Synchronize data from an Apsara PolarDB for MySQL cluster to an ApsaraDB RDS for MySQL instance - Data Migration or Synchronization| Alibaba Cloud Documentation Center

This topic describes how to synchronize data from an Apsara PolarDB for MySQL cluster to an ApsaraDB RDS for MySQL instance by using Data Transmission Service (DTS).

Prerequisites

An Apsara PolarDB for MySQL cluster is created. For more information, see Create an Apsara PolarDB for MySQL cluster.
The binary logging feature is enabled for the Apsara PolarDB for MySQL cluster. For more information, see Enable binary logging.

Precautions

DTS uses read and write resources of the source and destination databases during initial full data synchronization. This may increase the database load. If the database performance is unfavorable, the specification is low, or the data volume is large, database services may become unavailable. For example, DTS occupies a large amount of read and write resources in the following cases: a large number of slow SQL queries are performed on the source database, the tables have no primary keys, or a deadlock occurs in the destination database. Before synchronizing data, you must evaluate the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. For example, you can synchronize data when the CPU usage of the source and destination databases is less than 30%.
If you have selected one or more tables (not a database) for synchronization, do not use gh-ost or pt-online-schema-change to modify the tables during data synchronization. Otherwise, data synchronization may fail.

Notice To avoid synchronization failure, you can use Data Management (DMS) to perform online DDL schema changes during data synchronization. For more information, see Change the table schema without locking.
During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination cluster. After initial full data synchronization, the tablespace of the destination cluster is larger than that of the source instance.
The source database must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, duplicate data may exist in the destination database.

SQL operations that can be synchronized


Operation type	SQL statements
DML	INSERT, UPDATE, DELETE, and REPLACE
DDL	ALTER TABLE and ALTER VIEW CREATE FUNCTION, CREATE INDEX, CREATE PROCEDURE, CREATE TABLE, and CREATE VIEW DROP INDEX and DROP TABLE RENAME TABLE TRUNCATE TABLE

Supported synchronization topologies

One-way one-to-one synchronization
One-way one-to-many synchronization
One-way cascade synchronization
One-way many-to-one synchronization

For more information about synchronization topologies, see Synchronization topologies.

Limits

Incompatibility of triggers
If the object you want to synchronize is a database and the database contains a trigger that updates the synchronized table, the synchronized data may be inconsistent. For example, the source database contains Table A and Table B. If a data record is inserted into Table A, a trigger inserts a data record into Table B. In this case, after an INSERT operation is performed on Table A in the source instance, the data in Table B becomes inconsistent between the source and destination instances.

To avoid this situation, you must delete the trigger that is synchronized to the destination instance and select Table B as the object to be synchronized. For more information, see Configure synchronization when triggers exist.
Limits on RENAME TABLE operations
RENAME TABLE operations may cause data inconsistency between the source and destination databases. For example, if only Table A needs to be synchronized and it is renamed Table B, Table B cannot be synchronized to the destination database. To avoid this situation, you can select the database to which Table A and Table B belong as the object when configuring the data synchronization task.

Procedure

Purchase a data synchronization instance. For more information, see Purchase procedure.

Note On the buy page, set Source Instance to PolarDB, Target Instance to MySQL, and Synchronization Topology to One-Way Synchronization.
Log on to the DTS console.
In the left-side navigation pane, click Data Synchronization.
At the top of the Synchronization Tasks page, select the region where the destination instance resides.
Find the data synchronization instance and click Configure Synchronization Channel in the Actions column.

Configure the source and destination instances.


Section	Parameter	Description
N/A	Synchronization Task Name	DTS automatically generates a task name. We recommend that you use an informative name for easy identification. You do not need to use a unique task name.
Source Instance Details	Instance Type	The value of this parameter is set to PolarDB Instance and cannot be changed.
	Instance Region	The region of the source instance. The region is the same as the source region that you selected when you purchased the data synchronization instance. You cannot change the value of this parameter.
	PolarDB Instance ID	Select the ID of the source PolarDB cluster.
	Database Account	Enter the database account of the source PolarDB cluster.
	Database Password	Enter the password for the source database account.
Destination Instance Details	Instance Type	Select RDS instance.
	Instance Region	The region of the destination instance. The region is the same as the destination region that you selected when you purchased the data synchronization instance. You cannot change the value of this parameter.
	Database Account	Enter the database account of the destination RDS instance.
	Database Password	Enter the password for the destination database account. Note If the database engine of the destination RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to configure the database account or database password.
	Encryption	Select Non-encrypted or SSL-encrypted. If you want to select SSL-encrypted, you must enable SSL encryption for the RDS instance before configuring the data synchronization task. For more information, see Configure SSL encryption for an RDS for MySQL instance. Note The Encryption parameter is available only in mainland China and Hong Kong(China).

In the lower-right corner of the page, click Set Whitelist and Next.

Note The CIDR blocks of DTS servers are automatically added to the whitelists of the source PolarDB cluster and the destination RDS instance. This ensures that DTS servers can connect to the source PolarDB cluster and the destination RDS instance.

Configure the processing mode in existing destination tables and the objects to be synchronized.

Configure the processing mode and objects


Parameter	Description
Processing Mode In Existed Target Table	Pre-check and Intercept: checks whether the destination database contains tables that have the same names as tables in the source database. If the destination database does not contain tables that have the same names as tables in the source database, the precheck is passed. Otherwise, an error is returned during precheck and the data synchronization task cannot be started. Note If tables in the destination database have the same names as tables in the source database, and cannot be deleted or renamed, you can use the object name mapping feature. For more information, see Specify the name of an object in the destination instance. Ignore: skips the precheck for identical table names in the source and destination databases. Warning If you select Ignore, data consistency is not guaranteed and your business may be exposed to potential risks. DTS does not synchronize data records that have the same primary keys as data records in the destination database during initial data synchronization. This occurs if the source and destination databases have the same schema. However, DTS synchronizes these data records during incremental data synchronization. If the source and destination databases have different schemas, initial data synchronization may fail. In this case, only some columns are synchronized or the data synchronization task fails.
Objects	Select objects from the Available section and click the icon to move the objects to the Selected section. You can select tables and databases as the objects to be synchronized. Note If you select a database as the object to be synchronized, all schema changes in the database are synchronized to the destination database. After an object is synchronized to the destination database, the name of the object remains unchanged. You can change the name of an object in the destination PolarDB cluster by using the object name mapping feature. For more information about how to use this feature, see Specify the name of an object in the destination instance.

In the lower-right corner of the page, click Next.
Configure initial synchronization.

Note Initial synchronization includes initial schema synchronization and initial full data synchronization. Select both Initial Schema Synchronization and Initial Full Data Synchronization. Before synchronizing incremental data, DTS synchronizes the schemas and historical data of the required objects from the source database to the destination database.
In the lower-right corner of the page, click Precheck.
Note
- Before you can start the data synchronization task, a precheck is performed. You can start the data synchronization task only after the task passes the precheck.
- If the task fails to pass the precheck, click the icon next to each failed item to view details. Troubleshoot the issues based on the causes and run the precheck again.
Close the Precheck dialog box after the following message is displayed: The precheck is passed.
Wait until the initial synchronization is complete and the data synchronization task is in the Synchronizing state.

On the Synchronization Tasks page, view the status of the data synchronization task.