Sympathy for the daemon

Daemon is a linux process that runs in background; apache or mysql are daemons.
In a linux environment, we can run a background program  using cronjob, but it has some limitations, and in some scenarios it' s not a good idea.
For example, using cronjob, we can't control if the previously run has finished yet.
So often it's more convenient run a process as a daemon and this post describes how can be do this in python.
The main duty of a daemon consists to fork it self; it fires up a child process and detaches it. So parent process now can dies and it will give the console back. 

//Method

import os

try:
    ''' fork a process from parent processor 
        so father can exit and return control to the shell '''
    pid = os.fork()
except OSError, e:
    raise Exception, "%s [%d]" % (e.strerror, e.errno)
if (pid == 0):
    # this is first child

continue ...

Asyncronous function in Node.js

There are two kind of function in Node.js:

• Asyncronous, non blocking function 
• Syncronous, blocking function

The first type, receive a result throw a callback function:

test('x', function(err, data){
}); 

A common mistake that a node.js newbies developer commit is try to return a value from an asyncronous function. The right way to work with asyncronous function is follow example below:

// Method
function query (sQuery, callback){
     var query = connection.query(sQuery);
     query.on('result',function(row){
        callback(null, row);
     });
};

// Implementation
query("select * from test", function(err, result){
    console.log(err || result);
});

It's important to note that asyncronous functions don't return something explicitly (like return x ...), but they set output values through a callback function; in above example callback function sets err (to null) and output value (to row). There are some paradigms to implement asyncronous functions and this link shows some of these

npm and package.json

npm is official package manager for node.js.
It can be runs throw command line and manages dependencies for an application.
With npm we can install a node.js package hosted on a node.js repository.
In this post i explain a simple package.json file that i wrote to distribute a node.js module (https://github.com/sergioska/noodlejuice).

{

  "name": "noodlejuise",

  "description": "Mini toolkit for node.js",

  "author": "Sergio Sicari <sergiosicari[]gmail.com>",

  "dependencies": {

        "mysql": ">=2.0.0-alpha9",

        "socket.io": ">=0.9.16",

        "node-mysql": ">=0.3.4",

        "csv": ">=0.3.4"

  },

  "directories": {

          "lib": "./lib"

  },

  "repository": {

            "type": "git",

            "url": "git://github.com/sergioska/noodlejuice"

  },

  "engine": "node 0.11.7"

}

In dependencies section there are dependencies module required by own application; in this case mysql, socket.io, node-mysql, csv module.

In repository section there is a link of application repository.

In this way we can install this application only with this package.json file; in a directory that contains this file we run the following command:

npm install