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Contents
---
title: Rails日志实现探索（1）
layout: post
category: ruby
---
### 概述
我一直很好奇在Rails中，日志是如何记录一个Rails App运行的信息的。我查看了一下Rails的中间件，发现了`Rails::Rack::Logger`
```
# rake middleware
...
use Rack::MethodOverride
use Rails::Rack::Logger
use ActionDispatch::ShowExceptions
...
```
查看`Rails::Rack::Logger`中实现记录日志的主要代码，发现了两个奇怪的类：`ActiveSupport::Notifications`和`ActiveSupport::LogSubscriber`
```ruby
def call_app(request, env)
# ...
instrumenter = ActiveSupport::Notifications.instrumenter
instrumenter.start 'request.action_dispatch', request: request
logger.info { started_request_message(request) }
resp = @app.call(env)
resp[2] = ::Rack::BodyProxy.new(resp[2]) { finish(request) }
resp
rescue Exception
finish(request)
raise
ensure
ActiveSupport::LogSubscriber.flush_all!
end
```
### 日志功能实现
Rails中对日志的处理采用的是“消息-订阅”机制，各部分组件和功能如下：
* 消息发送：`ActiveSupport::Notifications`
- `instrument`: 通知subscribers
* 消息订阅：`ActiveSupport::LogSubscriber`
Version data entries
3 entries across 3 versions & 1 rubygems