Synchronize data from PolarDB for PostgreSQL to ApsaraDB for SelectDB - Data Transmission Service

This topic describes how to synchronize data from a PolarDB for PostgreSQL cluster to an ApsaraDB for SelectDB instance by using Data Transmission Service (DTS). In this way, you can analyze massive amounts of data.

Prerequisites

The destination ApsaraDB for SelectDB instance is created. The available storage space of this instance is larger than the total size of data in the source PolarDB for PostgreSQL cluster. For more information, see Create an instance.
The wal_level parameter of the source PolarDB for PostgreSQL cluster is set to logical. For more information, see Specify cluster parameters.

Usage notes

Type	Description
Limits on the source database	Requirements for the objects to be synchronized: The tables to be synchronized must have PRIMARY KEY or UNIQUE NOT NULL indexes: Make sure that all fields must be unique. Otherwise, the destination database may contain duplicate data. The objects to be synchronized must contain tables that do not have PRIMARY KEY or UNIQUE NOT NULL indexes: We recommend that you select Schema Synchronization for the Synchronization Types parameter and duplicate for the Engine parameter in the Configurations for Databases, Tables, and Columns step when you configure an instance. Note During schema synchronization, DTS automatically adds fields to the destination table. For more information, see the Additional columns section of this topic. If the source database has one or more long-running transactions and incremental data is synchronized in the data synchronization task, the write-ahead logging (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. If you want to perform a primary/secondary switchover on the source PolarDB for PostgreSQL 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. The logical subscription from source database has limits on the usage of DTS. If the size of a single data to be synchronized from the source database exceeds 256 MB upon incremental data changes, the data synchronization instance that is running fails to run and cannot be recovered. You need to configure the task again. During schema synchronization and full data synchronization, do not execute DDL statements to change the schemas of databases or tables. Otherwise, the data synchronization task fails.
Other limits	You can synchronize data to only tables that use the Unique Key model or Duplicate key model in ApsaraDB for SelectDB instances. If you synchronize data to the destination table that uses the Duplicate key model, and if one of the following operations occurs, duplicate data may exist in the destination database. In this case, you can deduplicate the data based on additional columns such as _is_deleted, _version, and _record_id. Retry operation occured in a data synchronization instance. Retry operation was executed in a data synchronization instance. Two or more DML operations were executed for the same row of data after a data synchronization instance started. Note When the destination table is the table that uses the Duplicate key model, DTS converts UPDATE or DELETE statements to INSERT statements. You can specify only the `bucket_count` parameter in the Selected Objects section. Note The `bucket_count` parameter value must be a positive integer. Default value: auto. ApsaraDB for SelectDB instances support only database names and table names that start with letters. If the name of a database or table that you want to synchronize does not start with a letter, you must use the object name mapping feature to rename the database or table. If the name of the object that you want to synchronize, such as a database, a table, or a column, contains Chinese characters, you must use the object name mapping feature to rename the object. For example, you can change the name from Chinese to English. Otherwise, the task may fail. A data synchronization instance can synchronize data from only one database. To synchronize data from multiple databases, you must create a data synchronization instance for each database. During initial full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. Before you synchronize data, assess the impact of data synchronization on the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours, such as when the CPU usage of the source and destination databases is lower than 30%. During data synchronization, do not add backend nodes to the ApsaraDB for SelectDB database. Otherwise, the task fails. You can restart the data synchronization instance to resume the data synchronization task that failed. During data synchronization, do not create clusters in the destination ApsaraDB for SelectDB instance. Otherwise, the data synchronization task fails. You can restart the data synchronization instance to resume the data synchronization task that failed. DTS does not check the validity of metadata, such as sequences. You must manually check the validity of metadata. DTS creates the following temporary tables in the source database to obtain the DDL statements of incremental data, the schemas of incremental tables, and the heartbeat information. During data synchronization, do not delete temporary tables in the source database. Otherwise, the data synchronization task fails. After the DTS instance is released, temporary tables are automatically deleted. `public.dts_pg_class`, `public.dts_pg_attribute`, `public.dts_pg_type`, `public.dts_pg_enum`, `public.dts_postgres_heartbeat`, `public.dts_ddl_command`, and `public.dts_args_session`. During data synchronization, DTS creates a replication slot in the source database to replicate data. The replication slot is prefixed with `dts_sync_`. DTS can obtain the incremental logs of the source database within the last 15 minutes by using this replication slot. Note If the data synchronization task is released or fails, DTS automatically deletes the replication slot. If a primary/secondary switchover is performed on the source database, you must log on to the secondary database to manually delete the replication slot. If a DTS task fails to run, DTS technical support will try to restore the task within 8 hours. During the restoration, the task may be restarted, and the parameters of the task may be modified. Note Only the parameters of the task may be modified. The parameters of databases are not modified. The parameters that may be modified include but are not limited to the parameters in the "Modify instance parameters" section of the Modify the parameters of a DTS instance topic.

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 support data synchronization

Operation type	SQL statements
DML	INSERT, UPDATE, DELETE
DDL	ADD COLUMN, DROP COLUMN

Permissions required for database accounts

Database	Required permission	References
Source PolarDB for PostgreSQL cluster	Permissions of a privileged account that is the owner of the database	Create a database account and Database management.
Destination ApsaraDB for SelectDB instance	The USAGE_PRIV permission and the following read and write permissions: SELECT_PRIV, LOAD_PRIV, ALTER_PRIV, CREATE_PRIV, and DROP_PRIV.	Manage the permissions to access clusters and Manage the basic permissions of users.

Procedure

Use one of the following methods to go to the Data Synchronization page and select the region in which the data synchronization instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Synchronization.
3. In the upper-left corner of the page, select the region in which the data synchronization instance resides.
DMS console
Note
The actual operations may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data + AI and choose DTS (DTS) > Data Synchronization.
3. From the drop-down list to the right of Data Synchronization Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.
Optional. Click New Configuration Page in the upper-right corner of the page.
Note
- Skip this step if the Back to Previous Version button is displayed in the upper-right corner of the page.
- Specific parameters in the new and previous versions of the configuration page may be different. We recommend that you use the new version of the configuration page.

Configure the source and destination databases. The following table describes the parameters.

Category	Configuration	Description
N/A	Task Name	The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select Existing Connection	The database that you want to use. You can choose whether to use an existing database based on your business requirements. If you select an existing database, DTS automatically populates the parameters for the database. If you do not select an existing database, you must configure the following database information. Note In the DTS console, register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	The type of the source database. Select PolarDB for PostgreSQL.
	Access Method	The access method of the source database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the source PolarDB for PostgreSQL cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Instance ID	Select the ID of the source PolarDB for PostgreSQL cluster.
	Database Name	The name of the database that stores the data to be synchronized in the source PolarDB for PostgreSQL cluster.
	Database Account	The database account of the source PolarDB for PostgreSQL cluster. For more 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 that is used to access the database.
Destination Database	Select Existing Connection	The database that you want to use. You can choose whether to use an existing database based on your business requirements. If you select an existing database, DTS automatically populates the parameters for the database. If you do not select an existing database, you must configure the following database information. Note In the DTS console, register a database with DTS on the Database Connections page or the new configuration page. For more information, see Manage database connections. In the DMS console, you can select an existing database from the Select a DMS database instance. drop-down list. You can also click Add DMS Database Instance or go back to the homepage of the DMS console to register a database with DMS. For more information, see Register an Alibaba Cloud database instance and Register a database hosted on a third-party cloud service or a self-managed database.
	Database Type	The type of the destination database. Select SelectDB.
	Access Method	The access method of the destination database. Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination ApsaraDB for SelectDB instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database of the current Alibaba Cloud account is used. Select No.
	Instance ID	The ID of the destination ApsaraDB for SelectDB instance cluster.
	Database Account	The database account of the destination ApsaraDB for SelectDB instance. 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 that is used to access the database.

In the lower part of the page, click Test Connectivity and Proceed.
Note
Make sure that the CIDR blocks of DTS servers can be automatically or manually added to the security settings of the source and destination databases to allow access from DTS servers. For more information, see Add the CIDR blocks of DTS servers.

Configure the objects to be synchronized.

In the Configure Objects step, configure the objects that you want to synchronize.

Configuration	Description
Synchronization Types	The synchronization types. 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. Important When you synchronize data from a PolarDB for PostgreSQL cluster to an ApsaraDB for SelectDB instance, data types are converted. If you do not select Schema Synchronization, you must create tables that use the Unique or Duplicate key model with corresponding schemas in the destination ApsaraDB for SelectDB instance in advance. For more information, see the Data type mapping and Additional columns sections of this topic and Data models.
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 If the source and destination databases contain tables with identical names and the tables in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the tables that are synchronized to the destination database. 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 schema and a data record in the destination database has the same primary key value or unique key value as a data record in the source 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. Proceed with caution.
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 ensure 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 schemas as objects to be synchronized.
Selected Objects	To rename an object to be synchronized in the destination database, right-click the object in the Selected Objects section. For more information, see Map object names. To remove a selected object, click the object in the Selected Objects section and then click the icon to move the object to the Source Objects section. If you select Schema Synchronization for the Synchronization Types parameter, you can select only tables as the objects to be synchronized and the `bucket_count` parameter must be set. Right-click a table to be migrated in the Selected Objects section, select Yes for the Enable Parameter Settings parameter in the Parameter Settings section that appears, specify Value based on your business requirements, and click OK. Note 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. To specify WHERE conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Specify filter conditions. To select SQL operations for incremental synchronization, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to synchronize.

Click Next: Advanced Settings to configure advanced settings.

Configuration	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data synchronization tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Retry Time for Failed Connections	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 this 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 specify different retry time ranges for multiple data synchronization tasks that have the same source or destination database, the shortest retry time range 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 your earliest opportunity after the source and destination instances are released.
Retry Time for Other Issues	The retry time range for other issues. For example, if the DDL or DML operations fail to be performed after the data synchronization task is started, DTS immediately retries the operations within the time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set this parameter to a value greater than 10. If the failed operations are successfully performed within the specified time range, DTS resumes the data synchronization task. Otherwise, the data synchronization task fails. Important The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Migration	During full data synchronization, DTS uses the read and write resources of the source and destination databases. This may increase the load on the database servers. You can configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters for full data synchronization tasks to reduce the load on the destination database server. Note This parameter is displayed only if Full Data Synchronization is selected for the Synchronization Types parameter.
Enable Throttling for Incremental Data Synchronization	Specifies whether to enable throttling for incremental data synchronization. You can enable throttling for incremental data synchronization based on your business requirements. To configure throttling, you must configure the RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s) parameters. This reduces the load on the destination database server.
Environment Tag	You can select an environment tag to identify the instance based on your actual situation. In this example, no environment tag is selected.
Configure ETL	Specifies whether to enable 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.
Monitoring and Alerting	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 enable alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see the "Configure monitoring and alerting when you create a DTS task" section of the Configure monitoring and alerting topic.

Optional: In the lower part of the page, click Next: Configure Database and Table Fields. In the dialog box that appears, specify Primary Key Column, Distribution Key, and Engine parameters for the tables that you want to synchronize.
Note
- This step is available only if you select Schema Synchronization for the Synchronization Types parameter. You can set Definition Status to All and modify the database and table fields.
- You can select multiple columns for the Primary Key Column parameter from the drop-down list. One or more columns specified for the Primary Key Column parameter can be selected for the Distribution Key parameter.
- If the table does not have primary keys or UNIQUE constraints, select duplicate for the Engine parameter. Otherwise, the data synchronization task may fail or data loss may occur.

Save the task settings and run a precheck.
- To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.
- If you do not need to view or have viewed the parameters, click Next: Save Task Settings and Precheck in the lower part of the page.
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 data synchronization task fails the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, rerun the precheck.
- If an alert is triggered for an item during the precheck:
  If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issue. Then, run a precheck again.
  If an alert item can be ignored, click Confirm Alert Details. 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.

Purchase an instance.

Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.

On the buy 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 a subscription when you create a data synchronization 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 billed 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 data synchronization instance, you can release the instance to reduce costs.
	Resource Group Settings	The resource group to which the data synchronization instance belongs. Default value: default resource group. For more information, see What is Resource Management?
	Instance Class	DTS provides instance classes that vary in synchronization speed. You can select an instance class based on your business requirements. For more information, see Instance classes of data synchronization instances.
	Subscription Duration	If you select the subscription billing method, specify the subscription duration and the number of data synchronization instances that you want to create. The subscription duration can be one to nine months, one year, two years, three years, or five years. Note This parameter is available only if you select the Subscription billing method.

Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the dialog box that appears, click OK.
You can view the progress of the task in the task list.

Data type mapping

Category	Data types in a PolarDB for PostgreSQL cluster	Data types in an ApsaraDB for SelectDB instance
NUMERIC	SMALLINT	SMALLINT
	INTEGER	INT
	BIGINT	BIGINT
	DECIMAL	DECIMAL
	NUMERIC	DECIMAL
	REAL	DOUBLE
	DOUBLE	DOUBLE
	SMALLSERIAL	SMALLINT
	SERIAL	INT
	BIGSERIAL	BIGINT
MONETARY	MONEY	STRING
CHARACTER	CHAR(n) VARCHAR(n)	VARCHAR Important To avoid data loss, data of the CHAR and VARCHAR(n) types is converted to VARCHAR(4*n) after the data is synchronized to the destination SelectDB instance. If you do not specify the length of data, the default value VARCHAR(65533) is used. If the data exceeds 65533 characters in length, the data is converted to strings after it is synchronized to SelectDB instance.
CHARACTER	TEXT	STRING
BINARY	BYTEA	STRING
DATE AND TIME	TIMESTAMP [(P)] [WITHOUT TIME ZONE]	DATETIMEV2
	TIMESTAMP [(P)] WITH TIME ZONE	DATETIMEV2
	DATE	DATEV2
	TIME [(P)] [WITHOUT TIME ZONE]	VARCHAR(50)
	TIME [(P)] WITH TIME ZONE	VARCHAR(50)
	INTERVAL [FIELDS] [(P)]	STRING
BOOLEAN	BOOLEAN	BOOLEAN
GEOMETRIC	POINT LINE LSEG BOX PATH POLYGON CIRCLE	STRING
NETWORK ADDRESS	CIDR INET MACADDR MACADDR8	STRING
TEXT SEARCH	TSVECTOR	STRING
XML	XML	STRING
JSON	JSON	JSON

Additional columns

Note

The following table provides additional columns that are automatically created by DTS or you manually add in the destination tables in the Duplicate key model.

Column	Type	Default value	Description
_is_deleted	Int	0	Specify whether the data is deleted. Insert: The data record is inserted. Default value: 0. Update: The data record is updated. Default value: 0. Delete: The data record is deleted. Default value: 1.
_version	Bigint	0	Full data synchronization: Default value: 0. Incremental data synchronization: The timestamp (seconds) of the source binary log file.
_record_id	Bigint	0	Full data synchronization: Default value: 0. Incremental data synchronization: The unique ID of the incremental log entry. Note The ID auto-increments for each new log entry.