This topic describes how to migrate data from an ApsaraDB RDS for PostgreSQL instance to an ApsaraDB RDS for MySQL instance by using Data Transmission Service (DTS). DTS supports full data migration and incremental data migration. When you configure a data migration task, you can select both supported migration types to ensure service continuity.

Supported source and destination databases

You can use DTS to migrate data between the following types of PostgreSQL and MySQL databases. This topic describes how to migrate data from an ApsaraDB RDS for PostgreSQL instance to an ApsaraDB RDS for MySQL instance. You can follow the procedure to configure data migration tasks for other scenarios.
  • Migrate data from an ApsaraDB RDS for PostgreSQL instance to an ApsaraDB RDS for MySQL instance
  • Migrate data from a self-managed PostgreSQL database to a self-managed MySQL database

Prerequisites

  • The source ApsaraDB RDS for PostgreSQL instance is created. For more information, see Create an ApsaraDB RDS for PostgreSQL instance.
  • The destination ApsaraDB RDS for MySQL instance is created. For more information, see Create an ApsaraDB RDS for MySQL instance.
  • The available storage space of the destination ApsaraDB RDS for MySQL instance is larger than the total size of the data in the source ApsaraDB RDS for PostgreSQL instance.

Usage notes

Category Description
Limits on the source database
  • The server to which the source database belongs must have sufficient outbound bandwidth. Otherwise, the data migration speed decreases.
  • The tables to be migrated must have PRIMARY KEY or UNIQUE constraints, and all fields must be unique. Otherwise, the destination database may contain duplicate data records.

    The name of the source database cannot contain hyphens (-). For example, dts-testdata is not allowed.

  • If you select tables as the objects to be migrated and you need to edit tables, such as renaming tables or columns in the destination database, up to 1,000 tables can be migrated in a single data migration task. If you run a task to migrate more than 1,000 tables, a request error occurs. In this case, we recommend that you configure multiple tasks to migrate the tables or configure a task to migrate the entire database.
  • If you need to migrate incremental data, make sure that the following requirements are met:
    • The value of the wal_level parameter must be set to logical.

    • For incremental data migration, the write-ahead logging (WAL) logs of the source database must be stored for more than 24 hours. For full data and incremental data migration, the WAL logs of the source database must be stored for at least seven days. Otherwise, DTS may fail to obtain the WAL logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. After full data migration is complete, you can set the retention period to more than 24 hours. Make sure that you set the retention period of WAL logs based on the preceding requirements. Otherwise, the service reliability and performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed.

  • Limits on operations to be performed on the source database:
    • If you perform a primary/secondary switchover on a self-managed PostgreSQL database, the data migration task fails.
    • If you need to perform a primary/secondary switchover on the source ApsaraDB RDS for PostgreSQL instance, the Logical Replication Slot Failover feature must be enabled. This prevents logical subscriptions from being interrupted and ensures that your data migration task can run as expected. For more information, see Logical Replication Slot Failover.
    • During full data migration, do not perform DDL operations to change the schemas of databases or tables. Otherwise, the data migration task fails.
    • If you perform only full data migration, do not write data to the source database during data migration. Otherwise, data inconsistency may occur between the source and destination databases. To ensure data consistency, we recommend that you select full data migration and incremental data migration as the migration types.
  • If the source database has long-running transactions and the task contains incremental data migration, the WAL logs that are generated before the long-running transactions are submitted may not be cleared and therefore pile up, resulting in insufficient storage space in the source database.
Other limits
  • Schema migration is not supported. Before you configure a data migration task, you must create databases and tables in the destination instance.
  • A data migration task can migrate data from only a single database. To migrate data from multiple databases, you must create a data migration task for each database.
  • If you select a schema as the object to be migrated and create a table in the schema or execute the RENAME statement to rename the table during incremental data migration, you must execute the ALTER TABLE schema.table REPLICA IDENTITY FULL; statement before you write data to the table.
    Note Replace the schema and table variables in the preceding statement with your schema name and table name.
  • DTS creates the following temporary tables in the source database to obtain the DDL statements of incremental data, the schemas of incremental tables, and the heartbeat information. During data migration, do not delete temporary tables in the source database. Otherwise, exceptions occur. After the DTS instance is released, temporary tables are automatically deleted.

    public.DTS_PG_CLASS, public.DTS_PG_ATTRIBUTE, public.DTS_PG_TYPE, public.DTS_PG_ENUM, public.DTS_POSTGRES_HEARTBEAT, public.DTS_DDL_COMMAND, and public.DTS_ARGS_SESSION.

  • To ensure that the latency of incremental data migration is accurate, DTS adds a heartbeat table to the source database. The name of the heartbeat table is dts_postgres_heartbeat.
  • During incremental data migration, DTS creates a replication slot for the source database. The replication slot is prefixed with dts_sync_. DTS automatically clears historical replication slots every 90 minutes to reduce storage usage.
    Note
    • After the DTS instance is released, the replication slot is automatically deleted. If you change the password of the source database or delete the IP address whitelist of DTS, the replication slot cannot be automatically deleted. In that case, you must delete it from the source database to prevent it from piling up.
    • If the data migration task is released or fails, DTS automatically clears the replication slot. If a primary/secondary switchover is performed on the source self-managed PostgreSQL database, you must log on to the secondary database to clear the replication slot.
    Replication slot information
  • Before you migrate data, evaluate the impact of data migration on the performance of the source and destination databases. We recommend that you migrate data during off-peak hours. 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.
  • During full data migration, concurrent INSERT operations cause fragmentation in the tables of the destination database. After full data migration is complete, the tablespace of the destination database is larger than that of the source database.
  • Make sure that the precision settings for columns of the FLOAT or DOUBLE data type meet your business requirements. DTS uses the ROUND(COLUMN,PRECISION) function to retrieve values from columns of the FLOAT or DOUBLE data type. If you do not specify a precision, DTS sets the precision for the FLOAT data type to 38 digits and the precision for the DOUBLE data type to 308 digits.
  • DTS attempts to resume data migration tasks that failed within the last seven days. Before you switch workloads to the destination database, stop or release the data migration task. You can also execute the REVOKE statement to revoke the write permissions from the accounts used by DTS to access the destination database. Otherwise, the data in the source database will overwrite the data in the destination database after the task is resumed.
Special cases

  • During data migration, do not modify the endpoint or zone of the source ApsaraDB RDS for PostgreSQL instance. Otherwise, the data migration task fails.

  • If you migrate data to an ApsaraDB RDS for MySQL instance, DTS automatically creates a destination database in the ApsaraDB RDS for MySQL instance. However, if the name of the source database is invalid, you must manually create a database in the ApsaraDB RDS for MySQL instance before you configure the data migration task. For more information, see Create a database.

Billing

Migration type Instance configuration fee Internet traffic fee
Full data migration Free of charge. Charged only when data is migrated from Alibaba Cloud over the Internet. For more information, see Billing overview.
Incremental data migration Charged. For more information, see Billing overview.

Migration types

  • Full data migration

    DTS migrates historical data of required objects from the source database to the destination database.

  • Incremental data migration

    After full data migration is complete, DTS migrates incremental data from the source database to the destination database. Incremental data migration allows you to ensure service continuity when you migrate data between self-managed databases.

SQL operations that can be migrated

Operation type SQL statement
DML INSERT, UPDATE, and DELETE

Permissions required for database accounts

Database Full data migration Incremental data migration References
ApsaraDB RDS for PostgreSQL instance SELECT permission on the objects to be migrated Permissions of a privileged account. The account must be the owner of the database.
Note If the version of the source ApsaraDB RDS for PostgreSQL instance is 9.4 and you migrate only DML operations, only the REPLICATION permission is required for the database account.
Create an account on an ApsaraDB RDS for PostgreSQL instance and Create a database on an ApsaraDB RDS for PostgreSQL instance
ApsaraDB RDS for MySQL instance Read and write permissions Create an account on an ApsaraDB RDS for MySQL instance and Modify the permissions of a standard account on an ApsaraDB RDS for MySQL instance

Procedure

  1. Go to the Data Migration Tasks page.
    1. Log on to the Data Management (DMS) console.
    2. In the top navigation bar, click DTS.
    3. In the left-side navigation pane, choose DTS (DTS) > Data Migration.
    Note You can also configure the settings by using the new DTS console.
  2. From the drop-down list to the right of Data Migration Tasks, select the region in which your data migration instance resides.
    Note If you use the new DTS console, select the region in which your data migration instance resides in the upper-left corner.
  3. Click Create Task. On the page that appears, configure the source and destination databases.
    Warning After you select the source and destination instances, we recommend that you read the limits displayed at the top of the page. This helps you create and run the data migration task.
    Section Parameter Description
    N/A Task Name

    The name of the task. DTS automatically generates a task name. We recommend that you specify an informative name to identify the task. You do not need to specify a unique task name.

    Source Database Database Type The type of the source database. Select PostgreSQL.
    Access Method The access method of the source database. Select Alibaba Cloud Instance.
    Instance Region The region in which the source ApsaraDB RDS for PostgreSQL instance resides.
    Instance ID The ID of the source ApsaraDB RDS for PostgreSQL instance.
    Database Name The name of the source database in the ApsaraDB RDS for PostgreSQL instance.
    Database Account The database account of the source ApsaraDB RDS for PostgreSQL instance. For information about the permissions that are required for the account, see Permissions required for database accounts.
    Database Password

    The password of the database account.

    Destination Database Database Type The type of the destination database. Select MySQL.
    Access Method The access method of the destination database. Select Alibaba Cloud Instance.
    Instance Region The region in which the destination ApsaraDB RDS for MySQL instance resides.
    RDS Instance ID The ID of the destination ApsaraDB RDS for MySQL instance.
    Database Account The database account of the destination ApsaraDB RDS for MySQL instance. For more information about the permissions that are required for the account, see Permissions required for database accounts.
    Database Password

    The password of the database account.

    Encryption

    Specifies whether to encrypt the connection to the source database. You can select Non-encrypted or SSL-encrypted based on your business requirements. If you want to select SSL-encrypted, you must enable SSL encryption for the ApsaraDB RDS for MySQL instance before you configure the data migration task. For more information, see Configure SSL encryption for an ApsaraDB RDS for MySQL instance.

  4. In the lower part of the page, click Test Connectivity and Proceed.
    Warning
    • If the source or destination database instance is an Alibaba Cloud database instance, such as an ApsaraDB RDS for MySQL or ApsaraDB for MongoDB instance, or is a self-managed database hosted on Elastic Compute Service (ECS), DTS automatically adds the CIDR blocks of DTS servers to the whitelist of the database instance or ECS security group rules. For more information, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases. If the source or destination database is a self-managed database on data centers or is from other cloud service providers, you must manually add the CIDR blocks of DTS servers to allow DTS to access the database.
    • If the CIDR blocks of DTS servers are automatically or manually added to the whitelist of the database, Alibaba Cloud database instance, or ECS security group rules, security risks may arise. Therefore, before you use DTS to migrate data, you must understand and acknowledge the potential risks and take preventive measures, including but not limited to the following measures: enhancing the security of your username and password, limiting the ports that are exposed, authenticating API calls, regularly checking the whitelist or ECS security group rules and forbidding unauthorized CIDR blocks, or connecting the database to DTS by using Express Connect, VPN Gateway, or Smart Access Gateway.
    • After the DTS task is complete or released, we recommend that you manually detect and remove the added CIDR blocks from the whitelist of the database, Alibaba Cloud database instance, or ECS security group rules.
  5. Select objects for the task and configure advanced settings.
    Parameter Description
    Task Stages
    • To perform only full data migration, select only Full Data Migration.
    • To ensure service continuity during data migration, select Full Data Migration and Incremental Data Migration.
    Note If Incremental Data Migration is not selected, we recommend that you do not write data to the source instance during data migration. This ensures data consistency between the source and destination instances.
    Processing Mode of Conflicting Tables

    • Precheck and Report Errors: checks whether the destination database contains tables that have the same names as tables in the source database. If the source and destination databases do not contain tables that have the same names, the precheck is passed. Otherwise, an error is returned during the precheck and the data migration task cannot be started.

      Note You can use the object name mapping feature to rename the tables that are migrated to the destination database. You can use this feature if the source and destination databases contain tables that have identical names and the tables in the destination database cannot be deleted or renamed. 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 consistency is not ensured, and your business may be exposed to potential risks.
      • If the source and destination databases have the same schema, DTS does not migrate data records that have the same primary keys as data records in the destination database.
      • If the source and destination databases have different schemas, only specific columns are migrated or the data migration task fails.
    Source Objects

    Select one or more objects from the Source Objects section and click the Rightwards arrow icon to add the objects to the Selected Objects section.

    Note You can select columns, tables, or schemas as the objects to be migrated. If you select tables or columns as the objects to be migrated, DTS does not migrate other objects such as views, triggers, or stored procedures to the destination database.
    Selected Objects
    • To rename an object that you want to synchronize to the destination instance, right-click the object in the Selected Objects section. For more information, see Map the name of a single object.
    • To rename multiple objects at a time, click Batch Edit in the upper-right corner of the Selected Objects section. For more information, see Map multiple object names at a time.
    Note
    • If you use the object name mapping feature to rename an object, other objects that are dependent on the object may fail to be migrated.
    • To specify WHERE conditions to filter data, right-click an object in the Selected Objects section. In the dialog box that appears, specify the conditions. For more information, see Use SQL conditions to filter data.
    • To select the SQL operations performed on a specific database or table, right-click an object in the Selected Objects section. In the dialog box that appears, select the SQL operations that you want to migrate. For more information about the SQL operations that can be migrated, see the SQL operations that can be migrated section of this topic.
    • Basic Settings
    • Advanced SettingsAdvanced Settings (Not Replicate Temporary Tables When DMS Performs DDL Operations)

  6. Parameter Description
    Set Alerts
    Specifies whether to set alerts for the data migration task. If the task fails or the migration latency exceeds the threshold, the alert contacts will receive notifications. Valid values:
    • No: does not set alerts.
    • Yes: sets alerts. If you select Yes, you must also set the alert threshold and alert contacts.
    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 time range. Valid values: 10 to 1440. Unit: minutes. Default value: 120. We recommend that you set the parameter to a value greater than 30. If DTS reconnects 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 set different retry time ranges for multiple data migration tasks that have the same source or destination database, the shortest retry time range that is set 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 your earliest opportunity after the source and destination instances are released.
    Configure ETL
    Specify whether to configure the extract, transform, and load (ETL) feature. For more information, see What is ETL?.
  7. In the lower part of the page, click Next: Save Task Settings and Precheck.
    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, you can click the Info icon icon next to each failed item to view details.
      • You can troubleshoot the issues based on the causes and run a precheck again.
      • If you do not need to troubleshoot the issues, you can ignore failed items and run a precheck again.
  8. Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.
  9. On the Purchase Instance page, specify the Instance Class parameter for the data migration instance. The following table describes the parameter.
    Section Parameter Description
    Parameters Instance Class

    DTS provides several instance classes that have different performance in migration speed. You can select an instance class based on your business scenario. For more information, see Specifications of data migration instances.

  10. Read and select Data Transmission Service (Pay-as-you-go) Service Terms.
  11. Click Buy and Start to start the data migration task. You can view the progress of the task in the task list.