From DDM to MySQL

Supported Source and Destination Databases

**Table 1** Supported databases
Source DB	Destination DB
DDM instances	RDS for MySQL

Prerequisites

You have logged in to the DRS console.

Suggestions

When a 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.
To keep data consistency before and after the synchronization, ensure that no data is written to the destination database during the synchronization.

The success of database synchronization depends on environment and manual operations. To ensure a smooth synchronization, perform a synchronization trial before you start the synchronization to help you detect and resolve problems in advance.
Start your synchronization task during off-peak hours. A less active database is easier to synchronize successfully. If the data is fairly static, there is less likely to be any severe performance impacts during the synchronization.
- If network bandwidth is not limited, the query rate of the source database increases by about 50 MB/s during full synchronization, and two to four CPUs are occupied.
- To ensure data consistency, tables to be synchronized without a primary key may be locked for 3s.
- The data being synchronized may be locked by other transactions for a long period of time, resulting in read timeout.
- When DRS concurrently reads data from a database, it will use about 6 to 10 sessions. The impact of the connections on services must be considered.
- If you read a table, especially a large table, during the full migration, the exclusive lock on that table may be blocked.
Data-Level Comparison
To obtain accurate comparison results, start data comparison at a specified time point during off-peak hours. If it is needed, select Start at a specified time for Comparison Time. Due to slight time difference and continuous operations on data, data inconsistency may occur, reducing the reliability and validity of the comparison results.

Precautions

Before creating a synchronization task, read the following notes:

**Table 2** Precautions
Type	Restrictions
Database permissions	The source database DDM account must have the SELECT permission. The physical sharded database account must have the following permissions: SELECT, SHOW VIEW, EVENT, LOCK TABLES, REPLICATION SLAVE and REPLICATION CLIENT. The destination database user must have the following permissions: SELECT, CREATE, DROP, DELETE, INSERT, and UPDATE. The root account of the RDS MySQL DB instance has the preceding permissions by default.
Synchronization object	Full synchronization supports the synchronization of data, table structures, and indexes. The source database cannot contain tables whose sharding keys are timestamp. The sharding key of the source table must be added to the primary key and unique key of the destination table, which means that the primary key and unique key columns of the destination table must contain the sharded columns of the source table to avoid data conflict and inconsistency.
Source database	The binlog of the MySQL source database must be enabled and use the row-based format. If the storage space is sufficient, store the source database binlog for as long as possible. The recommended retention period is three days. During an incremental synchronization, the server_id value of the MySQL source database must be set. If the source database version is MySQL 5.6 or earlier, the server_id value ranges from 2 to 4294967296. If the source database is MySQL 5.7 or later, the server_id value ranges from 1 to 4294967296. The database names and table names of the source database sharding middleware cannot contain the following characters: '<>/\ and non-ASCII characters. Enable skip-name-resolve for the MySQL source database to reduce the possibility of connection timeout. Enable the Global Transaction Identifier (GTID) of the source database.
Destination database	The destination DB instance is running properly. If the destination DB instance type is primary/standby, the replication status must also be normal. The destination DB instance must have sufficient storage space. If the destination database (excluding MySQL system database) has the same name as the source database, the table structures in the destination database must be consistent with those in the source database. The character set of the destination database must be the same as that of the source database. The time zone of the destination database must be the same as that of the source database. During a synchronization, a large amount of data is written to the destination database. If the value of the max_allowed_packet parameter of the destination database is too small, data cannot be written. You are advised to set the max_allowed_packet parameter to a value greater than 100 MB.
Precautions	If the data types are incompatible, the synchronization may fail. Resumable upload is supported, but data may be repeatedly inserted into a table that does not have a primary key when the server system breaks down. If the source database contains a duplicate primary key or unique key, the data synchronized to the destination database will be less than that in the source database. Therefore, you must check and correct the data before starting the synchronization task. If the destination DB instance is an RDS MySQL instance, tables encrypted using Transparent Data Encryption (TDE) cannot be synchronized. 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 cannot be null or have default values. If newly inserted data records are synchronized from the source to the destination, the extra columns will become null, which does not meet the requirements of the destination and will cause the task to fail. Assume that extra columns on the destination must be fixed at a default value and have a unique constraint. If newly inserted data records are synchronized from the source to the destination, the extra columns will contain multiple default values, which does not meet the unique constraint of the destination and will cause the task to fail. When creating a synchronization task, do not set the destination database to read-only. After a synchronization task is created, you are not allowed to add a schema or modify the association between the old schema and a new RDS DB instance. Otherwise, the synchronization task will fail. During synchronization, do not modify or delete the usernames, passwords, permissions, or ports of the source and destination databases. During the synchronization, do not change the sharding key of a table on the source DDM instance, or change an unsharded or broadcast table to a sharded table, or change a sharded table to an unsharded or broadcast table. During an incremental synchronization, do not perform the restoration operation on the source database. During incremental synchronization, some DDL operations are supported. DROP_DATABASE, DROP_TABLE, TRUNCATE_TABLE, CREATE_VIEW and DROP_VIEW are not supported. Online DDL is not supported. Tables can be created. For example: create table `ddl_test` (id int, c1 varchar(25), primary key(id)); create table `ddl_test_gho` like `ddl_test`; Tables can be renamed. Both the source and destination tables must be selected. For example: rename table `ddl_test` to `ddl_test_new`; Columns in a table can be added and modified, but cannot be deleted. For example: alter table `ddl_test` add column `c2` varchar(25); alter table `ddl_test` modify column `c1` varchar(50); alter table `ddl_test` alter c1 set default 'xxx'; Table indexes can be modified. For example: alter table `ddl_test` drop primary key; alter table `ddl_test` add primary key(id); alter table `ddl_test` add index `ddl_test_uk`(id); alter table `ddl_test` drop index `ddl_test_uk`; In table-level synchronization, you can add columns, modify columns, and add primary keys and normal indexes. During database-level synchronization, you can create tables, rename tables, add columns, modify columns, and add primary keys and normal indexes. The name of a table, column, or index to be added or modified cannot exceed 63 characters. Otherwise, the task fails. If a primary key is added to a table that does not have a primary key in the source database, the DDL operation must contain the first column. Otherwise, the task fails.

Procedure

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

On the Create Synchronization Instance page, specify the task name, description, and the synchronization instance details, and click Next.

**Table 3** Task and recipient description
Parameter	Description
Task Name	The task name consists of 4 to 50 characters, starts with a letter, and can contain only letters (case-insensitive), digits, hyphens (-), and underscores (_).
Description	The description consists of a maximum of 256 characters and cannot contain the following special characters: !=<>'&"\

**Table 4** Synchronization instance settings
Parameter	Description
Data Flow	Select To the cloud.
Source DB Engine	Select DDM.
Destination DB Engine	Select MySQL.
Network Type	The public network is used as an example. Available options: VPC, Public network and VPN or Direct Connect
Destination DB Instance	The RDS DB instance you created.
Synchronization Instance Subnet	Select the subnet where the synchronization instance is located. You can also click View Subnet 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 ensure that there are available IP addresses. To ensure that the synchronization instance is successfully created, only subnets with DHCP enabled are displayed.
Synchronization Mode	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.
Source DB Instance Quantity	The default minimum number of source DB instances is 2. You can set this parameter based on the number of source database shards.

On the Configure Source and Destination Databases page, wait until the synchronization instance is created. Then, 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.

**Table 5** Source database settings
Parameter	Description
Middleware 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
Middleware Username	The username of the source DDM instance.
Middleware Password	The password for the source DDM instance username.
SSL Connection	SSL encrypts the connections between the source and destination databases. If SSL is enabled, upload the SSL CA root certificate. NOTE: The maximum size of a single certificate file that can be uploaded is 500 KB. If the SSL certificate is not used, your data may be at risk.
DB Instance	The sharded database details.

The IP address, domain name, username, and password of the source database are encrypted and stored in DRS, and will be cleared after the task is deleted.

**Table 6** 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 database username and password are encrypted and stored in the system and will be cleared after the task is deleted.

On the Set Synchronization Task page, select the objects to be synchronized, and then click Next.

**Table 7** Synchronization mode and object
Parameter	Description
Synchronization Object	DRS supports table-level synchronization. You can select data for synchronization based on your service requirements. The search function can help you quickly select the required database objects.

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.
- 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, confirm that the configured information is correct, and click Submit to submit the task.

**Table 8** Task startup settings
Parameter	Description
Started 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.

After the 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.