE, and REFERENCES The root account of the RDS for MySQL DB instance has the preceding permissions by default. If the destination database version is in the range 8.0.14 to 8.0.18, the SESSION_VARIABLES_ADMIN permission is required.

You are advised to create an independent database account for DRS task connection to prevent task failures caused by database account password modification.
After changing the account passwords for the source and destination databases, modify the connection information in the DRS task as soon as possible to prevent automatic retry after a task failure. Automatic retry will lock the database accounts.

Supported Synchronization Objects

Table 3 lists the objects that can be synchronized in different scenarios. DRS will automatically check the objects you selected before the synchronization.

**Table 3** Supported synchronization objects
Type	Notes
Synchronization objects	Tables, primary key indexes, unique indexes, common indexes, stored procedures, views, and functions can be synchronized. Only MyISAM and InnoDB tables can be synchronized. Events and triggers cannot be synchronized. Full and incremental synchronizations do not support invisible columns. Invisible columns can be synchronized since MySQL 8.0.23. For example: CREATE TABLE `test11` ( `id` int NOT NULL, `c1` int DEFAULT NULL /!80023 INVISIBLE /, PRIMARY KEY (`id`));

Precautions

To ensure tasks can run normally, DRS provides automatic pre-check. Before starting a DRS task, DRS checks the configurations and conditions of the source and destination databases. For details about the main check items and handling suggestions, see Pre-check Items. In addition to the pre-check items, you need to pay attention to the items listed in Table 4.

**Table 4** Precautions
Type	Restrictions
Restrictions on the source database	The source database does not support the reset master or reset master to command, which may cause DRS task failures or data inconsistency. The source and destination databases cannot contain tables that have the same names but do not have primary keys. If the source MySQL database does not support TLS 1.2 or is a self-built database of an earlier version (earlier than 5.6.46 or between 5.7.0 and 5.7.28), you need to submit an O&M application for testing the SSL connection.
Restrictions on usage	General You are not advised to modify or delete the usernames, passwords, and permissions of the source and destination databases or change the ports of the source and destination databases. Do not perform operations (including but not limited to DDL and DML operations) on the destination database. Do not clear binlogs on the source and destination databases. If index synchronization is required for a DRS task, the destination database table cannot contain indexes with the same name but different columns. In the full phase, DRS ignores the existing indexes with the same name. In the incremental phase, DDL operations on indexes based on the index name trigger misplacement. Full synchronization When a DRS task is being started or in the full synchronization phase, do not perform DDL operations on the source database. Otherwise, the task may be abnormal. Modifying MyISAM tables may cause data inconsistency. Incremental synchronization If the session variable character_set_client is set to binary, some data may include garbled characters. You can add additional objects in the incremental synchronization phase of a forward task. Resumable upload is supported. However, data may be repeatedly inserted into a non-transactional table that does not have a primary key when the server system breaks down. Resetting a task is not supported for a two-way synchronization task. During incremental synchronization of a forward task, some DDL statements are supported. In one-to-one synchronization, the following DDL operations are synchronized by default: CREATE_TABLE, RENAME_TABLE, ADD_COLUMN, MODIFY_COLUMN, CHANGE_COLUMN, DROP_COLUMN, DROP_INDEX, ADD_INDEX, CREATE_INDEX, RENAME_INDEX, DROP_TABLE, TRUNCATE_TABLE, DROP_PARTITION, RENAME_COLUMN, DROP_PRIMARY_KEY and ADD_PRIMARY_KEY. You can select the DDL operations to be synchronized on the object selection page as required. Incremental synchronization supports table renaming. Ensure that both the source and destination tables are selected. DDL synchronization is not supported during incremental synchronization of a backward task. Do not perform the point-in-time recovery (PITR) operation on the source database. The destination database cannot be restored to a point in time when a full synchronization was being performed. Stopping a task Stop a task normally. Before a task is completed, ensure that the source and destination databases are connected and pay attention to the synchronization status reported by the synchronization logs. Forcibly stop a task. If you forcibly stop a task, DRS resources will be released. Troubleshooting If any problem occurs during task creation, startup, full synchronization, incremental synchronization, or completion, rectify the fault by referring to Troubleshooting.
Other restrictions	If the DCC does not support instances with 4 vCPUs and 8 GB of memory or higher instance specifications, the synchronization task cannot be created. During table name mapping, tables on which views, stored procedures, and functions depend cannot be synchronized, and foreign key constraints of tables cannot be synchronized. If a physically generated column in a table is generated based on a time type, the data in the column may be inconsistent. If the source database contains any table without a primary key defined, the table-level many-to-one synchronization is not supported. If the source and destination databases are the same RDS DB instance, real-time synchronization without database mapping is not supported. If the database table name contains characters other than letters, digits, and underscores (_), or the mapped database table name contains hyphens (-) and number signs (#), the name length cannot exceed 42 characters. Braces ({}) in a database or table name in the source database must be used in pairs. Otherwise, the table structure synchronization may fail. After data mapping, if braces ({}) in a database or name mapped to the destination database are not in pairs, the table structure may fail to be synchronized. The partitioned table does not support column mapping. If the source database contains non-standard floating-point data and the data can be written in loose mode but cannot be written in strict mode, there may be data inconsistency during synchronization. If the source and destination DB instances are RDS for MySQL instances, tables encrypted using Transparent Data Encryption (TDE) cannot be synchronized. Before creating a DRS task, if concurrency control rules of SQL statements are configured for the source or destination database, the DRS task may fail. When creating a task, ensure that the synchronization object dimensions (table-level, database-level, and object file import) selected for the forward task and backward task are the same. After a synchronization task is created, the source and destination databases cannot be set to read-only. The destination table can contain more columns than the source table. However, the following failures must be avoided: Assume that extra columns on the destination database cannot be null or have default values. If newly inserted data records are synchronized from the source database to the destination database, the extra columns will become null, which does not meet the requirements of the destination database and will cause the task to fail. Assume that extra columns on the destination database must be fixed at a default value and have a unique constraint. If newly inserted data records are synchronized from the source database to the destination database, the extra columns will contain multiple default values. That does not meet the unique constraint of the destination database and will cause the task to fail. For many-to-one synchronization tasks that involve the synchronization of the same table, DDL operations cannot be performed. Otherwise, all synchronization tasks fail. The DDL operation of renaming an unselected table is filtered out during the synchronization. As a result, the task may fail or data may be inconsistent. If you rename table A to the name of table B and tables A and B are selected for synchronization, this RENAME statement will not be filtered out. If you rename table A to the name of table B but table B is not synchronized, this RENAME statement will be filtered out. You are not advised to perform the rename operation in the many-to-one synchronization scenario. Otherwise, the task may fail or data may be inconsistent. Do not perform service operations on the same data record in the same table in the source database of the forward task and the source database of the backward task at the same time. Otherwise, data may be inconsistent.

Prerequisites

You have logged in to the DRS console.
Your account balance is greater than or equal to $0 USD.
For details about the DB types and versions supported by real-time synchronization, see Supported Databases.
If a subaccount is used to create a DRS task, ensure that an agency has been added. For details about how to create an agency, see Agency Management.

Procedure

This section describes how to create a two-way synchronization task from MySQL to RDS for MySQL.

On the Data Synchronization Management page, click Create Synchronization Task.

On the Create Synchronization Instance page, select a region and project, specify the task name, description, and the synchronization instance details, and click Create Now.

Task information description

Figure 1 Synchronization task information

**Table 5** Task information
Parameter	Description
Region	The region where the replication instance is deployed. You can change the region.
Project	The project corresponds to the current region and can be changed.
Task Name	The task name must start with a letter and consist of 4 to 50 characters. It can contain only letters, digits, hyphens (-), and underscores (_).
Description	The description consists of a maximum of 256 characters and cannot contain special characters !=<>'&"\

Synchronization instance details

Figure 2 Synchronization instance details

**Table 6** Synchronization instance settings
Parameter	Description
Synchronization Type	Select Two-way. A forward task and a backward task are created. The synchronization mode of the forward task is full+incremental synchronization, and the synchronization mode of the backward task is incremental synchronization.
Instance Role in Current Cloud	Specifies the role of the current cloud DB instance in the two-way synchronization. The value can be Active 1 and Active 2. For details, see How Do I Select Active Database 1 and 2 for Dual-Active DR? Active 1: Indicates that the DB instance serves as the source database of the forward task and contains initial data. Active 2: Indicates that the DB instance serves as the destination database of the forward task. Active 2 is used as an example.
Source DB Engine	Select MySQL.
Destination DB Engine	Select MySQL.
Network Type	Public network is used as an example. Available options: Public network, VPC, VPN or Direct Connect VPC is suitable for data synchronization between cloud databases of the same account in the same region. Public network is suitable for data synchronization from on-premises or external cloud databases to the destination database bound with an EIP. VPN or Direct Connect is suitable for data synchronization from on-premises databases to cloud databases, between databases of different accounts in the same region on the cloud, or between databases across regions on the cloud using a VPN, Direct Connect, Cloud Connect, VPCEP, or a VPC peering connection.
Destination DB Instance	Select an RDS DB instance you have created.
Synchronization Instance Subnet	Select the subnet where the synchronization instance is located. You can also click View Subnets to go to the network console to view the subnet where the instance resides. By default, the DRS instance and the destination DB instance are in the same subnet. You need to select the subnet where the DRS instance resides, and there are available IP addresses for the subnet. To ensure that the synchronization instance is successfully created, only subnets with DHCP enabled are displayed.
Security Group	Select a security group. You can use security group rules to allow or deny access to the instance.
Synchronization Mode	The synchronization mode supported by a DRS task. Full+Incremental is used as an example. For details about the underlying working principles for full or incremental synchronization, see Product Architecture and Function Principles. Full+Incremental This synchronization mode allows you to synchronize data in real time. After a full synchronization initializes the destination database, an incremental synchronization parses logs to ensure data consistency between the source and destination databases. NOTE: If you select Full+Incremental, data generated during the full synchronization will be continuously synchronized to the destination database, and the source remains accessible.
Enable Binlog Cleanup	Indicates whether to enable the function of quickly clearing binlogs of the destination database. After this function is enabled, binlog clearing is enabled for the destination database during the full synchronization and disabled during the incremental synchronization.
Specify EIP	This parameter is available when you select Public network for Network Type. Select an EIP to be bound to the DRS instance. DRS will automatically bind the specified EIP to the DRS instance and unbind the EIP after the task is complete.

Task type

Figure 3 Task type

**Table 7** Task type information
Parameter	Description
Specifications	DRS instance specifications. Different specifications have different performance upper limits. For details, see Real-Time Synchronization. NOTE: DRS allows you to upgrade specifications only for single-AZ synchronization tasks. Task specifications cannot be downgraded. For details, see Changing Specifications.
AZ	Select the AZ where you want to create the DRS task. Selecting the one housing the source or destination database can provide better performance.

Enterprise Project and Tags

Figure 4 Enterprise projects and tags

**Table 8** Enterprise Project and Tags
Parameter	Description
Enterprise Project	An enterprise project you would like to use to centrally manage your cloud resources and members. Select an enterprise project from the drop-down list. The default project is default. For more information about enterprise project, see Enterprise Management User Guide. To customize an enterprise project, click Enterprise in the upper right corner of the console. The Enterprise Project Management Service page is displayed. For details, see Creating an Enterprise Project in Enterprise Management User Guide.
Tags	Tags a task. This configuration is optional. Adding tags helps you better identify and manage your tasks. Each task can have up to 20 tags. If your organization has configured tag policies for DRS, add tags to tasks based on the policies. If a tag does not comply with the policies, task creation may fail. Contact your organization administrator to learn more about tag policies. After a task is created, you can view its tag details on the Tags tab. For details, see Tag Management.

If a task fails to be created, DRS retains the task for three days by default. After three days, the task automatically stops.

After the synchronization instance is created, the Configure Source and Destination Databases page of the forward task is displayed by default. On the Configure Source and Destination Databases page of the forward task, specify source and destination database information and click Test Connection for both the source and destination databases to check whether they have been connected to the synchronization instance. After the connection tests are successful, select the check box before the agreement and click Next.

The information to be entered for the source and destination databases is different for the role Active 1 and Active 2 of the DB instance on the current cloud.

Source database information of the forward task

Figure 5 Source database information

**Table 9** Source database settings
Parameter	Description
IP Address or Domain Name	The IP address or domain name of the source database.
Port	The port of the source database. Range: 1 – 65535
Database Username	The username for accessing the source database.
Database Password	The password for the database username. You can change the password if necessary. If the task is in the Starting, Full synchronization, Incremental synchronization, or Incremental synchronization failed status, in the Connection Information area on the Basic Information tab, click Modify Connection Details. In the displayed dialog box, change the password.
SSL Connection	If SSL connection is required, enable SSL on the source database, ensure that related parameters have been correctly configured, and upload an SSL certificate. NOTE: The maximum size of a single certificate file that can be uploaded is 500 KB. If SSL is disabled, your data may be at risk.

The IP address, port, username, and password of the source database are encrypted and stored in the database and the synchronization instance, and will be cleared after the task is deleted.

Destination database information of the forward task

Figure 6 Destination database information

**Table 10** Destination database settings
Parameter	Description
DB Instance Name	The RDS DB instance you selected when creating the synchronization task. This parameter cannot be changed.
Database Username	The username for accessing the destination database.
Database Password	The password for the database username. You can change the password if necessary. If the task is in the Starting, Full synchronization, Incremental synchronization, or Incremental synchronization failed status, in the Connection Information area on the Basic Information tab, click Modify Connection Details. In the displayed dialog box, change the password.
SSL Connection	If SSL connection is required, enable SSL on the destination database, ensure that related parameters have been correctly configured, and upload an SSL certificate. NOTE: The maximum size of a single certificate file that can be uploaded is 500 KB. If SSL is disabled, your data may be at risk.

The username and password of the destination database are encrypted and stored in the database and the synchronization instance during the synchronization. After the task is deleted, the username and password are permanently deleted.

On the Set Synchronization Task page, select the conflict policy and synchronization objects, and then click Next.

Figure 7 Synchronization mode

**Table 11** Synchronization mode and object
Parameter	Description
Flow Control	You can choose whether to control the flow. Flow Control takes effect in the full phase only. Yes You can customize the maximum synchronization speed. During the full synchronization, the synchronization speed of each task (or each subtask in multi-task mode) does not exceed the value of this parameter. In addition, you can set the time range based on your service requirements. The traffic rate setting usually includes setting of a rate limiting time period and a traffic rate value. Flow can be controlled all day or during specific time ranges. The default value is Always. A maximum of three time ranges can be set, and they cannot overlap. The flow rate must be set based on the service scenario and cannot exceed 9,999 MB/s. Figure 8 Flow control No The synchronization speed is not limited and the outbound bandwidth of the source database is maximally used, which will increase the read burden on the source database. For example, if the outbound bandwidth of the source database is 100 MB/s and 80% bandwidth is used, the I/O consumption on the source database is 80 MB/s. NOTE: The flow control mode takes effect only in the full synchronization phase. You can also change the flow control mode after creating a task. For details, see Modifying the Flow Control Mode.
Incremental Conflict Policy	The conflict policy refers to the conflict handling policy during incremental synchronization. By default, conflicts in the full synchronization phase are ignored. Select any of the following conflict policies: Ignore The system will skip the conflicting data and continue the subsequent synchronization process. If you select Ignore, data in the source database may be inconsistent with that in the destination database. Overwrite Conflicting data will be overwritten.
Synchronization Object	The left pane displays the source database objects, and the right pane displays the selected objects. You can select Tables, Import object file, or Databases for Synchronization Object as required. If the synchronization objects in source and destination databases have different names, you can map the source object name to the destination one in the right pane. For details, see Changing Object Names (Mapping Object Names). In a two-way synchronization scenario, many-to-one database and table name mapping is not supported. If the database table name contains characters other than letters, digits, and underscores (_), or the mapped database table name contains hyphens (-) and number signs (#), the name length cannot exceed 42 characters. If you want to change the saved database mapping name during table-level synchronization, you need to expand the database. For details about how to import an object file, see Importing Synchronization Objects. NOTE: To quickly select the desired database objects, you can use the search function. If there are changes made to the source databases or objects, click in the upper right corner to update the objects to be synchronized. If an object name contains spaces, the spaces before and after the object name are not displayed. If there are two or more consecutive spaces in the middle of the object name, only one space is displayed. The name of the selected synchronization object cannot contain spaces.

On the Check Task page, check the synchronization task.
- If any check fails, review the cause and rectify the fault. After the fault is rectified, click Check Again.
  For details about how to handle check failures, see Solutions to Failed Check Items in Data Replication Service User Guide.
- If all check items are successful, click Next.
  
  You can proceed to the next step only when all checks are successful. If there are any items that require confirmation, view and confirm the details first before proceeding to the next step.

On the displayed page, specify Start Time, Send Notifications, SMN Topic, Delay Threshold (s), and Stop Abnormal Tasks After, confirm that the configured information is correct, select the check box before the agreement, and click Submit to submit the task.

Figure 9 Task startup settings

**Table 12** Task startup settings
Parameter	Description
Start Time	Set Start Time to Start upon task creation or Start at a specified time based on site requirements. NOTE: After a synchronization task is started, the performance of the source and destination databases may be affected. You are advised to start a synchronization task during off-peak hours.
Send Notifications	This parameter is optional. After enabled, select a SMN topic. If the status, latency metric, or data of the migration task is abnormal, DRS will send you a notification.
SMN Topic	This parameter is available only after you enable Send Notifications and create a topic on the SMN console and add a subscriber. For details, see Simple Message Notification User Guide.
Delay Threshold (s)	During an incremental synchronization, a synchronization delay indicates a time difference (in seconds) of synchronization between the source and destination database. If the synchronization delay exceeds the threshold you specify, DRS will send alarms to the specified recipients. The value ranges from 0 to 3,600. To avoid repeated alarms caused by the fluctuation of delay, an alarm is sent only after the delay has exceeded the threshold for six minutes. NOTE: If the delay threshold is set to 0, no notifications will be sent to the recipient. In the early stages of an incremental synchronization, the synchronization delay is long because a large quantity of data is awaiting synchronization. In this case, no notifications will be sent. Before setting the delay threshold, enable Send Notifications.
Data Exception Notification	This parameter is optional. After enabled, DRS will send a notification if the task data is abnormal.
Stop Abnormal Tasks After	Number of days after which an abnormal task is automatically stopped. The value must range from 14 to 100. The default value is 14. NOTE: You can set this parameter only for pay-per-use tasks. Tasks in the abnormal state are still charged. If tasks remain in the abnormal state for a long time, they cannot be resumed. Abnormal tasks run longer than the period you set (unit: day) will automatically stop to avoid unnecessary fees.

After the forward task is submitted, wait until the forward task starts and enters the incremental synchronization state. After the latency is less than 60 seconds, click Edit of the backward task on the task list page to test connections of the backward task. After the connection tests are successful, click Next.
The connection information of the backward task cannot be edited. You can only test the connections.

On the Set Synchronization Task page of the backward task, select the conflict policy and synchronization objects, and then click Next.

Figure 10 Synchronization mode

**Table 13** Synchronization mode and object
Parameter	Description
Incremental Conflict Policy	The conflict policy refers to the conflict handling policy during incremental synchronization. By default, conflicts in the full synchronization phase are ignored. Select any of the following conflict policies: Ignore The system will skip the conflicting data and continue the subsequent synchronization process. If you select Ignore, data in the source database may be inconsistent with that in the destination database. Overwrite Conflicting data will be overwritten.
Synchronization Object	The left pane displays the source database objects, and the right pane displays the selected objects. You can select Tables, Import object file, or Databases for Synchronization Object as required. When selecting the synchronization object, ensure that the object dimensions (table-level, object file import, and database-level) selected for the forward task and backward task are the same. The synchronization object of the forward task must be consistent with that of the backward task. For example: If the synchronization object for the forward task is database B (table T1), the synchronization object for the backward task must be database B (table T1). If the synchronization objects in source and destination databases have different names, you can map the source object name to the destination one in the right pane. For details, see Changing Object Names (Mapping Object Names). In a two-way synchronization scenario, many-to-one database and table name mapping is not supported. In a backward task, the object mapping must be consistent with that in a forward task. For example: If the synchronization object for the forward task is database B (mapped to database B1) [table T1 (mapped to table T2), the synchronization object for the backward task must be database B1 (mapped to database B) [table T2 (mapped to table T1). If the database table name contains characters other than letters, digits, and underscores (_), or the mapped database table name contains hyphens (-) and number signs (#), the name length cannot exceed 42 characters. If you want to change the saved database mapping name during table-level synchronization, you need to expand the database. For details about how to import an object file, see Importing Synchronization Objects. NOTE: To quickly select the desired database objects, you can use the search function. If there are changes made to the source databases or objects, click in the upper right corner to update the objects to be synchronized. If an object name contains spaces, the spaces before and after the object name are not displayed. If there are two or more consecutive spaces in the middle of the object name, only one space is displayed. The name of the selected synchronization object cannot contain spaces.

On the Check Task page, check the synchronization task.
- If any check fails, review the cause and rectify the fault. After the fault is rectified, click Check Again.
  For details about how to handle check failures, see Solutions to Failed Check Items in Data Replication Service User Guide.
- If all check items are successful, click Next.
  
  You can proceed to the next step only when all checks are successful. If there are any items that require confirmation, view and confirm the details first before proceeding to the next step.
On the displayed page, specify Start Time, Send Notifications, SMN Topic, Delay Threshold (s), and Stop Abnormal Tasks After, confirm that the configured information is correct, select the check box before the agreement, and click Submit to submit the backward task.
After the backward task is submitted, you can view and manage it on the Data Synchronization Management page.
- You can view the task status. For more information about task status, see Task Statuses.
- You can click in the upper right corner to view the latest task status.
- You can switch over the forward task and backward task after the status of the backward task changes to Incremental. For details, see Exchanging the Direction for a Two-Way Synchronization Task.