How to synchronize data from PolarDB for PostgreSQL to RDS for PostgreSQL - Data Transmission Service

Data Transmission Service (DTS) lets you synchronize data from PolarDB for PostgreSQL to RDS for PostgreSQL.

Prerequisites

You have created a source PolarDB for PostgreSQL cluster. For more information, see Create a database cluster.
You have created a destination RDS for PostgreSQL instance. For more information, see Create an RDS for PostgreSQL instance.
You have set the wal_level parameter of the source PolarDB for PostgreSQL instance to logical. For more information, see Set cluster parameters.
The destination database must have more storage space than the source database.

Notes

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 replication 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. During schema synchronization and full data synchronization, do not perform Data Definition Language (DDL) operations that change the schema of databases or tables. Otherwise, the data synchronization task fails. Note During the full synchronization phase, DTS queries the source database, which acquires metadata locks. This 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 separate task for each database. DTS does not support synchronizing TimescaleDB extension tables, tables with cross-schema inheritance, or tables with unique indexes based on expressions. 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 in the source database before you write data to them. This ensures data consistency. Do not lock the tables while running this command to prevent deadlocks. If you skip the related precheck items, DTS automatically runs this command during the initialization of the instance. When the instance runs for the first time. When you select Schema as the granularity for object selection, and a new table is created in the schema or a table to be synchronized is rebuilt using the RENAME command. When you use the feature to modify synchronization objects. Note In the command, replace `schema` and `table` with the actual schema name and table name. We recommend that you perform this operation during off-peak hours. If a table to be synced 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 to run. Before you synchronize data, evaluate the performance of the source and destination databases. We recommend that you synchronize data during off-peak hours. Otherwise, initial full data synchronization consumes read and write resources on both the source and destination databases, which increases the database load. 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. For example, if you use Data Management Service (DMS) to perform online DDL operations while other data is being written to the destination database, data loss may occur in the destination database. 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 DDL statements for incremental data, the structure of incremental tables, and heartbeat information. Do not delete these temporary tables during synchronization. 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 `dts_sync_` prefix in the source database to replicate data. This replication slot allows DTS to obtain incremental logs from the source database within the last 15 minutes. When the data synchronization fails or the synchronization instance is released, DTS attempts to automatically clear the replication slot. Note If you change the password of the source database account used by the task or delete the DTS IP address from the whitelist of 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. This prevents the slot from continuously accumulating and consuming disk space, which can make the source database unavailable. If a failover occurs in the source database, you must log on to the secondary database to manually clear the slot. After you switch your business to the destination instance, new sequences do not increment from the maximum value of the source sequence. You must update the sequence value in the destination database before the business switchover. For more information, see Update the sequence value in the destination database. For a full or incremental synchronization task, if the tables to be synchronized in the source database contain foreign keys, triggers, or event triggers, DTS temporarily sets the `session_replication_role` parameter to `replica` at the session level if the destination database account is a privileged account or has superuser permissions. 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 (while `session_replication_role` is `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 `session_replication_role` parameter back to `origin`. If the task fails, DTS technical support will attempt to recover it within 8 hours. During the recovery process, operations such as restarting the task or adjusting its parameters may be performed. Note When parameters are adjusted, only DTS task parameters are modified. Database parameters remain unchanged.The parameters that may be modified include but are not limited to those described in Modify instance parameters. When you synchronize partitioned tables, you must include both the parent table and its child partitions as synchronization objects. Otherwise, data inconsistency may occur for the partitioned table. Note The parent table of a PostgreSQL partitioned table does not directly store data. All data is stored in the child partitions. The synchronization task must include the parent table and all its child partitions. Otherwise, data from the child partitions 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-way one-to-one synchronization
One-way one-to-many synchronization
One-way many-to-one synchronization

For more information about each synchronization topology, see Data synchronization topologies.

Supported objects to be synchronized

SCHEMA, TABLE
Note
This includes PRIMARY KEY, UNIQUE KEY, FOREIGN KEY, DATATYPE (built-in data types), and DEFAULT CONSTRAINT.
VIEW, PROCEDURE (PostgreSQL 11 or later), FUNCTION, RULE, SEQUENCE, EXTENSION, TRIGGER, AGGREGATE, INDEX, OPERATOR, DOMAIN

Supported SQL operations

Operation type	SQL operation statement
DML	INSERT, UPDATE, DELETE
DDL	Only data sync tasks created after October 1, 2020 support DDL operations. Important For data sync tasks created before May 12, 2023, create triggers and functions in the source database to catch DDL information before you configure the sync task. For more information, see Use triggers and functions to implement DDL incremental migration for PostgreSQL. Incremental data synchronization does not support the bit data type. If the source database account is a privileged account, the sync 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 database engine of the source PolarDB for PostgreSQL database must be PostgreSQL 11 or later) CREATE INDEX ON TABLE Important Additional information in DDL statements, such as CASCADE or RESTRICT, is not synchronized. DDL is not supported in a session that uses the `SET session_replication_role = replica` command. DDL statements that are executed by calling methods such as functions are not synchronized. If a single commit to the source database contains both DML and DDL statements, the DDL statements are not synchronized. If a single commit to the source database contains DDL statements for objects that are not configured for synchronization, those DDL statements are 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	Configuration	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	Database Type	Select PolarDB for PostgreSQL.
	Connection Type	Select Cloud Instance.
	Instance Region	Select the region where the PolarDB for PostgreSQL cluster resides.
	Cross-account	This example shows synchronization within the same Alibaba Cloud account. Select No.
	Instance ID	Select the ID of the source PolarDB for PostgreSQL cluster.
	Database Name	Enter the name of the PolarDB for PostgreSQL database.
	Database Account	Enter the database account of the PolarDB for PostgreSQL instance.
	Database Password	Enter the password for the specified database account.
Destination Database	Database Type	Select PostgreSQL.
	Connection Type	Select Cloud Instance.
	Instance Region	Select the region where the destination RDS for PostgreSQL instance resides.
	Instance ID	Select the ID of the destination RDS for PostgreSQL instance.
	Database Name	Enter the name of the database in the destination RDS for PostgreSQL instance to which the synchronization objects belong.
	Database Account	Enter the database account of the destination RDS for PostgreSQL instance, which must have owner permissions on the schema. For information about how to create and grant permissions to an account, see Create an account.
	Database Password	Enter the password for the specified database account.
	Encryption	Specifies whether to encrypt the connection to the source database. You can configure this parameter based on your business requirements. In this example, Non-encrypted is selected. If you want to establish an SSL-encrypted connection to the source database, perform the following steps: Select SSL-encrypted, upload CA Certificate, Client Certificate, and Private Key of Client Certificate as needed, and then specify Private Key Password of Client Certificate. Note If you set Encryption to SSL-encrypted for a self-managed PostgreSQL database, you must upload CA Certificate. If you want to use the client certificate, you must upload Client Certificate and Private Key of Client Certificate and specify Private Key Password of Client Certificate. For information about how to configure SSL encryption for an ApsaraDB RDS for PostgreSQL instance, see SSL encryption.

After completing the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
- Ensure that you add the CIDR blocks of the DTS servers (either automatically or manually) to the security settings of both the source and destination databases to allow access. For more information, see Add the IP address whitelist of DTS servers.
- If the source or destination is a self-managed database (i.e., the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the task objects.

On the Configure Objects page, specify the objects 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.
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.
Case Policy for Destination Object Names	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 The synchronization object granularity can be schema or table. If you select tables as synchronization objects, other objects such as views, triggers, and stored procedures are not synchronized to the destination database. If a table to be synchronized contains SERIAL data type, and you select Schema Synchronization as the Synchronization Types, we recommend that you also select Sequence or entire schema synchronization.
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.

Click Next: Advanced Settings.

Configuration	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. No selection is needed 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 notifications. 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.