Data synchronization between AnalyticDB for PostgreSQL instances - Data Transmission Service

This topic describes how to use Data Transmission Service (DTS) to synchronize data between AnalyticDB for PostgreSQL instances.

Important

This feature is in invitational preview and is available only to select users.

Prerequisites

You have created a destination AnalyticDB for PostgreSQL instance. The storage space of the destination instance must be larger than the data volume to be synchronized from the source AnalyticDB for PostgreSQL instance.
You have created a database in the destination AnalyticDB for PostgreSQL instance to receive the synchronized data.

Precautions

Source database limits

Bandwidth: The server that hosts the source database must have at least 100 Mbit/s of outbound bandwidth. Otherwise, the data synchronization speed will be affected.
The kernel version of the source AnalyticDB for PostgreSQL instance must be 7.2.1.4 or later.
Parameter settings for the source database:
- To enable logical parsing, the wal_level parameter must be set to logical.
- If the Instance Edition of the source AnalyticDB for PostgreSQL instance is High-availability Edition, set the hot_standby, hot_standby_feedback, and sync_replication_slots parameters to on. This prevents the logical subscription from being interrupted by a primary/secondary switchover.
Synchronization object requirements:
- The name of the database to be synchronized cannot contain hyphens (-), for example, dts-testdata.
- The tables to be synchronized must have a primary key or a UNIQUE constraint. The fields in the key or constraint must be unique. Otherwise, duplicate data may be written to the destination database.
- DTS does not support synchronizing tables with cross-schema inheritance, temporary tables, internal triggers, some functions (C-language functions and internal functions for PROCEDURE and FUNCTION), or extensions. DTS supports synchronizing some custom data types (COMPOSITE, ENUM, or RANGE) and constraints such as primary key, UNIQUE, and CHECK constraints.
- DTS does not support synchronizing the schema of partitioned tables. If a table to be synchronized is a partitioned table, its partition information is lost when the schema is synchronized. By default, the table is created as a non-partitioned table in the destination database.
- If you synchronize objects at the table level and need to edit them, such as by mapping table or column names, you must split the tables into multiple tasks if the number of tables exceeds 5,000. Alternatively, you can configure a task to synchronize the entire database. Otherwise, a request error may occur after you submit the task.
During schema synchronization and full data synchronization, do not run DDL statements to change the schema of a database or table. Otherwise, the data synchronization task fails.
If the source database has long-running transactions and the instance includes an incremental synchronization task, the Write-Ahead Logging (WAL) logs that are generated before the long-running transactions are committed cannot be cleared and may accumulate. This may cause insufficient disk space in the source database.

Other limits

A single data synchronization task can synchronize data from only one database. To synchronize data from multiple databases, you must configure a separate data synchronization task for each database.
DTS does not support synchronizing DDL operations from the source database. After a DTS task starts, new tables created in the source database and their subsequent data changes are not synchronized to the destination database. If needed, you must create a new synchronization task.
After the schema is synchronized to the destination database, the owner of the schema is the database account used for the task.
By default, the distribution key is retained during schema synchronization. If a source table has a primary key, the primary key column is retained when the table schema is synchronized. If a source table does not have a primary key, the distribution key is used as the primary key column by default when the table schema is synchronized.
If a table to be synchronized contains a field of the SERIAL type, the source database automatically creates a Sequence for that field. Therefore, when you configure Source Objects, if you select Schema Synchronization for the Synchronization Types, we recommend that you also select Sequence or synchronize the entire schema. Otherwise, the synchronization instance may fail.
In the following three scenarios, you must run the ALTER TABLE schema.table REPLICA IDENTITY FULL; command on the table to be synchronized in the source database before you write data to the table. This ensures data consistency. During the execution of this command, we recommend that you do not perform table locking operations. Otherwise, the table may be locked. If you skip the related check item in the precheck, DTS automatically runs this command when the instance is initialized.
- When the instance runs for the first time.
- When you synchronize data at the schema level, and a new table is created in the schema to be synchronized or a table to be synchronized is rebuilt using the RENAME command.
- When you use the feature of modifying synchronization objects.
Note
- In the command, replace schema and table with the schema name and table name of the data to be synchronized.
- We recommend that you perform this operation during off-peak hours.
DTS verifies data content. DTS does not verify metadata such as sequences. You must verify the metadata on your own.
After you switch your business to the destination database, the initial value of a new sequence does not increment from the maximum value of the corresponding sequence in the source database. Before the switchover, you must query the maximum value of the sequence in the source database and set it as the initial value for the corresponding sequence in the destination database. You can use the following command to query the sequence values in the source database:
```
do language plpgsql $$
declare
  nsp name;
  rel name;
  val int8;
begin
  for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S'
  loop
    execute format($_$select last_value from %I.%I$_$, nsp, rel) into val;
    raise notice '%',
    format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1);
  end loop;
end;
$$;
```
Note
The SQL statements that are returned by the preceding command include all sequences in the source database. You can run the statements in the destination database as needed.
To ensure the accuracy of the displayed synchronization latency, DTS adds a heartbeat table named dts_postgres_heartbeat to the source database.
DTS creates the following temporary tables in the source database to obtain the DDL statements of incremental data, the schemas of incremental tables, and heartbeat information. Do not delete these temporary tables from the source database during data synchronization. Otherwise, the DTS task may become abnormal. The temporary tables are automatically deleted after the DTS instance is released.
public.dts_pg_class, public.dts_pg_attribute, public.dts_pg_type, public.dts_pg_enum, public.dts_postgres_heartbeat, public.dts_ddl_command, public.dts_args_session, and public.aliyun_dts_instance.
During data synchronization, DTS creates a replication slot with the dts_sync_ prefix in the source database to replicate data. DTS can obtain the incremental logs of the source database within 15 minutes from this replication slot.
Note
- DTS automatically deletes the replication slot after the instance is released. If you change the password of the database account or delete the IP address of the DTS server from the whitelist of the source database during data synchronization, the replication slot cannot be automatically deleted. In this case, you must manually delete the replication slot from the source database to prevent it from occupying disk space and making the ApsaraDB RDS for PostgreSQL instance unavailable.
- When the synchronization task is released or fails, DTS automatically clears the replication slot. If a primary/secondary failover occurs on the ApsaraDB RDS for PostgreSQL instance, you must log on to the secondary database to manually clear the replication slot.
During initial full data synchronization, DTS consumes some read and write resources of the source and destination databases. This may increase the database load. Before you start data synchronization, evaluate the performance of the source and destination databases. You can perform data synchronization during off-peak hours. For example, you can perform the synchronization when the CPU load of both databases is below 30%.
Because initial full data synchronization performs concurrent INSERT operations, fragmentation occurs in the tables of the destination database. As a result, the storage space occupied by the tables in the destination database is larger than that in the source database.
While the synchronization instance is running:
- Do not change the endpoint or zone of the AnalyticDB for PostgreSQL instance. Otherwise, the synchronization instance will fail.
- If a data source other than DTS writes data to the destination database, data inconsistency may occur between the source and destination databases. This can even cause the synchronization instance to fail.
If the instance fails, DTS helpdesk will try to recover the instance within 8 hours. During the recovery process, the instance may be restarted and its parameters may be adjusted.
Note
When parameters are adjusted, only the parameters of the DTS instance are modified, not the parameters of the database. The parameters that can be modified include but are not limited to those described in Modify instance parameters.

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 objects

SCHEMA, TABLE
Note
Includes PRIMARY KEY, UNIQUE KEY, DATATYPE (built-in data types), and DEFAULT CONSTRAINT.
VIEW, INDEX, PROCEDURE, FUNCTION, RULE, SEQUENCE, AGGREGATE, OPERATOR, DOMAIN

Supported incremental SQL operations

Operation type	SQL operations
DML	INSERT, UPDATE, DELETE

Permissions for database accounts

Database	Required permissions	Account creation and authorization method
Source AnalyticDB for PostgreSQL instance	Read permissions on the objects to be synchronized and the `REPLICATION` permission. `ALTER USER your_user WITH REPLICATION;`.	Create and manage users Note You can grant the `REPLICATION` permission by running the `ALTER USER <username> WITH REPLICATION;` command.
Destination AnalyticDB for PostgreSQL instance	Read and write permissions on the destination database.	Create and manage users Note You can use the initial account or an account with the RDS_SUPERUSER permission.

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 task 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.

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

Category	Configuration	Description
None	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	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select AnalyticDB for PostgreSQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source AnalyticDB for PostgreSQL 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	Select the ID of the source AnalyticDB for PostgreSQL instance.
	Database Name	Enter the name of the database that contains the data to be synchronized in the source AnalyticDB for PostgreSQL instance.
	Database Account	Enter the database account of the source AnalyticDB for PostgreSQL instance.
	Database Password	The password that is used to access the database.
Destination Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select AnalyticDB for PostgreSQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination AnalyticDB for PostgreSQL instance resides.
	Instance ID	Select the ID of the destination AnalyticDB for PostgreSQL instance.
	Database Name	Enter the name of the database that is used to receive the synchronized data in the destination AnalyticDB for PostgreSQL instance.
	Database Account	Enter the database account of the destination AnalyticDB for PostgreSQL instance.
	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 DTS server IP addresses to a whitelist.

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.
DDL and DML Operations to Be Synchronized	Select the incremental SQL operations at the instance level as required. Note To select SQL operations for incremental synchronization at the schema or table level, right-click a synchronization object in the Selected Objects list and select the desired operations from the shortcut menu.
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 instance fails. Proceed with caution.
Storage Engine Type	Select the storage engine for the destination tables. The default value is Beam. Note This configuration item is available only if the target AnalyticDB for PostgreSQL instance uses kernel version v7.0.6.6 or later, and the Synchronization Types is set to Schema Synchronization.
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 objects to synchronize at the schema or table level.
Selected Objects	To configure the name of an object to be synchronized in the destination database or specify an object that receives data 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. 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 instances, 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 Synchronization	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 You can configure this parameter 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 as needed. This is not required for this example.
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 instance. 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: After you complete the previous configurations, click Next: Configure Database and Table Fields to configure the Type, Primary Key Column, and Distribution Key of the destination table in AnalyticDB for PostgreSQL.
Note
- This step is available only when you set the Synchronization Types to Schema Synchronization. You can then set Definition Status to All to make modifications.
- The Primary Key Column can be a composite primary key. You must select one or more columns from the Primary Key Column to serve as the Distribution Key. For more information, see Data Table Management and Table Distribution Definition.

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 the 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 task. 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.