Data synchronization between AnalyticDB for PostgreSQL instances-Data Transmission Service(DTS)-阿里云帮助中心

Prerequisites

A destination AnalyticDB for PostgreSQL instance has been created with enough storage to accommodate the data from the source AnalyticDB for PostgreSQL instance.
A database has been created in the destination AnalyticDB for PostgreSQL instance to receive the synchronized data.

Considerations

Source database limitations

Bandwidth requirement: The source database server must have at least 100 Mb/s of outbound bandwidth. Otherwise, data synchronization may be slow.
The source AnalyticDB for PostgreSQL instance must run kernel version 7.2.1.4 or later.
Source database parameter settings:
- Enable logical decoding by setting the wal_level parameter to logical.
- If the instance series of the source AnalyticDB for PostgreSQL instance is High-availability Edition, you must also set the hot_standby, hot_standby_feedback, and sync_replication_slots parameters to on. This ensures the synchronization task runs smoothly and prevents the logical subscription from being interrupted by a failover.
Synchronization object requirements:
- The name of the database to be synchronized cannot contain hyphens (-), such as dts-testdata.
- Each table to be synchronized must have a primary key or a unique constraint to prevent data duplication in the destination database.
- DTS does not support synchronizing the following objects: tables with cross-schema inheritance, temporary tables, internal system triggers (TRIGGER), extensions (EXTENSION), C-language functions, and internal functions (PROCEDURE and FUNCTION). DTS supports synchronizing some custom data types (COMPOSITE, ENUM, or RANGE) and the following constraints: primary key, unique, and CHECK.
- DTS cannot synchronize the structure of partitioned tables. If you synchronize a partitioned table, its partition information is lost during schema synchronization. By default, DTS creates it as a non-partitioned table in the destination.
- If you synchronize at the table level and need to edit objects (for example, mapping table or column names), split the task if it involves more than 5,000 tables. Alternatively, you can configure a task to synchronize the entire database. Otherwise, an error may occur when you submit the task.
Do not run DDL operations that change database or table schemas during schema synchronization or full synchronization. Otherwise, the synchronization task fails.

Note
During full synchronization, DTS queries the source database. This creates metadata locks that may block DDL operations on the source database.
If the source database has long-running transactions and the instance includes an incremental synchronization task, the write-ahead logs (WALs) generated before the long-running transactions are committed cannot be cleared and may accumulate. This can cause the disk space of the source database to become insufficient.

Other limitations

A single data synchronization task can synchronize data from only one database. To synchronize multiple databases, you must create a separate task for each.
DTS does not support synchronizing DDL operations from the source database. After a task starts, any new tables and their data changes are not synchronized. To include them, you must create a new synchronization task.
After schema synchronization, the destination database account used for the task becomes the owner of the synchronized objects.
By default, the distribution key is retained during schema synchronization. If a source table has a primary key, the destination table is created with the same primary key. If the source table has no primary key, DTS uses the distribution key as the primary key for the destination table.
If a table to be synchronized contains a SERIAL field, the source database automatically creates a Sequence for the field. Therefore, when you configure Source Objects, if you select Schema Synchronization for 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 tables to be synchronized before you write data to them. This ensures data consistency. Do not perform table locking operations during the execution of this command. Otherwise, the tables may be locked. If you skip the related check items in the precheck, DTS automatically runs this command during the initialization of the instance.
- When the instance runs for the first time.
- When the synchronization granularity is schema, 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 Modify Objects feature.
Note
- In the command, replace schema and table with the names of the schema and table to which the data to be synchronized belongs.
- Perform this operation during off-peak hours.
DTS validates data content but does not validate metadata such as sequences. You must validate the metadata yourself.
Before the switchover, query the maximum value of each sequence in the source database and set it as the starting value for the corresponding sequence in the destination database. The following command can be used 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 in the output of the preceding command contain all sequences from the source database. Run these statements on the destination database based on your business requirements.
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 structure of incremental tables, and heartbeat information. During synchronization, do not delete these temporary tables. Otherwise, the DTS task becomes 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 prefix dts_sync_ in the source database to replicate data. DTS uses this replication slot to obtain incremental logs from the source database within 15 minutes. When the data synchronization fails or the synchronization instance is released, DTS attempts to automatically clear this replication slot.
Note
- If you change the password of the database account used by the task or delete the DTS IP address whitelist from the source database during data synchronization, the replication slot cannot be automatically cleared. In this case, you must manually clear the replication slot in the source database to prevent it from accumulating and occupying disk space, which can make the source database unavailable.
- If a failover occurs on the source database, you must log on to the secondary database to manually clear the replication slot.
During full data initialization, DTS consumes read and write resources on both the source and destination databases, which may increase the database load. Before you start data synchronization, we recommend that you evaluate the performance of both databases and perform the synchronization during off-peak hours, for example, when the CPU load of both databases is below 30%.
Full initialization involves concurrent INSERT operations, which can cause table fragmentation in the destination database. Consequently, after full initialization, tables in the destination database may use more storage space than in the source database.
While the synchronization instance is running:
- Do not change the connection endpoint or Availability Zone of the AnalyticDB for PostgreSQL instance. Otherwise, the synchronization task may fail.
- If a data source other than DTS writes data to the destination database, data inconsistency may occur, and the synchronization task may even fail.
If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters.

Note
Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.

Billing

Synchronization type	Pricing
Schema synchronization and full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

Supported synchronization 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 SQL operations for incremental synchronization

Operation type	SQL operation
DML	INSERT, UPDATE, DELETE

Database account permissions

Database	Required permissions	Creation and authorization
Source AnalyticDB for PostgreSQL instance	Read permissions on the objects to be synchronized and the `REPLICATION` privilege. `ALTER USER your_user WITH REPLICATION;`.	Create and manage users Note You can grant the `REPLICATION` privilege 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 that has the `RDS_SUPERUSER` privilege.

Procedure

Go to the data synchronization task list page in the destination region. You can do this in one of two ways.
DTS console
1. Log on to the DTS console.
2. In the navigation pane on the left, click Data Synchronization.
3. In the upper-left corner of the page, select the region where the synchronization instance is located.
DMS console
Note
The actual steps may vary depending on the mode and layout of the DMS console. For more information, see Simple mode console and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top menu bar, choose Data + AI > DTS (DTS) > Data Synchronization.
3. To the right of Data Synchronization Tasks, select the region of the synchronization instance.
Click Create Task to open the task configuration page.

Configure the source and destination databases.

Category	Setting	Description
N/A	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select AnalyticDB for PostgreSQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region of the source AnalyticDB for PostgreSQL instance.
	Replicate Data Across Alibaba Cloud Accounts	For this example, select No, as the database instance belongs to the current Alibaba Cloud account.
	Instance ID	Select the ID of the source AnalyticDB for PostgreSQL instance.
	Database Name	Enter the name of the database in the source AnalyticDB for PostgreSQL instance that contains the data you want to synchronize.
	Database Account	Enter the database account for the source AnalyticDB for PostgreSQL instance.
	Database Password	Enter the password for the specified database account.
Destination Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select AnalyticDB for PostgreSQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region of the destination AnalyticDB for PostgreSQL instance.
	Instance ID	Select the ID of the destination AnalyticDB for PostgreSQL instance.
	Database Name	Enter the name of the database in the destination AnalyticDB for PostgreSQL instance that will receive the synchronized data.
	Database Account	Enter the database account for the destination AnalyticDB for PostgreSQL instance.
	Database Password	Enter the password for the specified database account.

After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
Ensure that the IP address blocks of the DTS service are added to the security settings of the source and destination databases, either automatically or manually, to allow access from DTS servers. For more information, see Add the IP address whitelist of DTS servers.

Configure the task objects.

On the Configure Objects page, specify the objects to synchronize.

Setting	Description
Synchronization Types	DTS always selects Incremental Data Synchronization. By default, you must also select Schema Synchronization and Full Data Synchronization. After the precheck, DTS initializes the destination cluster with the full data of the selected source objects, which serves as the baseline for subsequent incremental synchronization.
DDL and DML Operations to Be Synchronized	Based on your requirements, select the SQL operations for incremental synchronization at the instance level. Note To select incremental SQL operations at the schema or table level, right-click a synchronization object in the Selected Objects box and select the desired SQL operations in the dialog box that appears.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks for tables with the same names in the destination database. If any tables with the same names are found, an error is reported during the precheck and the data synchronization task does not start. Otherwise, the precheck is successful. Note If you cannot delete or rename the table with the same name in the destination database, you can map it to a different name in the destination. For more information, see Database Table Column Name Mapping. Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example: If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database: During full data synchronization, DTS retains the destination record and skips the source record. During incremental synchronization, DTS overwrites the destination record with the source record. If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.
Storage Engine Type	Select a storage engine type for the destination table based on your business requirements. The default value is Beam. Note This setting is available only if the destination AnalyticDB for PostgreSQL instance runs kernel version v7.0.6.6 or later and Schema Synchronization is selected for Synchronization Types.
Capitalization of Object Names in Destination Instance	Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.
Source Objects	In the Source Objects box, click the objects, and then click to move them to the Selected Objects box. Note You can select objects at the schema or table level.
Selected Objects	To set the name of a synchronization object in the destination instance, right-click the object in the Selected Objects box to modify it. For more information, see Database Table Column Name Mapping. To remove a synchronization object, click it in the Selected Objects box, and then click to move it to the Source Objects box. Note If you use the object name mapping feature, other objects that depend on the mapped object may fail to synchronize. To set WHERE conditions to filter data for a table, right-click the table in the Selected Objects box and set the filter conditions in the dialog box that appears. For more information, see Set filter conditions. To select SQL operations for incremental synchronization, right-click the synchronization object in the Selected Objects box and select the desired operations from the dialog box.

Click Next: Advanced Settings.

Setting	Description
Dedicated Cluster for Task Scheduling	By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.
Retry Time for Failed Connections	If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails. Note If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances. DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.
Retry Time for Other Issues	If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.
Enable Throttling for Full Data Synchronization	During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s). Note This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
Enable Throttling for Incremental Data Synchronization	You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).
Environment Tag	You can select an environment tag to identify the instance based on your business requirements. No selection is required for this example.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert contact. For more information, see Configure monitoring and alerting during task configuration.

Optional: After you complete the preceding configurations, click Next: Configure Database and Table Fields to set the Type, Primary Key Column, and Distribution Key for the synchronized tables in the destination AnalyticDB for PostgreSQL instance.
Note
- This step is available only if you select Schema Synchronization for Synchronization Types when configuring synchronization objects. You can set Definition Status to All to make modifications.
- The Primary Key Column can be a composite primary key consisting of multiple columns. You must select one or more columns from the Primary Key Column to serve as the Distribution Key. For more information, see Manage data tables and Table distribution.

Save the task and perform a precheck.
- To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.
- If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.
- If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.
- If the precheck generates warnings:
  For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.
  For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.

Purchase the instance.

When the Success Rate reaches 100%, click Next: Purchase Instance.

On the Purchase page, select the billing method and link specifications for the data synchronization instance. For more information, see the following table.

Category	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay upfront for a specific duration. This is cost-effective for long-term, continuous tasks. Pay-as-you-go: You are billed hourly for actual usage. This is ideal for short-term or test tasks, as you can release the instance at any time to save costs.
	Resource Group Settings	The resource group to which the instance belongs. The default is default resource group. For more information, see What is Resource Management?.
	Instance Class	DTS offers synchronization specifications at different performance levels that affect the synchronization rate. Select a specification based on your business requirements. For more information, see Data synchronization link specifications.
	Subscription Duration	In subscription mode, select the duration and quantity of the instance. Monthly options range from 1 to 9 months. Yearly options include 1, 2, 3, or 5 years. Note This option appears only when the billing method is Subscription.

Read and select the checkbox for Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start, and then click OK in the OK dialog box.
You can monitor the task progress on the data synchronization page.