Detailed introduction of express default log component morgan

This time I will bring you a detailed introduction to express's default log component morgan. What are the precautions when using express's default log component morgan? The following is a practical case, let's take a look.

Chapter Overview

Morgan is express's default logging middleware. It can also be used independently from express as a node.js logging component. This article goes from shallow to deep, and the content mainly includes:

1. Getting started with morgan examples

2. How to save logs to local files

3. Core API usage instructions and examples

4. Advanced usage: 1. Log splitting 2. Write logs to database

5. Source code analysis: Morgan’s log format and pre-compilation

##Getting started example

First ,

Initialization project.

npm install express morgan

Then, add the following code in basic.js.

var express = require('express');
var app = express();
var morgan = require('morgan');
app.use(morgan('short'));
app.use(function(req, res, next){
 res.send('ok');
});
app.listen(3000);

node basic.jsRun the program and access http://127.0.0.1:3000 in the browser. The print log is as follows

➜ 2016.12 .11-advanced-morgan git:(master) ✗ node basic.js

::ffff:127.0.0.1 - GET / HTTP/1.1 304 - - 3.019 ms
::ffff:127.0.0.1 - GET / favicon.ico HTTP/1.1 200 2 - 0.984 ms

Print the log to a local file

#morgan supports the stream configuration item, which can be used to implement The effect of log implementation, the code is as follows:

var express = require('express');
var app = express();
var morgan = require('morgan');
var fs = require('fs');
var path = require('path');
var accessLogStream = fs.createWriteStream(path.join(dirname, 'access.log'), {flags: 'a'});
app.use(morgan('short', {stream: accessLogStream}));
app.use(function(req, res, next){
 res.send('ok');
});
app.listen(3000);

Usage explanation

Core API

morgan has very few APIs, and the most frequently used one is morgan( ), its function is to return an express log middleware.

morgan(format, options)

The parameter description is as follows:

1. format: Optional, morgan has defined several log formats, each format has a corresponding name, such as combined, short etc. The default is default. The differences between different formats can be found here. The following will explain how to customize the log format.

2. options: Optional, configuration items, including stream (commonly used), skip, and immediate.

3. stream: Log output stream configuration, the default is process.stdout.

4. skip: Whether to skip logging, please refer to here for usage.

5. immediate: Boolean value, default is false. When true, the log will be logged as soon as the request is received; if false, the log will be logged after the request returns.

Custom log format

First understand the two concepts in morgan: format and token. Very simple:

1. format: log format, which is essentially a string representing the log format, such as :method :url :status :res[content-length] - :response-time ms.

2. Token: The component of format, such as :method and :url above are so-called tokens.

After clarifying the difference between format and token, you can take a look at the key APIs about custom log formats in morgan.

morgan.format(name, format); // 自定义日志格式
morgan.token(name, fn); // 自定义token

Custom format

is very simple, first define the log format named joke through morgan.format(), and then call it through morgan('joke') Can.

var express = require('express');
var app = express();
var morgan = require('morgan');
morgan.format('joke', '[joke] :method :url :status');
app.use(morgan('joke'));
app.use(function(req, res, next){
 res.send('ok');
});
app.listen(3000);

Let’s take a look at the running results

➜ 2016.12.11-advanced-morgan git:(master) ✗ node morgan.format.js

[joke] GET / 304
[joke] GET /favicon.ico 200

Custom token

The code is as follows, customize the token through morgan.token(), Then add the custom token to the custom format.

var express = require('express');
var app = express();
var morgan = require('morgan');
// 自定义token
morgan.token('from', function(req, res){
 return req.query.from || '-';
});
// 自定义format，其中包含自定义的token
morgan.format('joke', '[joke] :method :url :status :from');
// 使用自定义的format
app.use(morgan('joke'));
app.use(function(req, res, next){
 res.send('ok');
});
app.listen(3000);

Run the program and visit http://127.0.0.1:3000/hello?from=app and http://127.0.0.1:3000/hello?from=pc## in the browser. #

➜  2016.12.11-advanced-morgan git:(master) ✗ node morgan.token.js
[joke] GET /hello?from=app 200 app
[joke] GET /favicon.ico 304 -
[joke] GET /hello?from=pc 200 pc
[joke] GET /favicon.ico 304 -

高级使用

日志切割

一个线上应用，如果所有的日志都落地到同一个本地文件，时间久了，文件会变得非常大，既影响性能，又不便于查看。这时候，就需要用到日志分割了。

借助file-stream-rotator插件，可以轻松完成日志分割的工作。除了file-stream-rotator相关的配置代码，其余跟之前的例子差不多，这里不赘述。

var FileStreamRotator = require('file-stream-rotator')
var express = require('express')
var fs = require('fs')
var morgan = require('morgan')
var path = require('path')
var app = express()
var logDirectory = path.join(dirname, 'log')
// ensure log directory exists
fs.existsSync(logDirectory) || fs.mkdirSync(logDirectory)
// create a rotating write stream
var accessLogStream = FileStreamRotator.getStream({
 date_format: 'YYYYMMDD',
 filename: path.join(logDirectory, 'access-%DATE%.log'),
 frequency: 'daily',
 verbose: false
})
// setup the logger
app.use(morgan('combined', {stream: accessLogStream}))
app.get('/', function (req, res) {
 res.send('hello, world!')
})

日志写入数据库

有的时候，我们会有这样的需求，将访问日志写入数据库。这种需求常见于需要实时查询统计的日志系统。

在morgan里该如何实现呢？从文档上，并没有看到适合的扩展接口。于是查阅了下morgan的源码，发现实现起来非常简单。

回顾下之前日志写入本地文件的例子，最关键的两行代码如下。通过stream指定日志的输出流。

var accessLogStream = fs.createWriteStream(path.join(dirname, 'access.log'), {flags: 'a'});
app.use(morgan('short', {stream: accessLogStream}));

在morgan内部，大致实现是这样的（简化后）。

// opt为配置文件
var stream = opts.stream || process.stdout;
var logString = createLogString(); // 伪代码，根据format、token的定义，生成日志
stream.write(logString);

于是，可以用比较取巧的方式来实现目的：声明一个带write方法的对象，并作为stream配置传入。

var express = require('express');
var app = express();
var morgan = require('morgan');
// 带write方法的对象
var dbStream = {
 write: function(line){
 saveToDatabase(line); // 伪代码，保存到数据库
 }
};
// 将 dbStream 作为 stream 配置项的值
app.use(morgan('short', {stream: dbStream}));
app.use(function(req, res, next){
 res.send('ok');
});
app.listen(3000);

深入剖析

morgan的代码非常简洁，从设计上来说，morgan的生命周期包含：

token定义 --> 日志格式定义 -> 日志格式预编译 --> 请求达到/返回 --> 写日志

其中，token定义、日志格式定义前面已经讲到，这里就只讲下 日志格式预编译 的细节。

跟模板引擎预编译一样，日志格式预编译，也是为了提升性能。源码如下，最关键的代码就是compile(fmt)。

 function getFormatFunction (name) {
 // lookup format
 var fmt = morgan[name] || name || morgan.default
 // return compiled format
 return typeof fmt !== 'function'
 ? compile(fmt)
 : fmt
}

compile()方法的实现细节这里不赘述，着重看下compile(fmt)返回的内容：

var morgan = require('morgan');
var format = morgan['tiny'];
var fn = morgan.compile(format);
console.log(fn.toString());

运行上面程序，输出内容如下，其中tokens其实就是morgan。 

function anonymous(tokens, req, res
/**/) {
 return ""
 + (tokens["method"](req, res, undefined) || "-") + " "
 + (tokens["url"](req, res, undefined) || "-") + " "
 + (tokens["status"](req, res, undefined) || "-") + " "
 + (tokens["res"](req, res, "content-length") || "-") + " - "
 + (tokens["response-time"](req, res, undefined) || "-") + " ms";
}

看下morgan.token()的定义，就很清晰了

function token (name, fn) {
 morgan[name] = fn
 return this
}

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