Help Center/ Data Replication Service/ Real-Time Synchronization/ To the Cloud/ From MySQL to PostgreSQL
Updated on 2023-04-26 GMT+08:00
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From MySQL to PostgreSQL

Supported Source and Destination Databases

Table 1 Supported databases

Source DB

Destination DB
  • On-premises MySQL databases
  • MySQL databases on an ECS
  • MySQL databases on other clouds
  • RDS for MySQL
  • RDS for PostgreSQL

Prerequisites

  • You have logged in to the DRS console.
  • Your account balance is greater than or equal to $0 USD.
  • For details about the DB types and versions supported by real-time synchronization, see Supported Databases.
  • If a subaccount is used to create a DRS task, ensure that an agency has been added. To create an agency, see Agency Management.

Suggestions

  • When a task is being started or in the full synchronization phase, do not perform DDL operations on the source database. Otherwise, the task may be abnormal.
  • To keep data consistency before and after the synchronization, ensure that no data is written to the destination database during the synchronization.
  • The success of database synchronization depends on environment and manual operations. To ensure a smooth synchronization, perform a synchronization trial before you start the synchronization to help you detect and resolve problems in advance.
  • Start your synchronization task during off-peak hours. A less active database is easier to synchronize successfully. If the data is fairly static, there is less likely to be any severe performance impacts during the synchronization.
    • If network bandwidth is not limited, the query rate of the source database increases by about 50 MB/s during full synchronization, and two to four CPUs are occupied.
    • To ensure data consistency, tables to be synchronized without a primary key may be locked for 3s.
    • The data being synchronized may be locked by other transactions for a long period of time, resulting in read timeout.
    • Due to the inherent characteristics of MySQL, in certain scenarios the performance may be negatively affected. For example, if the CPU resources are insufficient and the storage engine is TokuDB, the read speed on tables may be decreased by 10%.
    • When DRS concurrently reads data from a database, it will use about 6 to 10 sessions. The impact of the connections on services must be considered.
    • If you read a table, especially a large table, during the full migration, the exclusive lock on that table may be blocked.
    • For more information about the impact of DRS on databases, see What Is the Impact of DRS on Source and Destination Databases?
  • Data-Level Comparison

    To obtain accurate comparison results, compare data at a specified time point during off-peak hours. If it is needed, select Start at a specified time for Comparison Time. Due to slight time difference and continuous operations on data, data inconsistency may occur, reducing the reliability and validity of the comparison results.

Precautions

Before creating a synchronization task, read the following notes:

  • You are advised to create an independent database account for DRS task connection to prevent task failures caused by database account password modification.
  • After changing the account passwords for the source or destination databases, modify the connection information in the DRS task as soon as possible to prevent automatic retry after a task failure. Automatic retry will lock the database accounts.
Table 2 Precautions

Type

Restrictions

Database permissions
  • The source database user must have the following minimum permissions.
    • Minimum permission for full synchronization: SELECT.
    • Minimum permission for full and incremental synchronization: SELECT, LOCK TABLES, REPLICATION SLAVE, and REPLICATION CLIENT
  • The destination database user must have the minimum permissions: the default permissions of an RDS PostgreSQL instance account

Synchronization object
  • Only tables, indexes, and (primary key, unique, null, not null) constraints can be synchronized. Views, foreign keys, stored procedures, triggers, functions, events, and virtual columns cannot be synchronized.
  • MySQL views support syntax "as select ... from a join b where ..." but PostgreSQL does not, which may lead to the synchronization task to fail.
  • The following data types are not supported: XML, geometry, point, lineString, polygon, geometrycollection, multipoint, multilinestring, multipolygon, and json.
  • Tables with storage engine different to MyISAM and InnoDB cannot be synchronized.
  • If there is a table containing fields of the longtext or longblob type in the synchronization object, you are advised to create a DRS task with large specifications. Otherwise, capture OOM may occur.

Source database
  • The names of the source databases and tables cannot contain non-ASCII characters, or special characters <'>.`/\"
  • During the incremental synchronization, the binlog of the source MySQL database must be enabled and use the row-based format.
  • If the storage space is sufficient, store the source database binlog for as long as possible. The recommended retention period is three days. If this period is set to 0, the synchronization may fail.
    • If the source database is an on-premises MySQL database, set expire_logs_days to specify the binlog retention period. Set expire_logs_day to a proper value to ensure that the binlog does not expire before data transfer resumes. This ensures that services can be recovered after interruption.
    • If the source database is an RDS for MySQL instance, set the binlog retention period by following the instructions provided in RDS User Guide.
  • GTID must be enabled for the source database. If GTID is not enabled for the source database, primary/standby switchover is not supported. DRS tasks will be interrupted and cannot be restored during a switchover.
  • The server_id value of the source MySQL database must be in the range from 1 to 4294967296.

Destination database
  • The destination DB instance is running properly. If the destination DB instance type is RDS primary/standby instance, the replication status must also be normal.
  • The destination DB instance must have sufficient storage space.
  • The time zone of the destination database must be the same as that of the source database.

Precautions
  • If the DCC does not support instances with 4 vCPUs and 8 GB memory or higher instance specifications, the synchronization task cannot be created.
  • Objects that have dependencies must be synchronized at the same time to avoid synchronization failure. Common associations: tables or views referenced by views
  • If the source and destination sides are RDS for MySQL instances, tables encrypted using TDE cannot be synchronized.
  • If the source MySQL database does not support TLS 1.2 or is a self-built database of an earlier version (earlier than 5.6.46 or between 5.7.0 and 5.7.28), you need to submit an O&M application for testing the SSL connection.
  • If the network is reconnected within 30 seconds, real-time synchronization will not be affected. If the network is interrupted for more than 30 seconds, the synchronization task will fail.
  • You can map multiple databases to one database for synchronization. Tables with the same name cannot exist in mapped databases. To use this function, submit a service ticket. In the upper right corner of the management console, choose Service Tickets > Create Service Ticket to submit a service ticket.
  • The table without a primary key lacks a unique identifier for rows. When the network is unstable, you may need to retry the task several times, or data inconsistency may occur.
  • Different types of indexes synchronized to the destination database will become B-Tree indexes.
  • If the character sets of the source and destination databases are different, data may be inconsistent or synchronization may fail.
  • If the data types are incompatible, the synchronization may fail.
  • The destination table can contain more columns than the source table. However, the following failures must be avoided:
    • Assume that extra columns on the destination cannot be null or have default values. If newly inserted data records are synchronized from the source to the destination, the extra columns will become null, which does not meet the requirements of the destination and will cause the task to fail.
    • Assume that extra columns on the destination must be fixed at a default value and have a unique constraint. If newly inserted data records are synchronized from the source to the destination, the extra columns will contain multiple default values. That does not meet the unique constraint of the destination and will cause the task to fail.
  • Only data that violates the non-null constraint and data of the char or varchar type that exceeds the field length limit can be recorded.
  • The source database cannot be restored.
  • Cascade operations cannot be performed on tables with foreign keys. If the foreign key index of a table is a common index, the table structure may fail to be created. You are advised to use a unique index.
  • Binlogs cannot be forcibly deleted. Otherwise, the synchronization task fails.
  • The source database does not support the reset master or reset master to command, which may cause DRS task failures or data inconsistency.
  • Do not use an imprecise value type as the primary key in the database. This feature affects the synchronization of UPDATE and DELETE statements in the DRS incremental scenario.
  • If tables without primary keys contain large fields (BLOB, TEXT, CLOB, NCLOB, or BYTEA), data of the large fields may be inconsistent during incremental synchronization.
  • If you change the port of the RDS source database and the synchronization task fails, retry the task.
  • During real-time synchronization, if the source is not RDS, the port cannot be changed.
  • During real-time synchronization, the IP address, account, and password cannot be changed.
  • DDL operations are not supported during full synchronization.
  • During incremental synchronization, some DDL operations are supported.
    • DROP_DATABASE, DROP_TABLE, TRUNCATE_TABLE, CREATE_VIEW and DROP_VIEW are not supported.
    • Online DDL is not supported.
    • Table fields can be added, deleted, and modified. For example: 
      alter table `ddl_test` add column `c2` varchar(25); 
alter table `ddl_test` modify column `c1` varchar(50);
alter table `ddl_test` alter c1 set default 'xxx';
    • Table indexes can be modified. For example: 
      alter table `ddl_test` drop primary key; 
alter table `ddl_test` add primary key(id); 
alter table `ddl_test` add index  `ddl_test_uk`(id);
alter table `ddl_test` drop index `ddl_test_uk`;
    • In table-level synchronization, you can add columns, modify columns, and add primary keys and normal indexes. If you rename a column in many-to-one synchronization, you must stop services. Otherwise, data inconsistency may occur. For example: 
      alter table `ddl_test` modify column `c1` varchar(50);
    • The name of a table, column, or index to be added or modified cannot exceed 63 characters. Otherwise, the task fails.
  • Set the expire_log_day parameter to a proper value to ensure that the binlog does not expire before data transfer resumes. This ensures that services can be recovered after interruption.
  • During a full synchronization, DRS writes large amount of data to the destination PostgreSQL database. As a result, the number of PostgreSQL WAL logs increases sharply, and the PostgreSQL disk space may be used up. You can disable the PostgreSQL log backup function before the full synchronization to reduce the number of WAL logs. After the synchronization is complete, enable the function.
    CAUTION:

    Disabling log backup will affect database disaster recovery. Exercise caution when performing this operation.

  • If the source table to be synchronized has the AUTO_INCREMENT attribute, DRS automatically updates the start value of the PostgreSQL auto-increment sequence corresponding to the integer sequence of the table when the task is complete. The updated value is the maximum value of the sequence plus 10,000.

Procedure

  1. On the Data Synchronization Management page, click Create Synchronization Task.
  2. On the Create Synchronization Instance page, select a region and project, specify the task name, description, and the synchronization instance details, and click Create Now.

    • Task information description
      Figure 1 Synchronization task information
      Table 3 Task and recipient description

      Parameter

      Description

      Region

      The region where the synchronization instance is deployed. You can change the region.

      Project

      The project corresponds to the current region and can be changed.

      Task Name

      The task name must start with a letter and consist of 4 to 50 characters. It can contain only letters, digits, hyphens (-), and underscores (_).

      Description

      The description consists of a maximum of 256 characters and cannot contain special characters !=<>'&"\
    • Synchronization instance details
      Figure 2 Synchronization instance details
      Table 4 Synchronization instance settings

      Parameter

      Description

      Data Flow

      Select To the cloud.

      Source DB Engine

      Select MySQL.

      Destination DB Engine

      Select PostgreSQL.

      Network Type

      Available options: VPC, Public network and VPN or Direct Connect. Public network is used as an example.

      Destination DB Instance

      The RDS PostgreSQL DB instance.

      Synchronization Instance Subnet

      Select the subnet where the synchronization instance is located. You can also click View Subnet to go to the network console to view the subnet where the instance resides.

      By default, the DRS instance and the destination DB instance are in the same subnet. You need to select the subnet where the DRS instance resides and ensure that there are available IP addresses. To ensure that the synchronization instance is successfully created, only subnets with DHCP enabled are displayed.

      Synchronization Mode
      • Full+Incremental

        This synchronization mode allows you to synchronize data in real time. After a full synchronization initializes the destination database, an incremental synchronization parses logs to ensure data consistency between the source and destination databases.

      • Full

        All objects and data in non-system databases are synchronized to the destination database at a time. This mode is applicable to scenarios where service interruption is acceptable.
    • DRS instance specifications
      Figure 3 Specifications
      Table 5 Specifications

      Parameter

      Description

      Specifications

      DRS instance specifications. Different specifications have different performance upper limits. For details, see Real-Time Synchronization.

      NOTE:

      Currently, DRS supports specification upgrade only in MySQL-to-MySQL synchronization tasks with single-node DRS instances configured. Task specifications cannot be downgraded. For details, see Changing Specifications.
    • Enterprise Projects and Tags
      Figure 4 Enterprise projects and tags
      Table 6 Enterprise Projects and Tags

      Parameter

      Description

      Enterprise Project

      An enterprise project you would like to use to centrally manage your Global Accelerator resources. Select an enterprise project from the drop-down list. The default project is default. For more information about enterprise project, see Enterprise Management User Guide.

      To customize an enterprise project, click Enterprise in the upper right corner of the console. The Enterprise Management page is displayed. For details, see Creating an Enterprise Project in Enterprise Management User Guide.

      Tags
      • This setting is optional. Adding tags helps you better identify and manage your tasks. Each task can have up to 10 tags.
      • After a task is created, you can view its tag details on the Tags tab. For details, see Tag Management.

    If a task fails to be created, DRS retains the task for three days by default. After three days, the task automatically ends.

  3. On the Configure Source and Destination Databases page, wait until the synchronization instance is created. Then, specify source and destination database information and click Test Connection for both the source and destination databases to check whether they have been connected to the synchronization instance. After the connection tests are successful, select the check box before the agreement and click Next.

    Figure 5 Source database information
    Table 7 Self-build on ECS - source database information

    Parameter

    Description

    IP Address or Domain Name

    The IP address or domain name of the source database.

    Port

    The port of the source database. Range: 1 – 65535

    Database Username

    The username for accessing the source database.

    Database Password

    The password for the database username.

    SSL Connection

    SSL encrypts the connections between the source and destination databases. If SSL is enabled, upload the SSL CA root certificate.

    NOTE:
    • The maximum size of a single certificate file that can be uploaded is 500 KB.
    • If SSL is not enabled, your data may be at risk.

    The IP address, domain name, username, and password of the source database are encrypted and stored in DRS, and will be cleared after the task is deleted.

    Figure 6 Destination database information
    Table 8 Destination database settings

    Parameter

    Description

    DB Instance Name

    The RDS PostgreSQL instance you selected when creating the migration task and cannot be changed.

    Database Username

    The username for accessing the destination database.

    Database Password

    The password for the database username.

    The username and password of the destination database are encrypted and stored in the database and the synchronization instance during the synchronization. After the task is deleted, the username and password are permanently deleted.

  4. On the Set Synchronization Task page, select the synchronization policy and synchronization object, and click Next.

    Figure 7 Synchronization mode
    Table 9 Synchronization Object

    Parameter

    Description

    Flow Control

    You can choose whether to control the flow.

    • Yes

      You can customize the maximum migration speed.

      In addition, you can set the time range based on your service requirements. The traffic rate setting usually includes setting of a rate limiting time period and a traffic rate value. Flow can be controlled all day or during specific time ranges. The default value is All day. A maximum of three time ranges can be set, and they cannot overlap.

      The flow rate must be set based on the service scenario and cannot exceed 9,999 MB/s.

      Figure 8 Flow control
    • No
      The synchronization speed is not limited and the outbound bandwidth of the source database is maximally used, which will increase the read burden on the source database. For example, if the outbound bandwidth of the source database is 100 MB/s and 80% bandwidth is used, the I/O consumption on the source database is 80 MB/s.
      NOTE:
      • The flow control mode takes effect only in the full synchronization phase.
      • You can also change the flow control mode after creating a task. For details, see Modifying the Flow Control Mode.

    Synchronization Object Type

    Available options: Table structure, Data, and Constraint

    • Data is selected by default.
    • If Table structure is selected, the destination database cannot contain tables whose names are the same as the source tables to be synchronized.
    • If Table structure is not selected, the destination database must have tables that match the source tables, and the table structure must be the same as the selected source table structures.

    Incremental Conflict Policy

    The conflict policy refers to the conflict handling policy during incremental synchronization. By default, conflicts in the full synchronization phase are ignored. Select any of the following conflict policies:

    • Ignore

      The system will skip the conflicting data and continue the subsequent synchronization process.

    • Report error

      The synchronization task will be stopped and fail.

    • Overwrite

      Conflicting data will be overwritten.

    Filter DROP DATABASE

    During real-time synchronization, executing DDL operations on the source database may affect the synchronization performance. To reduce the risk of synchronization failure, DRS allows you to filter out DDL operations. Currently, only the delete operations on databases can be filtered by default.

    • If you select Yes, the database deletion operation performed on the source database is not synchronized during data synchronization.
    • If you select No, related operations are synchronized to the destination database during data synchronization.

    Data Synchronization Topology

    This parameter is available when Incremental DDLs is selected for Synchronize. Data synchronization supports multiple synchronization topologies. You can plan your synchronization instances based on service requirements. For details, see Data Synchronization Topologies.

    NOTE:

    Data Synchronization Topology can be selected only for whitelisted users. You need to submit a service ticket to apply for this function. In the upper right corner of the management console, choose Service Tickets > Create Service Ticket to submit a service ticket.

    Synchronize DDLs

    You can select the DDL types for incremental synchronization as required. The DDL type depends on the application scenario.

    • One-to-one and one-to-many scenarios: If the DDL usage of the source and destination databases must be consistent, high-risk DDLs must be synchronized. If you do not want a high-risk DDL to be performed in the destination, deselect the high-risk DDL to protect destination data. However, this may cause the synchronization to fail. However, filtering DDL may cause synchronization to fail, for example, column deletion.
    • Many-to-one scenarios: Synchronize only the Add Column operation, or tasks may fail or data may be inconsistent due to changes in destination tables.
    NOTE:

    Only whitelisted users can synchronize incremental DDL operations. You need to submit a service ticket to apply for this function. In the upper right corner of the management console, choose Service Tickets > Create Service Ticket to submit a service ticket.

    Synchronize

    Incremental DDL synchronization is supported. You can determine whether to synchronize data based on service requirements.

    Synchronization Object

    The left pane displays the source database objects, and the right pane displays the selected objects. DRS supports table-level synchronization. You can select data for synchronization based on your service requirements.

    If the synchronization objects in source and destination databases have different names, you can map the source object name to the destination one. For details, see Mapping Object Names.

    NOTE:
    • To quickly select the desired database objects, you can use the search function.
    • If there are changes made to the source databases or objects, click in the upper right corner to update the objects to be synchronized.
    • If the object name contains spaces, the spaces before and after the object name are not displayed. If there are multiple spaces between the object name and the object name, only one space is displayed.
    • The name of the selected synchronization object cannot contain spaces.

  5. On the Check Task page, check the synchronization task.

    • If any check fails, review the cause and rectify the fault. After the fault is rectified, click Check Again.

      For details about how to handle check failures, see Checking Whether the Source Database Is Connected in Data Replication Service User Guide.

    • If all check items are successful, click Next.
      Figure 9 Pre-check

      You can proceed to the next step only when all checks are successful. If there are any items that require confirmation, view and confirm the details first before proceeding to the next step.

  6. On the displayed page, specify Start Time, Send Notification, SMN Topic, Synchronization Delay Threshold, and Stop Abnormal Tasks After, confirm that the configured information is correct, select the check box before the agreement, and click Submit to submit the task.

    Figure 10 Task startup settings
    Table 10 Task startup settings

    Parameter

    Description

    Started Time

    Set Start Time to Start upon task creation or Start at a specified time based on site requirements.

    NOTE:

    After a synchronization task is started, the performance of the source and destination databases may be affected. You are advised to start a synchronization task during off-peak hours.

    Send Notifications

    SMN topic. This parameter is optional. If an exception occurs during synchronization, the system will send a notification to the specified recipients.

    SMN Topic

    This parameter is available only after you enable Send Notifications and create a topic on the SMN console and add a subscriber.

    For details, see Simple Message Notification User Guide.

    Synchronization Delay Threshold

    During an incremental synchronization, a synchronization delay indicates a time difference (in seconds) of synchronization between the source and destination database.

    If the synchronization delay exceeds the threshold you specify, DRS will send alarms to the specified recipients. The value ranges from 0 to 3,600. To avoid repeated alarms caused by the fluctuation of delay, an alarm is sent only after the delay has exceeded the threshold for six minutes.

    NOTE:
    • If the delay threshold is set to 0, no notifications will be sent to the recipient.
    • In the early stages of an incremental synchronization, the synchronization delay is long because a large quantity of data is awaiting synchronization. In this case, no notifications will be sent.
    • Before setting the delay threshold, enable Send Notification.

    Stop Abnormal Tasks After

    Number of days after which an abnormal task is automatically stopped. The value must range from 14 to 100. The default value is 14.

    NOTE:

    Tasks in the abnormal state are still charged. If tasks remain in the abnormal state for a long time, they cannot be resumed. Abnormal tasks run longer than the period you set (unit: day) will automatically stop to avoid unnecessary fees.

  7. After the task is submitted, you can view and manage it on the Data Synchronization Management page.

    • You can view the task status. For more information about task status, see Task Statuses.
    • You can click in the upper-right corner to view the latest task status.
    • By default, DRS retains a task in the Configuration state for three days. After three days, DRS automatically deletes background resources, and the task status remains unchanged. When you restart the task configuration, DRS applies for resources again.

Parent topic: To the Cloud