Synchronize data from a Db2 for LUW database to a PolarDB for MySQL cluster - Data Transmission Service

This topic describes how to synchronize data from a Db2 for LUW database to a PolarDB for MySQL cluster by using Data Transmission Service (DTS).

Prerequisites

The source Db2 for LUW database and the destination PolarDB for MySQL cluster are created. For more information about the supported database versions, see Overview of data synchronization scenarios.
The available storage space of the destination database is larger than the total size of the data in the source database.

Usage notes

Note

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 database	Bandwidth requirements: The server to which the source database belongs must have sufficient outbound bandwidth. Otherwise, the data synchronization speed is affected. The tables to be migrated must have PRIMARY KEY or UNIQUE constraints and all fields must be unique. Otherwise, the destination database may contain duplicate data records. If you select tables as the objects to be synchronized and you need to modify the tables in the destination database, such as renaming tables or columns, you can synchronize up to 5,000 tables in a single data synchronization task. If you run a task to synchronize more than 5,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to synchronize the tables in batches or configure a task to synchronize the entire database. The data logging feature must be enabled. Otherwise, error messages are returned during precheck, and the data synchronization task cannot be started. Note If you perform only incremental data synchronization, the data logs of the source database must be stored for more than 24 hours. If you perform both full data synchronization and incremental data synchronization, the data logs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the data logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After the full data synchronization is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of data logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the SLA of DTS cannot be guaranteed.
Other limits	Schema synchronization is not supported. Before you configure a data synchronization task, you must create databases and tables in the destination instance based on the databases and tables that you want to synchronize. DTS synchronizes incremental data from a Db2 for LUW database to the destination database based on the CDC replication technology of Db2 for LUW. However, the CDC replication technology has its own limits. For more information, see General data restrictions for SQL Replication. Before you synchronize data, evaluate the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. During full data synchronization, DTS uses read and write resources of the source and destination databases. This may cause the loads of the database servers to increase. During initial full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. Therefore, after initial full data synchronization is complete, the size of the used tablespace of the destination database is larger than that of the source database. During data synchronization, we recommend that you use only DTS to write data to the destination database. This prevents data inconsistency between the source and destination databases. You can use Data Management (DMS) to perform online DDL operations after data synchronization is complete. For more information, see Perform lock-free DDL operations.
Special cases	You must take note of the following items because the source Db2 for LUW database is a self-managed database: If you perform a primary/secondary switchover on the source database when the data synchronization task is running, the task fails. DTS calculates synchronization latency based on the timestamp of the latest synchronized data in the destination database and the current timestamp in the source database. If no DML operation is performed on the source database for a long time, the synchronization latency may be inaccurate. If the latency of the synchronization task is too high, you can perform a DML operation on the source database to update the latency. Note If you select an entire database as the object to be synchronized, you can create a heartbeat table. The heartbeat table is updated or receives data every second.

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.

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 the synchronization topologies supported by DTS, see Synchronization topologies.

SQL operations that can be synchronized


Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE

Preparations

Create database accounts and grant permissions to the accounts


Database	Required permission	References
Db2 for LUW	The database administrator permissions (DBADM authority)	Creating group and user IDs for a Db2 database installation (Linux and UNIX) and Authorities overview
Destination PolarDB for MySQL cluster	Read and write permissions on the destination database to which the objects are synchronized	Create a database account

For more information about how to enable log archiving for a Db2 for LUW database, see logarchmeth1 - Primary log archive method configuration parameter and logarchmeth2 - Secondary log archive method configuration parameter.

Procedure

Go to the Data Synchronization Tasks page.
1. Log on to the Data Management (DMS) console.
2. In the top navigation bar, click DTS.
3. In the left-side navigation pane, choose DTS (DTS) > Data Synchronization.
Note
- Operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Configure the DMS console based on your business requirements.
- You can also go to the Data Synchronization Tasks page of the new DTS console.
From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Note If you use the new DTS console, you must select the region in which the data synchronization instance resides in the top navigation bar.

Click Create Task. On the page that appears, configure the source and destination databases.


Section	Parameter	Description
N/A	Task Name	DTS automatically generates a task name. We recommend that you specify an informative name to identify the task. You do not need to use a unique task name.
Source Database	Database Type	The type of the source database. Select DB2 for LUW.
	Access Method	The access method of the source database. Select Express Connect, VPN Gateway, or Smart Access Gateway.
	Instance Region	The region in which the source Db2 for LUW database resides.
	Replicate Data Across Alibaba Cloud Accounts	Specifies whether to synchronize data across Alibaba Cloud accounts. In this example, No is selected.
	Connected VPC	The ID of the virtual private cloud (VPC) that is connected to the Db2 for LUW database.
	IP Address	The server IP address of the Db2 for LUW database.
	Port Number	The service port number of the source Db2 for LUW database. Default value: 50000.
	Database Name	The name of the source Db2 for LUW database to which the objects to be synchronized belong.
	Database Account	The account of the source Db2 for LUW database. For more information about the permissions that are required for the account, see Preparations.
	Database Password	The password of the database account.
Destination Database	Database Type	The type of the destination database. Select PolarDB for MySQL.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination PolarDB for MySQL cluster resides.
	PolarDB Cluster ID	The ID of the destination PolarDB for MySQL cluster.
	Database Account	The database account of the destination PolarDB-X 2.0 instance. For more information about the permissions that are required for the account, see Preparations.
	Database Password	The password of the database account.

In the lower part of the page, click Test Connectivity and Proceed.
Warning
- If the source or destination database is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB, DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the instance. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. 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 need to manually add the CIDR blocks of DTS servers to the whitelist of the self-managed database on the ECS instance to allow DTS to 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.
- If the CIDR blocks of DTS servers are automatically or manually added to the whitelist or 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.
- After the DTS task is complete or released, we recommend that you manually remove the CIDR blocks of DTS servers from the whitelist or ECS security group rules. You must remove the IP address whitelist group whose name contains dts from the whitelist of the Alibaba Cloud database instance or the security group rules of the ECS instance. For more information about the CIDR blocks that you must remove from the whitelist of the self-managed databases that are deployed in data centers or databases that are hosted on third-party cloud services, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.

Select objects for the task and configure advanced settings.

Basic Settings


Parameter or setting	Description
Task Stages	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 the selected objects from the source database to the destination cluster. 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. Note You 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. Warning If 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. In this case, only some columns are synchronized or the data synchronization task fails.
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 make sure 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.
Source Objects	Select one or more objects from the Source Objects section and click the icon to add the objects to the Selected Objects section. Note You 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.
Selected Objects	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.

Advanced Settings


Parameter or setting	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. If you select Yes, you must also specify the alert threshold and alert contacts. For more information, see Configure monitoring and alerting when you create a DTS task.
Select the engine type of the destination database	Select the engine type of the destination database. InnoDB: the default storage engine. X-Engine: an online transaction processing (OLTP) database storage engine.
Retry Time for Failed Connection	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. Note If 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 the earliest opportunity after the source and destination instances are released.
Configure ETL	Specifies whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?. Valid values: Yes: configures the ETL feature. You can enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: does not configure the ETL feature.

In the lower part of the page, click Next: Save Task Settings and Precheck.
Note
- Before 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 analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
- If an alert is generated for an item during the precheck, perform the following operations based on the scenario:
  
  In scenarios where you cannot ignore the alert item, click View Details next to the failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
  
  In scenarios where you can ignore the alert item, click Confirm Alert Details next to the failed item. 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.
	Instance Class	DTS provides several instance classes that have different performance in synchronization speed. You can select an instance class based on your business scenario. For more information, see Specifications of data synchronization instances.
	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 or one to three years. Note This parameter is displayed only if you select the subscription billing method.

Read and select the check box for 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.