Configure one-way synchronization for PolarDB for PostgreSQL-Data Transmission Service(DTS)-阿里云帮助中心

Use Data Transmission Service (DTS) to configure one-way synchronization between PolarDB for PostgreSQL clusters.

Prerequisites

You have created the source and destination PolarDB for PostgreSQL database clusters. For more information, see Create a database cluster.
You have set the wal_level parameter to logical for the source and destination PolarDB for PostgreSQL instances. For more information, see Set cluster parameters.
The storage space of the destination database must be larger than the storage space used by the source database.

Limitations

Note

During schema synchronization, DTS synchronizes foreign keys from the source database to the destination database.
During full data synchronization and incremental data synchronization, DTS temporarily disables constraint checks and foreign key cascade operations at the session level. Data inconsistency may occur if cascade update or delete operations are performed on the source database while the task is running.

Type	Description
Source database limits	In a PolarDB for PostgreSQL cluster, the tables to be synchronized must have a primary key or a non-null unique index. If the source database has long-running transactions and the instance has an incremental synchronization task, the write-ahead log (WAL) that is generated before the long-running transactions are committed may accumulate. This can cause the disk space of the source database to become insufficient. To ensure that the synchronization task runs as expected and to prevent logical replication from being interrupted by a primary/secondary switchover, the PolarDB for PostgreSQL cluster must support and enable Logical Replication Slot Failover. Note If the source PolarDB for PostgreSQL cluster does not support Logical Replication Slot Failover (for example, if the Database Engine of the cluster is PostgreSQL 14), a high-availability (HA) switchover in the source database may cause the synchronization instance to fail and become unrecoverable. Due to the limits of logical subscription in the source database, if a single piece of data to be synchronized exceeds 256 MB after an incremental change, the synchronization instance may fail and cannot be recovered. You must reconfigure the synchronization instance. 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.
Other limits	A single data synchronization task can synchronize only one database. To synchronize multiple databases, you must configure a data synchronization task for each database. DTS does not synchronize TimescaleDB extension tables, tables with cross-schema inheritance, or tables that contain expression-based unique indexes. Schemas created by installing plugins cannot be synchronized. You cannot obtain information about these schemas in the console when you configure the task. 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. 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. Initial full data synchronization consumes read and write resources on both the source and destination databases, which increases the database load. Therefore, evaluate the performance of the source and destination databases before you synchronize data, and perform the synchronization during off-peak hours. Initial full data synchronization runs concurrent INSERT operations. This causes table fragmentation in the destination database. As a result, the table space in the destination instance is larger than that in the source instance after initial full data synchronization is complete. During DTS synchronization, do not write data to the destination database from sources other than DTS. Otherwise, data inconsistency between the source and destination databases may occur. 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. DTS validates data content but does not support validation for metadata such as sequences. You must validate metadata yourself. 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`. After the business is switched to the destination, new sequences do not use the maximum sequence value from the source database as the initial value for incrementing. You must update the sequence value of the destination database before the business switchover. For more information, see Update the sequence value of the destination database. For a task that performs full or incremental synchronization, if the tables to be synchronized in the source database contain foreign keys, triggers, or event triggers, and the destination database account is a privileged account or has superuser permissions, DTS temporarily sets the session_replication_role parameter to replica at the session level during synchronization. If the destination database account does not have these permissions, you must manually set the session_replication_role parameter to replica in the destination database. During this period (when session_replication_role is set to replica), if cascade update or delete operations occur in the source database, data inconsistency may occur. After the DTS task is released, you can change the value of the session_replication_role parameter back to origin. 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. When you synchronize partitioned tables, you must include both the parent table and its child tables as synchronization objects. Otherwise, data inconsistency may occur in the partitioned table. Note The parent table of a PostgreSQL partitioned table does not directly store data. All data is stored in the child tables. The synchronization task must include the parent table and all its child tables. Otherwise, data in the child tables may not be synchronized, leading to data inconsistency between the source and destination.

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 topologies

One-to-one one-way synchronization
One-to-many one-way synchronization
Many-to-one one-way synchronization

For information about each synchronization topology and its limitations, see Data synchronization topologies.

Supported synchronization objects

SCHEMA, TABLE

Note

Includes PRIMARY KEY, UNIQUE KEY, FOREIGN KEY, DATATYPE (built-in data types), and DEFAULT CONSTRAINT.
The list of supported features for this database type as a source may vary depending on the destination database type. The features displayed on the console page are definitive.

Supported SQL operations

Operation type	SQL statement
DML	INSERT, UPDATE, DELETE
DDL	DDL synchronization is supported only for data synchronization tasks created after October 1, 2020. Important For data synchronization tasks created before September 9, 2022, you must create triggers and functions in the source database to capture DDL information before you configure the task. For more information, see Implement PostgreSQL DDL incremental migration through triggers and functions. Incremental data synchronization does not support the BIT data type. If the source database uses a privileged account, the data synchronization task supports the following DDL operations: CREATE TABLE, DROP TABLE ALTER TABLE (including RENAME TABLE, ADD COLUMN, ADD COLUMN DEFAULT, ALTER COLUMN TYPE, DROP COLUMN, ADD CONSTRAINT, ADD CONSTRAINT CHECK, and ALTER COLUMN DROP DEFAULT) TRUNCATE TABLE (The source PolarDB for PostgreSQL cluster must use a database engine of PostgreSQL 11 or later.) CREATE INDEX ON TABLE Important Additional clauses in DDL statements, such as CASCADE or RESTRICT, are not synchronized. DDL statements are not synchronized from a session where the `SET session_replication_role = replica` command was executed. DDL statements executed by calling a `FUNCTION` are not synchronized. If a single commit in the source database contains both DML and DDL statements, the DDL statements are not synchronized. If a commit includes a DDL statement that targets an object not configured for synchronization, the statement is not synchronized.

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	Parameter	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 PolarDB for PostgreSQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB for PostgreSQL database cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	For this example, synchronization is performed within the same Alibaba Cloud account. Select No.
	Instance ID	Select the ID of the source PolarDB for PostgreSQL database cluster.
	Database Name	Enter the name of the database in the PolarDB for PostgreSQL cluster.
	Database Account	Enter the database account for the PolarDB for PostgreSQL cluster.
	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 PolarDB for PostgreSQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination PolarDB for PostgreSQL database cluster resides.
	Instance ID	Select the ID of the destination PolarDB for PostgreSQL database cluster.
	Database Name	Enter the name of the destination PolarDB for PostgreSQL database.
	Database Account	Enter the privileged account for the destination PolarDB for PostgreSQL cluster. For information about how to create an account and grant permissions, see Create a database account.
	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.

Parameter	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.
Synchronization Topology	Select One-way Synchronization.
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.
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 schemas or tables as the synchronization objects. If you select tables, other objects such as views, triggers, and stored procedures are not synchronized to the destination database. If a table to be synchronized contains a field of the SERIAL type and Synchronization Types is selected for Schema Synchronization, we recommend also selecting Sequence or the entire schema.
Selected Objects	To rename a single object in the destination instance, right-click the object in the Selected Objects box. For more information, see Map a single object name. To rename multiple objects in bulk, click Batch Edit in the upper-right corner of the Selected Objects box. For more information, see Map multiple object names in bulk. Note To select SQL operations at the schema or table level, right-click an object in the Selected Objects box and choose the desired SQL operations from the dialog box that appears. To set a WHERE clause to filter data, right-click a table in the Selected Objects box and set the filter condition in the dialog box that appears. For instructions, see Set filter conditions.

Click Next: Advanced Settings.

Parameter	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 needs. This example does not require this setting.
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.

Click Data Verification to configure a data verification task.
To use the data verification feature, see Configure data verification.

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.