Asynchronously replicating data from PostgreSQL to ClickHouse®

You can migrate a database from PostgreSQL to ClickHouse® using Yandex Data Transfer. To do this:

1. Set up the transfer.
2. Activate the transfer.
3. Test the replication process.
4. Select the data from the target.

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

Getting startedGetting started

For clarity, we will create all required resources in Yandex Cloud. Prepare the infrastructure:

Set up the transferSet up the transfer

1. Connect to the Managed Service for PostgreSQL cluster.

2. Create a table named x_tab in the db1 database and populate it with data:

   CREATE TABLE x_tab
   (
       id NUMERIC PRIMARY KEY,
       name CHAR(5)
   );
   CREATE INDEX ON x_tab (id);
   INSERT INTO x_tab (id, name) VALUES
     (40, 'User1'),
     (41, 'User2'),
     (42, 'User3'),
     (43, 'User4'),
     (44, 'User5');
   

3. Create a transfer:

   Manually

   Terraform

   1. Create a source endpoint of the PostgreSQL type and specify the cluster connection parameters in it:

      • Installation type: Managed Service for PostgreSQL cluster.
      • Managed Service for PostgreSQL cluster: <source_PostgreSQL_cluster_name> from the drop-down list.
      • Database: db1.
      • User: pg-user.
      • Password: <user_password>.

   2. Create a target endpoint of the ClickHouse type and specify the cluster connection parameters in it:

      • Connection type: Managed cluster.
      • Managed cluster: <target_ClickHouse®_cluster_name> from the drop-down list.
      • Database: db1.
      • User: ch-user.
      • Password: <user_password>.
      • Cleanup policy: DROP.

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

   1. In the postgresql-to-clickhouse.tf file, set the transfer_enabled parameter 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.

Activate the transferActivate the transfer

1. Activate the transfer and wait for its status to change to Replicating.

2. To check that the transfer has moved the replicated data to the target, connect to the target Yandex Managed Service for ClickHouse® cluster and make sure that the x_tab table in db1 has the same columns as the x_tab table in the source database, as well as the timestamp columns, __data_transfer_commit_time and __data_transfer_delete_time:

   SELECT * FROM db1.x_tab WHERE id = 41;
   

   ┌─id─┬──name──┬─── __data-transfer_commit_time─┬───__data-transfer-delete_time─┐
   │ 41 │  User2 │   1633417594957267000          │ 0                             │
   └────┴────────┴────────────────────────────────┴───────────────────────────────┘
   

Test the replication processTest the replication process

1. Connect to the source cluster.

2. In the x_tab table of the source PostgreSQL database, delete the row with the 41 ID and update the one with the 42 ID:

   DELETE FROM db1.public.x_tab WHERE id = 41;
   UPDATE db1.public.x_tab SET name = 'Key3' WHERE id = 42;
   

3. Check the changes in the x_tab table on the ClickHouse® target:

   SELECT * FROM db1.x_tab WHERE (id >= 41) AND (id <= 42);
   

   ┌─id─┬──name──┬─── __data-transfer_commit_time─┬───__data-transfer-delete_time─┐
   │ 41 │  User2 │   1633417594957267000          │ 1675417594957267000           │
   │ 42 │  Key3  │   1675417594957267000          │ 0                             │
   │ 42 │  User3 │   1633417594957268000          │ 1675417594957267000           │
   └────┴────────┴────────────────────────────────┴───────────────────────────────┘
   

Select the data from ClickHouse®Select the data from ClickHouse®

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

• __data_transfer_commit_time: Time when the was row updated to this value, in TIMESTAMP format.

• __data_transfer_delete_time: Row deletion time, 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 to work. If a record is deleted or updated, a new row is inserted with a value in this column. A query by a single primary key returns multiple records with different __data_transfer_commit_time values.

With the Replicating transfer status, the source data can be added or deleted. To ensure standard behavior of SQL commands when a primary key points to a single record, provide a clause to filter data by __data_transfer_delete_time when querying tables transferred to ClickHouse®. Here is an example of a query to the x_tab table:

SELECT * FROM x_tab FINAL
WHERE __data_transfer_delete_time = 0;

To simplify the SELECT queries, create a view with filtering by __data_transfer_delete_time and use it for querying. Here is an example of a query to the x_tab table:

CREATE VIEW x_tab_view AS SELECT * FROM x_tab FINAL
WHERE __data_transfer_delete_time == 0;

Delete the resources you createdDelete the resources you created

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

• Make sure the transfer has the Completed status and delete it.

• Delete the endpoints and clusters:

  Manually

  Terraform

  • Both the source and target endpoints.
  • Managed Service for PostgreSQL.
  • Managed Service for ClickHouse®.

  If you created your resources using Terraform:

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

  2. Delete the postgresql-to-clickhouse.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 the resources described in the postgresql-to-clickhouse.tf configuration file will be deleted.

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