Shantibhushan Naik

Tag: database

  • Spring IoC, DI quick tutorial

    There has been an (evident) craze amongst Java community with contrived terms such as IoC (Inversion of Control), DI (Dependency Injection) mainly through Spring. I initially found the connotation “Don’t call me, I’ll call you” a bit difficult (yeah, it’s bending your head upside down) to understand, but after spending few hours around Spring documentation, I get the gist.

    To a layman (Java layman, of course) it is helpful to picture that each Java program is a set of one or more classes, which act together. Essentially, this means classes are “dependent” on some classes in order to be fully functional. Usually, the class requiring functionality of another class instantiates the class, and uses it. This is called coupling because class instantiates the object of required class. What if we always got an “instantiated” instance of required class, and our class did not have to worry of instantiation? – This is called IoC (Inversion of Control) principle in Spring, and it achieves this by providing ready-to-use instance (injecting dependency) to your class. This has some important uses, since now you don’t worry of creating connections to databases, loggers to log4j, or sessions for JMS queues. Spring will create these for you, and your class can focus on the actual purpose – using the pre-instantiated ready-to-use object.

    Get it? OK, to simplify it further, let us assume you have a class A having one field – log. You want to use log to log information, but your class nowhere has the logic to instantiate or initialize log. You accept a pre-instantiated log through constructor, or getter/setter methods, and you will have a ready-to-use log instance passed to your class via Spring!

    I won’t go into details of Spring further, since Spring’s own documentation on http://static.springsource.org/spring/docs/2.5.x/reference/index.html is the definitive source to look into.

  • Whois purely in SSI

    It was fun learning SSI, and tweaking around to get useful information such as detecting your IP adddress, and further WHOIS information.

    See below in action

    http://www.naiksblog.info/ip2country.shtml

    Impressive how quickly a page could be constructed using SSI. Courtesy to Apache’s mod_include, IP to Country database, and APNIC references.

  • Racking brains – a Javascript string combination generator

    After racking my brains for almost 4 days (yeah I am a slow learner) I finally created a simple Javascript string combination generator

    See the demo http://naiksblog.info/stringcombinations.html

    I tried modeling the logic to how databases combine sets in a cross join. All rows from left side are combined with all rows from right side.

    The script is as below

    	function combine(a, b) {
		var r= new Array();
		for(var i= 0, k= 0; i < a.length; i++) {
			for(var j= 0; j < b.length; j++) {
				if(-1==a[i].indexOf(b[j])) r[k++]= a[i]+ b[j];
			}
		}

		return(r);
	}

    You can call this as below

    	function permute() {
		var a= "abcd";
		var p= new Array();

		for(var i= 0; i< a.length; i++) p[i]= a.charAt(i);
		var r= p;	// Input string as-is is first permutation
		for(var i= 1; i< a.length; i++) r= combine(r, p);	// Get the permutations
		// r.length - Get the combinations
		// r contains all combinations as an array
	}

    Some points worth noting

    • If any character is repeated, the combination does not happen successfully since the logic tries to remove same character matching elsewhere
    • The number of combinations increase by factorial of the number of characters, hence it will be a good idea to perform this on server side ideally otherwise javascript will hang for large strings
    • Logic could be optimized to generate combination in a different better way than using crossjoin strategy

  • SQL JOINS quick tutorial

    Table T1, Table T2

    INNER JOIN returns only matching rows from T1, T2

    LEFT OUTER JOIN returns (all T1 rows) X (matching T1 rows in T2)

    RIGHT OUTER JOIN returns (all T2 rows) X (matching T2 rows in T1)

    UNION returns only unique rows from T1, and T2.

    UNION ALL returns T1+T2 rows

  • Interesting alternatives to databases in opensource

    I stumbled upon this url. The list of various options on databases is pretty amazing.

    http://www.webresourcesdepot.com/25-alternative-open-source-databases-engines/

    It’s a definite read. I never knew so many existed.

  • Wikimedia Downloads

    Wikimedia Downloads

    The following kinds of downloads are available:

    Database backup dumps
    A complete copy of all Wikimedia wikis, in the form of wikitext source and metadata embedded in XML. A number of raw database tables in SQL form are also available.
    Static HTML dumps
    A copy of all pages from all Wikipedia wikis, in HTML form.
    DVD distributions
    Available for some Wikipedia editions.

    Wikimedia Downloads.

  • MySQL UTF-8

    A very informative page (in Japanese) about MySQL and issues to enable correct UTF-8 data handling with client applications.

    View original post

  • MySQL’s killer feature

    MySQL has a killer feature – you can have a timestamp field with DEFAULT as LOCALTIMESTAMP, so that when you insert a new record, the timestamp field will automatically have current timestamp inserted. But what about when the record is updated? Do you manually have to update the timestamp again? NO – MySQL allows you also specify another default on UPDATE so that whenever you update the record, it will automatically update the timestamp again.

    …
      `timestamp` timestamp NOT NULL default CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    …

    Now, isn’t that a neat feature!