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PostgreSQL Migration and Synchronization to ClickHouse

NineData data replication supports copying data from self-built PostgreSQL to ClickHouse, supporting both structure and full data replication.

Feature Introduction​

NineData data replication supports high-performance replication of structure, full data, and incremental data between data sources. For certain data sources, it also provides bidirectional replication, enabling the quick construction of geo-distributed active-active business architectures.

  • Structure: Supports the replication of object structures between homogeneous and heterogeneous data sources, greatly reducing the barrier to data replication between two sources.
  • Full Data: Achieves row-level concurrent batch replication through intelligent data sharding, effectively ensuring replication performance. The independently developed novel breakpoint resume technology ensures the accuracy of data in tables without primary keys.
  • Incremental Data: Supports DML and DDL incremental data replication for all object types. By leveraging technologies such as row-level concurrency and hotspot merging, it delivers robust replication performance.
  • Bidirectional Real-time Data Replication (only between MySQL instances): Direct bidirectional replication of data between multiple nodes ensures that all node data remains up-to-date.

With the above features, it is easy and efficient to achieve scenarios such as full|incremental data replication, full|incremental data migration, full|incremental data synchronization, data integration, and seamless migration without downtime, providing enterprises with flexible and reliable data replication solutions.

Prerequisites​

  • Both source and target data sources must be added to NineData. For instructions on how to add data sources, please refer to Adding Data Sources.

  • The versions of the source and target data sources are as follows:

    Source Data SourceTarget Data Source
    PostgreSQL 9 or aboveClickHouse 20.8 or above

  • For incremental replication, open the postgresql.conf file and configure the following parameters. If you cannot find the location of this file, you can execute the SHOW config_file; SQL command in the psql client to view it.

    • The wal_level parameter of the source data source must be logical. To confirm the current value, execute SHOW wal_level directly in the client.
    • Set the wal_sender_timeout parameter of the source data source to 0. This parameter is used to interrupt replication connections that have been stagnant for more than the specified number of milliseconds. The default value is 60000 milliseconds. Setting it to 0 will disable the timeout mechanism. To confirm the current value, execute SHOW wal_sender_timeout directly in the client.
    • The max_replication_slots parameter of the source data source must be greater than 1. This parameter specifies the maximum number of replication slots that the server can support. The default value is 10.
    • The max_wal_senders parameter of the source data source must be greater than 1. This parameter specifies the maximum number of concurrent connections. The default value is 10. To confirm the current value, execute SHOW max_wal_senders directly in the client.

  • The access_management parameter value for the account used to log in to ClickHouse must be 1.

    Configuration Method:

    Open the User.xml configuration file located in the /etc/clickhouse-server directory with an editor, find the target user, and add the parameter: <access_management>1</access_management>.

  • The following permissions are required for both the source and target data sources.

    Replication TypeSource Data Source PermissionsTarget Data Source Permissions
    Full replicationCONNECT, SELECTPermissions related to TABLE (CREATE, CREATE TABLE, ALTER, ALTER COLUMN, DROP, SELECT, INSERT)

Considerations​

  • If there are unsigned integer type fields in the source PostgreSQL data source, they will be mapped to Uint type in ClickHouse.

  • Typically, the replication scenario from PostgreSQL to ClickHouse needs to include structural replication, where the system will automatically migrate the table structure from PostgreSQL to ClickHouse and insert two system columns into the replicated tables in ClickHouse. If the replication type does not include structural replication, you need to manually ensure the following:

    • The table structures in ClickHouse data sources involved in replication must be consistent with those in PostgreSQL data sources.

    • Two system columns are manually inserted into the replicated tables in ClickHouse to record DML operations and the time of Binlog.

      Column NameData TypeDefault ValueDescription
      _jz_data_signInt8DEFAULT 1Records the type of DML operation to ensure consistency between data in ClickHouse and PostgreSQL.
      • INSERT operation: Record as 1.
      • DELETE operation: Record as -1.
      • UPDATE operation: Split into INSERT and DELETE two records.
      _jz_data_timeStringDEFAULT now()Records the time of Binlog.

      Example:

      lessCopy code
      CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
      (
          ...
          _jz_data_sign Int8 DEFAULT 1 COMMENT 'replication data update sign: add = 1, remove = -1',
          _jz_data_time String DEFAULT now() COMMENT 'replication data update time'
      ) ENGINE = engine

Usage Restrictions​

  • Evaluate the performance of both the source and target data sources before executing data synchronization. It is recommended to perform data synchronization during low traffic periods to avoid increasing the database load during full data initialization.
  • It is recommended to ensure that each table in the synchronization object has a primary key or unique constraint, and column names are unique to prevent duplicate synchronization of the same data.

Operation Steps​

Commercialization Notice

NineData’s data replication product has been commercialized. You can still use 10 replication tasks for free, with the following considerations:

  • Among the 10 replication tasks, you can include 1 "Incremental" task, with a specification of Micro.

  • Tasks with a status of Terminated do not count towards the 10-task limit. If you have already created 10 replication tasks and want to create more, you can terminate previous replication tasks and then create new ones.

  • When creating replication tasks, you can only select the Spec you have purchased. Specifications that have not been purchased will be grayed out and cannot be selected. If you need to purchase additional specifications, please contact us through the customer service icon at the bottom right of the page.

  1. Log in to the NineData Console.

  2. In the left navigation pane, click Replication > Data Replication.

  3. On the Replication page, click Create Replication in the upper-right corner.

  4. On the Source & Target tab, configure the parameters in the following table, and click Next.

    Parameter
    		Description
    NameEnter a name for the data synchronization task. To make the task easier to find and manage later, use a meaningful name. Up to 64 characters are supported.
    SourceThe data source that contains the objects to synchronize.
    TargetThe data source that receives the synchronized objects.
    Target Object NameSelect the case conversion rule for object names after they are migrated from the source to the target.
    • Convert all to Lowercase: Regardless of the naming rule on the source, all target names are lowercase.
    • Consistent with Source: Keep the naming rule of the source.
    • Convert all to Uppercase: Regardless of the naming rule on the source, all target names are uppercase.
    TypeSelect the replication type.
    • Schema: Synchronize only the database and table schemas of the source data source, without synchronizing data.
    • Full: Synchronize all objects and data from the source data source, namely full data replication.
    SpecThe specification of the replication task. A larger specification provides a higher replication rate. Hover over the details icon to view the rate and configuration information of each specification. If you configure the data replication task before purchasing resources, you can select the required specification here. If you purchase resources before configuring the task, the system selects the specification chosen during resource purchase, and you cannot change it in the task configuration.
    If target table already exists (Required when Schema is selected)
    • Pre-Check Error and Stop Task: Stop the task when a table with the same name is detected during the precheck stage.
    • Skip and Continue Task: When a table with the same name is detected during the precheck stage, display a prompt and continue the task. During schema replication, ignore the table with the same name. If you also perform data replication, data is appended to the table with the same name and existing data is not overwritten.
    • Delete Objects and Rewrite: When a table with the same name is detected during the precheck stage, display a prompt and continue the task. During schema replication, delete the table with the same name in the target database and replicate the table schema again based on the source database. If you also perform data replication, data is written after schema replication is complete.
    Target Table Exists Data (Required when Full is selected)
    • Pre-Check Error and Stop Task: Stop the task when data is detected in the target table during the precheck stage.
    • Ignore existing target data and append to it.: When data is detected in the target table during the precheck stage, ignore that data and append other data.
    • Clear target existing data before write: When data is detected in the target table during the precheck stage, delete that data and write it again.

  5. On the Objects tab, configure the following parameters, and click Next.

    Parameter
    		Description
    Replication ObjectsSelect the content to replicate. You can select All Objects to replicate all content in the source database, or select Customized Object, select the content to replicate in the Source Object list, and click > to add it to the Target Object list on the right.

    If you need to create multiple replication links with the same replication objects, you can create a configuration file and import it when creating a task. Click Import Config in the upper-right corner, click Download Template to download the configuration file template, edit the file, and then click Upload to upload it for batch import. Configuration file description:

    Parameter
    		Description
    source_table_nameThe source table name of the object to synchronize.
    destination_table_nameThe target table name that receives the synchronized object.
    source_schema_nameThe source schema name of the object to synchronize.
    destination_schema_nameThe target schema name that receives the synchronized object.
    source_database_nameThe source database name of the object to synchronize.
    target_database_nameThe target database name that receives the synchronized object.
    column_listThe list of columns to synchronize.
    extra_configurationAdditional configuration information. You can configure the following information here:
    • column_rules: Defines column mappings and value rules. Field descriptions:
      • column_name: Original column name.
      • destination_column_name: Specifies the target column name.
      • column_value: Specifies the column value, which can be an SQL function or a constant value.
    • filter_condition: Specifies row-level data filtering conditions. Only rows that meet the conditions are replicated.
    tip

    • The following is an example of extra_configuration:

      {
        "extra_config":{
          "column_rules":[
            {
              "column_name": "created_time", // Specifies the original column name to be mapped.
              "destination_column_name": "migrated_time", // Maps the target column name to "migrated_time".
              "column_value": "current_timestamp()" // Changes the column value to the current timestamp.
            }
          ],
          "filter_condition": "id != 0" // Only rows whose ID is not 0 are synchronized.
        }
      }

    • For a complete example of the configuration file, see the downloaded template.

  6. On the "Mapping" tab, choose different operations based on the selected replication type, and click Save and Pre-Check. If the source and target data sources are updated during mapping configuration, click Refresh Metadata in the upper-right corner of the page to refresh the source and target data source information.

    • Includes Schema: Configure the table name after synchronization to the target data source.

    • Does not include Schema: The system selects the database with the same name in the target data source by default. If no such database exists, you need to manually select the target database. The table names and column names in the target database must match the synchronization objects. If they do not match, you can manually map the table names and column names.

    You can also perform the following operations:

    • Click Mapping & Filtering on the right side of the page to customize the column names after synchronization to the target data source.
    • On the Mapping & Filtering page, enter a comparison expression in the text box below Data Filter as the filtering condition. Only data that meets the filtering condition is synchronized to the target data source. For example, if the filtering condition is set to emp_no>=10005, data whose emp_no column value is less than 10005 is not synchronized to the target data source.
    • Click the replace_tablename icon to the right of "Target Table" to search for a table name and replace it with the target name.
    • Enter a table name in the Search Table text box in the upper-right corner to quickly locate the target table.
    • Click Batch Configuration in the upper-right corner to define common rules in batches, such as table name and column name case conversion, prefix or suffix addition, and replacement, so that mapping configuration can be applied uniformly to a large number of tables and columns.

  1. On the Pre-check tab, wait for the system to complete the precheck. After the precheck is passed, click Launch.

    • You can select Enable data consistency comparison. After the synchronization task is complete, a data consistency comparison task based on the source data source automatically starts to ensure data consistency between both ends. Based on the selected Type, Enable data consistency comparison starts at the following times:

      • Schema: Starts after schema replication is complete.
      • Schema+Full: Starts after full replication is complete.
      • Full: Starts after full replication is complete.

    • If the precheck fails, click Details in the Actions column on the right of the failed check item, troubleshoot the cause, manually fix the issue, and then click Check Again to run the precheck again until it passes.

    • Check items whose Result is Warning can be fixed or ignored depending on the situation.

  2. On the Launch page, the Launch Successfully prompt appears, indicating that the synchronization task has started. You can then perform the following operations:

    • Click View Details to view the execution status of each stage of the synchronization task.

    • Click Back to list to return to the Replication task list page.

Viewing Synchronization Results​

  1. Log in to the NineData Console.

  2. Click on Replication > Data Replication in the left navigation pane.

  3. On the Replication page, click on the Task ID of the target synchronization task, and the page description is as follows.

    result_no_incre

    Number
    		Function
    		Description
    1Configure AlertsAfter configuring alerts, the system will notify you in the selected way when the task fails. For more information, please refer to Operational Monitoring Overview.
    2More
    • Pause: Pause the task. Only tasks with the status Running are selectable.
    • Duplicate: Create a new replication task with the same configuration as the current task.
    • Terminate: End tasks that are incomplete or in listening (i.e., in incremental synchronization). After terminating the task, it cannot be restarted, so please proceed with caution. If triggers are included in the synchronization object, trigger replication options will pop up, please choose as needed.
    • Delete: Delete the task. Once the task is deleted, it cannot be recovered, so please proceed with caution.
    3Structural Replication (Displayed in scenarios involving structural replication)Display the progress and details of structural replication.
    • Click on Log on the right side of the page: View the execution log of structural replication.
    • Click on refresh on the right side of the page: View the latest information.
    • Click on View DDL in the Actions column on the right side of the target object in the list: View SQL playback.
    4Full Replication (Displayed in scenarios involving full replication)Display the progress and details of full replication.
    • Click on Monitor on the right side of the page: View various monitoring indicators during full replication. During full replication, you can also click on Flow Control Settings on the right side of the monitoring indicator page to limit the rate of data written to the target data source per second. The unit is rows/second.
    • Click on Log on the right side of the page: View the execution log of full replication.
    • Click on refresh on the right side of the page: View the latest information.
    5Data ComparisonDisplay the comparison results between the source data source and the target data source. If you have not enabled data comparison, click Enable Comparison on the page.
    • Click on Re-compare on the right side of the page: Re-initiate the comparison of data between the current source and target ends.
    • Click on Stop on the right side of the page: After starting the comparison task, you can click this button to stop the comparison task immediately.
    • Click on Log on the right side of the page: View the execution log of consistency comparison.
    • Click on Monitor (only displayed in data comparison): View the trend chart of RPS (records per second compared) for comparison. Click on Details to view records from earlier times.
    • Click on details in the Actions column on the right side of the comparison list (displayed only under the Data tab when there is inconsistency): View detailed comparison between the source and target ends.
    • Click on sql in the Actions column on the right side of the comparison list (displayed only in case of inconsistency): Generate change SQL, which you can directly copy to the target data source to execute and modify the inconsistent content.
    6ExpandDisplay detailed information of the current replication task. Common Options:
    • Export table configuration: Export the current task's database and table configuration, allowing for quick import when creating a new replication task. This helps rapidly establish multiple replication links with the same replication objects.
    • Alert Rules: Configure the alarm strategy for the current task.

Appendix 1: Mapping Table of PostgreSQL and ClickHouse Data Types​

CategoryPostgreSQL Data TypeClickHouse Data Type
NumericSMALLINT/INT2INT16
INTEGER/INT4/INTINT32
BIGINT/INT8INT64
BITSTRING
BIT VARINGSTRING
DOUBLEFLOAT64
DOUBLE PRECISIONFLOAT64
REALFLOAT64
FLOAT4FLOAT64
FLOAT8FLOAT64
NUMERICDECIMAL
MOENYDECIMAL
BOOL/BOOLEANUINT8
DATE AND TIMEDATEDATE32
TIMESTAMP WITHOUT TIME ZONE
  • Version 20.3 and above: DATATIME64
  • Earlier than version 20.3
    • With time precision: DATETIME
    • Without time precision: STRING
TIMESTAMP WITH TIME ZONE/ TIMESTAMPTZ
  • Version 20.3 and above: DATATIME64
  • Earlier than version 20.3
    • With time precision: DATETIME
    • Without time precision: STRING
TIME WITH TIME ZONE/TIMETZSTRING
TIMESTAMP
  • Version 20.3 and above: DATATIME64
  • Earlier than version 20.3
    • With time precision: DATETIME
    • Without time precision: STRING
TIMESTRING
INTERVALSTRING
STRINGCHARSTRING
CHARACTER VARYINGSTRING
CHARACTERSTRING
TEXTSTRING
INETSTRING
CIDRSTRING
MACADDRSTRING
MACADDR8STRING
UUIDSTRING
RANGEINT4RANGESTRING
INT8RANGESTRING
NUMRANGESTRING
DATERANGESTRING
JSONJSONSTRING
JSONBSTRING
BINARYBYTEASTRING
SPATIALPOINTPOINT
LINESTRING
LSEGSTRING
BOXSTRING
PATHSTRING
POLYGONSTRING
CIRCLESTRING
XMLXMLSTRING

Appendix 2: Checklist Overview​

Check ItemCheck Content
Source datasource connection checkCheck the status of the gateway of the source datasource, database connectable, and verify the username and password
Target datasource connection checkCheck the status of the gateway of the target datasource, database connectable, and verify the username and password
Target databse privilege checkCheck whether the account privileges of the target database meet the requirements
Source databse privilege checkCheck whether the account privileges of the source database meet the requirements
Target database data existence checkCheck whether the object to be replicated not empty in the target database
Objects with the same name in the target database existence checkCheck whether the object to be replicated already exists in the target database

Introduction to Data Replication

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