Migrate PolarDB for PostgreSQL (Oracle compatible) to Message Queue for Kafka - Data Transmission Service

Prerequisites

In the source PolarDB for PostgreSQL (Oracle-compatible) cluster, the wal_level parameter must be set to logical to add the information required for logical decoding to write-ahead logging (WAL). For more information, see Set cluster parameters.
Create a destination Alibaba Cloud Message Queue for Kafka instance with more storage space than is used by the source PolarDB for PostgreSQL (Compatible with Oracle) instance.

Note
For information about the supported versions for the source and destination, see Migration solutions overview.
Create a topic in the destination Alibaba Cloud Message Queue for Kafka instance to receive migrated data. For instructions, see Step 1: Create a topic.

Limitations

Type	Description
Source database limitations	The server that hosts the source database must have sufficient outbound bandwidth. Insufficient bandwidth can affect the migration speed. Each table to be migrated must have a primary key or a unique constraint. All fields in the key or constraint must be unique to prevent duplicate data in the destination database. If you migrate objects at the table level and need to perform edits such as column name mapping, a single migration task supports a maximum of 1,000 tables. If you exceed this limit, an error is reported when you submit the task. In this case, you can split the tables into multiple tasks or configure a task to migrate the entire database. For incremental migration, you must configure write-ahead logging (WAL): The WAL feature must be enabled. For an incremental migration-only task, Data Transmission Service (DTS) requires the WAL logs of the source database to be retained for at least 24 hours. For a task that includes both full and incremental migration, DTS requires the logs to be retained for at least 7 days. You can change the retention period back to at least 24 hours after the full migration is complete. Setting a shorter retention period can cause the task to fail if DTS cannot obtain the required WAL logs, potentially leading to data inconsistency or data loss. The DTS Service Level Agreement (SLA) does not cover issues caused by a WAL retention period shorter than the requirement. Operational limitations on the source database: During the schema migration and full data migration phases, do not perform any DDL operations that alter the schema of databases or tables. Otherwise, the migration task will fail. If you perform only full data migration, do not write new data to the source instance to avoid data inconsistency. To maintain real-time data consistency, we recommend selecting schema migration, full data migration, and incremental data migration. To ensure the migration task runs smoothly and to prevent logical subscription interruptions caused by a primary/secondary switchover, the source PolarDB for PostgreSQL(Compatible with Oracle) cluster must support logical replication slot failover and have it enabled. Note If the source PolarDB for PostgreSQL(Compatible with Oracle) cluster does not support logical replication slot failover (for example, if the cluster's database engine is Oracle syntax compatibility 2.0), a high-availability (HA) switchover on the source database can cause the migration instance to fail and become unrecoverable. Due to the limitations of logical subscriptions, if a migration instance that includes incremental data migration is running and the size of a single row to be migrated exceeds 256 MB after an incremental change, the migration instance fails irrecoverably, and you must reconfigure it. If long-running transactions exist in the source database during an incremental migration, write-ahead logging (WAL) records generated before these transactions commit cannot be cleared. This can cause WAL files to accumulate and exhaust the disk space of the source database.
Other limitations	A single data migration task can migrate only one database. To migrate multiple databases, you must configure a separate migration task for each one. DTS does not support migrating TimescaleDB extension tables, tables with cross-schema inheritance, or tables with expression-based unique indexes. These schemas will not be visible in the console when you configure the task. Migration of the following object types is not supported: INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, and foreign keys (FK). For tasks that include incremental data migration, you must run the `ALTER TABLE schema.table REPLICA IDENTITY FULL;` command on the tables to be migrated in the source database before you write data to them. This ensures data consistency for the tables in the following two scenarios. To prevent deadlocks, avoid table-locking operations while this command runs. If you skip the related checks during the precheck, DTS automatically runs this command when it initializes the instance. When the instance runs for the first time. When the migration object granularity is set to Schema and a new table is created in the schema or an existing table is rebuilt by using the RENAME command. Note In the command, replace `schema` and `table` with the schema name and table name of the data to be migrated. Perform this operation during off-peak hours. DTS creates the following temporary tables in the source database to obtain information such as DDL statements for incremental data, the structure of incremental tables, and heartbeats. Do not delete these temporary tables during the migration, as this will cause the task to fail. The 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`. To ensure the accuracy of latency metrics for incremental data migration, DTS creates a heartbeat table named `dts_postgres_heartbeat` in the source database. During incremental data migration, DTS creates a replication slot prefixed with `dts_sync_` in the source database to replicate data. This replication slot allows DTS to obtain incremental logs from the source database from the last 15 minutes. When a data migration task fails or the migration instance is released, DTS attempts to automatically clean up the replication slot. Note If you change the password of the source database account used by the task or remove the DTS IP addresses from the IP address whitelist of the source database during migration, the replication slot cannot be automatically cleaned up. In this case, you must manually clean up the replication slot in the source database to prevent log accumulation, which can exhaust disk space and cause the source database to become unavailable. If a primary/secondary switchover occurs on the source database, you must log on to the secondary database to manually clean up the replication slot. If the destination Kafka cluster is scaled up or down during data migration, you must restart the migration instance. Before you start the migration, evaluate the performance of both the source and destination databases. We recommend that you run the migration during off-peak hours. During full data migration, DTS consumes read and write resources on both databases, which can increase their load. DTS attempts to resume a failed migration task for up to seven days. Before you switch business workloads to the destination instance, you must end or release the task, or revoke the write permissions of the account that DTS uses to access the destination instance. This prevents an automatically resumed task from overwriting data in the destination instance. 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 migrating partitioned tables, you must include the parent table and all its child partitions as migration objects to prevent data inconsistency. Important In PolarDB for PostgreSQL(Compatible with Oracle), the parent table of a partitioned table does not store data directly. Data is stored in the child partitions. The migration task must include the parent table and all its child partitions to prevent data loss and inconsistency between the source and destination databases. Migration of partitioned tables or inherited tables (parent and child tables) that are distributed across different databases is not supported. Ensure that the partitioned tables and all their partitions, as well as parent tables and all their child tables, are in the same database.

Billing

Migration type	Task configuration fee	Internet traffic fee
Schema migration and full data migration	Free of charge.	DTS charges an Internet traffic fee when the Access Method of the destination database is set to Public IP Address. Billing overview.
Incremental data migration	Charged. Billing overview.

Supported SQL operations for incremental migration

Type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	CREATE TABLE, ALTER TABLE, DROP TABLE, RENAME TABLE, and TRUNCATE TABLE CREATE VIEW, ALTER VIEW, and DROP VIEW CREATE PROCEDURE, ALTER PROCEDURE, and DROP PROCEDURE CREATE FUNCTION and DROP FUNCTION CREATE INDEX and DROP INDEX Important Additional clauses in DDL statements, such as CASCADE or RESTRICT, are not migrated. DDL statements from a session where the`SET session_replication_role = replica` command is executed are not migrated. DDL statements called from a function cannot be migrated. If a commit to the source database contains both DML and DDL statements, the DDL statements are not migrated. If a commit to the source database contains DDL statements for objects not selected for migration, those statements are not migrated.

Database account permissions

Database	Permissions	References
PolarDB for PostgreSQL (Compatible with Oracle) cluster	privileged account	Create and manage a database account

Procedure

Navigate to the migration task list page for the destination region using one of the following methods.
From the DTS console
1. Log on to the Data Transmission Service (DTS) console.
2. In the navigation pane on the left, click Data Migration.
3. In the upper-left corner of the page, select the region where the migration instance is located.
From the DMS console

Note
The actual operations may vary based 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 Data Management (DMS) console.
2. In the top menu bar, choose Data + AI > Data Transmission (DTS) > Data Migration.
3. To the right of Data Migration Tasks, select the region where the migration instance is located.
Click Create Task to navigate to the task configuration page.

Configure the source and destination databases.

Note

For information about how to obtain the parameters of the destination Message Queue for Apache Kafka instance, see Configure parameters for a Message Queue for Apache Kafka instance.

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	To use a database instance that has been added to the system (created or saved), select the desired database instance from the drop-down list. The database information below will be automatically configured. Note In the DMS console, this parameter is named Select a DMS database instance.. If you have not registered the database instance with the system, or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select PolarDB (Compatible with Oracle).
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB (Compatible with Oracle) cluster is located.
	Replicate Data Across Alibaba Cloud Accounts	In this example, data is migrated within the same Alibaba Cloud account. Select No.
	Instance ID	Select the instance ID of the source PolarDB (Compatible with Oracle) cluster.
	Database Name	Enter the name of the database in the source PolarDB (Compatible with Oracle) cluster that contains the objects to migrate.
	Database Account	Enter the database account for the source PolarDB (Compatible with Oracle) cluster. For information about the required permissions, see Required permissions for the database account.
	Database Password	Enter the password for the specified database account.
Destination Database	Select Existing Connection	To use a database instance that has been added to the system (created or saved), select the desired database instance from the drop-down list. The database information below will be automatically configured. Note In the DMS console, this parameter is named Select a DMS database instance.. If you have not registered the database instance with the system, or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select Kafka.
	Access Method	Select Express Connect, VPN Gateway, or Smart Access Gateway. Note In this step, the Message Queue for Apache Kafka instance is configured as a self-managed Kafka database for the migration instance.
	Instance Region	Select the region where the destination Message Queue for Apache Kafka instance is located.
	Connected VPC	Select the ID of the Virtual Private Cloud (VPC) where the destination Message Queue for Apache Kafka instance is located.
	Domain Name or IP	Enter any IP address from the Default Endpoint of the destination Message Queue for Apache Kafka instance.
	Port Number	Enter the service port of the destination Message Queue for Apache Kafka instance. The default value is 9092.
	Database Account	You do not need to configure these parameters in this example.
	Database Password
	Kafka Version	Select the version that corresponds to your Kafka instance.
	Encryption	Based on your business and security requirements, select Non-encrypted or SCRAM-SHA-256.
	Topic	From the drop-down list, select the Topic to receive data.
	Use Kafka Schema Registry	Kafka Schema Registry is a service layer for your metadata. It provides a RESTful API to store and retrieve Avro schemas. No: Do not use Kafka Schema Registry. Yes: Use Kafka Schema Registry. In the URL or IP Address of Schema Registry field, enter the URL or IP address that is registered in Kafka Schema Registry for your Avro schema.

After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
- Ensure that the IP address segment of the DTS service is 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.
- If the source or destination database is a self-managed database (the Access Method is not Alibaba Cloud Instance), you must also click Test Connectivity in the CIDR Blocks of DTS Servers dialog box that appears.

Configure the task objects.

On the Configure Objects page, configure the objects that you want to migrate.

Parameter	Description
Migration Types	If you need to perform only full data migration, select both Schema Migration and Full Data Migration. For zero-downtime migration, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If the Access Method for the destination Kafka instance is Alibaba Cloud Instance, Schema Migration is not supported. If you do not select Incremental Data Migration, do not write new data to the source instance during data migration to ensure data consistency.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks whether tables with the same names exist in the destination database. If no tables with the same names exist, the precheck is passed. If tables with the same names exist, an error is reported during the precheck, and the data migration task does not start. Note If a table in the destination database has the same name but cannot be easily deleted or renamed, you can change the name of the table in the destination database. For more information, see Object name mapping. Ignore Errors and Proceed: Skips the check for tables with the same names. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and business risks. For example: If the table schemas are consistent and a record in the destination database has the same primary key value as a record in the source database: During full migration, DTS keeps the record in the destination database. The record from the source database is not migrated. During incremental migration, DTS does not keep the record in the destination database. The record from the source database overwrites the record in the destination database. If the table schemas are inconsistent, only some columns of data may be migrated, or the migration may fail. Proceed with caution.
Data Format in Kafka	Select the format for storing data in the Kafka instance based on your business requirements. If you select Canal JSON, see Canal JSON for parameter descriptions and examples. Note Currently, only the China (Qingdao) and China (Beijing) regions support the Canal JSON format. If you select DTS Avro, you must parse the data based on the DTS Avro schema definition. For more information, see DTS Avro schema definition and DTS Avro deserialization example. If you select Shareplex JSON, see Shareplex JSON for parameter descriptions and examples.
Kafka Data Compression Format	Select a compression format for Kafka messages based on your requirements. LZ4 (default): low compression ratio, high compression speed. GZIP: high compression ratio, low compression speed. Note High CPU consumption. Snappy: medium compression ratio, medium compression speed.
Policy for Shipping Data to Kafka Partitions	Select a policy based on your business requirements.
Message acknowledgement mechanism	Select a message acknowledgment mechanism based on your business requirements.
Topic That Stores DDL Information	Select the topic for storing DDL information. Note If no topic is selected, DTS stores DDL information in the data topic by default.
Capitalization of Object Names in Destination Instance	You can configure the case sensitivity policy for the names of migrated objects, such as databases, tables, and columns, in the destination instance. By default, DTS default policy is selected. You can also choose to keep the case sensitivity consistent with the default policy of the source or destination database. For more information, see Case sensitivity of object names in the destination database.
Source Objects	In the Source Objects box, click the objects to migrate, and then click to move them to the Selected Objects box. Note You can select tables as the migration objects.
Selected Objects	No additional configurations are required in this example. You can use the mapping feature to set the Topic name, number of partitions in the Topic, and partition key for the source table in the destination Kafka instance. For more information, see Mapping information. Note If you use the object name mapping feature, other objects that depend on the mapped object may fail to be migrated. To select the SQL operations for incremental migration, right-click a migration object in the Selected Objects box, and select the desired SQL operations in the dialog box that appears.

Click Next: Advanced Settings to configure advanced parameters.

Parameter	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules tasks on a shared cluster. You do not need to select one. If you want more stable tasks, you can purchase a dedicated cluster to run DTS migration tasks.
Retry Time for Failed Connections	After the migration task starts, if the connection to the source or destination database fails, 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 1440 minutes. We recommend that you set the duration to more than 30 minutes. If DTS reconnects to the source and destination databases within the specified duration, the migration task automatically resumes. Otherwise, the task fails. Note For multiple DTS instances that share the same source or destination, the network retry time is determined by the setting of the last created task. Because you are charged for the task during the connection retry period, we recommend that you customize the retry time based on your business needs, or release the DTS instance as soon as possible after the source and destination database instances are released.
Retry Time for Other Issues	After the migration task starts, if a non-connectivity issue, such as a DDL or DML execution exception, occurs in the source or destination database, DTS reports an error and immediately begins to retry the operation. The default retry duration is 10 minutes. You can customize the retry time to a value from 1 to 1440 minutes. We recommend that you set the duration to more than 10 minutes. If the related operations succeed within the specified retry duration, the migration task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than the value of Retry Time for Failed Connections.
Enable Throttling for Full Data Migration	During full migration, DTS consumes read and write resources on the source and destination databases, which may increase the database load. If required, you can enable throttling for the full migration task. You can set Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) to reduce the load on the destination database. Note This configuration item is available only if you select Full Data Migration for Migration Types. You can also adjust the full migration speed after the migration instance is running.
Enable Throttling for Incremental Data Migration	If required, you can also choose to set speed limits for the incremental migration task. You can set RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) to reduce the load on the destination database. Note This configuration item is available only if you select Incremental Data Migration for Migration Types. You can also adjust the incremental migration speed after the migration instance is running.
Environment Tag	You can select an environment tag to identify the instance based on your business requirements. No tag needs to be selected in 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	Select whether to set alerts and receive alert notifications based on your business needs. No: Does not set an alert. Yes: Configure alerts by setting an alert threshold and an alert notifications. If a migration fails or the latency exceeds the threshold, the system sends an alert notification.

Save the task and run a precheck.
- To view the parameters for configuring this instance when you call the API operation, move the pointer over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the bubble that appears.
- If you do not need to view or have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before the migration task starts, DTS performs a precheck. The task starts only after it passes the precheck.
- If the precheck fails, click View Details next to the failed check item, fix the issue based on the prompt, and then run the precheck again.
- If a warning is reported during the precheck:
  
  For check items that cannot be ignored, click View Details next to the failed item, fix the issue based on the prompt, and then run the precheck again.
  
  For check items that can be ignored, you can click Confirm Alert Details, Ignore, OK, and Precheck Again to skip the alert item and run the precheck again. If you choose to ignore a warning, it may cause issues such as data inconsistency and pose risks to your business.

Purchase the instance.

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

On the Purchase page, select the link specification for the data migration instance. For more information, see the following table.

Category	Parameter	Description
New Instance Class	Resource Group Settings	Select the resource group to which the instance belongs. The default value is default resource group. For more information, see What is Resource Management?
New Instance Class	Instance Class	DTS provides migration specifications with different performance levels. The link specification affects the migration speed. You can select a specification based on your business scenario. For more information, see Data migration link specifications.

After the configuration is complete, read and select Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start. In the OK dialog box that appears, click OK.

You can view the progress of the migration task on the Data Migration Tasks list page.
Note
- If the migration task does not include incremental migration, it stops automatically after the full migration is complete. After the task stops, its Status changes to Completed.
- If the migration task includes incremental migration, it does not stop automatically. The incremental migration task continues to run. While the incremental migration task is running, the Status of the task is Running.

Mapping information

In the Selected Objects area, hover over the target topic name (at the table level).
Click Edit next to the target topic name.

In the Edit Table dialog box, configure the mapping information.