[PoC] Test clickhouse-activerecord gem

What does this MR do and why?

This MR attempts to integrate the clickhouse-activerecord gem with the GitLab application.

Setup

GDK supports ClickHouse but it needs to be installed automatically. The gdk.yml can be used to run the service:

---
clickhouse:
  bin: "/usr/bin/clickhouse"
  data_dir: "/gitlab-development-kit/clickhouse/data"
  dir: "/gitlab-development-kit/clickhouse"
  enabled: true
  http_port: 8123
  interserver_http_port: 9009
  log_dir: "/gitlab-development-kit/clickhouse/log"
  log_level: trace
  max_memory_usage: 1000000000
  max_server_memory_usage: 2000000000
  max_thread_pool_size: 1000
  tcp_port: 9001
gdk:
  experimental:
    ruby_services: true

Running gdk reconfigure will make the clickhouse service available within GDK.

What works

• Creating and running migrations
• Tracking schema version
• Creating AR models
• Querying the database
• Inserting to the database

How to test it

This MR includes a test migration that creates the issues table on CH.

1. Ensure that the Clickhouse DB is runnig.
2. Configure CH in the config/database.yml file
3. Run the migration: rails db:migrate:clickhouse_analytics
4. Start a rails console: rails c
5. Create a record: Clickhouse::Issue.create!(title: 'My issue', project_id: 15)
6. Query the record: Clickhouse::Issue.where(project_id: 15)

Identified quirks/issues

• We patch ActiveRecord::Base a lot. This is not great because we add a lot of PG-specific functionality to the base class which prevents using a different DB adapter.
  • This MR introduces a base class for PG based models, we can apply extensions to this class.
  • Ideally, all models (including external models coming from gems) should inherit from PostgresqlRecord.
• Query analyzers for cross DB modifications and migrations are off when the adapter is `Clickhouse. Clickhouse queries might not always be parseable by PG.
• Our Database module contains a lot of PG-specific methods.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.