Migrate data from PostgreSQL to an ApsaraDB for ClickHouse cluster - Data Transmission Service

Data Transmission Service (DTS) lets you migrate data from a PostgreSQL database to an ApsaraDB for ClickHouse cluster. This topic describes how to migrate data from a source ApsaraDB RDS for PostgreSQL instance.

Prerequisites

You must create a destination ApsaraDB for ClickHouse cluster. The disk space of the destination cluster must be larger than the data volume to be migrated.
Note
For more information about the supported versions of the source and destination databases, see Migration solutions.
If you want to perform incremental data migration, you must set the wal_level parameter of the source ApsaraDB RDS for PostgreSQL instance to logical. For more information, see Modify the parameters of an ApsaraDB RDS for PostgreSQL instance.

Precautions

Source database limits

Bandwidth: The server that hosts the source database must have an outbound bandwidth of 100 Mbit/s or higher. Otherwise, the data migration speed is affected.
Migration objects:
- The tables to be migrated must have primary keys or UNIQUE constraints, and the values in the columns with UNIQUE constraints must be unique. Otherwise, data may be duplicated in the destination database.
  Note
  If the target table is not created by DTS (the Schema Migration option is not selected for Migration Types), you must ensure that this table has the same primary key or a non-null UNIQUE constraint as the table to be migrated in the source database. Otherwise, duplicate data may appear in the target database.
  The name of the database to be migrated cannot contain a hyphen (-), such as dts-testdata.
- DTS does not support migrating TimescaleDB extension tables or tables with cross-schema inheritance relationships.
- If you migrate tables and need to edit the tables, such as mapping table and column names, a single data migration task can migrate a maximum of 1,000 tables. If the number of tables exceeds the limit, an error is reported after you submit the task. In this case, we recommend that you split the tables to be migrated and configure multiple tasks to migrate them in batches, or configure a task to migrate the entire database.
If you perform incremental migration, take note of the following limits on Write-Ahead Logging (WAL):
- WAL must be enabled. To enable WAL, set the wal_level parameter to logical.
- For an incremental migration task, DTS requires that WAL logs in the source database be retained for more than 24 hours. For a task of full migration and incremental migration, DTS requires that WAL logs in the source database be retained for at least 7 days. You can set the log retention period to more than 24 hours after full migration is complete. Otherwise, the DTS task may fail because DTS cannot obtain the WAL logs. In extreme cases, data inconsistency or loss may occur. Issues that are caused because the WAL log retention period that you specified is shorter than the period required by DTS are not covered by the DTS Service-Level Agreement (SLA).
Restrictions:
- If you perform DDL operations that use non-standard syntax on the migration objects, the migration task may fail or data may be lost.
- If the source database undergoes a major version upgrade while the migration instance is running, the migration instance will fail and cannot be recovered. You need to reconfigure the migration instance.
- During full migration, do not perform DDL operations to change the schema of the database or table. Otherwise, the data migration task fails.
- Due to the limits on logical subscriptions in the source database, if a single piece of data to be migrated from the source database exceeds 256 MB after an incremental change during the running of a migration instance that includes an incremental task, the migration instance may fail and cannot be recovered. You need to reconfigure the migration instance.
- If you perform only full data migration, do not write new data to the source database. Otherwise, data inconsistency between the source and destination databases occurs. To ensure real-time data consistency, we recommend that you select Full Data Migration and Incremental Data Migration.
To ensure that the data migration task runs as expected and to prevent logical subscription interruptions caused by primary/secondary failover, you must enable Logical Replication Slot Failover for the ApsaraDB RDS for PostgreSQL instance. For more information about how to enable this feature, see Logical Replication Slot Failover.
If the source database has long-running transactions and the instance includes an incremental migration task, the Write-Ahead Logging (WAL) logs generated before the long-running transactions are committed may not be cleared and may accumulate. This can cause insufficient disk space in the source database.

Other limits

A single data migration task can migrate data from only one database. To migrate data from multiple databases, you must configure a data migration task for each database.
The time-related data types in ApsaraDB for ClickHouse have range limits. If the time data to be migrated is outside the supported range, the time data migrated to ApsaraDB for ClickHouse is incorrect. For more information about the range limits, see Time information.
The Partition Key cannot be a nullable field. Otherwise, the migration task fails.
Note
The partition key supports only the BIGINT, INT, TIMESTAMP, DATETIME, and DATE data types.
The number of databases to be migrated must not exceed the limit of ApsaraDB for ClickHouse, which is 256.
The names of the databases, tables, and columns to be migrated must comply with the naming conventions of ApsaraDB for ClickHouse. For more information about the naming conventions, see Object naming conventions.
During initial schema synchronization, DTS adds the _sign, _is_deleted, and _version fields to the destination table. If you do not select Schema Migration when you configure Migration Types, you must manually create a table in the destination database that includes these required fields. For more information about the table and field requirements, see Table and field information.
If the migration instance includes an incremental data migration task, you must run the ALTER TABLE schema.table REPLICA IDENTITY FULL; command on the table to be migrated in the source database before you write data to it in the following two scenarios. This ensures data consistency for the table migration. During the execution of this command, we recommend that you do not perform table locking operations. Otherwise, the table will be locked. If you skip the relevant check in the precheck, DTS automatically runs this command when the instance is initialized.
- When the instance runs for the first time.
- When the migration object granularity is Schema, and a new table is created in the schema to be migrated or a table to be migrated is rebuilt 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.
- We recommend that you perform this operation during off-peak hours.
DTS creates the following temporary tables in the source database to obtain information such as the DDL statements of incremental data, the structure of incremental tables, and heartbeat information. Do not delete these temporary tables from the source database during migration. 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 incremental data migration, DTS creates a replication slot with the prefix dts_sync_ in the source database to replicate data. DTS can use this replication slot to obtain incremental logs from the source database within 15 minutes.
Note
- DTS automatically deletes the replication slot after the instance is released. If you change the database password or delete the DTS IP address from the whitelist during migration, the replication slot cannot be automatically deleted. In this case, you must manually delete the replication slot from the source database to prevent it from accumulating and occupying disk space, which can make the ApsaraDB RDS for PostgreSQL instance unavailable.
- When a migration task is released or fails, DTS automatically clears the replication slot. If a primary/secondary failover occurs on the ApsaraDB RDS for PostgreSQL instance, you need to log on to the secondary database to manually clear the replication slot.
During full data migration, DTS consumes some read and write resources of the source and destination databases, which may increase the database load. Before you perform data migration, you should evaluate the performance of the source and destination databases. We recommend that you perform data migration during off-peak hours, for example, when the CPU load of both databases is below 30%.
Because full migration involves concurrent INSERT operations, fragmentation may occur in the destination table. After the full migration is complete, the storage space occupied by the destination table is larger than that of the source table.
If data is written to the destination database from sources other than DTS while the migration instance is running, data inconsistency may occur between the source and destination databases. In some cases, the migration instance may even fail.
DTS validates data content. DTS does not support the validation of metadata such as sequences. You must validate the metadata yourself.
If an instance fails, DTS helpdesk will try to recover the instance within 8 hours. During the recovery process, operations such as restarting the instance and adjusting parameters may be performed.
Note
When parameters are adjusted, only the parameters of the DTS instance are modified. The parameters of the database are not modified. The parameters that may be modified include but are not limited to those described in Modify instance parameters.
When you migrate a partitioned table, you must include both the parent table and its child tables 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 tables. The synchronization task must include both the parent table and all its child tables. Otherwise, the data in the child tables may not be synchronized, which leads to data inconsistency between the source and destination databases.

Special cases

If the source instance is an ApsaraDB RDS for PostgreSQL instance, do not change its endpoint or zone during migration. Otherwise, the migration fails.
If the source instance is a self-managed PostgreSQL database, you must make sure that the values of the max_wal_senders and max_replication_slots parameters are greater than the sum of the number of used replication slots in the current database and the number of DTS instances to be created with this self-managed PostgreSQL database as the source.
When the source instance is Google Cloud Platform Cloud SQL for PostgreSQL, you must specify an account that has the `cloudsqlsuperuser` permission for the Database Account of the source database. When you select the objects to migrate, you must select objects that this account has permission to manage, or grant the Owner permission on the objects to be migrated to this account (for example, by running the GRANT <owner_of_the_object_to_be_migrated> TO <source_database_account_used_by_the_task> command to allow this account to perform operations as the owner of the objects).
Note
An account with cloudsqlsuperuser permissions cannot manage data whose owner is another account with cloudsqlsuperuser permissions.

Billing

Migration type	Link Configuration Fees	Data transfer cost
Schema migration and full data migration	Free of charge.	This example is free of charge.
Incremental data migration	Charged. For more information, see Billing overview.	This example is free of charge.

SQL operations supported for incremental migration

Operation type	SQL statement
DML	INSERT, UPDATE, DELETE
DDL	CREATE TABLE TRUNCATE TABLE ALTER TABLE ADD COLUMN MODIFY COLUMN DROP TABLE, DROP COLUMN

Data type mappings

The data types supported by ApsaraDB RDS for PostgreSQL instances and ApsaraDB for ClickHouse clusters are different, which prevents a one-to-one mapping. During initial schema synchronization, Data Transmission Service (DTS) maps the data types based on the types supported by the destination database.

Permissions required for database accounts

Database	Required permissions	How to grant permissions
Source ApsaraDB RDS for PostgreSQL instance	A privileged account that owns the database to be migrated.	Create an account and Create a database.
Destination ApsaraDB for ClickHouse cluster	For versions 22.8 and later: Read and write permissions on the destination database. A privileged account has the required permissions. For version 21.8: Read, Write, and Settings Permissions and DDL Allowed.	Manage accounts of an ApsaraDB for ClickHouse Community-compatible Edition cluster.

Procedure

Use one of the following methods to go to the Data Migration page and select the region in which the data migration instance resides.
DTS console
1. Log on to the DTS console.
2. In the left-side navigation pane, click Data Migration.
3. In the upper-left corner of the page, select the region in which the data migration instance resides.
DMS console
Note
The actual operation may vary based on the mode and layout of the DMS console. For more information, see Simple mode and Customize the layout and style of the DMS console.
1. Log on to the DMS console.
2. In the top navigation bar, move the pointer over Data + AI > DTS (DTS) > Data Migration.
3. From the drop-down list to the right of Data Migration Tasks, select the region in which the data synchronization instance resides.
Click Create Task to go to the task configuration page.

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

Category	Configuration	Description
None	Task Name	The name of the DTS task. DTS automatically generates a task name. We recommend that you specify a descriptive name that makes it easy to identify the task. You do not need to specify a unique task name.
Source Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select PostgreSQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source ApsaraDB RDS for PostgreSQL instance resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database instance of the current Alibaba Cloud account is used. Select No.
	Instance ID	Select the ID of the source ApsaraDB RDS for PostgreSQL instance.
	Database Name	Enter the name of the database that contains the objects to be migrated in the source ApsaraDB RDS for PostgreSQL instance.
	Database Account	Enter the database account of the source ApsaraDB RDS for PostgreSQL instance. For information about the permission requirements, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.
Destination Database	Select Existing Connection	If you use a database instance that is registered with DTS, select the instance from the drop-down list. DTS automatically populates the following database parameters for the instance. For more information, see Manage database connections. Note In the DMS console, you can select the database instance from the Select a DMS database instance drop-down list. If you fail to register the instance with DTS, or you do not need to use the instance that is registered with DTS, you must configure the following database information.
	Database Type	Select ClickHouse.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination ApsaraDB for ClickHouse cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, a database instance of the current Alibaba Cloud account is used. Select No.
	Cluster Type	Select the type of the ApsaraDB for ClickHouse cluster as needed.
	Cluster ID	Select the ID of the destination ApsaraDB for ClickHouse cluster.
	Database Account	Enter the database account of the destination ApsaraDB for ClickHouse cluster. For information about the permission requirements, see Permissions required for database accounts.
	Database Password	The password that is used to access the database instance.

In the lower part of the page, click Test Connectivity and Proceed.
Note
- Make sure that the CIDR blocks of DTS servers can be automatically or manually added to the security settings of the source and destination databases to allow access from DTS servers. For more information, see Add DTS server IP addresses to a whitelist.
- If the source or destination database is a self-managed database and its Access Method is not set to Alibaba Cloud Instance, click Test Connectivity in the CIDR Blocks of DTS Servers dialog box.

Configure the objects to be migrated.

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

Configuration	Description
Migration Types	To perform only full data migration, select Schema Migration and Full Data Migration. To ensure service continuity during data migration, select Schema Migration, Full Data Migration, and Incremental Data Migration. Note If you do not select Schema Migration, make sure a database and a table are created in the destination database to receive data and the object name mapping feature is enabled in Selected Objects. If you do not select Incremental Data Migration, we recommend that you do not write data to the source database during data migration. This ensures data consistency between the source and destination databases.
Processing Mode of Conflicting Tables	Precheck and Report Errors: checks whether the destination database contains tables that use the same names as tables in the source database. If the source and destination databases do not contain tables that have identical table names, the precheck is passed. Otherwise, an error is returned during the precheck and the data migration task cannot be started. Note If the source and destination databases contain tables with identical names and the tables in the destination database cannot be deleted or renamed, you can use the object name mapping feature to rename the tables that are migrated to the destination database. For more information, see Map object names. Ignore Errors and Proceed: skips the precheck for identical table names in the source and destination databases. Warning If you select Ignore Errors and Proceed, data inconsistency may occur and your business may be exposed to the following potential risks: If the source and destination databases have the same schema, and a data record has the same primary key as an existing data record in the destination database, the following scenarios may occur: During full data migration, DTS does not migrate the data record to the destination database. The existing data record in the destination database is retained. During incremental data migration, DTS migrates the data record to the destination database. The existing data record in the destination database is overwritten. If the source and destination databases have different schemas, only specific columns are migrated or the data migration task fails. Proceed with caution.
Capitalization of Object Names in Destination Instance	The capitalization of database names, table names, and column names in the destination instance. By default, DTS default policy is selected. You can select other options to make sure that the capitalization of object names is consistent with that of the source or destination database. For more information, see Specify the capitalization of object names in the destination instance.
Source Objects	Select one or more objects from the Source Objects section. Click the icon to add the objects to the Selected Objects section. Note In the Source Objects box, you can select migration objects at the schema or table level.
Selected Objects	To specify the name of an object to be migrated in the destination instance or specify the object to receive data in the destination instance, right-click the object in the Selected Objects section. For more information, see Map object names. To remove one or more objects that are selected for migration, click the objects in the Selected Objects section and then click the icon to move the objects to the Source Objects section. Note If you use the object name mapping feature to rename an object, other objects that depend on the object may fail to be migrated. To specify WHERE conditions to filter data, right-click a table in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Specify filter conditions. To select the SQL operations for incremental migration, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to migrate.

Click Next: Advanced Settings to configure advanced settings.

Configuration	Description
Dedicated Cluster for Task Scheduling	By default, DTS schedules the data migration task to the shared cluster if you do not specify a dedicated cluster. If you want to improve the stability of data migration tasks, purchase a dedicated cluster. For more information, see What is a DTS dedicated cluster.
Retry Time for Failed Connections	The retry time range for failed connections. If the source or destination database fails to be connected after the data migration task is started, DTS immediately retries a connection within the retry time range. Valid values: 10 to 1,440. Unit: minutes. Default value: 720. We recommend that you set the parameter to a value greater than 30. If DTS is reconnected to the source and destination databases within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails. Note If you specify different retry time ranges for multiple data migration tasks that share the same source or destination database, the value that is specified later takes precedence. When DTS retries a connection, you are charged for the DTS instance. We recommend that you specify the retry time range based on your business requirements. You can also release the DTS instance at the earliest opportunity after the source database and destination instance are released.
Retry Time for Other Issues	The retry time range for other issues. For example, if DDL or DML operations fail to be performed after the data migration task is started, DTS immediately retries the operations within the retry time range. Valid values: 1 to 1440. Unit: minutes. Default value: 10. We recommend that you set the parameter to a value greater than 10. If the failed operations are successfully performed within the specified retry time range, DTS resumes the data migration task. Otherwise, the data migration task fails. Important The value of the Retry Time for Other Issues parameter must be smaller than the value of the Retry Time for Failed Connections parameter.
Enable Throttling for Full Data Migration	Specifies whether to enable throttling for full data migration. During full data migration, DTS uses the read and write resources of the source and destination databases. This may increase the loads of the database servers. You can enable throttling for full data migration based on your business requirements. To configure throttling, you must configure the Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s) parameters. This reduces the loads of the destination database server. Note You can configure this parameter only if you select Full Data Migration for the Migration Types parameter.
Enable Throttling for Incremental Data Migration	Specifies whether to enable throttling for incremental data migration. To configure throttling, you must configure the RPS of Incremental Data Migration and Data migration speed for incremental migration (MB/s) parameters. This reduces the loads of the destination database server. Note You can configure this parameter only if you select Incremental Data Migration for the Migration Types parameter.
Environment Tag	You can select an environment tag for the instance as needed. In this example, you do not need to select a tag.
Configure ETL	Specifies whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: configures the ETL feature. You can enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: does not configure the ETL feature.
Monitoring and Alerting	Specifies whether to configure alerting for the data migration task. If the task fails or the migration latency exceeds the specified threshold, the alert contacts receive notifications. Valid values: No: does not configure alerting. Yes: configures alerting. In this case, you must also configure the alert threshold and alert notification settings. For more information, see the Configure monitoring and alerting when you create a DTS task section of the Configure monitoring and alerting topic.

Click Next: Configure Database and Table Fields and then configure the Type, Primary Key Column, Sort Key, Distribution Key, and Partition Key parameters for the table that is migrated.
- DTS provides a default configuration. You can set Definition Status to All and then modify the configuration.
- The Primary Key Column and Sort Key can be composite keys. You can select multiple fields from the corresponding drop-down lists for the Primary Key Column or Sort Key. You must select one or more columns from the Primary Key Column to serve as the Partition Key. You can select only one field as the Distribution Key. For more information about primary key columns, sort keys, and partition keys, see CREATE TABLE.
  Note
  Setting the Partition Key is optional. However, if you set a partition key, it cannot be a nullable field. Otherwise, the migration task fails.
  The partition key supports only the BIGINT, INT, TIMESTAMP, DATETIME, and DATE data types. For information about the calculation logic, see Calculation logic for partition keys.

Save the task settings and run a precheck.
- To view the parameters to be specified when you call the relevant API operation to configure the DTS task, move the pointer over Next: Save Task Settings and Precheck and click Preview OpenAPI parameters.
- If you do not need to view or have viewed the parameters, click Next: Save Task Settings and Precheck in the lower part of the page.
Note
- Before you can start the data migration task, DTS performs a precheck. You can start the data migration task only after the task passes the precheck.
- If the task fails to pass the precheck, click View Details next to each failed item. After you analyze the causes based on the check results, troubleshoot the issues. Then, run a precheck again.
- If an alert is triggered for an item during the precheck:
  If an alert item cannot be ignored, click View Details next to the failed item and troubleshoot the issues. Then, run a precheck again.
  If the alert item can be ignored, click Confirm Alert Details. In the View Details dialog box, click Ignore. In the message that appears, click OK. Then, click Precheck Again to run a precheck again. If you ignore the alert item, data inconsistency may occur, and your business may be exposed to potential risks.

Purchase the instance.

Wait until Success Rate becomes 100%. Then, click Next: Purchase Instance.

On the Purchase Instance page, configure the Instance Class parameter for the data migration instance. The following table describes the parameters.

Section	Parameter	Description
New Instance Class	Resource Group	The resource group to which the data migration instance belongs. Default value: default resource group. For more information, see What is Resource Management?
New Instance Class	Instance Class	DTS provides instance classes that vary in the migration speed. You can select an instance class based on your business scenario. For more information, see Instance classes of data migration instances.

Read and agree to Data Transmission Service (Pay-as-you-go) Service Terms by selecting the check box.
Click Buy and Start. In the message that appears, click OK.
You can view the progress of the task on the Data Migration page.
Note
- If a data migration task cannot be used to migrate incremental data, the task automatically stops. The Completed is displayed in the Status section.
- If a data migration task can be used to migrate incremental data, the task does not automatically stop. The incremental data migration task never stops or completes. The Running is displayed in the Status section.

Appendix

Time information

Data type	Minimum value	Maximum value
Date	1970-01-01 00:00:00	2149-06-06 00:00:00
Date32	1925-01-01 00:00:00	2283-11-11 00:00:00
DateTime	1970-01-01 08:00:00	2106-02-07 14:28:15
DateTime64	1925-01-01 08:00:00	2283-11-12 07:59:59

Table and field information

Tables

If you do not use the object name mapping feature, make sure that the tables you create meet the following requirements:

Important

If a destination table contains the ENGINE parameter, the parameter must be set to ENGINE = ReplicatedReplacingMergeTree(_version, _is_deleted). Otherwise, data inconsistency may occur.

If the destination database is an ApsaraDB for ClickHouse Community-compatible Edition cluster, you must create a local table and a distributed table. The name of the distributed table must be the same as that of the source table. The name of the local table is in the <Distributed table name>+_local format.
If the destination database is an ApsaraDB for ClickHouse Enterprise Edition cluster, you must create a table with the same name as the source table.

Fields

Note

You can execute the select * from table_name final where _sign>0; statement to query data that is synchronized to an ApsaraDB for ClickHouse cluster. In this statement, the WHERE clause can be used to filter the deleted data, and the final field behind the table name can be used to filter the data with the same sort keys.

Version	Field	Data type	Default value	Description
ApsaraDB for ClickHouse Community-compatible Edition cluster that runs ClickHouse V23.8 or earlier	_sign	Int8	1	The type of the DML operation. Valid values: INSERT: 1. UPDATE: 1. DELETE: -1.
	_version	UInt64	1	The timestamp when data is written to the ClickHouse cluster.
ApsaraDB for ClickHouse Enterprise Edition cluster or ApsaraDB for ClickHouse Community-compatible Edition cluster that runs CLickHouse V23.8 or later	_sign	Int8	1	The type of the DML operation. Valid values: INSERT: 1. UPDATE: 1. DELETE: -1.
	_is_deleted	UInt8	0	Specifies whether the data record is deleted. Valid values: Insert: 0. Delete: 1. Update: 0.
	_version	UInt64	1	The timestamp when data is written to the ClickHouse cluster.

Calculation logic for partition keys

Data type of source field	Calculation logic of partition key
BIGINT	`intDiv(" + tablePartKey + ", 18014398509481984)`
INT	`intDiv(" + tablePartKey + ", 4194304)`
TIMESTAMP	`toYYYYMM(" + tablePartKey + ")`
DATETIME
DATE