Web Rambling

There are couple of things you got to keep in mind when you are using django to build a large scale applications, Django ORM is super cool and awesome but its not designed for large scale applications. Django ORM is written generic to suit different database and you have to careful in using ORM. It tend to fire multiple queries which can be ideally be done in one, when you indented to scaling a app the first and fore most you have to things and plan is the database. Its OK to have de-normalized design, very frequently tables should avoid multiple joins

If you are not sure about queries django is firing make sure you check connection.queries.

Django ORM executed 2 queries first query to select with the username and second to insert damn! sounds ridicules doesn't it? If you are using MySQL there is a better way to do INSERT IF NOT EXIST

connection.queries tells the last executed query, there are quite a few tools which would help in knowing the performance Cprofile, If you are building a really scalable website I would suggest you taking a look at David Carmer talk about high performance django application.

Hope you might have read my previous article Associate array are bad in Javascript, let me get in one step further beside associate array, what are the bad ways to loop a DOM, array, node list in JavaScript. Some of the Bad ways:

Length of a HTML collection is expensive

Remember every DOM operation that you make is expensive so think of ways you can minimize. When are trying to do DOM operation like looping on a HTML collection with length its live during your looping user might make some change which might change the length of the collection. So, the better way is to avoid the length.

There are rumors that apple is going to make some big announcement tomorrow(9th September) about media event and few new products. So, what are the products likely to get something new or a upgrade?

I am facing this strange memory leak problem when I open any of the google's heavy JavaScript application in firefox.

whenever i open gmail or google reader the memory usage of firefox goes up to 100%. But this doesn't happen either in safari or opera. Is firefox the culprit? Is anyone is facing this problem?

Ok Its not just me or the mac many of them are facing the same problem in almost all the operating systems.

Hurry!, I bought a new macbook white poly carbonate.

Node list is a collection of DOM Nodes. Most of them who code in Firebug tend to consider the node list as array because it looks like any other array. Node list has just two properties length and index. If you want to iterate on that you can use the old school way of incrementing the index or do any of the Enumerable functions by passing node list to call method. Let me show how to do that.

So what does call do? Call allows you to invoke a function as if it were a method of some other object. On above example foo is a node list its not a array but still I could able to splice that. ECMAScript specifies two methods call and apply. Both does relatively same job except the type of the argument. Apply takes a array as a argument.

I incline this post more towards google app engine or application that runs on python/Django. There are quite a few tools which will analyze your website and suggest the possible idea to improve the performance of your website. I use Yslow. The beauty about yslow is it grades your website and suggest the changes to improve it.

Implementation in python

Combine Files:

If your website has multiple JavaScript or CSS files you can combine them into one.

CSS Sprites

CSS Sprites is a good idea to combine all the images into one so that you can save the number of HTTP request back and forth to the server. You can either use GIMP or use sprites generator. Sprites generator takes up the images combine them into one and the beauty is it gives the position so that you can directly copy(Specially if you are a n00b in tools like GIMP which I am :p. So I choose to use csssprites).

Cache static files

Static file like CSS, JavaScript and Images can be cached on the browser. So that browser doesn't send request back every time when the page is hit. If you are using google app engine you can add a line in your app.yml like this "default_expiration: '365d'". If you are combining files this rules doesn't apply since you will be pointing to a script file. So can set the header like "self.response.headers['Cache-Control'] = 'public max-age=31536000'" this sets the cache expiry to one year from now. So If you are updating these static files you can suffix the files with some unique number ("blog_2.1.js"), usually its better to have version number. When make updations to your static files since browser doesn't request for these files your new code will have no effect. So its important to change the name of these files.

Minify

You can save some size by stripping of the space and newline in your CSS and Javascript. JSMin was written by Crockford it has a python implementation as well. Refer to my JSMin class at the first section of this post.

Associate array in javascript: An array is a linear allocation of memory in which elements are accessed by integers that are used to compute offsets. Arrays can be very fast data structures. Unfortunately, JavaScript does not have anything like this kind of array. Instead, JavaScript provides an object that has some array-like characteristics. It converts array subscripts into strings that are used to make properties.

You can call foo as anything you like (Boolean, or Date, or String) all will work. It works because all you are doing is setting properties on an object in JavaScript foo['bar'] and foo.bar is one and same. When you iterate on the foo all you do is iterating on the properties. Remember all data type in JavaScript are object. Use array as a object and don't store key/value pairs.

My law "When you don't know what you are doing sit back read and then code."

Most of the people start writing JavaScript without learning it( ridicules!). Like much of the language JavaScript also has good and bad parts. If you start doing something without learning it you will end up writing something non-standard or bad practices.

JavaScript despite having few bad parts its has some extra ordinary good parts which makes the language cool, beautiful, elegant, highly expressive language. So, to be a better programmer in JavaScript learn the good things, know the bad things and code the good things avoiding the bad part. Its very important to learn good things when you learn a language because its hard to unlearn the bad things.

Why JavaScript?

JavaScript is THE language of web browser, sun came up java applet gave a try to override JavaScript ended in a failure. Applet is just crap(You are a java lover you will find hard time reading my blog because I bash Java a lot :) and you thing java and JavaScript are one and the same I am gonna kick your ass get lost. Let me write about the name history behind JavaScript in a another post.). I call JavaScript as THE language because you don't have any other option. I see JavaScript is often blamed for browser hang probably because of you are doing extensive DOM operation. This is not the fault with JavaScript, DOM API very awful and JavaScript is unfairly blamed.

Good and Bad:

Good parts of JavaScript are loose typing, dynamic object, include functions and very bad part of JavaScript is using global variables. Some of the other good parts are lexical scoping, lambda(these is not unique, it resembles more of lisp). JavaScript is object oriented and class free. So inheritance happens between objects. Most of them who are from a strong typed languages feel bit hard to get used to it. People ask how would you identify a variable's type. I learned programming using Lisp, Python so I am much used to it and trust it is a good.

JavaScript offers two form of comments:

1. Block comments /* */
2. Line ending comments //

Comments are pretty important for readability but obsolete comments are worse than no comments. The /* */ form of block comments came from a language called PL/I. PL/I chose those strange pairs as the symbols for comments because they were unlikely to occur in that language's programs, except perhaps in string literals. In JavaScript, those pairs can also occur in regular expression literals, so block comments are not safe for commenting out blocks of code.

In the above case the block comment close on the match which was not the intention. So better way is to do line ending comment(//).