Migrate data from a PolarDB for MySQL cluster to a MaxCompute project - Data Transmission Service

This topic describes how to migrate data from a PolarDB for MySQL cluster to a MaxCompute project by using Data Transmission Service (DTS).

Prerequisites

MaxCompute is activated and a MaxCompute project is created. See Activate MaxCompute and DataWorks and Creates a MaxCompute project.
An IP address whitelist is configured for the MaxCompute project to allow access from DTS to MaxCompute. See Configure an IP address whitelist to allow access from Alibaba Cloud services to MaxCompute.
An AccessKey pair is created for the Alibaba Cloud account that owns the destination MaxCompute project. For more information, see Create an AccessKey pair.
Note
You can Create a RAM user and Set a RAM user as the super administrator for a MaxCompute project.

Precautions

Note

DTS does not migrate foreign keys in the source database to the destination database. Therefore, the cascade and delete operations of the source database are not migrated to the destination database.

Limit type	Description
Limits on source database	The server on which the source database is deployed 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. If you select tables as the objects to be migrated and you need to edit the tables in the destination database, such as renaming tables or columns, you can migrate up to 1,000 tables 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: The binary logging feature must be enabled and the loose_polar_log_bin parameter must be set to on. Otherwise, error messages are returned during the precheck and the data migration task fails to be started. For more information, see Enable binary logging and Modify parameters. Note If you enable the binary logging feature for a PolarDB for MySQL cluster, you are charged for the storage space that is occupied by binary logs. The binary logs of a PolarDB for MySQL cluster must be stored for at least three days. We recommend that you store the binary logs seven days. Otherwise, DTS may fail to obtain the binary logs and the task may fail. In exceptional circumstances, data inconsistency or loss may occur. Make sure that you configure the retention period of binary logs based on the preceding requirements. Otherwise, the service reliability or performance stated in the Service Level Agreement (SLA) of DTS may not be guaranteed. Note For more information about how to set retention period of the binary logs of a PolarDB for MySQL cluster, see Modify the retention period. Limits on operations to be performed on the source database: During schema migration and full data migration, do not execute DDL statements 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 between the source and destination databases occurs. To ensure data consistency, we recommend that you select schema migration, full data migration, and incremental data migration as the migration types.
Other limits	DTS does not migrate the read-only nodes of the source PolarDB for MySQL cluster. DTS does not migrate Object Storage Service (OSS) external tables from the source PolarDB for MySQL cluster. 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 size of used tablespace of the destination database is larger than that of the source database. DTS attempts to resume data migration tasks that failed within the last seven days. Before you switch workloads to the destination database, you must stop or release the failed tasks. You can also execute the `REVOKE` statement to revoke the write permissions from the accounts that are used by DTS to access the destination database. Otherwise, the data in the source database overwrites the data in the destination database after a failed task is resumed. During data migration, do not use tools such as pt-online-schema-change to perform online DDL operations on the objects to be migrated. Otherwise, the data migration task fails. Data inconsistency between the source and destination databases may occur if data from other sources is written to the destination database during data migration. MaxCompute does not support PRIMARY KEY constraints. If network errors occur, DTS may migrate duplicate data records to the MaxCompute project. 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.

Billing

Migration type	Task configuration fee	Internet traffic fee
Schema migration and full data migration	Free of charge	No fees are charged in this example. Note If the Access Method parameter of the destination database is set to Public IP Address, you are charged for internet traffic fees. For more information, see Billing overview.
Incremental data migration	Charged. For more information, see Billing overview.

Migration types

Schema migration
DTS migrates the schemas of the required objects from the source database to MaxCompute. During schema synchronization, DTS adds the _base suffix to the end of the source table name. For example, if the name of the source table is customer, the name of the table in MaxCompute is customer_base.
Full data migration
DTS migrates the historical data of the table from the source database to the destination table in MaxCompute. For example, the customer table in the source database is migrated to the customer_base table in MaxCompute. The data is the basis for subsequent incremental synchronization.
Note
The destination table that is suffixed with _base is known as a full baseline table.
Incremental data migration
DTS creates an incremental data table in MaxCompute. The name of the incremental data table is the name of the destination table suffixed with _log, such as customer_log. Then, DTS migrates incremental data from the source database to this table in real time.
Note
For more information about the structure of incremental data tables, see Structure of incremental data tables.

SQL operations that can be incrementally migrated

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	ADD COLUMN Note The ADD COLUMN operation that contains attribute columns cannot be migrated.

Permissions required for database accounts

Database type	Required permissions	Creation and Authorization Methods
PolarDB for MySQL	The read permissions on the objects to migrate.	Create and manage database accounts and Manage database account passwords.

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.

Section	Parameter	Description
N/A	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 PolarDB for MySQL.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	The region in which the source PolarDB for MySQL cluster resides.
	Replicate Data Across Alibaba Cloud Accounts	In this example, data is migrated within the same Alibaba Cloud account. Select No.
	PolarDB Cluster ID	The ID of the source PolarDB for MySQL cluster.
	Database Account	The database account of the source PolarDB for MySQL cluster. For more information about the permissions that are required for the database account, see Permissions required for database accounts.
	Database Password	The password that is used to access the database.
	Encryption	Specifies whether to encrypt the connection to the source database. You can configure this parameter based on your business requirements. For more information about the SSL encryption feature, see Configure SSL encryption.
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 MaxCompute.
	Access Method	Select Alibaba Cloud Instance.
	Instance Region	The region in which the destination MaxCompute project resides.
	Project	The name of the destination MaxCompute project.
	AccessKey ID of Alibaba Cloud Account	Enter the AccessKey pair that you prepared in the Prerequisites section.
	AccessKey Secret of Alibaba Cloud Account

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.
Click OK to grant permissions on the MaxCompute project to the built-in account created by DTS. Then, click Test Connectivity and Proceed.

Configure the objects to be migrated.

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

Parameter	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.
Naming Rules of Additional Columns	After DTS migrates data to MaxCompute, DTS adds additional columns to the destination table. If an additional column has the same name as an existing column in the destination table, the data migration task fails. You need to select New Rule or Previous Rule based on your business requirements. Warning Before you specify this parameter, check whether additional columns and existing columns in the destination table have name conflicts. Otherwise, the task may fail or data loss may occur. For more information about the naming rules for additional columns, see Naming rules for additional columns.
Partition Definition of Incremental Data Table	Select partition names based on your business requirements. For more information about partitions, see Partition.
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 You can select tables as the objects to be migrated.
Selected Objects	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 To change a table name to be used in the destination instance, right-click the table in the Selected Objects section. For more information, see Map object names. To filter data in a table to be migrated, right-click the table in the Selected Objects section. In the dialog box that appears, configure filter conditions. For more information, see Specify filter conditions. If you use the object name mapping feature to rename an object, other objects that are dependent on this object may fail to be migrated.

Click Next: Advanced Settings to configure advanced settings.

Parameter	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.
Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Specifies whether to write SQL operations on heartbeat tables to the source database while the DTS instance is running. Valid values: Yes: does not write SQL operations on heartbeat tables. In this case, a latency of the DTS instance may be displayed. No: writes SQL operations on heartbeat tables. In this case, features such as physical backup and cloning of the source database may be affected.
Environment Tag	The environment tag that is used to identify the DTS instance. You can select an environment tag based on your business requirements. In this example, you do not need to configure this parameter.
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.

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 an 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.

Structure of incremental data tables

Note

You must run the set odps.sql.allow.fullscan=true; command in MaxCompute to allow full table scan for the MaxCompute project.

DTS migrates incremental data that is generated in the source MySQL database to the incremental data table in MaxCompute. The incremental data table stores incremental data and specific metadata. The following table shows the schema of an incremental data table.

Field	Description
record_id	The record ID of the incremental log, which is the unique identifier of the log. Note The ID auto-increments for each new log entry. If an UPDATE operation is performed, DTS generates two incremental log entries to record the pre-update and post-update values. The values of the record_id field in the two incremental log entries are the same.
operation_flag	The operation type. Valid values: I: an INSERT operation. D: a DELETE operation. U: a UPDATE operation.
utc_timestamp	The operation timestamp in UTC. It is also the timestamp of the binary log file.
before_flag	Indicates whether the column values are pre-update values. Valid values: Y and N.
after_flag	Indicates whether the column values are post-update values. Valid values: Y and N.