Synchronize data from a PolarDB for PostgreSQL (compatible with Oracle) cluster to an AnalyticDB for MySQL V3.0 cluster - Data Transmission Service

This topic describes how to synchronize data from a PolarDB for PostgreSQL(Compatible with Oracle) cluster to an AnalyticDB for MySQL V3.0 cluster by using Data Transmission Service (DTS).

Prerequisites

The destination AnalyticDB for MySQL V3.0 cluster is created. For more information, see Create a cluster.
The value of the wal_level parameter is set to logical for the source PolarDB for PostgreSQL(Compatible with Oracle) cluster. This setting ensures that logical encoding is supported in write-ahead logging (WAL). For more information, see Configure cluster parameters.
The available storage space of the destination AnalyticDB for MySQL V3.0 cluster is larger than the total size of the data in the source PolarDB for PostgreSQL(Compatible with Oracle) cluster.

Limits

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 synchronized 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, up to 1,000 tables can be synchronized in a single data synchronization task. If you run a task to synchronize more than 1,000 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 WAL feature must be enabled. If you perform only incremental data synchronization, the WAL 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 WAL logs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the WAL logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After 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 WAL logs based on the preceding requirements. Otherwise, the Service Level Agreement (SLA) of DTS does not guarantee service reliability or performance. If one or more long-running transactions exist in the source database and incremental data is synchronized in the data synchronization task, the WAL logs generated before the long-running transactions in the source database are committed may be accumulated. As a result, the disk space of the source database may be insufficient. Limits on operations to be performed on the source database: During schema synchronization and full data synchronization, do not perform DDL operations to change the schemas of databases or tables. Otherwise, the data synchronization task fails. If you perform only full data synchronization, do not write data to the source database during data synchronization. Otherwise, data inconsistency may occur between the source and destination databases. To ensure data consistency, we recommend that you select schema synchronization, full data synchronization, and incremental data synchronization as the synchronization types. If you need to perform a primary/secondary switchover on the source PolarDB for PostgreSQL(Compatible with Oracle) cluster, the logical replication slot failover feature must be enabled. This prevents logical subscriptions from being interrupted and ensures that your data synchronization task can run as expected. For more information, see Logical replication slot failover.
Other limits	A single data synchronization task can synchronize data from only one database. To synchronize data from multiple databases, you must create a data synchronization task for each database. During incremental data synchronization, if you select a schema as the object to be synchronized and create a table in the schema or execute the RENAME statement to rename a table in the schema, you must execute the `ALTER TABLE schema.table REPLICA IDENTITY FULL;` statement before you write data to the table. Note Replace the `schema` and `table` variables in the preceding statement with your schema name and table name. To ensure that the latency of incremental data synchronization is accurate, DTS adds a heartbeat table named `dts_postgres_heartbeat` to the source database. The following figure shows the schema of the heartbeat table. 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. For example, you can synchronize data when the CPU utilization of the source and destination databases is less than 30%. During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads on the database servers. During full data synchronization, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data synchronization is complete, the tablespace of the destination database is larger than that of the source database. DTS uses the `ROUND(COLUMN,PRECISION)` function to retrieve values from columns of the FLOAT or DOUBLE data type. If you do not specify a precision, DTS sets the precision for columns of the FLOAT data type to 38 digits and the precision for columns of the DOUBLE data type to 308 digits by default. You must check whether the precision settings meet your business requirements. DTS attempts to resume data synchronization tasks that failed within the last seven days. Before you switch workloads to the destination database, stop or release the failed tasks. You can also execute the `REVOKE` statement to revoke the write permissions from the accounts that are used by DTS to access the destination database. Otherwise, the data in the source database overwrites the data in the destination database after a failed task is resumed.

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.

SQL operations that can be synchronized


Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE Note When you write data to an AnalyticDB for MySQL V3.0 cluster, the UPDATE statement is automatically converted to the REPLACE INTO statement.

Permissions required for database accounts


Database	Required permission	References
Source PolarDB for PostgreSQL(Compatible with Oracle) cluster	Permissions of a privileged account	Create database accounts
Destination AnalyticDB for MySQL V3.0 cluster	Read and write permissions on the destination cluster to which the objects are to be synchronized	Create a database account

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	Select Instance	Select whether to use an existing instance. If you select an existing instance, DTS automatically applies the parameter settings of the instance. You do not need to configure the corresponding parameters again. If you do not use an existing instance, you must configure parameters for the source database.
	Database Type	The type of the source database. Select PolarDB (Compatible with Oracle).
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source PolarDB for PostgreSQL(Compatible with Oracle) cluster resides.
	Instance ID	The ID of the source PolarDB for PostgreSQL(Compatible with Oracle) cluster.
	Database Name	The name of the source database in the PolarDB for PostgreSQL(Compatible with Oracle) cluster.
	Database Account	The database account of the source PolarDB for PostgreSQL(Compatible with Oracle) cluster. For information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
	Database Password	The password of the database account.
Destination Database	Select Instance	Select whether to use an existing instance. If you select an existing instance, DTS automatically applies the parameter settings of the instance. You do not need to configure the corresponding parameters again. If you do not use an existing instance, you must configure parameters for the source database.
	Database Type	The type of the destination database. Select AnalyticDB for MySQL 3.0.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination AnalyticDB for MySQL V3.0 cluster resides.
	Instance ID	The ID of the destination AnalyticDB for MySQL V3.0 cluster.
	Database Account	The database account of the destination AnalyticDB for MySQL V3.0 cluster. For information about the permissions that are required for the account, see the Permissions required for database accounts section of this topic.
	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.


Parameter	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.
DDL and DML Operations to Be Synchronized	The SQL operations that you want to synchronize during incremental data synchronization at the instance level. For more information, see the SQL operations that can be synchronized section of this topic. Note To select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize.
Merge Tables	Yes: In online transaction processing (OLTP) scenarios, sharding is implemented to speed up the response to business tables. In online analytical processing (OLAP) scenarios, you can store a large volume of data in a single table. This makes your SQL queries more efficient. You can merge multiple source tables that have the same schema into a single destination table. This feature allows you to synchronize data from multiple tables in the source database to a single table in the destination database. For more information, see Enable the multi-table merging feature. Note After you select multiple tables from the source database, you must change the names of these tables to the name of the destination table. To do this, you can use the object name mapping feature. For more information, see Map object names. DTS adds a column named `__dts_data_source` to the destination table. This column is used to record the data source. The data type of this column is TEXT. DTS specifies the column values based on the following format: `<DTS instance ID>:<Database name>:<Schema name>:<Table name>`. Such column values allow DTS to identify each source table. For example, `dts******:dtstestdata:testschema:customer1` indicates that the source table is customer1. If you set this parameter to Yes, all the selected source tables in the task are merged into a single destination table. If you do not need to merge specific source tables, you can create a separate data synchronization task for these tables. Warning** We recommend that you do not perform DDL operations to change the schemas of source databases or tables. Otherwise, data inconsistency may occur or the data synchronization task fails. No: This is the default value.
Capitalization of Object Names in Destination Instance	The capitalization of database names, table names, and column names in the destination cluster. 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. If you select a database as the object to be synchronized, DTS synchronizes data based on the following default rules: If the table to be synchronized in the source database has a primary key, such as a single-column primary key or a composite primary key, the primary key columns are specified as the distribution keys. If the table to be synchronized in the source database does not have a primary key, an auto-increment primary key column is automatically generated in the destination table. This may cause data inconsistency between the source and destination databases.
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. Note To select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize. For more information, see the SQL operations that can be synchronized section of this topic. To specify WHERE conditions to filter data, right-click an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Use SQL conditions to filter data. If you use the object name mapping feature to rename an object, other objects that are dependent on the object may fail to be synchronized.

Click Next: Advanced Settings to configure 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. 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.
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: Configure Database and Table Fields. On the page that appears, set the primary key columns and distribution key columns of the tables that you want to synchronize to the destination AnalyticDB for MySQL cluster.
Note
- If you select Schema Synchronization as Task Stages, you must configure the Type, Primary Key Column, and Distribution Key parameters for the tables that you want to synchronize to the AnalyticDB for MySQL V3.0 cluster. For more information, see CREATE TABLE.
- In the Primary Key Column field, you can specify multiple columns to form a composite primary key. In this case, you must specify one or more primary key columns as the distribution keys.
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.