This topic describes how to migrate schemas, full data, and incremental data between ApsaraDB RDS for PostgreSQL instances by using Data Transmission Service (DTS). When you migrate data between ApsaraDB RDS for PostgreSQL instances, you can select all the supported migration types to ensure service continuity.

Prerequisites

  • The source and destination ApsaraDB RDS for PostgreSQL instances are created. For more information, see Create an ApsaraDB RDS for PostgreSQL instance.
    Note To ensure compatibility, the database engine version of the destination ApsaraDB RDS for PostgreSQL instance is the same as or later than the database engine version of the source ApsaraDB RDS for PostgreSQL instance.

    If the version of the destination database is earlier than the version of the source database, database compatibility issues may occur.

  • The available storage space of the destination ApsaraDB RDS for PostgreSQL instance is larger than the total size of data in the source ApsaraDB RDS for PostgreSQL instance.

Limits

Category Description
Limits on the source database
  • 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:
    • During schema migration and 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 schema migration, 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
  • 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.
  • 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 does not check the validity of metadata such as sequences. You must manually check the validity of metadata.
  • After your workloads are switched to the destination database, newly written sequences do not increment from the maximum value of the sequences in the source database. Therefore, you must query the maximum value of the sequences in the source database before you switch your workloads to the destination database. Then, you must specify the queried maximum value as the starting value of the sequences in the destination database. You can run the following statements to query the maximum value of the sequences in the source database: 
    do language plpgsql $$
declare
  nsp name;
  rel name;
  val int8;
begin
  for nsp,rel in select nspname,relname from pg_class t2 , pg_namespace t3 where t2.relnamespace=t3.oid and t2.relkind='S'
  loop
    execute format($_$select last_value from %I.%I$_$, nsp, rel) into val;
    raise notice '%',
    format($_$select setval('%I.%I'::regclass, %s);$_$, nsp, rel, val+1);
  end loop;
end;
$$;
  • 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.

Migration types

  • Schema migration

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

  • 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 incrementally migrated

Operation type SQL statement
DML INSERT, UPDATE, and DELETE

Permissions required for database accounts

Database Schema migration Full data migration Incremental data migration
Source ApsaraDB RDS for PostgreSQL instance USAGE permission on the pg_catalog schema 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.
Destination ApsaraDB RDS for PostgreSQL instance CREATE and USAGE permissions on the objects to be migrated Permissions of the schema owner

For more information about how to create accounts and grant permissions to accounts for an ApsaraDB RDS for PostgreSQL instance, see Create an account on an ApsaraDB RDS for PostgreSQL instance and Create a database on an ApsaraDB RDS for PostgreSQL instance.

Procedure

  1. Go to the Data Migration page of the new DTS console.
    Note You can also log on to the Data Management (DMS) console. In the top navigation bar, click DTS. Then, in the left-side navigation pane, choose DTS (DTS) > Data Migration.
  2. In the upper-left corner of the page, select the region in which the data migration instance resides.
    Select a region
  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 Select Instance
    The instance that you want to use. You can choose whether to use an existing instance based on your business requirements.
    • If you use an existing instance, DTS automatically applies the parameter settings of the instance.
    • If you do not use an existing instance, you must set parameters for the 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 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.

    Destination Database Select Instance
    The instance that you want to use. You can choose whether to use an existing instance based on your business requirements.
    • If you use an existing instance, DTS automatically applies the parameter settings of the instance.
    • If you do not use an existing instance, you must set parameters for the source database.
    Database Type The type of the destination database. Select PostgreSQL.
    Access Method The access method of the destination database. Select Alibaba Cloud Instance.
    Instance Region The region in which the destination ApsaraDB RDS for PostgreSQL instance resides.
    Instance ID The ID of the destination ApsaraDB RDS for PostgreSQL instance.
    Database Name The name of the destination database in the ApsaraDB RDS for PostgreSQL instance.
    Database Account The database account of the destination ApsaraDB RDS for PostgreSQL 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.

  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.
    • Basic Settings
      Parameter Description
      Task Stages
      • 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 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.
      Select 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.
      Rename Databases and Tables
      • To rename an object that you want to migrate 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.
      Filter data

      You can specify WHERE conditions to filter data. For more information, see Use SQL conditions to filter data.

      Select the SQL operations to be migrated In the Selected Objects section, right-click an object to be migrated. In the dialog box that appears, select the SQL operations that you want to migrate. For more information, see SQL operations that can be incrementally migrated.
    • Advanced SettingsAdvanced Settings
      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.
      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 ensure that the capitalization of object names is consistent with that in the source or destination database. For more information,see Specify the capitalization of object names in the destination instance.

      Retry Time for Failed Connections
      The retry time range for failed connections. Unit: minutes. Valid values: 10 to 720. Default value: 720. We recommend that you set the retry time range to more than 30 minutes. If DTS reconnects to the source and destination databases within the specified 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 value that is set 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 and destination instances are released.
  6. 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.
  7. Wait until the Success Rate becomes 100%. Then, click Next: Purchase Instance.
  8. 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.

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