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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
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。
