Getting Started with Rails & TiDB

Getting Started with Rails & TiDB

也许是第一份 Rails + TiDB集成的资料，网上新手入门方面的文章太少了，而且ActiveRecord这种复杂的ORM和TiDB集成还确实有一些门槛，所以就写了这么一个入门教程。

搭建本地TiDB开发环境

安装TiUP

TiUP 安装过程十分简洁，无论是 Darwin 还是 Linux 操作系统，执行一行命令即可安装成功：

curl --proto '=https' --tlsv1.2 -sSf https://tiup-mirrors.pingcap.com/install.sh | sh

本机启动集群

tiup playground

输出

tiup playground
Starting component `playground`: /Users/hooopo/.tiup/components/playground/v1.4.1/tiup-playground
Use the latest stable version: v5.0.0

    Specify version manually:   tiup playground <version>
    The stable version:         tiup playground v4.0.0
    The nightly version:        tiup playground nightly

Playground Bootstrapping...
Start pd instance
Start tikv instance
Start tidb instance
Waiting for tidb instances ready
127.0.0.1:4000 ... Done
Start tiflash instance
Waiting for tiflash instances ready
127.0.0.1:3930 ... Done
CLUSTER START SUCCESSFULLY, Enjoy it ^-^
To connect TiDB: mysql --host 127.0.0.1 --port 4000 -u root -p (no password)
To view the dashboard: http://127.0.0.1:2379/dashboard
To view the Prometheus: http://127.0.0.1:9090
To view the Grafana: http://127.0.0.1:3000

dashboard

详细文档

• https://docs.pingcap.com/zh/tidb/stable/tiup-overview

Rails & TiDB 配置

创建Rails项目

rails new myapp --database=mysql

database.yml配置

default: &default
  adapter: mysql2
  encoding: utf8mb4
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  port: 4000
  username: root
  password:
  host: 127.0.0.1
  variables:
    tidb_enable_noop_functions: ON

注意，tiup启动的本地集群默认端口是4000，设置数据库connection变量tidb_enable_noop_functions: ON，因为Rails需要使用get_lock函数，tidb里默认是关闭的。

如果你使用URI的方式配置数据库链接，也是类似：

default: &default
  adapter: mysql2
  encoding: utf8mb4
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  url: <%= ENV.fetch("DB_URL") || "mysql2://root:pass@localhost:4000/myapp" %>
  variables:
    tidb_enable_noop_functions: ON

主键、自增、唯一索引

创建一个users表：

class CreateUsers < ActiveRecord::Migration[6.1]
  def change
    create_table :users do |t|
      t.string :email
      t.string :password
      t.string :username

      t.timestamps
    end
  end
end

加一个唯一索引

class AddUniqueIndexForEmail < ActiveRecord::Migration[6.1]
  def change
    add_index :users, :email, unique: true
  end
end

和使用单机MySQL没有任何区别，TiDB已经兼容的很好了，相比其他分布式数据库上手起来容易很多，一些分布式数据库主键、自增、唯一索引这些功能都是不兼容的，需要额外处理。

看看生成的数据表：

mysql> show create table users;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                             |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| users | CREATE TABLE `users` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `email` varchar(255) DEFAULT NULL,
  `password` varchar(255) DEFAULT NULL,
  `username` varchar(255) DEFAULT NULL,
  `created_at` datetime(6) NOT NULL,
  `updated_at` datetime(6) NOT NULL,
  PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */,
  UNIQUE KEY `index_users_on_email` (`email`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30001 |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

savepoint patch

TiDB和ActiveRecord结合的唯一障碍就是TiDB不支持savepoint，我写了个简单的patch来解决：

# https://github.com/rails/rails/blob/6-1-stable/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb#L313
require 'active_record/connection_adapters/abstract/database_statements.rb'


module DisableSavepoint
  def transaction(requires_new: nil, isolation: nil, joinable: true)
    if requires_new
      requires_new = nil
      Rails.logger.warn "savepoint statement was used, but your db not support, ignored savepoint."
      Rails.logger.warn caller
      super(requires_new: requires_new, isolation: isolation, joinable: joinable)
    else
      super(requires_new: requires_new, isolation: isolation, joinable: joinable)
    end
  end
end

ActiveRecord::ConnectionAdapters::DatabaseStatements.send(:prepend, DisableSavepoint)

原理就是，Rails里只有在transaction传参requires_new为true的时候才会引入savepoint，通过patch把requires_new为true的地方变成nil，再输出日志来迁移。我的经验大部分Rails项目用到savepoint的地方不多，如果想迁移不是很难。跑migration的时候会引入savepoint，但在没有并发执行migration的场景直接去掉也没有什么影响。

Links

• What’s New in TiDB 5.0
• tidb_enable_noop_functions-new-in-v40
  
• 与 MySQL 兼容性对比
• rails-tidb demo 完整源码