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Loading data from MySQL® to a ClickHouse® data mart

Written by
Updated at September 27, 2024

With Data Transfer, you can migrate your database from a MySQL® source cluster to ClickHouse®.

To transfer data:

  1. Prepare the source cluster.
  2. Prepare and activate the transfer.
  3. Test the transfer.
  4. Select the data from ClickHouse®.

If you no longer need the resources you created, delete them.

Getting started

Prepare the infrastructure:

  1. Create a Managed Service for MySQL® source cluster with any suitable configuration. To connect to the cluster from the user's local machine rather than doing so from the Yandex Cloud cloud network, enable public access to the cluster when creating it.

  2. Create a Managed Service for ClickHouse® target cluster in any suitable configuration with the following settings:

    • Number of ClickHouse® hosts: At least two, which is required to enable replication in the cluster.
    • Database name: Same as in the source cluster.
    • To connect to the cluster from the user's local machine rather than doing so from the Yandex Cloud cloud network, enable public access to the cluster when creating it.

  3. If you are using security groups in your clusters, configure them so that you can connect to the clusters from the internet:

  1. If you do not have Terraform yet, install it.

  2. Get the authentication credentials. You can add them to environment variables or specify them later in the provider configuration file.

  3. Configure and initialize a provider. There is no need to create a provider configuration file manually, you can download it.

  4. Place the configuration file in a separate working directory and specify the parameter values. If you did not add the authentication credentials to environment variables, specify them in the configuration file.

  5. Download the data-transfer-mmy-mch.tf configuration file to the same working directory.

    This file describes:

    • Network.
    • Subnet.
    • Security group and the rule required to connect to a Managed Service for MySQL® cluster.
    • Managed Service for MySQL® source cluster.
    • Managed Service for ClickHouse® target cluster.
    • Source endpoint.
    • Target endpoint.
    • Transfer.

  6. In the data-transfer-mmy-mch.tf file, specify:

    • The Managed Service for MySQL® source cluster parameters that will also be used as the source endpoint parameters:

      • source_mysql_version: MySQL® version.
      • source_db_name: MySQL® database name to use as the Managed Service for ClickHouse® database name.
      • source_user and source_password: Database owner username and password.

    • The Managed Service for ClickHouse® target cluster parameters that will also be used as the target endpoint parameters:

      • target_user and target_password: Database owner username and password.

  7. Make sure the Terraform configuration files are correct using this command:

    terraform validate

    If there are any errors in the configuration files, Terraform will point them out.

  8. Create the required infrastructure:

    1. Run the command to view planned changes:

      terraform plan

      If the resource configuration descriptions are correct, the terminal will display a list of the resources to modify and their parameters. This is a test step. No resources are updated.

    2. If you are happy with the planned changes, apply them:

      1. Run the command:

        terraform apply

      2. Confirm the update of resources.

      3. Wait for the operation to complete.

    All the required resources will be created in the specified folder. You can check resource availability and their settings in the management console.

Prepare the source cluster

  1. If you created the infrastructure manually, prepare the source cluster.

  2. Connect to the Managed Service for MySQL® source cluster.

  3. Add test data to the database.

    1. Create a table named x_tab:
    CREATE TABLE x_tab
(
    id INT,
    name TEXT,
    PRIMARY KEY (id)
);
    1. Populate the table with data:
    INSERT INTO x_tab (id, name) VALUES
    (40, 'User1'),
    (41, 'User2'),
    (42, 'User3'),
    (43, 'User4'),
    (44, 'User5');

Prepare and activate the transfer

  1. Create a source endpoint:

    • Database type: MySQL®

    • Endpoint parameters → Connection settings: Managed Service for MySQL cluster

      Select a source cluster from the list and specify its connection settings.

  2. Create a target endpoint:

    • Database type: ClickHouse

    • Endpoint parameters → Connection settings: Managed cluster

      Select a target cluster from the list and specify its connection settings.

  3. Create a transfer of the Snapshot and replication type that will use the created endpoints.

  4. Activate your transfer.

  1. In the data-transfer-mmy-mch.tf file, set the transfer_enabled variable to 1.

  2. Make sure the Terraform configuration files are correct using this command:

    terraform validate

    If there are any errors in the configuration files, Terraform will point them out.

  3. Create the required infrastructure:

    1. Run the command to view planned changes:

      terraform plan

      If the resource configuration descriptions are correct, the terminal will display a list of the resources to modify and their parameters. This is a test step. No resources are updated.

    2. If you are happy with the planned changes, apply them:

      1. Run the command:

        terraform apply

      2. Confirm the update of resources.

      3. Wait for the operation to complete.

    Once created, your transfer will be activated automatically.

Test the transfer

  1. Wait for the transfer status to change to Replicating.

  2. Make sure the data from the source Managed Service for MySQL® cluster has been moved to the Managed Service for ClickHouse® database:

    1. Connect to the cluster using clickhouse-client:

    2. Run the following query:

      SELECT * FROM <ClickHouse®_database_name>.x_tab

      Result:

      ┌─id─┬─name──┬─__data_transfer_commit_time─┬─__data_transfer_delete_time─┐
│ 40 │ User1 │         1661952756538347180 │                           0 │
│ 41 │ User2 │         1661952756538347180 │                           0 │
│ 42 │ User3 │         1661952756538347180 │                           0 │
│ 43 │ User4 │         1661952756538347180 │                           0 │
│ 44 │ User5 │         1661952756538347180 │                           0 │
└────┴───────┴─────────────────────────────┴─────────────────────────────┘

      The table also contains columns with timestamps such as __data_transfer_commit_time and __data_transfer_delete_time.

  3. In the x_tab table of the MySQL® source database, delete the row where id is 41 and update the row where id is 42:

    1. Connect to the Managed Service for MySQL® source cluster.

    2. Run the following queries:

      DELETE FROM x_tab WHERE id = 41;
UPDATE x_tab SET name = 'Key3' WHERE id = 42;

  4. Make sure that you see the changes in the x_tab table on the ClickHouse® target:

    SELECT * FROM <ClickHouse®_database_name>.x_tab WHERE id in (41,42);

    Result:

    ┌─id─┬─name──┬─__data_transfer_commit_time─┬─__data_transfer_delete_time─┐
│ 41 │ User2 │         1661952756538347180 │                           0 │
│ 42 │ User3 │         1661952756538347180 │                           0 │
└────┴───────┴─────────────────────────────┴─────────────────────────────┘
┌─id─┬─name─┬─__data_transfer_commit_time─┬─__data_transfer_delete_time─┐
│ 41 │ ᴺᵁᴸᴸ │         1661953256000000000 │         1661953256000000000 │
└────┴──────┴─────────────────────────────┴─────────────────────────────┘
┌─id─┬─name─┬─__data_transfer_commit_time─┬─__data_transfer_delete_time─┐
│ 42 │ Key3 │         1661953280000000000 │                           0 │
└────┴──────┴─────────────────────────────┴─────────────────────────────┘

Select the data from ClickHouse®

For table recovery, the ClickHouse® target with replication enabled uses the ReplicatedReplacingMergeTree and the ReplacingMergeTree engines. The following columns are added automatically to each table:

  • __data_transfer_commit_time: Time for the row to change to this value, in TIMESTAMP format.

  • __data_transfer_delete_time: Time for deleting the row, in TIMESTAMP format, if the row was deleted in the source. If the row was not deleted, the value is set to 0.

    The __data_transfer_commit_time column is required for the ReplicatedReplacedMergeTree engine. If a record is deleted or updated, a new row is inserted with a value in this column. When running a query by a single primary key, it will return multiple records with different values from the __data_transfer_commit_time column.

With the Replicating transfer status, the source data can be added or deleted. To ensure standard SQL commands behavior when a primary key points to a single record, add a construction filtering data by the __data_transfer_delete_time column to your queries to the tables moved to ClickHouse®. For example, for the x_tab table:

SELECT * FROM <ClickHouse®_database_name>.x_tab FINAL
WHERE __data_transfer_delete_time = 0;

To make it easier to run a SELECT query, create a view with filtering by the __data_transfer_delete_time column and access it. For example, for the x_tab table:

CREATE VIEW x_tab_view AS SELECT * FROM <ClickHouse®_database_name>.x_tab FINAL
WHERE __data_transfer_delete_time == 0;

Delete the resources you created

Note

Before deleting the created resources, deactivate the transfer.

Some resources are not free of charge. To avoid paying for them, delete the resources you no longer need:

  1. In the terminal window, go to the directory containing the infrastructure plan.

  2. Delete the data-transfer-mmy-mch.tf configuration file.

  3. Make sure the Terraform configuration files are correct using this command:

    terraform validate

    If there are any errors in the configuration files, Terraform will point them out.

  4. Confirm updating the resources.

    1. Run the command to view planned changes:

      terraform plan

      If the resource configuration descriptions are correct, the terminal will display a list of the resources to modify and their parameters. This is a test step. No resources are updated.

    2. If you are happy with the planned changes, apply them:

      1. Run the command:

        terraform apply

      2. Confirm the update of resources.

      3. Wait for the operation to complete.

    All resources described in the data-transfer-mmy-mch.tf configuration file will be deleted.

ClickHouse® is a registered trademark of ClickHouse, Inc.

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